geologist e-notes

Earth from space: Giant iceberg enters Nares Strait

2010-10-01T10:28:00.000-07:00

ESA's Envisat satellite has been tracking the progression of the giant iceberg that calved from Greenland's Petermann glacier on 4 August 2010. This animation shows that the iceberg, the largest in the northern hemisphere, is now entering Nares Strait - a stretch of water that connects the Lincoln Sea and Arctic Ocean with Baffin Bay.

The Petermann glacier in northern Greenland is one of the largest of the country's glaciers - and until August it had a 70 km tongue of floating ice extending out into the sea. The glacier regularly advances towards the sea at about 1 km per year.

Earlier this year, satellite images revealed that several cracks had appeared. Envisat radar images showed that the ice tongue was still intact on 3 August but, on 4 August, a huge chunk had detached.

Calvings from the Petermann glacier are quite common, but one of this magnitude is rare. Less significant events took place in 2001, in 2008 when a 27 sq km iceberg made its way south to Davis Strait, and in 2009.

This iceberg is about 30 km long and 15 km wide at its foot and almost 7 km wide at its head, covering an area of around 245 sq km. By 22 August this giant mass of ice had been carried about 22 km from its birth place.

On 1 September imagery showed that the iceberg had travelled almost another 6 km from the edge of the glacier and rotated westward (about 39°), just tipping into Nares Strait. The animation also shows that the iceberg hit a small island, which may delay further progression for a short while and may also cause the iceberg to break.

It is expected that the iceberg will soon be fully in Nares Strait, but its course depends on winds blowing off the glacier and currents in the strait, as well as sea ice that could block its path.

The animation was generated from 21 Envisat Advanced Synthetic Aperture Radar (ASAR) Wide Swath Mode (spatial resolution 150 m × 150 m) and three ASAR Image Mode (spatial resolution 30 m × 30 m) images.

Note: This story has been adapted from a news release issued by the European Space Agency

A tectonic zip

2010-09-25T10:26:00.000-07:00

The complex fracture pattern created by the earthquake in Concepción (Chile) on 27 February 2010 was to a certain extent predictable. GPS observations from the years before the earthquake showed the pattern of stresses that had accumulated through the plate movements during the past 175 years in this area. The stress distribution derived from the observations correlates highly with the subsequent fracture distribution. In all likelihood the tremor removed all the stress that had accumulated since the last earthquake in this region, which was observed by Charles Darwin in 1835. An earthquake of similar magnitude in this area is therefore unlikely in the near future. This result was presented by scientists of the GFZ German Centre for Geosciences (Helmholtz Association) in the latest edition of the scientific journal Nature (09 September 2010).

"The Maule earthquake near Concepción, Chile, on the 27 Februar registered with a momentum magnitude of 8.8, makes it one of the largest earthquakes to have been recorded in its entirety via a modern network of space-geodetic and geophysical instruments on the ground," says Professor Onno Oncken, head of the Department "Geodynamics" at GFZ. "It thus offers a unique opportunity to compare detailed observations prior to the earthquake with those taken during and after it, and to re-evaluate hypotheses regarding the predictability of such events."

Measurements using the satellite navigation system GPS showed that the seafloor of the Nazca plate in the Pacific Ocean does not slide evenly under the western boundary of the South American continent. Rather, it appears from the GPS measurements that some parts of the ocean floor got locked with the subsurface of the continent. In the gaps, however, the Nazca plate continued to push under South America. The resulting uneven stress pattern was released by the earthquake of the 27 February in such a way that, just like a zipper, the locked patches were ruptured one after the other. As a result, this seismic gap off the Chilean westcoast is now closed, one last gap remains in northern Chile. Here, the GFZ scientists set up a plate boundary observatory, in order to make use of the entire range of geoscientific instruments to record the conditions before, during and after an earthquake- an important step in understanding the processes of plate tectonics.

Modern Earth science may still not be able to predict the location, time and magnitude of an earthquake. But the present study offers an optimistic perspective concerning the predictability of possible fracture patterns and magnitudes of expected earthquakes.

Note: This story has been adapted from a news release issued by the Helmholtz Association of German Research Centres

Glaciers help high-latitude mountains grow taller

2010-09-21T10:19:00.000-07:00

Glaciers can help actively growing mountains become higher by protecting them from erosion, according to a University of Arizona-led research team.

The finding is contrary to the conventional view of glaciers as powerful agents of erosion that carve deep fjords and move massive amounts of sediment down mountains. Mountains grow when movements of the Earth's crust push the rocks up.

The research is the first to show that the erosion effect of glaciers - what has been dubbed the "glacial buzzsaw" - reverses on mountains in colder climates.

The researchers were surprised, said first author Stuart N. Thomson, a research scientist in the UA department of geosciences. "We were expecting to see the buzzsaw."

The team discovered the protective effects of glaciers by studying the Andes Mountains in the southernmost region of South America, known as Patagonia.

UA co-author Peter W. Reiners said, "It's been thought that glaciers limit the height of mountain ranges worldwide."

The key is climate. Glaciers atop mountains in temperate latitudes flow downhill, scouring away the surface of the mountain. Over millennia, such erosion can reduce the height and width of a mountain range by miles.

However in very cold climates such as the Patagonian Andes, rather than scraping away the surface of the mountain, the team found that glaciers protect the mountain top and sides from erosion.

The team dubs the action of the cold-climate glaciers "glacial armoring."

"Climate, especially through glaciers, has a really big impact on how big mountains get," said Reiners, a UA professor of geosciences.

"What we're seeing is that below certain latitudes, glacial buzzsaws clearly and efficiently operate, but south of about 45 degrees, it not only doesn't work - it has the opposite effect," he said. "The glaciers actually protect the surface and allow the mountains to grow higher."

He and his colleagues anticipate that glacial armoring also occurs on cold-climate mountains very far north, such as those in Alaska.

A Grain of Sand Picture Gallery

2010-07-08T04:18:00.000-07:00

The tip of a spiral shell has broken off and become a grain of sand. It is opalescent from the repeated tumbling action of the surf. Surrounding the shell fragment are five other sand grains, from top middle clockwise, (1) a pink shell fragment, (2) a foram, (3) a microscopic shell, (4) a volcanic melt, and (5) a bit of coral. Image Copyright © 2008 Dr. Gary Greenberg, All Rights Reserved.

Sand from Skeleton Beach in Namibia contains rounded and polished pink-and-red garnet. Image Copyright © 2008 Dr. Gary Greenberg, All Rights Reserved.

Sand from Zushi Beach, Japan, contains what looks like a sapphire crystal. The crystal is larger than the surrounding grains and has survived eroding because of its hardness and quality. Image Copyright © 2008 Dr. Gary Greenberg, All Rights Reserved.

The glacially deposited sands around Lake Winnibigoshish, Minnesota, contain abundant sediments from the igneous and metamorphic minerals of the Lake Superior basin. A sample includes pink garnets, green epidote, iron-rich red agates, black magnetite, and hematite. Image Copyright © 2008 Dr. Gary Greenberg, All Rights Reserved.

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2010-06-28T18:55:00.000-07:00

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Six Things to Know About the Earth's Mantle

2010-06-28T18:11:00.000-07:00

The rocks and continents of the world belong to the Earth's crust. Beneath that crust is the Earth's mantle.

The mantle is the thick layer of hot, solid rock between the crust and the molten iron core. The mantle makes up the bulk of the Earth, accounting for two-thirds of its mass. The mantle starts about 30 kilometers down and is about 2900 kilometers thick.

Let's take a look at six different aspects of the mantle. Each of these six items links to an article with more detail.

1. Samples from the Mantle

Earth has the same recipe of elements as the Sun and the other planets (ignoring hydrogen and helium, which have escaped Earth's gravity). Subtracting the iron in the core, we can calculate that the mantle is a mix of magnesium, silicon, iron, and oxygen that roughly matches the composition of garnet.

Exactly what mix of minerals is present at a given depth is an intricate question that is not firmly settled. It helps that we have samples from the mantle, chunks of rock carried up in certain volcanic erup

tions, from as deep as about 300 kilometers. These show that the uppermost part of the mantle consists of the rock types peridotite and eclogite. But the most exciting thing we get from the mantle is diamonds.

2. Activity in the Mantle

The uppermost mantle is slowly stirred by the plate motions going on above it. The main activities are the downward motion of subducting plates and the upward motion of mantle rock at spreading centers. All this commotion does not mix the upper mantle thoroughly, however, and geochemists think of the upper mantle as a rocky version of marble cake.

The world's patterns of volcanism faithfully reflect plate tectonics, but not the centers of eruptive action called hotspots. Hotspots may be a clue to the rise and fall of material much deeper in the mantle, even from its very bottom. Or they may not. There is a vigorousscientific discussion about hotspots these days.

3. Exploring the Mantle with Earthquake Waves

Our most powerful tool for exploring the mantle is to monitor seismic waves from the world's earthquakes. The two different kinds of seismic wave, P waves (analogous to sound waves) and S waves (like the waves in a shaken rope), respond to the physical properties of the rocks they go through. Like light waves, they reflect off density boundaries and refract in rocks of different density. We use these effects to map the Earth's insides.

Our tools are good enough to treat the Earth's mantle the way doctors make ultrasound pictures of their patients. After a century of collecting earthquakes, we're able to makesome impressive maps of the mantle.

4. Modeling the Mantle in the Laboratory

With the human body, ultrasound images are just shadows unless we have hands-on knowledge of what is beneath the skin. The same is true of seismic mantle maps. Minerals and rocks change under high pressure. For instance, the common mantle mineral olivinechanges to different crystal forms at depths of around 410 and 660 kilometers.

We study the behavior of minerals under conditions in the mantle with two methods: calculations based on the equations of mineral physics and laboratory experiments. Thus mantle studies are a three-way conversation of seismologists, computer programmers and lab researchers who can now reproduce conditions anywhere in the mantle with high-pressure laboratory equipment like the diamond-anvil cell.

5. The Mantle's Layers and Internal Boundaries

Over the last century, seismic data has let us fill some of the blanks in the mantle. It has three main layers. The upper mantle extends from the base of the crust down to 660 kilometers depth. Many workers distinguish the transition zone between 410 and 660 kilometers, two depths at which major physical changes occur to minerals.

The lower mantle extends from 660 down to about 2700 kilometers, a point where seismic waves are affected so strongly that most researchers believe the rocks beneath are chemically different. This controversial layer at the bottom of the mantle, about 200 kilometers thick, has the odd name "D-double-prime." Read more of what we've learnedabout these layers and the crucial boundaries between them.

6. Why Earth's Mantle Is Special

Because the mantle is the bulk of the Earth, its story is fundamental to geology. The mantle began, during Earth's birth, as an ocean of magma atop the iron core. As it solidified, elements that didn't fit into the major minerals collected as a scum on top—the crust. After that the mantle began the slow circulation it has had for the last 4 billion years, with at least the upper part being cooled, stirred and hydrated by the tectonic motions of the surface plates.

At the same time, we have learned a great deal about the structure of Earth's sister planets Mercury, Venus and Mars. Compared to them, Earth has an active, lubricated mantle that is very special thanks to the same ingredient that distinguishes its surface: water.

The Earth Core

2010-06-15T17:56:00.000-07:00

A century ago, science barely knew that the Earth even has a core. Today we are tantalized by the core and its connections with the rest of the planet. Indeed, we're at the start of a golden age of core studies.

The Core's Gross Shape

We knew by the 1890s, from the Earth's response to the gravity of the Sun and Moon, that the planet has a dense core, probably iron. In 1906 Richard Dixon Oldham found that earthquake waves move through the Earth's center much slower than through the mantle around it—because the center is liquid.

In 1936 Inge Lehmann reported that something reflects seismic waves from within the core. It became clear that the core consists of a thick shell of liquid iron—the outer core—with a smaller, solid inner core at its center. It's solid because at that depth the high pressure overcomes the effect of high temperature.

In 2002 Miaki Ishii and Adam Dziewonski of Harvard University published evidence of an "innermost inner core" some 600 kilometers across. In 2008 Xiadong Song and Xinlei Sunproposed a different inner inner core about 1200 km across (here's a condensed version in scientific jargon). Not much can be made of these ideas until others confirm the work.

Whatever we learn raises new questions. The liquid iron must be the source of Earth's geomagnetic field—the geodynamo—but how does it work? Why does the geodynamo flip, switching magnetic north and south, over geologic time? What happens at the top of the core, where molten metal meets the rocky mantle? Answers began to emerge during the 1990s.

Studying the Core

Our main tool for core research has been earthquake waves, especially those from large events like the 2004 Sumatra quake. The ringing "normal modes," which make the planet pulsate with the sort of motions you see in a large soap bubble, are useful for examining large-scale deep structure.

But a big problem is nonuniqueness—any given piece of seismic evidence can be interpreted more than one way. A wave that penetrates the core also traverses the crust at least once and the mantle at least twice, so a feature in a seismogram may originate in several possible places. Many different pieces of data must be cross-checked.

The barrier of nonuniqueness faded some as we began to simulate the deep Earth in computers with realistic numbers, and as we reproduced high temperatures and pressures in the laboratory with the diamond-anvil cell. These tools have let us peer through the layers of the Earth until at last we can contemplate the core.

What the Core Is Made Of

Considering that the whole Earth on average consists of the same mixture of stuff we see elsewhere in the solar system, the core must be iron metal along with some nickel. But it's less dense than pure iron, so about 10 percent of the core must be something lighter. Sulfur and oxygen are the two best candidates, though hydrogen has also been considered. But the money is still on one of the other two, with oxygen looking slightly more probable, or maybe both.

Seismologists continue to probe the inner core. The core's eastern hemisphere appears to differ from the western hemisphere in the way the iron crystals are aligned. The problem is hard to attack because seismic waves have to go pretty much straight from an earthquake, right through the Earth's center, to a seismograph. Events and machines that happen to be lined up just right are rare. And the effects are subtle.

Core Dynamics

In 1996, Xiadong Song and Paul Richards confirmed a prediction that the inner core rotates slightly faster than the rest of the Earth. The magnetic forces of the geodynamo seem to be responsible.

Over geologic time, the inner core grows as the whole Earth cools. Iron crystals freeze out at the top of the outer core and rain onto the inner core. At the base of the outer core, the iron freezes under pressure taking much of the nickel with it. The remaining liquid iron is lighter and rises. These rising and falling motions, interacting with geomagnetic forces, stir the whole outer core at a speed of 20 kilometers a year or so.

The planet Mercury also has a large iron core and a magnetic field, though much weaker than Earth's. Recent research hints that Mercury's core is rich in sulfur and that a similar freezing process stirs it, with "iron snow" falling and sulfur-enriched liquid rising.

Core studies surged in 1996 when computer models by Gary Glatzmaier and Paul Roberts first reproduced the behavior of the geodynamo, including spontaneous reversals. Hollywood gave Glatzmaier an unexpected audience when it used his animations in the action movie The Core.

Recent high-pressure lab work by Raymond Jeanloz, Ho-Kwang (David) Mao and others has given us hints about the core-mantle boundary, where liquid iron interacts with silicate rock. The experiments show that core and mantle materials undergo strong chemical reactions. This is the region where many think mantle plumes originate, rising to form places like the Hawaiian Islands chain, Yellowstone, Iceland, and other surface features. The more we learn about the core, the closer it becomes.

The Pacific Ocean Tsunami Threat is Higher Than Expected

2010-05-17T16:44:00.000-07:00

Potential of Future Tsunamis

The potential for a huge Pacific Ocean tsunami on the West Coast of North America may be greater than previously thought.

The new study of geological evidence along the Gulf of Alaska coast suggests that future tsunamis could reach a scale far beyond that suffered in the tsunami generated by the great 1964 Alaskan earthquake. Official figures put the number of deaths caused by the earthquake at around 130: 114 in Alaska and 16 in Oregon and California. The tsunami killed 35 people directly and caused extensive damage in Alaska, British Columbia, and the US Pacific region*.

The 1964 Alaskan earthquake - the second biggest recorded in history with a magnitude of 9.2 - triggered a series of massive waves with run up heights of as much as 12.7 metres in the Alaskan Gulf region and 52 metres in the Shoup Bay submarine slide in Valdez Arm.

The study suggests that rupture of an even larger area than the 1964 rupture zone could create an even bigger tsunami. Warning systems are in place on the west coast of North America but the finding

Evaluation of Ancient Earthquakes

The research team from Durham University in the UK, the University of Utah and Plafker Geohazard Consultants, gauged the extent of earthquakes over the last 2,000 years by studying subsoil samples and sediment sequences at sites along the Alaskan coast. The team radiocarbon-dated peat layers and sediments, and analysed the distribution of mud, sand and peat within them. The results suggest that earthquakes in the region may rupture even larger segments of the coast and sea floor than was previously thought.

The study published in the academic journal Quaternary Science Reviews and funded by the National Science Foundation, NASA, and the US Geological Survey shows that the potential impact in terms of tsunami generation, could be significantly greater if both the 800-km-long 1964 segment and the 250-km-long adjacent Yakataga segment to the east were to rupture simultaneously.

Larger Ancient Earthquakes

Lead author, Professor Ian Shennan, from Durham University's Geography Department said: "Our radiocarbon-dated samples suggest that previous earthquakes were fifteen per cent bigger in terms of the area affected than the 1964 event. This historical evidence of widespread, simultaneous plate rupturing within the Alaskan region has significant implications for the tsunami potential of the Gulf of Alaska and the Pacific region as a whole."

"Peat layers provide a clear picture of what's happened to the Earth. Our data indicate that two major earthquakes have struck Alaska in the last 1,500 years and our findings show that a bigger earthquake and a more destructive tsunami than the 1964 event are possible in the future. The region has been hit by large single event earthquakes and tsunamis before, and our evidence indicates that multiple and more extensive ruptures can happen."

Causes of a Tsunami

Tsunamis can be created by the rapid displacement of water when the sea floor lifts and/or falls due to crustal movements that accompany very large earthquakes. The shallow nature of the sea floor off the coast of Alaska could increase the destructive potential of a tsunami wave in the Pacific.

Earthquake behaviour is difficult to predict in this region which is a transition zone between two of the world's most active plate boundary faults; the Fairweather fault, and the Aleutian subduction zone. In 1899 and 1979, large earthquakes occurred in the region but did not trigger a Tsunami because the rupturing was localized beneath the land instead of the sea floor.

Prof Ron Bruhn from the University of Utah said: "If the larger earthquake that is suggested by our work hits the region, the size of the potential tsunami could be signficantly larger than in 1964 because a multi-rupture quake would displace the shallow continental shelf of the Yakutat microplate."

"In the case of a multi-rupture event, the energy imparted to the tsunami will be larger but spread out over a longer strike distance. Except for the small communities at the tsunami source in Alaska, the longer length will have more of an effect on areas farther from the source such as southeastern Alaska, British Columbia, and the US west coast from Washington to California."

Tsunami Warning Systems

Warning systems have been in place on the US western seaboard and Hawaii since the 1946 Aleutian Islands tsunami. Improvements were made following the 2004 earthquake under the Indian Ocean that triggered the most deadly tsunami in recorded history, killing more than 230,000 people.

Prof Shennan said: "Earthquakes can hit at any time of the day or night, and that's a big challenge for emergency planners. A tsunami in this region could cause damage and threaten life from Alaska to California and beyond; in 1964 the effects of the tsunami waves were felt as far away as southern California and were recorded on tide gages throughout the Pacific Ocean."

Dr George Plafker from Plafker Geohazard Consultants said: "A large scale earthquake will not necessarily create a large wave. Tsunami height is a function of bathymetry, and the amount of slip and dip of the faults that take up the displacement, and all these factors can vary greatly along the strike."

"Tsunamis will occur in the future. There are issues in warning and evacuating large numbers of people in coastal communities quickly and safely. The US has excellent warning systems in place but awareness is vital."

Mega Tsunamis

2010-05-14T16:50:00.000-07:00

How Mega Tsunamis Are Formed?

An "Ultrasound" of the Nankai Trough

Research by a team of United States and Japanese geoscientists may help explain why part of the seafloor near the southwest coast of Japan is particularly good at generating devastating tsunamis, such as the 1944 Tonankai event, which killed at least 1,200 people. The findings will help scientists assess the risk of mega tsunamis in other regions of the world.

Geoscientists from The University of Texas at Austin and colleagues used a commercial ship to collect three-dimensional seismic data that reveals the structure of Earth’s crust below a region of the Pacific seafloor known as the Nankai Trough. The resulting images are akin to ultrasounds of the human body.

The results, published in the journal Science, address a long standing mystery as to why earthquakes below some parts of the seafloor form large tsunamis while earthquakes in other regions do not.

How Mega Tsunamis Are Formed

The 3D seismic images allowed the researchers to reconstruct how layers of rock and sediment have cracked and shifted over time. They found two things that contribute to mega tsunamis. First, they confirmed the existence of a major fault that runs from a region known to unleash earthquakes about 10 kilometers (6 miles) deep right up to the seafloor. When an earthquake happens, the fault allows it to reach up and move the seafloor up or down, carrying a column of water with it and setting up a series of tsunami waves that spread outward.

Second, and most surprising, the team discovered that the recent fault activity, probably including the slip that caused the 1944 event, has shifted to landward branches of the fault, becoming shallower and steeper than it was in the past.

“That leads to more direct displacement of the seafloor and a larger vertical component of seafloor displacement that is more effective in forming tsunamis,” said Nathan Bangs, senior research scientist at the Institute for Geophysics at The University of Texas at Austin who was co-principal investigator on the research project and co-author on the Science article.

The Nankai Trough

The Nankai Trough is a subduction zone, an area where tectonic plates converge with one sinking as it passes below the other. Where steady movement is somehow impeded, elastic rock strains build up to the point that rupture occurs with the release of seismic energy. Subduction zones are the sites of the world's largest earthquakes.

In 2002, a team of researchers led by Jin-Oh Park at Japan Marine Science and Technology Center (JAMSTEC) had identified the fault, known as a megathrust or megasplay fault, using less detailed two-dimensional geophysical methods. Based on its location, they suggested a possible link to the 1944 event, but they were unable to determine where faulting has been recently active.

Fault Slip & the 1944 Tonankai Earthquake

“What we can now say is that slip has very recently propagated up to or near to the seafloor, and slip along these thrusts most likely formed the large tsunami during the 1944 Tonankai 8.1 magnitude event,” said Bangs.

The images produced in this project will be used by scientists in the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), an international effort designed to, for the first time, “drill, sample and instrument the earthquake-causing, or seismogenic portion of Earth’s crust, where violent, large-scale earthquakes have occurred repeatedly throughout history.”

Determining Tsunami Potential

“The ultimate goal is to understand what’s happening at different margins,” said Bangs. “The 2004 Indonesian tsunami was a big surprise. It’s still not clear why that earthquake created such a large tsunami. By understanding places like Nankai, we’ll have more information and a better approach to looking at other places to determine whether they have potential. And we’ll be less surprised in the future.”

Bangs’ co-principal investigator was Gregory Moore at JAMSTEC in Yokohama and the University of Hawaii, Honolulu. The other co-authors are Emily Pangborn at the Institute for Geophysics at The University of Texas at Austin, Asahiko Taira and Shin'ichi Kuramoto at JAMSTEC and Harold Tobin at the University of Wisconsin, Madison. Funding for the project was provided by the National Science Foundation, Ocean Drilling Program and Japanese Ministry of Education, Culture, Sports and Technology.

The causes of tsunami

2010-05-05T20:17:00.000-07:00

What causes a tsunami?... A tsunami is a large ocean wave that is caused by sudden motion on the ocean floor. This sudden motion could be an earthquake, a powerful volcanic eruption, or an underwater landslide. The impact of a large meteorite could also cause a tsunami. Tsunamis travel across the open ocean at great speeds and build into large deadly waves in the shallow water of a shoreline.

Subduction Zones are Potential Tsunami Locations

Most tsunamis are caused by earthquakes generated in a subduction zone, an area where an oceanic plate is being forced down into the mantle by plate tectonic forces. The friction between the subducting plate and the overriding plate is enormous. This friction prevents a slow and steady rate of subduction and instead the two plates become "stuck".

Image by USGS

Accumulated Seismic Energy

As the stuck plate continues to descend into the mantle the motion causes a slow distortion of the overriding plage. The result is an accumulation of energy very similar to the energy stored in a compressed spring. Energy can accumulate in the overriding plate over a long period of time - decades or even centuries.

Image by USGS

Earthquake Causes Tsunami

Energy accumulates in the overriding plate until it exceeds the frictional forces between the two stuck plates. When this happens, the overriding plate snaps back into an unrestrained position. This sudden motion is the cause of the tsunami - because it gives an enormous shove to the overlying water. At the same time, inland areas of the overriding plate are suddenly lowered.

Image by USGS

Tsunami Races Away From the Epicenter

The moving wave begins travelling out from where the earthquake has occurred. Some of the water travels out and across the ocean basin, and, at the same time, water rushes landward to flood the recently lowered shoreline.

Image by USGS

Tsunamis Travel Rapidly Across Ocean Basis

Tsunamis travel swiftly across the open ocean. The map below shows how a tsunami produced by an earthquake along the coast of Chile in 1960 traveled across the Pacific Ocean, reaching Hawaii in about 15 hours and Japan in less than 24 hours.

Image by USGS

Tsunami "Wave Train"

Many people have the mistaken belief that tsunamis are single waves. They are not. Instead tsunamis are "wave trains" consisting of multiple waves. The chart below is a tidal gauge record from Onagawa, Japan beginning at the time of the 1960 Chile earthquake. Time is plotted along the horizontal axis and water level is plotted on the vertical axis. Note the normal rise and fall of the ocean surface, caused by tides, during the early part of this record. Then recorded are a few waves a little larger than normal followed by several much larger waves. In many tsunami events the shoreline is pounded by repeated large waves.

Image by USGS

The material above describes how tsunamis are generated and how they travel rapidly across an ocean basin. For more detailed information on this topic the following websites are recommended.

Sedimentary structures and bedding style (part II)

2010-04-08T17:33:00.000-07:00

Graded bedding. Graded bedding results from a rapid decrease in flow velocity that causes sediment to drop out of suspension. Larger particles settle fastest, therefore they accumulate at the bottom of the bed. Houcheng Formation, Jurassic, Hebei Province, China.

Graded beds, flame structures. Rapid sediment fallout from suspension often loads underlying fine-grained sediment to the point of failure, causing foundering of the overlying sediment and formation of structures termed flames (for obvious reasons!). Permian, Inyo County, California.

Flame Structure. The flame structures underlying this thick sandstone bed are evidence for rapid sand depostion; these sandstone beds are in fact sublacustrine turbidites. Cretaceous, Hebei Province, China.

Imbrication. Disk-shaped gravel clasts that are transported by rolling often "shingle" themselves to form a fabric termed imbrication. Individual clasts dip upflow. Current in this photo is from left to right (click to enlarge!). Owens Valley, California.

Graded and imbricated bed. OK, now I'm just getting redundant, but here you are anyways. Trail Canyon alluvial fan, Death Valley, California.

Flute casts. Flutes are caused by erosional eddies at the base of a turbulent flow. The deepest scour is on the upstream end of the flute, and the scours widen and become shallower downflow. Shiguai Formation, Inner Mongolia, China.

Gutter casts. Gutter casts are formed by entrained particles dragging along the base of a bed and leaving a track in their wake. These gutters were formed by wood fragments draggeing along the base of a river bed.Chengde Formation, Jurassic, Hebei province, China.

Dessication cracks. Mudcracks can become very large, which may make them difficult to recognize in small outcrops. Person for scale. Jixian Formation, Proterozoic, China.

Dessication cracks. A modern example. These are up to 1 m deep, and form polygonal networks that are often difficult to see except from the air! Panamint Valley, California.

Linguloid Ripples. These mud-draped ripples are linguloid in form, indicating shallow, rapid flow within the ripple stability field.Permian, Inner Mongolia, China.

Debris flow deposit. This type of non-erosive, unstructured and unsorted bed is typical of cohesive debris flow deposits. Coffin Canyon Fan, Death Valley, California.

Debris flow deposit. Oversized clasts "floating" in a mud-rich matrix are also typical of such flows. Coffin Canyon Fan, Death Valley, California.

Slump Deposit. Chaotic deposits such as these indicate slope failure. Cretaceous, Hebei province, China.

Sedimentary structures and bedding style (part I)

2010-04-05T17:17:00.000-07:00

another picture of sedimentary structure and bedding style:

Wedge bedding. Successive stack of erosive-based channels creates wedge-shaped bedding cross-sections. Shiguai Formation, Inner Mongolia, China.

Channelized bedding. Erosive scours that backfill with sand may give rise to isolated channelized sandbodies such as this. Mesa Verde Group, Utah.

Tabular cross-bedding. These steep foresets are typical of eolian deposition. Late Jurassic, Liaoning Province, China.

Trough cross-bedding. Trough cross bedding is produced by the downflow migration of lunate dunes in both subaqueous and subaerial environments. Triassic, Hebei Province, China.

Trough cross-bedding. This unusual view is of mud-draped gravel foresets cut by a joint that is oriented parallel to bedding. We are looking down at a cut-away view of an originally horizontal surface. The arcuate surfaces shown were mud drapes on the downstream sides of gravel lunate dunes. Hammer handle points downflow. Note also the wood fragment oriented parallel to flow (just to the left of the hammer handle), and a second set of mud-draped forsets that is cut off by the bottom edge of the photograph.

Wave Ripples. Cross-sectional view of ripple cross-laminated sandstone, showing bi-directional cross laminae indicative of a wave origin. Entrada Formation, Jurassic, San Rafael Swell, Utah.

Ripple Marks. A pile of eroded, rippled beds that all contain gorgeous ripple marks. If you click on nothing else, click to enlarge this one!Carmel Formation, Utah.

Herrigbone cross-stratification. Bi-directional cross beds such as these are indicative of a tidal origin. Curtis Formation, Jurassic, Utah.

Gilbert Delta foresets. These are composite, large-scale foresets that indicate depostion into still water. The height of the foresets indicates the still water depth. Mecca Formation, California.

Climbing current ripples. Ripple foresets that "climb" on the backs of their predecessors are indicative of waning flow conditions and rapid sediment fallout, such that sediment drops out of suspension as fast as it can be molded into a bedform. Entrada Formation, Jurassic, Utah.

Climbing current ripples and convolute lamination. The result of rapid sediment fallout is often instability due to liquefaction, leading to disruption of laminae by water escape. Such disruption is termed convolute lamination. Modern Colorado River, Utah.

Sedimentary Structures

2010-04-03T16:57:00.000-07:00

some kinds of sedimentary structures:

1. Deformational Structures

- Load cast: are rounded or bulbous protrusions formed during compaction. Where these grooves and depressions become filled with sediment, especially if the surface is mud, the marks may be preserved.

- Convolute Bedding: appears as highly contorted, folded and disrupted layers.

- flame structure: deformed clay or silt laminations with curved, pointed ends that project into the overlying bed and resemble a flame blown in the wind.

2. Current Flow Structures

- Parallel Lamination : (where the upper and lower contacts are approximately parallel) can result from

settling from suspension
deposition in a low energy environment
laminar flow (water flowing in a planar way)

- Ripple Marks : formed when the current flow energy increases.

- Cross Bedding : continuous erosion of the stoss side of the ripples and deposition on the leeward side causes

- Graded Bedding: As sediment is carried from a high energy environment into a low-energy environment, the heaviest particles will be the first to settle out, followed by progressively lighter and lighter particles.

Sedimentary Rocks

2010-04-02T17:45:00.000-07:00

Sedimentary rocks are formed by the accumulation of sediments. There are three basic types of sedimentary rocks: 1) clastic sedimentary rocks such as breccia, conglomerate, sandstone and shale, that are formed from mechanical weathering debris; 2) chemical sedimentary rocks such as rock salt and some limestones, that form when dissolved materials precipitate from solution; and, 3)organic sedimentary rocks such as coal and some limestones which form from the accumulation of plant or animal debris. Pictures and brief descriptions of some common sedimentary rock types are shown below.

Breccia

Breccia is a clastic sedimentary rock that is composed of large (over two millimeter diameter) angular fragments. The spaces between the large fragments can be filled with a matrix of smaller particles or a mineral cement which binds the rock together. The specimen shown above is about two inches (five centimeters) across.

Chert

Chert is a microcrystalline or cryptocrystalline sedimentary rock material composed of silicon dioxide (SiO₂). It occurs as nodules and concretionary masses and less frequently as a layered deposit. It breaks with a conchoidal fracture, often producing very sharp edges. Early people took advantage of how chert breaks and used it to fashion cutting tools and weapons. The specimen shown above is about two inches (five centimeters) across.

Coal

Coal is an organic sedimentary rock that forms mainly from plant debris. The plant debris usually accumulates in a swamp environment. Coal is combustible and is often mined for use as a fuel. The specimen shown above is about two inches (five centimeters) across.

Conglomerate

Conglomerate is a clastic sedimentary rock that contains large (greater then two millimeters in diameter) rounded particles. The space between the pebbles is generally filled with smaller particles and/or a chemical cement that binds the rock together. The specimen shown above is about two inches (five centimeters) across.

Iron Ore

Iron Ore is a chemical sedimentary rock that forms when iron and oxygen (and sometimes other substances) combine in solution and deposit as a sediment. Hematite (shown above) is the most common sedimentary iron ore mineral. The specimen shown above is about two inches (five centimeters) across.

Limestone

Limestone is a rock that is composed primarily of calcium carbonate. It can form organically from the accumulation of shell, coral, algal and fecal debris. It can also form chemically from the precipitation of calcium carbonate from lake or ocean water. Limestone is used in many ways. Some of the most common are: production of cement, crushed stone and acid neutralization. The specimen shown above is about two inches (five centimeters) across.

Rock Salt

Rock Salt is a chemical sedimentary rock that forms from the evaporation of ocean or saline lake waters. It is also known by the mineral name "halite". It is rarely found at Earth's surface, except in areas of very arid climate. It is often mined for use in the chemical industry or for use as a winter highway treatment. Some halite is processed for use as a seasoning for food. The specimen shown above is about two inches (five centimeters) across.

Sandstone

Sandstone is a clastic sedimentary rock made up mainly of sand-size (1/16 to 2 millimeter diameter) weathering debris. Environments where large amounts of sand can accumulate include beaches, deserts, flood plains and deltas. The specimen shown above is about two inches (five centimeters) across.

Shale

Shale is a clastic sedimentary rock that is made up of clay-size (less then 1/256 millimeter in diameter) weathering debris. It typically breaks into thin flat pieces. The specimen shown above is about two inches (five centimeters) across.

Siltstone

Sedimentary Rocks

Picture Gallery of the Most Common Rock Types

Sedimentary rocks are formed by the accumulation of sediments. There are three basic types of sedimentary rocks: 1) clastic sedimentary rocks such as breccia, conglomerate, sandstone and shale, that are formed from mechanical weathering debris; 2) chemical sedimentary rocks such as rock salt and some limestones, that form when dissolved materials precipitate from solution; and, 3)organic sedimentary rocks such as coal and some limestones which form from the accumulation of plant or animal debris. Pictures and brief descriptions of some common sedimentary rock types are shown below.

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Sedimentary Rock Types Menu
Breccia	Chert	Coal	Conglomerate	Iron Ore
Limestone	Rock Salt	Sandstone	Shale	Siltstone

Breccia

Chert

Coal

Conglomerate

Iron Ore

Limestone

Rock Salt

Sandstone

Shale

Siltstone

Siltstone is a clastic sedimentary rock that forms from silt-size (between 1/256 and 1/16 millimeter diameter) weathering debris. The specimen shown above is about two inches (five centimeters) across.

Metamorphic Rocks

2010-03-28T16:05:00.000-07:00

Metamorphic rocks have been modified by heat, pressure and chemical process usually while buried deep below Earth's surface. Exposure to these extreme conditions has altered the mineralogy, texture and chemical composition of the rocks. There are two basic types of metamorphic rocks: 1) foliated metamorphic rocks such as gneiss, phyllite, schist and slate which have a layered or banded appearance that is produced by exposure to heat and directed pressure; and, 2) non-foliated metamorphic rocks such as marble and quartzite which do not have a layered or banded appearance. Pictures and brief descriptions of some common types of metamorphic rocks are provided below.

there's the picture some metamorphic rock:

Amphibolite

Amphibolite is a non-foliated metamorphic rock that forms through recrystallization under conditions of high viscosity and directed pressure. It is composed primarily of amphibole and plagioclase, usually with very little quartz. The specimen shown above is about two inches (five centimeters) across.

Gneiss

Gneiss is foliated metamorphic rock that has a banded appearance and is made up of granular mineral grains. It typically contains abundant quartz or feldspar minerals. The specimen shown above is about two inches (five centimeters) across.

Hornfels

Hornfels is a fine-grained nonfoliated metamorphic rock with no specific composition. It is produced by contact metamorphism. Hornfels is a rock that was "baked" while near a heat source such as a magma chamber, sill or dike. The specimen shown above is about two inches (five centimeters) across.

Marble

Marble is a non-foliated metamorphic rock that is produced from the metamorphism of limestone. It is composed primarily of calcium carbonate. The specimen shown above is about two inches (five centimeters) across.

Phyllite

Phyllite is a foliate metamorphic rock that is made up mainly of very fine-grained mica. The surface of phyllite is typically lustrous and sometimes wrinkled. It is intermediate in grade between slate and schist. The specimen shown above is about two inches (five centimeters across).

Quartzite

Quartzite is a non-foliated metamorphic rock that is produced by the metamorphism of sandstone. It is composed primarily of quartz. The specimen above is about two inches (five centimeters) across.

Schist (chlorite)

Schist is metamorphic rock with well developed foliation. It often contains significant amounts of mica which allow the rock to split into thin pieces. It is a rock of intermediate metamorphic grade between phyllite and gneiss. The specimen shown above is a "chlorite schist" because it contains a significant amount of chlorite. It is about two inches (five centimeters) across.

Schist (garnet)

Schist is metamorphic rock with well developed foliation. It often contains significant amounts of mica which allow the rock to split into thin pieces. It is a rock of intermediate metamorphic grade between phyllite and gneiss. The specimen shown above is a "garnet schist" because it contains a significant amount of garnet. The small crystals visible in the rock are small red garnets. It is about two inches (five centimeters) across.

Slate

Slate is a foliated metamorphic rock that is formed through the metamorphism of shale. It is a low grade metamorphic rock that splits into thin pieces. The specimen shown above is about two inches (five centimeters) across.

Volcanic Hazards

2010-03-21T16:13:00.000-07:00

Volcanic Hazards

Volcanoes can be exciting and fascinating, but also very dangerous. Any kind of volcano is capable of creating harmful or deadly phenomena, whether during an eruption or a period of quiescence. Understanding what a volcano can do is the first step in mitigating volcanic hazards, but it is important to remember that even if scientists have studied a volcano for decades, they do not necessarily know everything it is capable of. Volcanoes are natural systems, and always have some element of unpredictability.

Volcanologists are always working to understand how volcanic hazards behave, and what can be done to avoid them. Here are a few of the more common hazards, and some of the ways that they are formed and behave. (Please note that this is intended as a source of basic information only, and should not be treated as a survival guide by those who live near a volcano. Always listen to the warnings and information issued by your local volcanologists and civil authorities.)

Lava Flows

Lava is molten rock that flows out of a volcano or volcanic vent. Depending on its composition and temperature, lava can be very fluid or very sticky (viscous). Fluid flows are hotter and move the fastest; they can form streams or rivers, or spread out across the landscape in lobes. Viscous flows are cooler and travel shorter distances, and can sometimes build up into lava domes or plugs; collapses of flow fronts or domes can form pyroclastic density currents (discussed later).

Most lava flows can be easily avoided by a person on foot, since they don’t move much faster than walking speed, but a lava flow usually cannot be stopped or diverted. Because lava flows are extremely hot - between 1,000-2,000°C (1,800 - 3,600° F) - they can cause severe burns and often burn down vegetation and structures. Lava flowing from a vent also creates enormous amounts of pressure, which can crush or bury whatever survives being burned.

Pyroclastic Density Currents

Pyroclastic density currents are an explosive eruptive phenomenon. They are mixtures of pulverized rock, ash, and hot gases, and can move at speeds of hundreds of miles per hour. These currents can be dilute, as in pyroclastic surges, or concentrated, as in pyroclastic flows. They are gravity-driven, which means that they flow down slopes.

A pyroclastic surge is a dilute, turbulent density current that usually forms when magma interacts explosively with water. Surges can travel over obstacles like valley walls, and leave thin deposits of ash and rock that drape over topography. A pyroclastic flow is a concentrated avalanche of material, often from a collapse of a lava dome or eruption column, which creates massive deposits that range in size from ash to boulders. Pyroclastic flows are more likely to follow valleys and other depressions, and their deposits infill this topography. Occasionally, however, the top part of a pyroclastic flow cloud (which is mostly ash) will detach from the flow and travel on its own as a surge.

Pyroclastic density currents of any kind are deadly. They can travel short distances or hundreds of miles from their source, and move at speeds of up to 1,000 kph (650 mph). They are extremely hot – up to 400°C (750°F). The speed and force of a pyroclastic density current, combined with its heat, mean that these volcanic phenomena usually destroy anything in their path, either by burning or crushing or both. Anything caught in a pyroclastic density current would be severely burned and pummeled by debris (including remnants of whatever the flow traveled over). There is no way to escape a pyroclastic density current other than not being there when it happens!

One unfortunate example of the destruction caused by pyroclastic density currents is the abandoned city of Plymouth on the Caribbean island of Montserrat. When the Soufrière Hills volcano began erupting violently in 1996, pyroclastic density currents from eruption clouds and lava dome collapses traveled down valleys in which many people had their homes, and inundated the city of Plymouth. That part of the island has since been declared a no-entry zone and evacuated, although it is still possible to see the remains of buildings which have been knocked over and buried, and objects that have been melted by the heat of the pyroclastic density currents.

Pyroclastic Falls

Pyroclastic falls, also known as volcanic fallout, occur when tephra - fragmented rock ranging in size from mm to tens of cm (fractions of inches to feet) - is ejected from a volcanic vent during an eruption and falls to the ground some distance away from the vent. Falls are usually associated with Plinian eruptive columns, ash clouds or volcanic plumes. Tephra in pyroclastic fall deposits may have been transported only a short distance from the vent (a few meters to several km), or, if it is injected into the upper atmosphere, may circle the globe. Any kind of pyroclastic fall deposit will mantle or drape itself over the landscape, and will decrease in both size and thickness the farther away it is from its source.

Tephra falls are usually not directly dangerous unless a person is close enough to an eruption to be struck by larger fragments. The effects of falls can be, however. Ash can smother vegetation, destroy moving parts in motors and engines (especially in aircraft), and scratch surfaces. Scoria and small bombs can break delicate objects, dent metals and become embedded in wood. Some pyroclastic falls contain toxic chemicals that can be absorbed into plants and local water supplies, which can be dangerous for both people and livestock. The main danger of pyroclastic falls is their weight: tephra of any size is made up of pulverized rock, and can be extremely heavy, especially if it gets wet. Most of the damage caused by falls occurs when wet ash and scoria on the roofs of buildings causes them to collapse.

Pyroclastic material injected into the atmosphere may have global as well as local consequences. When the volume of an eruption cloud is large enough, and the cloud is spread far enough by wind, pyroclastic material may actually block sunlight and cause temporary cooling of the Earth’s surface. Following the eruption of Mount Tambora in 1815, so much pyroclastic material reached and remained in the Earth’s atmosphere that global temperatures dropped an average of about 0.5 °C (~1.0 °F). This caused worldwide incidences of extreme weather, and led 1816 to be known as ‘The Year Without A Summer’.

Lahars

Lahars are a specific kind of mudflow made up of volcanic debris. They can form in a number of situations: when small slope collapses gather water on their way down a volcano, through rapid melting of snow and ice during an eruption, from heavy rainfall on loose volcanic debris, when a volcano erupts through a crater lake, or when a crater lake drains because of overflow or wall collapse.

Lahars flow like liquids, but because they contain suspended material, they usually have a consistency similar to wet concrete. They flow downhill and will follow depressions and valleys, but they can spread out if they reach a flat area. Lahars can travel at speeds of over 80 kph (50 mph) and reach distances dozens of miles from their source. If they were generated by a volcanic eruption, they may retain enough heat to still be 60-70°C (140-160°F) when they come to rest.

Lahars are not as fast or hot as other volcanic hazards, but they are extremely destructive. They will either bulldoze or bury anything in their path, sometimes in deposits dozens of feet thick. Whatever cannot get out of a lahar’s path will either be swept away or buried. Lahars can, however, be detected in advance by acoustic (sound) monitors, which gives people time to reach high ground; they can also sometimes be channeled away from buildings and people by concrete barriers, although it is impossible to stop them completely.

Gases

Volcanic gases are probably the least showy part of a volcanic eruption, but they can be one of an eruption’s most deadly effects. Most of the gas released in an eruption is water vapor (H₂O), and relatively harmless, but volcanoes also produce carbon dioxide (CO₂), sulfur dioxide (SO₂), hydrogen sulfide (H₂S), fluorine gas (F₂), hydrogen fluoride (HF), and other gases. All of these gases can be hazardous – even deadly – in the right conditions.

Carbon dioxide is not poisonous, but it displaces normal oxygen-bearing air, and is odorless and colorless. Because it is heavier than air, it collects in depressions and can suffocate people and animals who wander into pockets where it has displaced normal air. It can also become dissolved in water and collect in lake bottoms; in some situations, the water in those lakes can suddenly ‘erupt’ huge bubbles of carbon dioxide, killing vegetation, livestock and people living nearby. This was the case in the overturn of Lake Nyos in Cameroon, Africa in 1986, where an eruption of CO₂ from the lake suffocated more than 1,700 people and 3,500 livestock in nearby villages.

Sulfur dioxide and hydrogen sulfide are both sulfur-based gases, and unlike carbon dioxide, have a distinct acidic, rotten-egg smell. SO₂can combine with water vapor in the air to form sulfuric acid (H₂SO₄), a corrosive acid; H₂S is also very acidic, and extremely poisonous even in small amounts. Both acids irritate soft tissues (eyes, nose, throat, lungs, etc.), and when the gases form acids in large enough quantities, they mix with water vapor to form vog, or volcanic fog, which can be dangerous to breathe and cause damage to the lungs and eyes. If sulfur-based aerosols reach the upper atmosphere, they can block sunlight and interfere with ozone, which have both short and long-term effects on climate.

One of the nastiest, although less common gases released by volcanoes is fluorine gas (F₂). This gas is yellowish brown, corrosive and extremely poisonous. Like CO₂, it is denser than air and tends to collect in low areas. Its companion acid, hydrogen fluoride (HF), is highly corrosive and toxic, and causes terrible internal burns and attacks calcium in the skeletal system. Even after visible gas or acid has dissipated, fluorine can be absorbed into plants, and may be able to poison people and animals for long periods following an eruption. After the 1783 eruption of Laki in Iceland, fluorine poisoning and famine caused the deaths of more than half the country’s livestock and almost a quarter of its population.

The Sliding Rocks of Racetrack Playa

2010-03-16T16:42:00.000-07:00

One of the most interesting mysteries of Death Valley National Park is the sliding rocks at Racetrack Playa (a playa is a dry lake bed). These rocks can be found on the floor of the playa with long trails behind them. Somehow these rocks slide across the playa, cutting a furrow in the sediment as they move.

Some of these rocks weigh several hundred pounds. That makes the question: "How do they move?" a very challenging one.

The truth: No one knows for sure exactly how these rocks move - although a few people have come up with some pretty good explanations. The reason why their movement remains a mystery: No one has ever seen them in motion!

Let's learn how they are thought to move....

About Racetrack Playa

Racetrack playa is lake bed that is almost perfectly flat and almost always dry. It is about 4 kilometers long (2.5 miles - north to south) and about 2 kilometers wide (1.25 miles - east to west). The surface is covered with mudcracks and the sediment is made up mainly of silt and clay.

The climate in this area is arid. It rains just a couple of inches per year. However, when it rains, the steep mountains which surround Racetrack Playa produce a large amount of runoff that converts the playa floor into a broad shallow lake. When wet, the surface of the playa is transformed into a very soft and very slippery mud.

Are They Moved by People or Animals?

The shape of trails behind the rocks suggest that they move during times when the floor of Racetrack Playa is covered with a very soft mud. A lack of disturbed mud around the rock trails eliminates the possibility of a human or animal pushing or assisting the motion of the rocks.

Are They Moved by Wind?

This is the favorite explanation. The prevailing winds that blow across Racetrack Playa travel from southwest to northeast. Most of the rock trails are parallel to this direction. This is strong evidence that wind is the prime mover or at least involved with the motion of the rocks.

Strong wind gusts are thought to nudge the rocks into motion. Once the rock begins to move a wind of much lower velocity can keep the rock in motion as it slides across the soft and very slippery mud. Curves in the rock trails are explained by shifts in wind direction or in how the wind interacts with an irregularly shaped rock.

Are They Moved by Ice?

A few people have reported seeing Racetrack Playa covered by a thin layer of ice. One idea is that water freezes around the rocks and then wind, blowing across the top of the ice, drags the ice sheet with its embedded rocks across the surface of the playa.

Some researchers have found highly congruent trails on multiple rocks that strongly support this movement theory. However, the transport of a large ice sheet might be expected to mark the playa surface in other ways - these marks have not been found.

Other researchers experimented with stakes that would be disturbed by ice sheets. The rocks moved without disturbing the stakes. The evidence for ice-sheet transport is not consistent.

Wind is the Favored Mover!

All of the best explanations involve wind as the energy source behind the movement of the rocks. The question remains is do they slide while encased in an ice sheet or do they simply side over the surface of the mud? Perhaps each of these methods is responsible for some rock movement?

Perhaps this story will remain more interesting if the real answer is never discovered!

Photos of Sliding Rocks Below!

Movement of a large rock across a barren surface is almost impossible to believe. However, good photographs can serve as evidence for those who can not travel to Death Valley National Park. Thanks to Steve Geer, Stephan Hoerold, Skye Bajoul, sartriano, John Alcorn and Mike Nortan for the great images used here.

A sliding rock that has left a long track across the surface of Racetrack Playa. Some tracks are hundreds of feet long! (See below for several more sliding rock photos.) © iStockphoto / Steve Geer

Landsat image of Racetrack Playa. It is the flat white area in the center of the image.
(click for more detail)

Igneous Rock

2010-02-28T21:56:00.000-08:00

Andesite

Andesite is a fine-grained, extrusive igneous rock composed mainly of plagioclase with other minerals such as hornblende, pyroxene and biotite. The specimen shown is about two inches (five centimeters) across.

Basalt

Basalt is a fine-grained, dark-colored extrusive igneous rock composed mainly of plagioclase and pyroxene. The specimen shown is about two inches (five centimeters) across.

Gabro

Gabbro is a coarse-grained, dark colored, intrusive igneous rock that contains feldspar, augite and sometimes olivine. The specimen shown above is about two inches (five centimeters) across.

Granite

Granite is a coarse-grained, light colored, intrusive igneous rock that contains mainly quartz and feldspar minerals. The specimen above is about two inches (five centimeters) across.

Diorite

Diorite is a coarse-grained, intrusive igneous rock that contains a mixture of feldspar, pyroxene, hornblende and sometimes quartz. The specimen shown above is about two inches (five centimeters) across.

Obsidian

Obsidian is a dark-colored volcanic glass that forms from the very rapid cooling of molten rock material. It cools so rapidly that crystals do not form. The specimen shown above is about two inches (five centimeters) across.

Pegmatite

Pegmatite is a light-colored, extremely coarse-grained intrusive igneous rock. It forms near the margins of a magma chamber during the final phases of magma chamber crystallization. It often contains rare minerals that are not found in other parts of the magma chamber. The specimen shown above is about two inches (five centimeters) across.

Peridotite

Peridotite is a coarse-grained intrusive igneous rock that is composed almost entirely of olivine. It may contain small amounts of amphibole, feldspar, quartz or pyroxene. The specimen shown above is about two inches (five centimeters) across

Pumice

Pumice is a light-colored vesicular igneous rock. It forms through very rapid solidification of a melt. The vesicular texture is a result of gas trapped in the melt at the time of solidification. The specimen shown above is about two inches (five centimeters) across.

Ryolite

Rhyolite is a light-colored, fine-grained, extrusive igneous rock that typically contains quartz and feldspar minerals. The specimen shown above is about two inches (five centimeters) across.

Scoria

Scoria is a dark-colored, vesicular, extrusive igneous rock. The vesicles are a result of trapped gas within the melt at the time of solidification. It often forms as a frothy crust on the top of a lava flow or as material ejected from a volcanic vent and solidifying while airborne. The specimen shown above is about two inches (five centimeters) across

Tuff

Welded Tuff is a rock that is composed of materials that were ejected from a volcano, fell to Earth, and then lithified into a rock. It is usually composed mainly of volcanic ash and sometimes contains larger size particles such as cinders. The specimen shown above is about two inches (five centimeters) across.

Diamond = The Mineral

2010-02-26T16:26:00.000-08:00

The world's most popular gemstone. -- The hardest natural substance known.

Diamond is a fascinating mineral. It is chemically resistant and it is the hardest known natural substance. These properties make it suitable for use as a cutting tool and for other uses where durability is required. Diamond also has special optical properties such as a high index of refraction, high dispersion and high luster. These properties help make diamond the world's most popular gemstone.

What is Diamond?

Diamond is a rare, naturally occurring mineral composed of carbon. Each carbon atom in a diamond is surrounded by four other carbon atoms and connected to them by strong covalent bonds. This simple, uniform, tightly bonded arrangement yields one of the most durable substances known.

Where Do Diamonds Form?

Diamonds are not native to Earth's surface. Instead they form at high temperatures and pressures that occur in Earth's mantle about 100 miles down. Diamonds are brought to Earth's surface by volcanic eruptions. Instead of melting and being transported to the surface as a melt, the diamonds are carried to the surface in large pieces of mantle rock known as xenoliths. The diamonds are produced either by mining the rock which contains the xenoliths or by mining the soils and sediments that formed as the diamond-bearing rock weathered away.

Gem Diamonds vs. Industrial Diamonds

Gemstone diamonds are stones with color and clarity that make them suitable for jewelry or investment use. These stones are especially rare and make up a minor portion of worldwide diamond production. Gemstone diamonds are sold for their beauty and quality.

Industrial diamonds are mostly used in cutting, grinding, drilling and polishing procedures. Here, hardness and heat conductivity characteristics are the qualities being purchased. Size and other measures of quality relevant to gemstones are not important. Industrial diamonds are often crushed to produce micron-sized abrasive powders. Large amounts of diamonds that are gemstone quality but too small to cut are sold into the industrial diamond trade.

Diamond as a Gemstone

Diamonds are the world's most popular gemstones. More money is spent on diamonds than on all other gemstones combined. Part of the reason for diamond's popularity is a result of its optical properties - or how it reacts with light. Other factors include fashion, custom and marketing.

Diamonds have a very high luster. The high luster is a result of a diamond reflecting a high percentage of the light that strikes its surface. This high luster is what gives diamonds their pleasing "sparkle".

Diamond also has a high dispersion. As white light passes through a diamond this high dispersion causes that light to separate into its component colors. Dispersion is what enables a prism to separate white light into the colors of the spectrum. This property of dispersion is what gives diamonds their colorful "fire".

Diamond Gemstone Quality

The quality of a diamond gemstone is primarily determined by four factors: color, cut, clarity and carats.

Color: Most gem quality diamonds range from colorless to yellow. The most highly regarded stones are those that are completely colorless. These are the ones sold for the highest prices. However, another category of diamond gemstone is increasing in popularity. These are the "fancy" diamonds, which occur in a variety of colors including, red, pink, yellow, purple, blue and green. The value of these stones is based upon their color intensity, rarity and popularity.

Cut: The quality of workmanship in a diamond has a large impact upon its quality. This influences not only the geometric appearance of the stone but also the stone's luster and fire. Ideal stones are perfectly polished to be highly reflective and emit a maximum amount of fire. The faceted faces are equal in size and identical in shape. And, the edges of each faceted face meets perfectly with each of its neighbors.

Clarity: The ideal diamond is free from internal flaws and inclusions (particles of foreign material within the stone). These detract from the appearance of the stone and interfere with the passage of light through the stone. When present in large numbers or sizes they can also reduce the strength of the stone.

Carat: Diamonds are sold by the carat (a unit of weight equal to 1/5th of a gram or 1/142nd of an ounce). Small diamonds cost less per carat than larger stones of equal quality. This is because very small stones are very common and large stones are especially rare.

Diamonds Used as an Abrasive

Because diamonds are very hard they are often used as an abrasive. Most industrial diamonds are used for these purposes. Small particles of diamond are embedded in a saw blade, a drill bit or a grinding wheel for the purpose of cutting, drilling or grinding. They might also be ground into a powder and made into a diamond paste that is used for polishing or for very fine grinding.

There is a very large market for industrial diamonds. Demand for them exceeds the supply obtained through mining. Synthetic diamonds are being produced to meet this industrial demand. They can be produced at a low cost per carat and perform well in industrial use.

Other Uses of Diamonds

Most industrial diamonds are used as abrasives. However, small amounts of diamond are used in other applications.

Diamond windows

are made from thin diamond membranes and used to cover openings in lasers, x-ray machines and vacuum chambers. They are transparent, very durable and resistant to heat and abrasion.

Diamond speaker domes

enhance the performance of high quality speakers. Diamond is a very stiff material and when made into a thin dome it can vibrate rapidly without the deformation that would degrade sound quality.

Heat sinks

are materials that absorb or transmit excess heat. Diamond has the highest thermal conductivity of any material. It is used to conduct heat away from the heat sensitive-parts of high performance microelectronics.

Low friction microbearings

are needed in tiny mechanical devices. Just as some watches have jewel bearings in their movements diamonds are used where extreme abrasion resistance and durability are needed.

Wear-resistant parts

can be produced by coating surfaces with a thin coating of diamond. In this process, diamond is converted into a vapor that deposits on the surface of parts prone to wear.

Fault Behaviour

2010-01-14T20:48:00.000-08:00

Unusual Fault Behavior

Some geologic faults that appear strong and stable, slip and slide like weak faults. Now an international team of researchers has laboratory evidence showing why some faults that 'should not' slip are weaker than previously thought. Their findings are detailed in the December 17, 2009 edition of the journal Nature.

The Case of Low-Angle Normal Faults

"Low-angle normal faults--faults that dip less than 45 degrees--are a problem," said Chris Marone, professor of geosciences, Penn State. "Standard analysis shows that these faults should not slip because it is easier to form a new fault than to slip on this orientation."

However, field evidence shows that low-angle normal faults do slip. One explanation is that they act more like weak faults, slipping and sliding. Previous laboratory experiments indicated that the fabric of the rocks in the fault had too much friction for the faults to slide easily. The researchers wanted to test the material in the fault in a form closer to what occurs naturally.

Fault Friction - Laboratory Testing

"The standard way to test the friction of the rocks in a fault is to take some of the rock and grind it up into a powder," said Marone. "The powder is then tested in an apparatus that applies shear forces to the materials measuring the amount of force it would take to move sides of the fault."

These conventional measurements indicated that low-angle normal faults have too much friction to move, but the reality is that they do move.

The Zuccale Fault

Cristiano Collettini, researcher at Geologia Strutturale e Geofisica, Universita degli Studi di Perugia, Italy, got his way because of an unusual low-angle normal fault on the Isle of Elba. The Zuccale fault sits exposed on the beach so it is easy to gather large amounts of rock.

"Cristiano was insistent on checking the shear forces under conditions as close to the natural situation as possible," said Marone. "I thought what he wanted to do was impossible because the rocks cannot easily be cut into a shape that we can work with. We need a prismatic wafer of the material."

The researchers first ground the material and tested the mechanical properties of the powder in the conventional way. The ground powder did produce sufficient frictional forces to prevent slipping. This is what they expected.

"Normally the rock we use from fault zones comes from below the surface and we only get small amounts to work with," said Marone. "With the samples from Elba we could use a had rotary cutter and carve a wafer from the rock with the same orientation that would slip in the ground."

The wafers were about two by two inches square and an eighth of an inch thick. The researchers found that material prepared in this way was very weak when sheared in one direction moving almost like a deck of cards pushed in opposing directions.

The Role of Talc and Montmorillonite Clays

The reason for the low friction is small patches of talc and clays like montmorillonite that allow the material to slide.

The researchers, who also include Andre Niemeijer, former Penn State postdoctoral fellow now at Instituto Nazionale Di Geofisica Vulcanologia, Rome, and Cecilia Viti, Universita degli Studi di Siena, Italy, note, in the Dec. 17 issue of Nature that "fault weakness can occur in cases where weak mineral phases constitute only a small percentage of the total fault rock and that low friction results from slip on a network of weak phyllosilicate-rich surfaces that define the rock fabric."

Talc and many chemical varieties of clay minerals these clays are often found in fault materials. In areas where connected layers of weak minerals occur, geologists have concluded that the connections create weak fabric that enables easy slippage. But when clays or talc create intermittent flake-like surface coatings, they provide far more slip than when they are simply powdered. The discontinuous flakes and coatings that the researchers found were previously considered insufficiently complete to weaken the fault.

"These low-angle normal faults do not look like they will do anything but creep along, but they could have earthquakes," said Marone. "There are places in central Italy, for example, where faults like this have had small earthquakes."

Application to Other Faults

"These findings may be widely applicable to other fault settings, depending on the local stresses creating the faults," said James Dunlap, program manager in NSF's Geosciences Directorate. "In this way, this work is exciting and timely for those in the earthsciences community, as it may well prove advantageous to society, especially for those communities, on or near fault lines, most susceptible to earthquakes."

Carbonate Lava

2010-01-03T20:52:00.000-08:00

Carbon Dioxide Becomes Solid at Surface of Oldoinyo Lengai

East African Rift Volcanoes

Science has unearthed the secret to what might have been alchemy at Oldoinyo Lengai volcano in Tanzania.

There, in the ancient East African Rift at a place known to local Maasai people as the Mountain of God, Oldoinyo Lengai spews forth carbon dioxide-laden lavas called carbonatites. The carbonatites line the volcano's flanks like snowballs.

The World's Only Carbonatite Eruptions

Oldoinyo Lengai is the only place on Earth where carbonatites currently erupt--and where carbon dioxide from a volcano doesn't vanish into thin air as a gas.

In a paper published this week in the journal Nature, scientists report the results of a study of Oldoinyo Lengai's volcanic gas emissions, sampled by the team during a carbonatite lava eruption.

How Carbonatite Magma is Produced

"We now know the origin of one of the most peculiar magmas on Earth," said William Leeman, program director in the National Science Foundation (NSF)'s Division of Earth Sciences, which funded the research. "These scientists have found that, based on new studies of the chemistry of gas emissions at Oldoinyo Lengai, a very small amount of melting of Earth's mantle, akin to that beneath mid-ocean ridges, can produce carbonatites."

The carbonatites consist of high amounts of carbon dioxide, some 30 percent. Unlike most lavas that are liquid at temperatures above 900 degrees Celsius (1,652 F), carbonatites are much cooler and erupt at only 540 degrees Celsius (1,004 F). However, they're extremely fluid, with a viscosity like that of motor oil.

Sampling Volcanic Gases

"We were able to collect pristine samples of the volcanic gases because Oldoinyo Lengai was erupting and under tremendous magma pressure at the time," said Tobias Fischer, a volcanologist at the University of New Mexico and lead author of the paper. "There was minimal air contamination."

"The gases reveal that the carbon dioxide comes directly from the upper mantle, just below the East African Rift," said David Hilton, a geochemist at the Scripps Institution of Oceanography.

"These samples of mantle gases allow us to infer the carbon content of the upper mantle where the carbonatites are produced."

It's about 300 parts per million, a concentration virtually identical to that measured below mid-ocean ridges.

Nephelinite Magmas

The finding is significant, said geochemist Bernard Marty of the CNRS-CRPG (Centre National de la Recherche Scientifique-Centre de Recherches Pe'trographiques et Ge'ochimiques in France), "because it shows that these extremely bizarre lavas and their parent magmas, called nephelinites, were produced by melting of typical upper mantle minerals--which don't have a high carbon dioxide content."

Previous research, mainly based on laboratory experiments, suggested that a higher carbon dioxide content is needed to produce nephelinites and carbonatites.

High Sodium Magmas

"Oldoinyo Lengai magmas also contain an unusually high amount of sodium, up to about 35 percent," said Pete Burnard, a geochemist at CNRS-CRPG.

"It's this sodium content that makes the Lengai carbonatites solid rather than gas at the surface. At all other volcanoes on Earth, carbon dioxide 'degasses' into the atmosphere without forming the sodium-rich carbonatite magmas of Oldoinyo Lengai."

Not all Oldoinyo Lengai's carbon dioxide becomes carbonatite, however. Like other volcanoes, Oldoinyo Lengai does emit carbon dioxide into the atmosphere as a gas.

The scientists conclude that the upper mantle below the continents and the oceans is a uniform and well-mixed reservoir in which the compositions and abundances of carbon dioxide and other gases like nitrogen, argon and helium are essentially the same.

Hilton, Marty and Burnard are co-authors of the Nature paper.

The University of New Mexico Research Allocations Committee (RAC), Institut National des Sciences de l'Univers, Centre National de la Recherche Scientifque (INSU-CNRS), and Programme Intérieur de la Terre also funded the research.

Earth Core and Magnetic Field

2009-12-24T20:37:00.000-08:00

Earth: Our Dynamic Planet

"Terra firma." It's Latin for "solid Earth." Most of the time, at least from our perspective here on the ground, Earth seems to be just that: solid. Yet the Earth beneath our feet is actually in constant motion. It moves through time and space, of course, along with the other objects in the universe, but it moves internally as well. The powerful forces of wind, water and ice constantly erode its surface, redistributing Earth's mass in the process.

Within Earth's solid crust, faulting literally creates and then moves mountains. Hydrological changes, such as the pumping of groundwater for use by humans, cause the ground beneath us to undulate. Volcanic processes deform our planet and create new land. Landslides morph and scar the terrain. Entire continents can even rise up, rebounding from the weight of massive glaciers that blanketed the land thousands of years ago.

A Need for Indirect Measurements

Indeed, the outermost layers of the celestial blue onion that is Earth-its crust and upper mantle-aren't very solid at all. But what happens if we peel back the layers and examine what's going on deep within Earth, at its very core? Obviously, Earth's core is too deep for humans to observe directly. But scientists can use indirect methods to deduce what's going on down there.

A new study in the journal Geophysical Research Letters, by Jean Dickey of NASA's Jet Propulsion Laboratory, Pasadena, California and co-author Olivier deViron of the Institut de Physique du Globe de Paris, University Paris Diderot, Centre National de la Recherche Scientifique, Paris, has confirmed previous theoretical predictions that the churning cauldron of molten metals that make up Earth's liquid outer core is slowly being stirred by a very complex but predictable series of periodic oscillations. The findings give scientists unique insights into Earth's internal structure, the strength of the mechanisms responsible for generating Earth's magnetic field and its geology.

Understanding Earth's "Layers"

In order to better understand what's going on inside our planet, it helps to first get a lay of the land, so to speak.

Earth has several distinct layers, each with its own properties. At the outermost layer of our planet is the crust, which comprises the continents and ocean basins. Earth's crust varies in thickness from 35 to 70 kilometers (22 to 44 miles) in the continents and 5 to 10 kilometers (3 to 6 miles) in the ocean basins. The crust is mainly composed of alumino-silicates.

Next comes the mantle. The mantle is roughly solid, though very slow motion can be observed inside of it. It is about 2,900 kilometers (1,800 miles) thick, and is separated into an upper and lower mantle. It is here where most of Earth's internal heat is located. Large convective cells in the mantle circulate heat and drive the movements of Earth's tectonic plates, upon which our continents ride. The mantle is mainly composed of ferro-magnesium silicates.

Earth's innermost layer is the core, which is separated into a liquid outer core and a solid inner core. The outer core is 2,300 kilometers (1,429 miles) thick, while the inner core is 1,200 kilometers (746 miles) thick. The outer core is mainly composed of a nickel-iron alloy (liquid iron), while the inner core is almost entirely composed of a pure solid iron body.

A Magnetic Field from a Churning Core

Scientists believe Earth's magnetic field results from movements of molten iron and nickel within its liquid outer core. These flows, which are caused by interactions between Earth's core and its mantle, are neither even, nor evenly distributed. The electrical currents generated by these flows result in a magnetic field, which is similarly uneven, moves around in location and varies in strength over time. Earth's magnetic field is also slightly tilted with respect to Earth's axis. This causes Earth's geographic north and south poles to not line up with its magnetic north and south poles--they currently differ by about 11 degrees.

Magnetic Reversals

In just the last 200 million years alone, Earth's magnetic poles have actually reversed hundreds of times, with the most recent reversal taking place about 790,000 years ago. Scientists are able to reconstruct the chronology of these magnetic pole reversals by studying data on the spreading of the seafloor at Earth's mid-oceanic ridges. Unlike the doomsday scenario popularized by Hollywood in the movie "2012," however, such reversals don't occur over days, but rather on geologic timescales spanning hundreds to thousands of years-very short in geologic time but comparatively long in human time. The time span between pole reversals is even longer, ranging from 100,000 to several million years.

Earth's Magnetic "Shield"

Earth's magnetic field is essential for life on Earth. Extending thousands of kilometers into space, it serves as a shield, deflecting the constant bombardment of charged particles and radiation known as the solar wind away from Earth. These solar winds would otherwise be fatal to life on Earth. At Earth's poles, the perpendicular angle of the magnetic field to Earth there allows some of these particles to make it into our atmosphere. This results in the Northern Lights in the northern hemisphere and the Southern Lights in the southern hemisphere.

Exploiting the Magnetic Field

Here on the ground, Earth's magnetic field has many practical applications to our everyday lives. It allows people to successfully navigate on land and at sea, making it a critical tool for commerce. Hikers use it to find their way. Archaeologists use it to deduce the age of ancient artifacts such as pottery, which, when fired, assumes the magnetic field properties that were present at the time of its creation. Similarly, the field of paleomagnetism uses magnetism to give scientists glimpses into Earth's remote past. In addition, geophysicists and geologists use geomagnetism as a tool to investigate Earth's structure and changes taking place in the Earth.

Indirectly Measuring Earth's Magnetic Field

Since Earth's liquid core is the primary source of Earth's magnetic field, scientists can use observations of the magnetic field at Earth's surface and its variability over time to mathematically calculate and isolate the approximate motions taking place within the core.

That's what Dickey and deViron did. They combined measurements of Earth's magnetic field taken by observatory stations on land and ships at sea dating back to 1840 with those of the Danish Oersted and German CHAMP geomagnetic satellite missions, both of which were supported by NASA investments. These measurements were then used as inputs for a complex model that employs statistical time series analyses to determine how fast liquid iron is flowing within Earth's core.

"Although we do not observe the core directly, it's amazing how much we can learn about Earth's interior using magnetic field observations," said Dickey.

Visualizing the Motion in Earth's Core

In order to approximate the flow of liquid in the core, the scientists visualized its motion as a set of 20 rigid cylinders, each rotating about a common point that represents Earth's axis. "Imagine that each cylinder is slowly rotating at a different speed, and you'll get a sense of the complex churning that's taking place within Earth's core," Dickey said.

The scientists analyzed the data to identify common patterns of movement among the different cylinders. These patterns represent how momentum and energy are transferred from the liquid core-mantle interface inward through the liquid core toward the inner core with diminishing amplitudes.

Oscillations in the Core

Their analyses isolated six slow-moving oscillations, or waves of motion, occurring within the liquid core. The oscillations originated at the boundary between Earth's core and its mantle and traveled inward toward the inner core with decreasing strength. Four of these oscillations were robust, occurring at periods of 85, 50, 35 and 28 years. Since the scientist's data set goes back to 1840, the recurrence period of the longest oscillation (85 years) is less well determined than the other oscillations. The last two oscillations identified were weaker and will require further study.

The 85- and 50-year oscillations are consistent with a 1997 study by researchers Stephen Zatman and Jeremy Bloxham of Harvard University, Cambridge, Mass., who used a different analysis technique. A later purely theoretical study by Harvard researcher Jon Mound and Bruce Buffett of the University of Chicago in 2006 showed that there should be several oscillations of this type; their predicted periods agree with the first four modes identified in Dickey and deViron's study.

"Our satellite-based results are in excellent agreement with the previous theoretical and other studies in this field, providing a strong confirmation of the existence of these oscillations," said Dickey. "These results will give scientists confidence in using satellite measurements in the future to deduce long-term changes taking place deep within our restless planet."

African Rift

2009-12-19T20:36:00.000-08:00

Part I. The East African Rift System

The East African Rift System (EARS) is one the geologic wonders of the world, a place where the earth's tectonic forces are presently trying to create new plates by splitting apart old ones. In simple terms, a rift can be thought of as a fracture in the earth's surface that widens over time, or more technically, as an elongate basin bounded by opposed steeply dipping normal faults. Geologists are still debating exactly how rifting comes about, but the process is so well displayed in East Africa (Ethiopia-Kenya-Uganda-Tanzania) that geologists have attached a name to the new plate-to-be; the Nubian Plate makes up most of Africa, while the smaller plate that is pulling away has been named the Somalian Plate (Figure 1). These two plates are moving away form each other and also away from the Arabian plate to the north. The point where these three plates meet in the Afar region of Ethiopia forms what is called a triple-junction. However, all the rifting in East Africa is not confined to the Horn of Africa; there is a lot of rifting activity further south as well, extending into Kenya and Tanzania and Great Lakes region of Africa. The purpose of this paper is to discuss the general geology of these rifts are and highlight the geologic processes involved in their formation.

What is the East Africa Rift System?

The oldest and best defined rift occurs in the Afar region of Ethiopia and this rift is usually referred to as the Ethiopian Rift. Further to the South a series of rifts occur which include a Western branch, the "Lake Albert Rift" or "Albertine Rift" which contains the East African Great Lakes, and an Eastern branch that roughly bisects Kenya north-to-south on a line slightly west of Nairobi (Figure 2). These two branches together have been termed the East African Rift (EAR), while parts of the Eastern branch have been variously termed the Kenya Rift or the Gregory Rift (after the geologist who first mapped it in the early 1900's). The two EAR branches are often grouped with the Ethiopian Rift to form the East Africa Rift System (EARS). The complete rift system therefore extends 1000's of kilometers in Africa alone and several 1000 more if we include the Red Sea and Gulf of Aden as extensions. In addition there are several well-defined but definitely smaller structures, called grabens, that have rift-like character and are clearly associated geologically with the major rifts. Some of these have been given names reflecting this such as the Nyanza Rift in Western Kenya near Lake Victoria. Thus, what people might assume to be a single rift somewhere in East Africa is really a series of distinct rift basins which are all related and produce the distinctive geology and topography of East Africa.

How did these Rifts form?

The exact mechanism of rift formation is an on-going debate among geologists and geophysicists. One popular model for the EARS assumes that elevated heat flow from the mantle (strictly the asthenosphere) is causing a pair of thermal "bulges" in central Kenya and the Afar region of north-central Ethiopia. These bulges can be easily seen as elevated highlands on any topographic map of the area (Figure 1). As these bulges form, they stretch and fracture the outer brittle crust into a series of normal faults forming the classic horst and graben structure of rift valleys (Figure 3). Most current geological thinking holds that bulges are initiated by mantle plumes under the continent heating the overlying crust and causing it to expand and fracture. Ideally the dominant fractures created occur in a pattern consisting of three fractures or fracture zones radiating from a point with an angular separation of 120 degrees. The point from which the three branches radiate is called a "triple junction" and is well illustrated in the Afar region of Ethiopia (Figure 4), where two branches are occupied by the Red Sea and Gulf of Aden, and the third rift branch runs to the south through Ethiopia.

The stretching process associated with rift formation is often preceded by huge volcanic eruptions which flow over large areas and are usually preserved/exposed on the flanks of the rift. These eruptions are considered by some geologists to be "flood basalts" - the lava is erupted along fractures (rather than at individual volcanoes) and runs over the land in sheets like water during a flood. Such eruptions can cover massive areas of land and develop enormous thicknesses (the Deccan Traps of India and the Siberian Traps are examples). If the stretching of the crust continues, it forms a "stretched zone" of thinned crust consisting of a mix of basaltic and continental rocks which eventually drops below sea level, as has happened in the Red Sea and Gulf of Aden. Further stretching leads to the formation of oceanic crust and the birth of a new ocean basin.

Part II. The East African Rift

If the rifting process described occurs in a continental setting, then we have a situation similar to what is now occurring in Kenya where the East African/Gregory Rift is forming. In this case it is referred to as "continental rifting" (for obvious reasons) and provides a glimpse into what may have been the early development of the Ethiopian Rift.

As mentioned in Part I, the rifting of East Africa is complicated by the fact that two branches have developed, one to the west which hosts the African Great Lakes (where the rift filled with water) and another nearly parallel rift about 600 kilometers to the east which nearly bisects Kenya north-to-south before entering Tanzania where it seems to die out (Figure 2). Lake Victoria sits between these two branches. It is thought that these rifts are generally following old sutures between ancient continental masses that collided billions of years ago to form the African craton and that the split around the Lake Victoria region occurred due to the presence of a small core of ancient metamorphic rock, the Tanzania craton, that was too hard for the rift to tear through. Because the rift could not go straight through this area, it instead diverged around it leading to the two branches that can be seen today.

As is the case in Ethiopia, a hot spot seems to be situated under central Kenya, as evidenced by the elevated topographic dome there (Figure 1). This is almost exactly analogous to the rift Ethiopia, and in fact, some geologists have suggested that the Kenya dome is the same hotspot or plume that gave rise to the initial Ethiopian rifting. Whatever the cause, it is clear that we have two rifts that are separated enough to justify giving them different names, but near enough to suggest that they are genetically related.

Other Points of Interest:

What else can we say about the Ethiopian and Kenya Rifts? Quite a lot actually; even though the Eastern and Western branches were developed by the same processes they have very different characters. The Eastern Branch is characterized by greater volcanic activity while the Western Branch is characterized by much deeper basins that contain large lakes and lots of sediment (including Lakes Tanganyika, the 2nd deepest lake in the world, and Malawi).

Recently, basalt eruptions and active crevice formation have been observed in the Ethiopian Rift which permits us to directly observe the initial formation of ocean basins on land. This is one of the reasons why the East African Rift System is so interesting to scientists. Most rifts in other parts of the world have progressed to the point that they are now either under water or have been filled in with sediments and are thus hard to study directly. The East African Rift System however, is an excellent field laboratory to study a modern, actively developing rift system.

This region is also important for understanding the roots of human evolution. Many hominid fossil finds occur within the rift, and it is currently thought that the rift's evolution may have played an integral role in shaping our development. The structure and evolution of the rift may have made East Africa more sensitive to climate changes which lead to many alternations between wet and arid periods. This environmental pressure could have been the drive needed for our ancestors to become bipedal and more brainy as they attempted to adapt to these shifting climates (see Geotimes 2008 articles: Rocking the Cradle of Humanity by Beth Christensen and Mark Maslin, and Tectonic Hypotheses of Human Evolution by M. Royhan Gani and Nahid DS Gani).

Conclusions:

The East African Rift System is a complicated system of rift segments which provide a modern analog to help us understand how continents break apart. It is also a great example of how many natural systems can be intertwined - this unique geological setting may have altered the local climate which may have in turn caused our ancestors to develop the skills necessary to walk upright, develop culture and ponder how such a rift came to be. Just like the Grand Canyon, the East African Rift System should be high on any geologist's list of geologic marvels to visit.

Question about "The Cracks" south of Holbrook

2009-12-15T10:49:00.000-08:00

One of my blog posts from a number of weeks ago prompted this inquiry:
"About 15 miles south of Holbrook, AZ there are extremely deep fissures (referred to locally as "the Cracks") They range from just a few inches wide to several feet wide, but some seem to go down for a hundred feet. When you drop a rock in the crack you can hear it bouncing down the hole for an extremely long distance. Are these formed from seizmic [sic] activity?"

Actually, this area is pretty quiet seismically. But it is underlain by extensive salt and other evaporate minerals. As these deposits were dissolved over time, the overlying rocks dropped down, creating a broad anticlinal fold. This resulted in extension cracks over the top of the folded layers which have eroded into The Sinks and other similar features in the region.

We have a publication that describes the fractures and related topics with striking photos including the one above.

World's Biggest Tsunami

2009-11-27T20:23:00.000-08:00

The largest recorded tsunami was a wave 1720 feet tall in Lituya Bay, Alaska

On the night of July 9, 1958 an earthquake along the Fairweather Fault in the Alaska Panhandle loosened about 40 million cubic yards (30.6 million cubic meters) of rock high above the northeastern shore of Lituya Bay. This mass of rock plunged from an altitude of approximately 3000 feet (914 meters) down into the waters of Gilbert Inlet (see map below). The impact generated a local tsunami that crashed against the southwest shoreline of Gilbert Inlet. The wave hit with such power that it swept completely over the spur of land that separates Gilbert Inlet from the main body of Lituya Bay. The wave then contiuned down the entire length of Lituya Bay, over La Chaussee Spit and into the Gulf of Alaska. The force of the wave removed all trees and vegetation from elevations as high as 1720 feet (524 meters) above sea level. Millions of trees were uprooted and swept away by the wave. This is the highest wave that has ever been known.

Detail Map: Lituya Bay, Alaska

Lituya Bay is an ice-scoured tidal inlet on the northeast shore of the Gulf of Alaska. It is about seven miles long (11.3 kilometers) and up to two miles wide (3.2 kilometers). It has a maximum depth of about 720 feet (219 meters) but a sill of only 32 feet (9.7 meters) in depth separates it from the Gulf of Alaska between La Chaussee Spit and Harbor Point.

The Fairweather Fault trends across the northeast end of the Bay and is responsible for the T-shape of the bay. Glacial scour has exploited the weak zone along the fault to produce a long linear trough known as the Fairweather Trench. The Lituya Glacier and North Crillon Glacier have scoured portions of the Fairweather Trench in the area of Lituya Bay. Gilbert Inlet and Crillon Inlet occupy the Fairweather Trench on the northeast end of Lituya Bay.

The rock fall of July 9, 1958 occurred on steep cliffs above the northeast shore of Gilbert Inlet. It is marked on the map above in red. The rocks fell from an elevation of about 3000 feet (914 meters). The impact of 40 million cubic yards (30.6 million cubic meters) of rock hitting the water produced a local tsunami that swept the entire length of the Lituya Bay and over the La Chaussee Spit. This wave stripped all vegetation and soil from along the edges of the bay. This damaged area is shown in yellow on the map above. The numbers are elevations (in feet) of the upper edge of the wave damage area and represent the approximate elevation of the wave as it traveled through the bay. Map redrawn from data included in United States Geological Survey Professional Paper 354-C.

geologist e-notes

Earth from space: Giant iceberg enters Nares Strait

A tectonic zip

Glaciers help high-latitude mountains grow taller

A Grain of Sand Picture Gallery

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Six Things to Know About the Earth's Mantle

1. Samples from the Mantle

2. Activity in the Mantle

3. Exploring the Mantle with Earthquake Waves

4. Modeling the Mantle in the Laboratory

5. The Mantle's Layers and Internal Boundaries

6. Why Earth's Mantle Is Special

The Earth Core

The Pacific Ocean Tsunami Threat is Higher Than Expected

Potential of Future Tsunamis

Evaluation of Ancient Earthquakes

Causes of a Tsunami

Tsunami Warning Systems

Mega Tsunamis

How Mega Tsunamis Are Formed

The Nankai Trough

Fault Slip & the 1944 Tonankai Earthquake

Determining Tsunami Potential

The causes of tsunami

Subduction Zones are Potential Tsunami Locations

Accumulated Seismic Energy

Earthquake Causes Tsunami

Tsunami Races Away From the Epicenter

Tsunamis Travel Rapidly Across Ocean Basis

Tsunami "Wave Train"

Sedimentary structures and bedding style (part II)

Sedimentary structures and bedding style (part I)

Sedimentary Structures

Sedimentary Rocks

Sedimentary Rocks

Picture Gallery of the Most Common Rock Types

Sedimentary Rock Types Menu

Metamorphic Rocks

Volcanic Hazards

Volcanic Hazards

Lava Flows

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Pyroclastic Falls

Lahars

Gases

The Sliding Rocks of Racetrack Playa

About Racetrack Playa

Are They Moved by People or Animals?

Are They Moved by Wind?

Are They Moved by Ice?

Wind is the Favored Mover!

Photos of Sliding Rocks Below!

Igneous Rock

Diamond = The Mineral

What is Diamond?

Where Do Diamonds Form?

Gem Diamonds vs. Industrial Diamonds

Diamond as a Gemstone

Diamond Gemstone Quality

Diamonds Used as an Abrasive

Other Uses of Diamonds

Diamond windows

Diamond speaker domes

Heat sinks

Low friction microbearings

Wear-resistant parts

Fault Behaviour

Carbonate Lava

Earth Core and Magnetic Field

Earth: Our Dynamic Planet

A Need for Indirect Measurements

Understanding Earth's "Layers"

A Magnetic Field from a Churning Core

Magnetic Reversals

Earth's Magnetic "Shield"

Exploiting the Magnetic Field

Indirectly Measuring Earth's Magnetic Field

Visualizing the Motion in Earth's Core

Oscillations in the Core