Stories tagged Earth Structure and Processes


Vivian Leung with tree roots: Vivian Leung in the field researching woody debris
Vivian Leung with tree roots: Vivian Leung in the field researching woody debrisCourtesy Vivian Leung
Every once in a while, I come across an area of scientific inquiry and I wonder what it's all about. I mean, why do scientists study what they study? For instance, why would one study the dead, upended tree roots that have become lodged in stream beds and stream banks?

Well, Vivian Leung, a Ph.D. student at the University of Washington, is doing just that, and she is so clearly fascinated by her work that I am fascinated with her.

Vivian studies woody debris (think dead logs sticking out into a stream) and how the roots interact with water flow and the movement of sediment to affect stream bed shape. The shape of a stream’s bed influences the presence of important fish habitat and the mobility of potentially dangerous hazards (like logs that can become dislodged and move downstream).

Whether or not roots are present on woody debris seems to be especially influential in determining stream bed shape. “Field studies have shown that having roots makes a big difference, I wanted to know which part of the roots is important” said Vivian.

So, down three flights of stairs at the University of Minnesota’s Saint Anthony Falls Laboratory (SAFL), steps from where the Mississippi River laps outside the door, lies an experimental flume that Vivian traveled across the country to work on. Vivian is using the flume to research how the presence, size, and permeability of roots influence the distribution of sediment on a stream bed. As I mentioned above, sediment distribution on the stream bed helps determine whether mobile elements stay put or dislodge and move down stream. It also helps determine whether there is suitable habitat for fish in the stream, an issue especially important to fishermen.

Vivian Leung's root research: Image of Vivian Leung's experiment about how tree roots influence the shape of a stream bed.
Vivian Leung's root research: Image of Vivian Leung's experiment about how tree roots influence the shape of a stream bed.Courtesy Vivian Leung
In a previous experiment, Vivian found that increased density of roots could potentially increase wood stability and decrease the likelihood that the woody debris would break free and move downstream. Through participation with the National Center for Earth-surface Dynamics (NCED), which is housed at SAFL, Vivian is going to do more experiments, this time using a flume with a mobile sediment bed to look at the fluid flow conditions under which model trees begin to be dislodged from the flume bed.

I think it is so neat that studying dead wood debris can help us understand things about fish habitat or potential natural hazards.


A whole lotta shakin' goin' on!: Recent study reports toads can sense pre-seismic activities.
A whole lotta shakin' goin' on!: Recent study reports toads can sense pre-seismic activities.Courtesy Bartosz Kosiorek via Wikimedia Commons
Scientists aren’t sure why but a recent study reports that toads seem to be able to sense early warning signs of impending earthquakes. Back in 2009, biologist Rachel Grant from the UK’s Open University, in Milton Keynes noticed that common toads she happened to be studying in Italy suddenly abandoned their breeding grounds in droves just days before an earthquake struck nearby. Somehow, the amphibians sensed danger and stopped spawning between the first tremor and the last aftershock, even though their colony was located over 40 miles from the earthquake’s epicenter.

“Our findings suggest that toads are able to detect pre-seismic cues such as the release of gases and charged particles, and use these as a form of earthquake early warning system." – Dr Rachel Grant.

Similar strange behaviors have been witnessed in frogs during an earthquake in China , but Grant’s study is the first to document such odd amphibian behaviors, before, during, and after a temblor. The study appeared in the Journal of Zoology.


A river scientist takes two measures of the rate at which sediment moves along the bed of a river (the sediment transport rate). The measurements are taken a few minutes apart but yield wildly different results. What went wrong? Why measure sediment transport in the first place?

Early in fall of 2008, Arvind Singh, a Ph.D. student with the National Center for Earth-surface Dynamics (NCED), used a flume (think water moving through a channel) experiment at the U of MN's Saint Anthony Falls Laboratory (SAFL) to demonstrate a phenomenon that had long plagued river scientists: field measurements of sediment transport rates are typically highly variable, depending on the location or amount of time over which the sample is collected. Arvind and his collaborators wanted to find a way to make the measurement reliable.

Not being a river scientist, it was a little unclear to me why accurate measurements of sediment transport are so important, but it turns out that understanding sediment transport is essential for almost any river management endeavor. Algal growth, fish habitat, and river flooding may all be influenced by small- or large-scale fluctuations of sediment transport. Arvind and colleagues wanted to determine the time that an individual would need to sample to obtain a representative average sediment flux, or, for that matter, whether it was even sensible to try to characterize such a measurement for a real river.

From his 2008 work at SAFL, Arvind was able to demonstrate that measurements of bed topography and turbulence can be used to obtain reliable measurements of sediment transport: both topography and turbulence will indicate the sampling time interval that is adequate to get a measurement of the average sediment transport rate for a given river.

From his 2008 work, Arvind also gained insight into the importance of variability in characterizing sediment transport. Since most sediment transport models employed in the field incorporated measures of mean sediment transport but not of variability in sediment transport processes, Arvind set out to characterize the randomness of sediment transport.

Arvind’s current experimental work involves measuring sediment transport under laminar flow conditions to remove the effects of turbulence and reveal the effect that particle-particle interactions have. He plans to run his flume experiments, with help from SAFL intern Kristin Sweeney, through April and analyze the data this spring to reveal more about the randomness involved in sediment transport.


A dam is removed from a mountain waterway. What happens to the 750,000m3 of sediment that has accumulated behind that dam? Chuck Podolak, a Ph.D. student with the National Center for Earth-surface Dynamics (NCED), is using a flume experiment at the U of M's Saint Anthony Falls Laboratory (SAFL) to help him understand just that question.

In 2007, the Marmot Dam was removed from the Sandy River in Oregon, releasing 750,000m3 of sand and gravel that had accumulated behind the dam. At the time, the breach represented the greatest release of sediment from any U.S. dam removal. Chuck's goal is to understand how all that sediment will affect the river’s bottom.

Chuck’s work will help predict how bed topography in mountain rivers like the Sandy will respond to dam removal, or how large-scale gravel additions that are occasionally employed downstream of dams will affect bed forms that are beneficial for fish spawning and habitat.

One arm of Chuck’s Ph.D. research endeavor is his recent flume experiment at SAFL. Peter Wilcock, Chuck’s adviser, described the work to me as a Sisyphean task, and he did so with good reason. With help from SAFL staff and researchers, Chuck spent several weeks removing sediment from the bottom of an indoor 9 ft x 250 ft x 6 ft flume (SAFL’s “main channel” facility), shoveling tons (literally) of it back into the flume, collecting it at the flume’s mouth, and adding it to the channel again.

Chuck is using a customized instrument set-up to take laser scans and make centimeter-scale maps of the flume bottom. The measurements will give him an extremely detailed look at the flume bed, tracking down where the added sediment ended up: in pools, on top of bars, or in new bars. After the experiment’s conclusion, Podolak will combine its revelations with data gathered from field studies to run computer experiments that will shed new light on bed topography dynamics in gravel-bedded rivers.

Here is a movie I made about Chuck's experiment:

Chuck also created time series renderings of the topography in the main channel flume at SAFL during his Feb-Mar 2010 sediment addition experiment. Orange background indicates times of sediment additions:


Strike-slip fault
Strike-slip faultCourtesy USGS
Last weekend’s massive magnitude 8.8 earthquake off the coast of Chile released 500 times the energy generated by the magnitude 7 earthquake that hit in Haiti the month before. Damage is extensive in both countries but so far Haiti seems to have taken a worse hit than Chile, despite suffering a less-powerful quake. In fact, there's news today that the death toll in Chile has been lowered, which is unusual with earthquake tolls. They're usual revised upward. So why the discrepancy between the two quakes? There are several reasons but a big one is the types of earthquakes involved.

The Haiti tremblor occurred along a strike-slip fault where stress is created as two tectonic plates (in Haiti’s case, the Caribbean plate and North American plate) scrape and grind past each other in opposite directions, like two cars trying to squeeze past each other on a single lane bridge. Tension builds as the plates catch and grind and energy is released in fits and starts in the form of tremors. California’s San Andreas fault is a classic strike-slip boundary. There the Pacific plate is moving in a northwesterly direction beside the North American plate.

Subduction zone: Megathrust earthquakes occur near subductions zones.
Subduction zone: Megathrust earthquakes occur near subductions zones.Courtesy USGS
With a subduction zone megathrust quake – like that which occurred in Chile - it’s more like a head-on collision, where a lighter oceanic crust slams into a heavier continental crust and pushes (or is pulled) beneath it. This creates tremendous tension which eventually gets released, and when it is does, megathrust earthquakes can sometimes occur. They don’t occur all the time, in fact, megathrust earthquakes are rare – only fourteen have been recorded in history – but they only happen in subduction zones, like the one along the coast of Chile where the oceanic Nazca plate is subducting beneath the continental South American plate. Chile’s Andes mountain range rose up as a result of this subduction.

To give an idea of the incredibly huge amount of energy involved with the Chilean quake, it’s been estimated the jolt shifted Earth’s axis 3 inches, caused the planet’s entire mass to contract, become denser, and it’s rotation to speed up, thereby shortening the length of a day by 1.26 milliseconds! (see story)

The nearness and depth of an earthquake’s epicenter is another factor in the amount of perceived intensity and actual damage (measured using the modified Mercalli Intensity Scale), and this figures in the Haiti-Chile comparison. Haiti’s tremblor occurred six miles below the surface, and within ten miles of the severely over-populated capital of Port-au-Prince. That’s a fairly shallow earthquake, so the intensity level was high. Chile’s earthquake was centered 22 miles underground and five miles offshore, more than 70 miles from the nearest large population center (Concepcion).

Haiti’s capital is also built on loose soil that’s been eroded and carried down from the hillsides, and since earthquakes are rare in the region, the poor island nation is ill-prepared and under-equipped to deal with them. Building construction is flimsy, and collapsed easily when shaken, even by many less intense after-shocks. Chile, however, is on solid bedrock, both geologically and in regards to their central government. The country has a long history of dealing with the many quakes that occur there (the normal-sized ones anyway), and has had building codes in place since the 1920s.

More earthquake information
USGS earthquake page


The Enriquillo fault: Only the western half of the Enriquillo fault ruptured during Haiti's January earthquake.
The Enriquillo fault: Only the western half of the Enriquillo fault ruptured during Haiti's January earthquake.Courtesy Mikenorton
Haiti’s January 12th earthquake occurred on a strike-slip fault that runs in an east/west direction through the country. The fault, known as the Enriquillo fault, is where the North American and Caribbean tectonic plates meet. The jostling of these plates caused a quake that measured 7.0 on the Richter scale, killed approximately 217,000 people, destroyed 280,000 residences and commercial buildings, and left over one million people homeless. It has been deemed the most destructive natural disaster that a single nation has endured. Unfortunately, geologists believe that another, equally destructive quake could occur in Haiti within the next 20-30 years. Using high-resolution radar images of the Enriquillo fault after the quake, geologists from the University of Miami found that only half (the western half) of the fault had surged. They speculate that the remaining energy still locked up in the earth is what will cause the next quake on the eastern portion of the fault.

The radar images also showed that the earthquake produced a lot of vertical motion, not typical in strike-slip faults. This vertical motion, say geologists, explains how such a small fault movement could cause such a large earthquake. From their analysis, geologists are recommending that Haiti move all of its essential infrastructure (schools, hospitals, etc.) north, out of the fault zone.

This is a great example of science being used to help avoid future devastation, or at least lessen future destruction. Knowing that there is still potential danger along the Enriqillo fault allows people to plan accordingly (i.e. building or rebuilding in a safer location). However, this is also a case where I hope science is wrong.


A satellite image of the East Siberian Sea from USGS
A satellite image of the East Siberian Sea from USGSCourtesy United States Geological Survey
When I read this story the other day, I thought to myself: why didn't I think of that? Or maybe I did think of it, but as usual no one was listening when I pitched the idea for an action-packed spy movie about climate change. Or were they?

The Central Intelligence Agency does have a bunch of high-powered satellites and other "classified" instruments, so it's possible they've been using them to eavesdrop on my conversations with friends about possible sci-fi movie plots.

What's more likely: they figured out on their own that intelligence-gathering instruments could be really helpful to scientists, who can read detailed pictures of melting sea ice, growing desserts and other phenomena to better understand how climate is changing the planet.

The C.I.A. recently confirmed that it had revived this controversial data-sharing program known as Madea, which stands for Measurements of Earth Data for Environmental Analysis. If you decode that C.I.A. code name, it means that government spies are working with climate scientists to gather images and data about environmental change, as well as its impact on human populations.

Not everyone is convinced that climate change is a real threat to national security, and so some complainers are complaining that this collaboration between scientists and the C.I.A. is a misuse of resources, but what do they know?

Really? What do they know? So much of what happens over at C.I.A. headquarters is top-secret.

Maybe the whole thing doesn't sound that action packed, but I'm telling you, if you had the right actors playing the scientists, it could be a blockbuster. And if you have the right scientists analyzing the data, it might provide really valuable insights into global environmental change.


GOCE Satellite: The Gravity field and steady-state Ocean Circulation Explorer
GOCE Satellite: The Gravity field and steady-state Ocean Circulation ExplorerCourtesy ESA
Can it be true? Yes, for a mere $5,544 dollars round-trip airfare to Greenland! In March 2009, the European Space Agency launched the Gravity field and steady-state Ocean Circulation Explorer (GOCE) into orbit around our planet, which is now transmitting detailed data about the Earth’s gravity. The GOCE satellite uses a gradiometer to map tiny variations in the Earth’s gravity caused by the planet’s rotation, mountains, ocean trenches, and interior density. New maps illustrating gravity gradients on the Earth are being produced from the information beamed back from GOCE. Preliminary data suggests that there is a negative shift in gravity in the northeastern region of Greenland where the Earth’s tug is a little less, which means you might weigh a fraction of a pound lighter there (a very small fraction, so it may not be worth the plane fare)!

In America, NASA and Stanford University are also working on the gravity issue. Gravity Probe B (GP-B) is a satellite orbiting 642 km (400 miles) above the Earth and uses four gyroscopes and a telescope to measure two physical effects of Einstein’s Theory of General Relativity on the Earth: the Geodetic Effect, which is the amount the earth warps its spacetime, and the Frame-Dragging Effect, the amount of spacetime the earth drags with it as it rotates. (Spacetime is the combination of the three dimensions of space with the one dimension of time into a mathematical model.)

Quick overview time. The Theory of General Relativity is simply defined as: matter telling spacetime how to curve, and curved spacetime telling matter how to move. Imagine that the Earth (matter) is a bowling ball and spacetime is a trampoline. If you place the bowling ball in the center of the trampoline it stretches the trampoline down. Matter (the bowling ball) curves or distorts the spacetime (trampoline). Now toss a smaller ball, like a marble, onto the trampoline. Naturally, it will roll towards the bowling ball, but the bowling ball isn’t ‘attracting’ the marble, the path or movement of the marble towards the center is affected by the deformed shape of the trampoline. The spacetime (trampoline) is telling the matter (marble) how to move. This is different than Newton’s theory of gravity, which implies that the earth is attracting or pulling objects towards it in a straight line. Of course, this is just a simplified explanation; the real physics can be more complicated because of other factors like acceleration.

Albert Einstein
Albert EinsteinCourtesy none
So what is the point of all this high-tech gravity testing? First of all, our current understanding of the structure of the universe and the motion of matter is based on Albert Einstein’s Theory of General Relativity; elaborate concepts and mathematical equations conceived by a genius long before we had the technology to directly test them for accuracy. The Theory of General Relativity is the cornerstone of modern physics, used to describe the universe and everything in it, and yet it is the least tested of Einstein’s amazing theories. Testing the Frame-Dragging Effect is particularly exciting for physicists because they can use the data about the Earth’s influence on spacetime to measure the properties of black holes and quasars.

Second, the data from the GOCE satellite will help accurately measure the real acceleration due to gravity on the earth, which can vary from 9.78 to 9.83 meters per second squared around the planet. This will help scientists analyze ocean circulation and sea level changes, which are influenced by our climate and climate change. The information that the GOCE beams back will also assist researchers studying geological processes such as earthquakes and volcanoes.

So, as I gobble down another mouthful of leftover turkey and mashed potatoes, I can feel confident that my holiday weight gain and the structure of the universe are of grave importance to the physicists of the world!