Volume 4, Issue 28 May 2007
Crystals Reveal Earth's Hidden History
Rudy Wenk's interest in geology stems from his love of climbing mountains. Photo courtesy of Rudy Wenk.
Rudy Wenk studies crystals. But you shouldn't get the wrong idea. A UC Berkeley professor of geology, Wenk is not interested in the sparkly gems revered in alternative healing classes. His crystals have a far more serious job: revealing the structure of the earth hundreds of miles beneath our feet.
Wenk's research centers around a property of crystals and rocks known as anisotropy. When earthquakes rumble through the planet, scientists have observed that they propagate faster in some directions than in others. This is due to the crystalline structure of rocks within the Earth. At these high pressures the atomic lattice of crystals within rocks will deform and align in a preferred direction, much the way tugging on a sagging tennis net will create an orderly, directional grid of cells. A crystal's alignment affects how fast seismic waves can pass through it. In some directions, a seismic wave may be slowed by a factor of two relative to other directions. The degree to which crystals are aligned in a rock determines this response.
"We can use the anisotropy seismologists observe and interpret from that how the material within the Earth has been deformed," Wenk says.
Scientists have already used seismic wave data to confirm the general structure of the Earth. So far, studies suggest it consists of a thin crust; a slightly thicker and mostly solid upper mantle; a viscous lower mantle that makes up about 70 percent of Earth's volume; and an iron core that is liquid on the outside and crystalline on the inside.
Wenk uses a diamond anvil cell to simulate the crushing pressures of earth's interior in his laboratory. Photo courtesy of Rudy Wenk.
Now scientists would like to improve the resolution of their deep earth images. Doing so would essentially allow them to peer into the mantle and witness the processes underlying mountain building, basin sinking, and the rise of islands such as Hawaii. "The lower mantle is the largest volume of the Earth. Whatever happens there has an impact on the structures which we observe on the surface," Wenk suggests.
This is where Wenk's research comes in. He studies crystal deformation and alignment under deep earth conditions. By doing so, he hopes to gain a better understanding of how to interpret both seismic wave data and surface rock samples. "If you take a rock sample from a mountain and look at the alignment of crystals within it, this tells us something about the history of how those rocks were deformed. This information can help us unravel the deformation history of the mountain range," Wenk says.
Wenk simulates conditions in Earth's interior by using a contraption called a diamond anvil cell. The cell consists of two gem-grade diamonds placed on either side of a sealed chamber. Wenk then fills the chamber with crystals he'd like to study under deep earth conditions, and squeezes the diamonds together. The anvil can produce more than 360 gigapascals of pressure – equivalent to conditions in the center of the Earth. As the cell is squeezed, Wenk bombards it with powerful synchotron x-rays to watch the crystals deform.
Wenk's research reveals how rocks are deformed below earth's surface. This image shows colder rock (blue) sinking into the Earth's mantle. Hot areas of upwelling rock are shown in red and yellow. This process, known as convection, is responsible for the upwelling of mountains and the sinking of basins on Earth's surface. Photo courtesy of Rudy Wenk.
"We can get an idea of the changes in the anisotropy pattern in the earth as material gets subducted into the lower mantle by observing deformation in the laboratory under extreme conditions," Wenk says.
He and his students are now developing a means to heat the interior of diamond anvil cells using lasers at the Advanced Light Source at Lawrence Berkeley Laboratories. They are able to cook their samples to 2,000 Kelvin, approximating the infernal temperatures found in Earth's interior.
Wenk also approaches the problem from the other direction, by analyzing the crystal orientation patterns of rocks samples thought to originate from the deep earth. With this method, he can use a small sample of rock to get an idea of the larger geological processes at work in an entire region.
By combining the experimental results, sample analysis, seismic results and information about large-scale convection, Wenk can model how anisotropy develops. For example, in one recent paper he follows the crystalline deformation likely to occur as a slab of rock sinks toward the core-mantle boundary, and upwells toward the surface again in a cycle that takes hundreds of millions of years. In the process, Wenk is revealing the dynamics of hidden subterranean earth.