March 5, 2014

Understanding The Forces Behind Tectonic Plates

Lee Rannals for - Your Universe Online

A new study published in the journal Science reveals new information about the forces behind earthquakes.

Geoscientists from the University of California, Los Angeles used a technique known as seismic tomography to study the structure of the Pacific Plate. This technique helped the team determine the plate’s thickness and image the interior of the plate and the underlying mantle, which they were able to relate to the direction of flow of rocks in the mantle.

"Rocks deform and flow slowly inside the Earth's mantle, which makes the plates move at the surface," Caroline Beghein, assistant professor of earth, planetary and space sciences in UCLA's College of Letters and Science, said in a statement. "Our research enables us to image the interior of the plate and helps us figure out how it formed and evolved." The findings might apply to other oceanic plates as well.

Beghein, lead author of the paper, said even with the findings, the fundamental properties of the plates are still difficult to understand.

Computed tomography (CT) scans used for medical imaging work similar to seismic tomography, the difference being that seismic tomography uses seismic waves generated by earthquakes instead of X-rays.

Seismologists use seismic data to help detect seismic waves that bounce off the interface that separates two layers. The team compared the layering observed by using seismic tomography with the layers revealed by these other types of data. The team said that comparing the results from these different methods is a challenge for geoscientists, but it is important in helping to understand the Earth’s structure.

"We overcame this challenge by trying to push the observational science to the highest resolutions, allowing us to more readily compare observations across datasets," said Nicholas Schmerr, the study's co-author and an assistant research scientist in geology at the University of Maryland.

The team was the first to discover that the Pacific Plate is formed by a combination of processes. As the mantle cools, it helps to thicken the plate and the chemical makeup of the rocks begins to change.

"By modeling the behavior of seismic waves in Earth's mantle, we discovered a transition inside the plate from the top, where the rocks didn't deform or flow very much, to the bottom of the plate, where they are more strongly deformed by tectonic forces," Beghein said. "This transition corresponds to a boundary between the layers that we can image with seismology and that we attribute to changes in rock composition."

The team’s research helps to advance our understanding of how oceanic plates form and evolve as they age by using and comparing two sets of seismic data. According to the findings, the compositional boundary inside the plate appears to be linked to the formation of the plate itself.

Image 2 (below): Asthenosphere and lithospheric plate. Credits: UCLA