Wednesday, March 5, 2014

New insights on plate tectonics

Asthenosphere and lithospheric plate: The Earth’s outer layer is broken into moving, interacting plates whose motion at the surface generates most earthquakes, creates volcanoes and builds mountains. 

In this image, the orange layer represents the deformable, warm asthenosphere in which there is active mantle flow. 

The green layer is the lithospheric plate, which forms at the mid ocean ridge, then cools down and thickness as it moves away from the ridge. 

The cooling of the plate overprints a compositional boundary that forms at the ridge by dehydration melting and is preserved as the plate ages. 

The more easily deformable, hydrated rocks align with mantle flow. 

The directions of past and present-day mantle flow can be detected by seismic waves, and changes in the alignment of the rocks inside and at the bottom of the plate can be used to identify layering.

The Earth's outer layer is made up of a series of moving, interacting plates whose motion at the surface generates earthquakes, creates volcanoes and builds mountains.

Geoscientists have long sought to understand the plates' fundamental properties and the mechanisms that cause them to move and drift, and the questions have become the subjects of lively debate.

A study published online Feb. 27 by the journal Science is a significant step toward answering those questions.

Caroline Beghein
Researchers led by Caroline Beghein, assistant professor of earth, planetary and space sciences in UCLA's College of Letters and Science, used a technique called seismic tomography to study the structure of the Pacific Plate—one of eight to 12 major plates at the surface of the Earth.

The technique enabled them to determine the plate's thickness, and to image the interior of the plate and the underlying mantle (the layer between the Earth's crust and outer core), 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," said Beghein, the paper's lead author.

"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. Even with the new findings, Beghein said, the fundamental properties of plates "are still somewhat enigmatic."

Beghein and her research team advanced our understanding of how oceanic plates form and evolve as they age by using and comparing two sets of seismic data; the study revealed the presence of a compositional boundary inside the plate that appears to be linked to the formation of the plate itself.

More information: "Changes in Seismic Anisotropy Shed Light on the Nature of the Gutenberg Discontinuity." Caroline Beghein, Kaiqing Yuan, Nicholas Schmerr, and Zheng Xing. Science 1246724. Published online 27 February 2014. [DOI: 10.1126/science.1246724]

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