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22 hours ago
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Scientists have discovered evidence that California is 'peeling apart' beneath Earth's surface.
They found dense rocks under the Sierra Nevada mountain range are detaching and sinking deeper into the mantle.
While the process, known as foundering, sparks terror that California's landscape could be separating, researchers from the University of Boulder said their discovery sheds light on how our planet's continental crust formed.
Scientists have long speculated that foundering was behind the geological changes about four billion years ago, but the discovery in California proved it to be true.
They used imagery techniques to map the lower crust and uppermost mantle of the Sierra Nevada,allowing them to see changes in how seismic waves as they moved beneath the surface.
The new study not only determined that areas of the Sierra Nevada separated several million years ago, but found it is currently taking place in the central part of the mountain range and has yet to occur in the northern part.
'We have therefore captured snapshots of a fundamental continent-building process,' the researchers wrote in their report.
Using nearly four decades of seismic data collected from stations around the Sierra Nevada mountain range, which spans California's eastern border, geologists uncovered an ongoing separation between the Earth's crust and the underlying mantle
Geologists Vera Schulte-Pelkum and Debora Klib used 'receiver function analysis' to image the lower crust and uppermost mantle beneath the Sierra Nevada.
This imaging technique uses seismic waves that change as they cross structures beneath the surface. Researchers can then analyze the changes to gain a clear picture of what lies deep underground.
Schulte-Pelkum and Klib also looked at earthquake data from the Advanced National Seismic System Comprehensive Earthquake Catalog, or ComCat.
This analysis revealed a band of seismic activity in the central Sierra Nevada made up of small earthquakes ranging from magnitude 1.9 to 3.2 that occurred at unusually low depths of roughly 25 miles or greater.
The presence of such deep earthquakes in this region can also be explained by the presence of a slab of colder continental lithosphere that cracks rather than stretching and flowing like the molten material usually found at such depths, according to the researchers.
However, they found no evidence of this layer in the northern Sierra, which suggests this region has not experienced foundering yet.
The researchers published their findings in the journal Geophysical Research Letters.
Based on these observations, Schulte-Pelkum and Klib concluded that foundering has been taking place beneath the Sierra Nevada mountains for at least three million years, and it may be progressing northward.
Scientists have long thought foundering might explain how the Earth's relatively light continental crust forms from the denser basaltic rocks of the upper mantle. But evidence to support this theory has been difficult to find
But perhaps more importantly, their investigation of this region provides evidence of a process of differentiation, or the separation of different materials within the Earth's lithosphere based on their density.
This may help confirm confirm geologists' long-held suspicions that foundering allows the planet to generate its lighter continental crust from the denser mix of minerals beneath.
The Earth's continental crust — the land we live on — is made up of solid rock that contains relatively high proportions of minerals, including silicates, aluminum and potassium.
By comparison, the planet's oceanic crust — which lies at the bottom of ocean basins — is denser because it is mostly made of basalt, a volcanic rock composed of magnesium and iron.
Both of these very different crusts formed from the same source: Earth's mantle. But how did the continental crust end up so dissimilar to its parent material, while oceanic crust retained the mantle's high density and basalt content?
One explanation could be differentiation through the process of foundering.
In other words, the heavier elements in the upper mantle (like those that make up basalt) separate themselves out of the mix as they melt, allowing them to form distinct sheets.
Because these sheets are so dense, the lower layer might then 'peel' away and slowly sink deeper into the molten part of the mantle.
The removal of these denser materials is what allows the lighter continental crust to form and rise above the denser oceanic crust, resulting in the land we live on.
This explanation has been little more than a theory for years, but now, Schulte-Pelkum and Klib have uncovered evidence to suggest it is a real process that takes place all over the world.
