Unveiling California’s Hidden Secrets: The Surprising Truth About Earth’s Crust Peeling Away

Under California’s Sierra Nevada mountains, something interesting is happening beneath the surface. The process, known as lithospheric foundering, might be a key part of how continents formed long ago. Continental crust is lighter and sits higher than oceanic crust. Lithospheric foundering helps separate lighter materials from heavier ones, which could be how the land we know came to be.

A recent study revealed that this process is ongoing under the Sierra Nevada mountains. In the southern part of the range, the lithosphere—made up of the upper mantle and crust—has already detached and sunk deeper into the mantle. The central part is currently experiencing this peeling, while the northern section hasn’t shown signs of it yet.

“You could be fishing in the Sierras, completely unaware that a huge layer is peeling off beneath you,” explains Vera Schulte-Pelkum, a geoscientist at the University of Colorado Boulder.

Interestingly, there’s no visible sign of this process on the surface. Researchers noted deep earthquakes in the Sierras, with tremors occurring over 25 miles (40 kilometers) down. Typically, rocks at such depths are hot and pressured, causing them to deform rather than break and produce seismic waves. This scenario puzzled Schulte-Pelkum and her co-author, Deborah Kilb, a seismologist at the University of California San Diego.

From 1985 to 2023, they studied earthquake records to gain insight into the deep crust and upper mantle below the mountains. They focused on a measurement called anisotropy, which shows how seismic waves travel differently depending on their direction. This information helps reveal the orientation of the rock layers.

Their findings uncovered a layer between 25 and 43 miles (40 to 70 kilometers) deep where rocks are separating from the crust above. In southern Sierra, the layer is completely gone, while in the north, around Lake Tahoe, it still exists. However, in the central Sierra, specifically under Yosemite National Park, this layer is currently falling into the mantle.

Earlier studies suggested that this peeling might have happened underneath southern Sierra 3 to 4 million years ago. “Now, we’re witnessing it still happening,” Schulte-Pelkum noted, excited to catch it in action.

The researchers published their findings in December in the journal Geophysical Research Letters. Schulte-Pelkum believes similar processes could be occurring in other parts of the world, like New Zealand, Turkey’s Anatolian plateau, and the Carpathian Mountains in Eastern Europe.

“We could investigate other areas where the lithosphere might have been thicker and has since peeled off,” she said, hinting at the possibility of more discoveries ahead.

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