In the summer of 2024, seismologists from CIRES and Scripps Institution of Oceanography were studying earthquakes in California when they found something interesting. The researchers were looking at how faults that cause major earthquakes, like the San Andreas, deform the rocks around them as they move. 

Coincidentally, they found evidence that dense portions of Earth's lithosphere (its top layer of rock) are peeling off and dropping into the mantle below the Sierra Nevada mountains. While this may seem to be an obscure finding, it's actually evidence for how continents are built all over the planet. 

Continents sit above sea level because they have a large proportion of low-density minerals. Ocean floor, by contrast, has a much higher concentration of dense minerals. To make a continent, you need to remove the denser portion of rocks from the lithosphere to allow the lighter layer to sit higher. One proposed way this could happen is if the bottom portion of the lithosphere, containing more of the denser minerals, were to peel off and drop into the mantle below. 

That is exactly what the researchers saw happening below the Sierra Nevada. They found evidence that a dense portion of rock peeled off from the lithosphere several million years ago under the southern part of the mountain range and is still peeling under the central portion, causing very small, deep earthquakes. The animation above illustrates this process. 

“It is very exciting to take pictures of this fundamental process happening in real time,” said Vera Shulte-Pelkum, CIRES seismologist and lead author of a new study detailing the findings.