In a new paper published in the journal Geology, scientists looked at how subducted sediment rose back up from deep in the crust to end up on the surface in California’s Mojave Desert and in Arizona. The best way to describe the process is like a lava lamp.

For those of you who didn’t experience any part of the seventies or haven’t paid attention to the resurgence of these popular items, here’s how they work. You take a rather plastic substance, something nice and stretchy like wax, and you add it to a liquid. Then you put a hot lamp at the bottom which heats the liquid and the plastic, but not at the same temperature because of the difference in density. The plastic heats and rises, stretching into blobs. As it bobs up, it begins to cool and drop back down, creating an ever-changing system of shapes, moving up and down. It’s mesmerizing, really. Beth has one on the dining room table, and they frequently get distracted by watching it.

Well, it turns out that something similar happened to subducted sand and mud about 75 million years ago off of the coast of California. Lead author Jay Chapman explains: The rocks started their lives as sediment eroded from the Sierra Nevada Mountains and carried by rivers and streams down to the ocean, where they ended up deposited in a subduction trench, similar to the modern-day Marianas trench. Then, they were carried about 20 miles deep into the Earth by a subducting oceanic plate, where the sediments were metamorphosed into a rock called schist. That in and of itself is pretty amazing, but the truly special thing about these rocks is that they didn’t stay subducted, but somehow made their way back up to the surface, where you can go stand on them today. 

Like the lava lamp, the bits of sediment were heated by the mantle and rose up to attach to the bottom of the crust in blobs. Add in some time and weathering, and voila, you can now stand on once-subducted rocks. Geology is amazing.

More Information

University of Wyoming press release

“Diapiric relamination of the Orocopia Schist (southwestern U.S.) during low-angle subduction,” James B. Chapman, 2021 April 22, Geology