Jay Chapman, a professor at the University of Wyoming, has used computer modeling to propose that sand and mud subducted of the coast of California around 75 million years ago returned to the Earth’s crust by rising through the mantle as enormous lava lamp like blobs.
These blobs are not found at the Earth’s surface, far inland from the coast, in places including western Arizona and the Mojave Desert.
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,” Chapman says. “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 and stand on them today.”
“The prevailing theory is that the sediments were smeared against and plastered to the base of the North American tectonic plate, forming a sheet-like layer,” Chapman says. “However, the density of these sediments is much lower than rocks in the mantle or lower crust and, over millions of years, computer modeling predicts that the sediments will flow and buoyantly ascend, like hot wax in a lava lamp.”
https://phys.org/news/2021-08-lava-lamp-tectonics-giant-blobs.html