Numerical modeling of sediment subducted beneath North American crust suggests that the sediment may buoyantly rise up through the mantle, like a lava lamp blob, and become attached to the base of the crust.
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.
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. 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.
The research has implications for understanding subduction zone processes and the distribution of natural resources. Geoscientists around the world are working to understand what gives continental crust its unique composition, and subduction and reincorporation of sediment are a popular hypothesis. In addition, many researchers are now wondering whether fluids and elements released from the subducted sediments may have contributed to the concentration of economically important minerals and metals.
James B. Chapman, Diapiric relamination of the Orocopia Schist (southwestern U.S.) during low-angle subduction, Geology (2021). DOI: 10.1130/G48647.1