A new investigation on how Earth forms new continental crust shows far more continental rock under the Andean Plateau in South America.
Jonathan Delph, the co-author of a new study from Rice University, explained that the new findings suggest that mountain forming regions called orogenic plateaus could produce larger volumes of continental rock in less time than previously believed.
“When crust from an oceanic tectonic plate plunges beneath a continental tectonic plate, as it does beneath the Andean Plateau, it brings water with it and partially melts the mantle, the layer below Earth’s crust,” Delph said in a statement. “The less dense melt rises and one of two things happens: It either stalls in the crust to crystallize in formations called plutons or reaches the surface through volcanic eruptions.”
The Andean Plateau—which has an average height of more than 12,000 feet—covers much of Bolivia and parts of Peru, Chile, and Argentina.
Kevin Ward, a postdoctoral researcher at the University of Utah explained how they discovered the additional material beneath the plateau.
“When we compared the amount of trapped plutonic rock beneath the plateau with the amount of erupted volcanic rock at the surface, we found the ratio was almost 30:1,” said Ward. “That means 30 times more melt gets stuck in the crust than is erupted, which is about six times higher than what’s generally believed to be the average.
“That’s a tremendous amount of new material that has been added to the crust over a relatively short time period.”
The majority of rocks that form the Earth’s crust initially came from partial melts of the mantle and if the melt erupts quickly, it forms basalt that makes up the crust beneath the oceans. However, it is still unknown how continental crust—which is more buoyant than oceanic crust—is formed.
The researchers were able to calculate the plutonic-to-volcanic ratio by comparing the volume of regions where seismic waves travel extremely slowly beneath volcanically active regions, indicating some melt is present, with the volume of rock deposited on the surface by volcanoes.
“Orogenic oceanic-continental subduction zones have been common as long as modern plate tectonics have been active,” Delph said. “Our findings suggest that processes similar to those we observe in the Andes, along with the formation of supercontinents, could have been a significant contributor to the episodic formation of buoyant continental crust.”
