Rain of Rocks Help Build Earth's Mystery Moho: Page 2

A geophysical map showing the position of the Moho discontinuity across the Earth.
AllenMcC, Christian-Albrechts University of Kiel, Germany via Wikimedia Commons

"It's like icebergs, but the stuff that's actually dropping off is actually underwater," Jagoutz said.

A schematic illustration of the properties of rocks from Pakistan and Alaska used to develop a new model for how the Moho forms.

Removing these dense rock leaves lighter, silica-rich materials behind — like the rocks found in continents, Jagoutz said. [Granite: Bedrock of the Earth]

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Sinking down

The researchers think the Moho starts to appear with big changes in volcanism, such as when melting stops or subduction shuts off. Because volcanic island chains appear above subduction zones, where a tectonic plate sinks into the mantle and releases fluids that trigger melting, new magma will rise upwards and replace the missing crust. But without new magma replenishing the crystalline rain, eventually a sharp boundary will appear between lighter material in the crust and the dense mantle below.

"When this happens, the mantle will remain relatively hot for a while and the material will continue to sink back down," Jagoutz said.

Geologist Suzanne Kay of Cornell University, one of the original proponents of crustal delamination in island arcs, said the study was "an interesting paper" but doesn't cover significant new ground.

"The idea of delamination in oceanic and continental arcs and the link with the composition of the continental crust by delamination have been around for more than 20 years, and others are also thinking of the ultimate fate of the delaminated material," Kay said in an email interview.

Original article on LiveScience's OurAmazingPlanet.

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