How a Mysterious Moho Forms Beneath Earth's Crust

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

(Image credit: Nature)

A dense crystalline "rain" falling into Earth's mantle could explain how a mysterious seismic boundary forms beneath the crust, according to a study published today (Dec. 4) in the journal Nature.

The model, based on rock evidence from volcanic islands that smashed into Asia and Alaska, confirms long-standing ideas about how continents are born.

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The seismic boundary investigated by Jagoutz and co-author Mark Behn, of the Woods Hole Oceanographic Institution in Woods Hole, Mass., is called the Moho, after Croatian seismologist Andrija Mohorovicic. In 1909, Mohorovicic realized earthquake waves suddenly sped up at a sharply defined boundary that hovers about 25 miles (40 kilometers) beneath continents. The discovery revealed Earth was divided, with a lighter crust and denser mantle where the seismic waves traveled faster.

One enduring puzzle has been the missing Moho — the boundary's absence beneath volcanic island chains, such as Japan's Izu-Bonin islands, that rise above colliding tectonic plates. Because these "island arcs" are the building blocks of continents, the missing Moho is a mystery. For example, the East Coast of North America has a clear, crisp Moho, but it is also quilted from scores of volcanic chains slamming into the continent's edge several hundred million years ago. Another problem is the rocks in continents are about 10 percent richer in silica than oceanic crust, which is the source of magma that feeds volcanic island chains. [Infographic: Tallest Mountain to Deepest Ocean Trench]

"If we want to produce continental crust in arcs, we are left with two problems," Jagoutz told LiveScience's OurAmazingPlanet. "The rocks we find on the surface of continents all resemble lavas that are erupted in subduction zones, but there needs to be a mechanism that brings the melt from 50 to 60 percent [richer in silica]," he said. "Another problem we have is the structural problem. Somehow we need to introduce this major structural discontinuity, the Moho, that we don't have in arcs but we have in continents."

The Pakistan rocks resemble modern island arc settings. There's no sharp density contrast that would produce a Moho boundary. The layers reveal a thick, continuous section of rocks of similar density, such as gabbros, at the depth of the Moho. But in Alaska, these rocks are missing. Instead, at the depth where the Moho would sit, there's a sharp density increase in the rock layers, with rocks called harzburgites and dunites instead of gabbros.

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.

Becky Oskin covers Earth science, climate change and space, as well as general science topics. Becky was a science reporter at Live Science and The Pasadena Star-News; she has freelanced for New Scientist and the American Institute of Physics. She earned a master's degree in geology from Caltech, a bachelor's degree from Washington State University, and a graduate certificate in science writing from the University of California, Santa Cruz.