Some of Earth's Rocky Plates Are Gooey on the Inside
An image of the Nazca slab subducting beneath the South American plate
Credit: University of Southampton

Tectonic plates may be similar to chocolate candies: Stiff on the outside, but as soft as marshmallow fluff on the inside.

That is the conclusion of a new study that suggests at least some of the rigid plates that cover the Earth's surface may be stretchier than thought.

The plate tectonics findings, which were described today (Nov. 23) in the journal Nature Geoscience, are based on investigations of the region under Peru, where the Nazca Plate is diving beneath the continental South American Plate. [Infographic: Tallest Mountain to Deepest Ocean Trench]

New plate formation

The results could shed light on the mysterious process that recycles ocean crust at mid-ocean ridges, which are like mountain chains along the seafloor. At these spots, two plates move away from each other; as the plates pull apart, old crust is buried in the mantle, while new magma seeps into the now-empty spaces to form new crust.

"The process of consuming old seafloor at subduction zones, where great slabs of oceanic material are swallowed up, drives circulation in the Earth's interior and keeps the planet going strong. One of the most crucial but least known aspects of this process is the strength and behavior of oceanic slabs once they sink below the Earth's surface," study co-author Caroline Eakin, a researcher at the University of Southampton in England, said in a statement. "Our findings provide some of the first direct evidence that subducted slabs are not only weaker and softer than conventionally envisioned, but also that we can peer inside the slab and directly witness their behavior as they sink."

As new seafloor forms, it pulls olivine, Earth's most abundant mineral, with it. The atoms in the olivine form a regular, repeating pattern, called its crystal structure, and as the plates move over the Earth's surface, the crystal structure shifts, orienting itself in the direction of the plate's growth. This plate motion also fixes the olivine crystal structure in place. 

Seismic waves travel through the crust at different speeds depending on which way they travel through the olivine crystal structure, allowing researchers to understand how the plates are deformingbased on how the olivine crystal is aligned.

Hard exterior, squishy interior

Past studies suggested that the bottom plate (the diver) would remain rigid in these subduction zones. To see whether that was the case, researchers measured how fast seismic waves traveled in different directions within the Nazca Plate over 2.5 years, pulling data from 15 local stations and seven distant ones located on other continents.

Normally, studying the entire plate's structure is difficult because the plates extend deep into the bowels of the Earth. But unlike most subduction zones, where plates dive beneath each other at a steep angle, the South American plate is stacked almost horizontally atop the Nazca Plate as it slides, a process called flat-slab subduction. This flat-slab subduction meant the team was able to recreate a picture of the Nazca Plate going 125 miles (200 kilometers) deep into the slab, using only land-based instruments.

Weirdly, the seismic wave speed suggested that in some spots, the olivine had switched orientations.

The only explanation for this olivine orientation flip-flop is that the Nazca Plate was deformed enough during the process to erase the original olivine orientation and replace it with a new one.

The new discovery implies that tectonic plates can be less rigid than previously thought. It also suggests their structure can change in the blink of an eye, geologically speaking.

"Imaging Earth's plates once they have sunk back into the Earth is very difficult," Lara Wagner, a researcher from the Carnegie Institution for Science in Baltimore, said in a statement. "It's very exciting to see results that tell us more about their ultimate fate, and how the materials within them are slowly reworked by the planet's hot interior. The original fabric in these plates stays stable for so long at the Earth's surface, that it is eye-opening to see how dramatically and quickly that can change."

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