We Were Totally Wrong About What's Happening Inside Earth's Mantle

By Maddie Stone on at

For the first time, geologists have compiled a global map of the wave-like motions called “convective currents” inside Earth’s mantle. They found that those convective currents are moving roughly ten times faster than previously thought. The discovery can help explain everything from how Earth’s surface changes over time to the formation of fossil fuel deposits to long-term climate change.

“In geological terms, the Earth’s surface bobs up and down like a yo-yo,” geologist Mark Hoggard of Cambridge University said in a statement. Hoggard is lead author on a paper published today in Nature Geoscience.

Our planet’s deep interior is an enduring scientific mystery. Having never drilled more than a few miles beneath the surface of the Earth, geologists rely on indirect measurements and models to get a sense of what’s happening further down. The mantle is a nearly 3000 kilometre (2000 mile) layer of gooey, compressed rock, and convective activity within it has a big impact on Earth’s surface.

“In addition to the normal plate tectonics, the interior of the plates which should be quite boring are being forced up and down by mantle convection,” Hoggard told Gizmodo. “People have known that this occurs for a long time, but for the past 30 years we haven’t had the data to measure it.”

That’s changing, thanks to new high-resolution seismic reflection profiles created by the oil industry. Seismic reflection profiling is a technique geologists use to peer deep into Earth’s crust, by measuring the reflection and refraction of seismic waves as they travel downwards. The method can reveal fine-scale changes in the thickness of the crust, which in turn relates to mantle convection.

By analysing over 2,000 seismic reflection measurements taken across the world’s oceans, Hoggard and his colleagues constructed the first global database of mantle convection. They were surprised to discover frequent changes in the thickness of seafloor crust, indicating that mantle convection is occurring far more frequently than we thought—think a vigorously bubbling pot of water instead of a slow-churning soup.

This insight into Earth’s deep interior can help explain all sorts of things closer to home. The formation of oil reserves, for instance, relies on the burial and compression of sediments that are chock full of decaying organic matter. “These motions help control how quickly rocks containing organics are buried and cooked into oil,” Hoggard said.

Mantle convection can also have a surprising impact on Earth’s climate, by affecting the large-scale ocean circulation patterns that move heat around the world. The Gulf Stream, for instance, carries warm water from the Gulf of Mexico to the coast of western Europe, before chilling out and sinking around Iceland.

“There are these narrow channels around Iceland that allow water to sink,” Hoggard explained. “If you elevate or depress them, you could really affect ocean circulation.” (Free plot idea for anyone looking to write a 2-in-1 sequel to the The Day After Tomorrow and The Core!)

Finally, mantle convection is responsible for forming geothermal systems, like Yellowstone, and island archipelagos, like Hawaii, that crop up in the middle of tectonic plates. Hoggard’s findings will shed light on how and why parts of the crust located far from plate boundaries are rising, falling, and cooking.

“It’s really a shift in view point,” he said. “A lot of geologists will look at places far away from plate boundaries and think they should be very stable. What we’ve shown is that regions that are often ignored are probably very active.”

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