Earth’s Mantle Moves Up and Down “Like a Yo-Yo”

May 10, 2016 | Joanne Kennell

Cross section of planet Earth

Scientists have been wrong about what’s happening inside Earth for over 30 years.

In a first, geologists have compiled a set of global observations of the movement of the Earth’s mantle — the 3,000-kilometer-thick layer of hot silicate rocks between the crust and the core. To their surprise, it looks very different from predictions made by geologists over the past 30 years.

The research team from the University of Cambridge analyzed more than 2,000 observations from seismic surveys of the world’s oceans in order to get a glimpse beneath the Earth’s crust and observe the mantle flow, which forces the surface above it up and down. By examining variations in the depth of the ocean floor, the researchers were able to construct the mantle’s movements.

These movements have a huge influence on the way that the Earth looks today. In fact, it causes the formation of mountains, volcanoes, and earthquakes. However, it turns out the mantle’s wave-like movements are occurring at a rate that is an order of magnitude faster than had been predicted.

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"Although we're talking about timescales that seem incredibly long to you or me, in geological terms, the Earth's surface bobs up and down like a yo-yo," said Mark Hoggard of Cambridge's Department of Earth Sciences, the paper's lead author, in a press release. "Over a period of a million years, which is our standard unit of measurement, the movement of the mantle can cause the surface to move up and down by hundreds of metres."

Plate tectonics, or the movement of the continental plates, is a result of the flow of the mantle. But in addition to these plate motions, convective currents inside the mantle push the surface up or down. For example, although the Hawaiian Islands lie in the middle of a tectonic plate, their volcanic activity is not caused by the movement of the plates, but rather the upward flow of the mantle below.

"We've never been able to accurately measure these movements before — geologists have essentially had to guess what they look like," said Hoggard. "Over the past three decades, scientists had predicted that the movements caused continental-scale features which moved very slowly, but that's not the case."

It turns out that the mantle convects in a very chaotic manner, but with length scales on the order of 1,000 kilometers, instead of the 10,000 kilometers that had been predicted.

The results, reported in the journal Nature Geoscience, have consequences for many fields of science, including the study of ocean circulation and past climate change.

"These results will have wider reaching implications, such as how we map the circulation of the world's oceans in the past, which are affected by how quickly the sea floor is moving up and down and blocking the path of water currents," said Hoggard. "Considering that the surface is moving much faster than we had previously thought, it could also affect things like the stability of the ice caps and help us to understand past climate change."

However, the movement is not only of interest to geologists, but to the oil and gas sector as well because the motions affect the rate at which sediment is shifted and hydrocarbons — the chief components of petroleum and natural gas — are generated.

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