Plate tectonics is the widely accepted theory that the Earth’s crust is divided into several sections that float around on the mantle — the mostly solid layer between the core and the crust. But how they move around has been a subject of much debate among scientists since the theory was first accepted in the 1950s.
The sections, known as plates, move at a rate of about 2 to 5 centimeters per year, which is a similar speed to how fast your fingernails grow. When they grind past each other, it’s called a transform plate boundary, which can cause earthquakes. When they move away from each other, it’s known as a divergent boundary, where lava spews out, and volcanoes can form. Sometimes, when they meet each other head on, one plate starts moving upwards to form mountains, while the other is pushed downwards where it melts. This is a convergent boundary.
Scientists have wondered for many decades whether the plates are being passively moved around by the mantle, or are the active drivers of movement themselves, dragging the mantle along with them.
A new study, published in the journal Science Advances, has shown the additional force of heat drawn from Earth’s core plays a part in plate dynamics. The team observed the East Pacific Rise, which is a divergent tectonic plate boundary which lies along the Pacific Ocean floor, and also made models of the mantle flow beneath the surface.
During their observations, they concluded that the movement of the East Pacific Rise could not be completely explained by subduction — when one plate moves under the other — and other forces had to be at play. In the paper, they state that buoyancy is created by heat rising up from deep within the Earth’s core.
The estimate is 50% of plate tectonic movement is driven by this heat, and about 20 terawatts of heat flows between the core and the mantle. That’s just slightly more than the average total power consumption of humans on Earth every year.
“We see strong support for significant deep mantle contributions of heat-to-plate dynamics in the Pacific hemisphere,” said Professor David B. Rowley, an expert of geophysical sciences at the University of Chicago and lead author of the paper, in a statement. “Heat from the base of the mantle contributes significantly to the strength of the flow of heat in the mantle and to the resultant plate tectonics.”
When the case on the mantle is heated, this reduces the density of the material, making it buoyant, which causes it to rise through the mantle. Plates also cool at the surface, creating something called negative buoyancy, which is another way of describing an object that sinks. These two forces move the plates around.
In other words, hot parts rise and cool parts sink, creating a sort of churning movement in the mantle that the plates ride on.
“Based on our models of mantle convection, the mantle may be removing as much as half of Earth’s total convective heat budget from the core,” Rowley said. “The implication of our work is that textbooks will need to be rewritten.”
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