Rocks in Earth's middle mantle flow slowly because an important mineral becomes harder and more resistant to deformation at the high temperatures and pressures found in this region, according to a new study published in the journal *Science*.

This finding could help scientists better understand the movement of tectonic plates, which are driven by the convection of heat and rock in the mantle. The mantle is the layer of rock that lies beneath the crust and makes up about 84% of Earth's volume.

"We found that a change in the crystal structure of the mineral bridgmanite makes it much stronger than previously thought," said lead author Oliver Tschauner, a professor of mineralogy and petrology at the University of Nevada, Las Vegas. "This means that the mantle is more resistant to deformation, and it explains why rocks flow so slowly in the middle mantle."

Bridgmanite is the most abundant mineral in Earth's mantle. It is a form of magnesium-iron silicate that is only stable at very high pressures and temperatures. In the middle mantle, the pressure can reach up to 2.5 million atmospheres (about 2.5 billion times the pressure at sea level), and the temperature can reach up to 2,000 degrees Celsius (about 3,600 degrees Fahrenheit).

At these extreme conditions, bridgmanite undergoes a change in its crystal structure, becoming more compact and denser. This change makes it harder for the mineral to deform, and it slows down the flow of rocks in the middle mantle.

"Our understanding of the flow of rocks in the mantle is important because it helps us understand the movement of tectonic plates," said co-author Stephen Jacobsen, a professor of geochemistry at Northwestern University. "The movement of tectonic plates is responsible for many of the features on Earth's surface, such as mountains, oceans, and earthquakes."

The new findings could also help scientists better understand the formation of diamonds. Diamonds are formed when carbon is subjected to extremely high pressures and temperatures. In the middle mantle, the pressure and temperature are high enough to transform carbon into diamonds. However, the slow flow of rocks in the middle mantle means that it takes a long time for diamonds to form.

"Our findings provide a new understanding of the conditions under which diamonds form," said Tschauner. "This could lead to new ways to find diamonds and other valuable minerals in the Earth's mantle."

The study was funded by the National Science Foundation, the Deep Carbon Observatory, and the Alfred P. Sloan Foundation.