Surprising behavior of minerals inside the Earth

Jan. 13 () –

Mineral processes deep within the planet’s interior actually occur in a completely opposite way to what had been previously theorized.

This is illustrated by a new and detailed model developed by Caltech researchers that has been published in the journal Nature.

“Despite the enormous size of the planet, the deepest parts are often overlooked because they are literally out of reach: we can’t sample them,” says study author and Jennifer M.Jackson Professor of Mineral Physics. it’s a statement. “In addition, these processes are so slow that they seem imperceptible to us. But the lower mantle flow communicates with everything it touches; it is a deep engine that affects plate tectonics and can control volcanic activity.”

The planet’s lower mantle is solid rock, but over hundreds of millions of years it slowly oozes out, like thick candy, transporting heat throughout the interior of the planet in a process called convection.

Many questions remain to be answered about the mechanisms that allow this convection to occur. The extreme temperatures and pressures of the lower mantle -up to 135 gigapascals and thousands of degrees- difficult to simulate in the laboratory.

For reference, the pressure in the lower mantle is nearly a thousand times the pressure at the deepest point in the ocean. Thus, although many laboratory experiments in mineral physics have provided hypotheses about the behavior of rocks in the lower mantle, the processes that occur on geologic time scales to drive the slow convective flow of the lower mantle have been uncertain.

The lower mantle is composed primarily of a magnesium silicate called bridgmanite, but also includes a small but significant amount of a magnesium oxide called periclase mixed between bridgmanite, plus small amounts of other minerals.

Laboratory experiments had previously shown that periclase is weaker than bridgmanite and deforms more easily, but these experiments did not take into account how minerals behave on a time scale of millions of years. By incorporating these time scales into a complex computational model, Jackson and his colleagues discovered that the periclase grains are actually stronger than the surrounding bridgmanite.

“We can use the boudinage analogy in the rocky record, where boudins, which is French for sausage, develop in a layer of rigid, ‘stronger’ rock, between less competent, ‘weaker’ rocksJackson says.

“As another analogy, think about chunky peanut butter,” Jackson explains. “For decades we had thought that periclase was the ‘oil’ in peanut butter, acting as a lubricant between the harder bridgmanite grains. Based on this new study, it turns out that the periclase grains act like the ‘nuts’ in chunky peanut butter. The periclase grains simply follow the flow but do not affect viscous behavior, except in circumstances where the grains are strongly concentrated. We show that, under pressure, mobility is much slower in periclase than in bridgmanite. There is a reversal of behavior: periclase hardly deforms, while the majority phase, bridgmanite, controls deformation in the Earth’s deep mantle.”

Understanding these extreme processes that take place deep below our feet is important to creating accurate four-dimensional simulations of our planet, and it also helps us better understand other planets. Thousands of exoplanets have already been confirmed, and discover more about the physics of minerals in extreme conditions provides new insights into the evolution of planets radically different from ours.