More than 400 miles below us is a massive world of extreme temperatures and pressures that has been churning and evolving for longer than humans have been on the planet. A new model from Caltech researchers shows that the processes involved are actually opposite to what had been previously theorized.
“Despite the enormous size of the planet, the deeper parts are often overlooked because they’re literally out of reach—we can’t sample them,” said team scientist Jennifer Jackson. “Additionally, these processes are so slow they seem imperceptible to us. But the flow in the lower mantle communicates with everything it touches; it’s a deep engine that affects plate tectonics and may control volcanic activity.”
The lower mantle of the earth is solid rock, but over hundreds of millions of years it slowly oozes, like a thick caramel, carrying heat throughout the planet’s interior in a process called convection.
There are many unknowns about the mechanisms that allow this convection to happen. The extreme pressures and temperatures at the lower mantle – up to 135 gigapascals and thousands of degrees Fahrenheit – make it difficult to simulate in the laboratory. As a comparison, the pressure at the lower mantle is almost a thousand times the pressure at the deepest point in the ocean.
The lower mantle is largely made up of a magnesium silicate called bridgmanite but also includes a small but significant amount of magnesium oxide called periclase mixed in among the bridgmanite in addition to small amounts of other minerals. Laboratory experiments had previously shown that periclase is weaker than bridgmanite and deforms easier, but these experiments did not take into account how minerals behave on a timescale of millions of years.
“As another analogy, think about chunky peanut butter,” said Jackson. “We had thought for decades that periclase was the ‘oil’ in peanut butter, and acted as the lubricant between the harder grains of bridgmanite. Based on this new study, it turns out that periclase grains act as the ‘nuts’ in chunky peanut butter.”
Understanding these extreme processes is important for creating accurate four-dimensional simulations of our planet and helps us comprehend more about other planets as well.
https://phys.org/news/2023-01-results-reveal-behavior-minerals-deep.html