As you read this, more than 400 miles below you is a massive world of extreme temperatures and pressures that has been churning and evolving for longer than humans have been on the planet. Now, a detailed new model from Caltech researchers illustrates the surprising behavior of minerals deep in the planet’s interior over millions of years, and shows that the processes actually occur in a manner completely opposite to what was previously theorized .
The research was conducted by an international team of scientists, including Jennifer M. Jackson, the William E. Leonhard Professor of Mineral Physics. A paper describing the study appears in the journal The nature on January 11.
“Despite the planet’s enormous size, the deep parts are often overlooked because they’re literally out of reach—we can’t sample them,” says Jackson. “Furthermore, these processes are so slow that they seem imperceptible to us. But the flow in the lower mantle 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 flows slowly, like thick caramel, carrying heat into the planet’s interior in a process called convection.
Many questions remain unanswered about the mechanisms that enable this convection. The extreme temperatures and pressures in the lower mantle—up to 135 gigapascals and thousands of degrees Fahrenheit—make it difficult to simulate in the laboratory.
For reference, the pressure in the lower mantle is almost a thousand times the pressure in the deepest point of the ocean. Thus, while many laboratory experiments on mineral physics have provided hypotheses about the behavior of rocks in the lower mantle, the processes that occur on geologic timescales to drive the slow flow of lower mantle convection have been uncertain.
The lower mantle is mostly composed of a magnesium silicate called bridgmanite, but also includes a small but significant amount of magnesium oxide called periclase mixed in with the bridgmanite, in addition to small amounts of other minerals. Laboratory experiments have 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. When they incorporated these timescales into a complex computational model, Jackson and his colleagues found that the periclase grains were actually stronger than the bridgmanite around them.
“We can use the analogy of boudinage in the rock record [image at right]where boudins, which means sausage in French, develop in a stiff, ‘stronger’ layer of rock among less competent, ‘weaker’ rocks,” says Jackson.
“As another analogy, think of thick peanut butter,” Jackson explains. “We thought for decades that periclase was the ‘oil’ in the peanut butter and acted as a lubricant between the harder grains of bridgmanite. Based on this new study, it has been shown that periclase grains act as the “nuts” in fat peanuts. butter. Periclase grains only go with the flow, but do not affect viscous behavior except in circumstances where the grains are highly concentrated. We show that under pressure, mobility is much slower in periclase compared to bridgmanite. There is a reversal of behavior: periclase hardly deforms, while the major phase, bridgmanite, controls deformation in the Earth’s deep mantle.”
Understanding these extreme processes happening far below our feet is important to creating accurate four-dimensional simulations of our planet and helping us understand more about other planets. Thousands of exoplanets (planets outside our solar system) have now been confirmed, and the discovery of more about the physics of minerals under extreme conditions offers new insights into the evolution of planets radically different from our own.
Patrick Cordier et al., Periclase deforms more slowly than bridgmanite under mantle conditions, The nature (2023). DOI: 10.1038/s41586-022-05410-9
Provided by the California Institute of Technology
Citation: New results reveal surprising behavior of deep Earth minerals (2023, January 12) Retrieved January 12, 2023 from https://phys.org/news/2023-01-results-reveal-behavior-minerals-deep.html
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