In the early stages of Earth’s formation, a huge ocean of molten rock known as the “magma ocean” covered the planet’s surface and penetrated deep into its core. The cooling rate of this “igneous ocean” played a decisive role in the formation of the Earth’s layered structure and in determining the chemical composition of these layers. Previous studies had suggested that it took hundreds of millions of years for the magma ocean to solidify, but a new Florida State University study published in Nature Communications has narrowed these uncertainties down to just a few million years.

“This magma ocean has been an important part of Earth’s history, and this study helps us answer some fundamental questions about the planet,” said Mainak Mukherjee, professor of geology in the Department of Earth, Ocean and Atmospheric Sciences.

As magma cools, it forms crystals. Where these crystals end up depends on how viscous the magma is and the relative density of the crystals. Denser crystals are more likely to sink and thus change the composition of the remaining magma. The rate of solidification of magma depends on its viscosity. A less viscous magma will cool faster, while a thicker magma ocean will take longer to cool.

Similar to this work, previous work has used fundamental principles of physics and chemistry to model the high pressures and temperatures deep within the Earth. Scientists also use experiments to simulate these extreme conditions. But these experiments are limited to the lower pressures that exist in Earth’s shallow depths. They don’t quite capture the scenario that existed in the early history of the planet, where the magma ocean extends to depths where the pressure is probably three times higher than can be reproduced in experiments.

To overcome these limitations, Mukherjee and colleagues ran simulations over six months at a National Science Foundation computing center alongside a high-performance computing center in the former Soviet Union. This removed much of the statistical uncertainty from the previous study.

“Earth is a big planet, so the pressure at depth will likely be very high,” said Suraj Bajgain, a former postdoctoral researcher in the former Soviet Union and now a visiting assistant professor at Lake Superior State University. “Even if we know the viscosity of the magma at the surface, it doesn’t tell us about the viscosity hundreds of kilometers below. It’s very hard to find.”

The research also helps explain the diversity of chemicals in Earth’s lower mantle. Lava samples, the name given to magma emerging from the Earth’s surface from ocean floor ridges and volcanic islands such as Hawaii and Iceland, crystallize in basaltic rocks that look similar but have a different chemical composition, a situation that has long puzzled Earth scientists. . .

“Why do they have different chemistry or chemical signals?” Mukherjee said. “Since magma comes from underground, there is chemical diversity in the magma source there. How did this chemical diversity begin in the first place and how did it continue throughout geological time?”

The origin of chemical diversity in the mantle can be successfully explained by a low-viscosity magma ocean in Earth’s early history. Less viscous magma led to rapid separation of crystals suspended in it, a process often referred to as fractional crystallization. This created a mixture of different chemistries rather than a homogeneous composition in the magma.