Previous research estimated that it took hundreds of million years for early Earth’s magma ocean to solidify, but new research narrows these large uncertainties down to less than just a couple of million years.
An illustration of Earth as it existed during part of its formation billions of years ago, when an ocean of magma covered its surface and stretched thousands of km deep into the core; a typical cell from a simulation conducted by Bajgain et al. with the relative positions of atoms are shown in the left. Image credit: Suraj Bajgain / Lake Superior State University.
“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 Dr. Mainak Mookherjee, a researcher at Florida State University.
“When magma cools, it forms crystals. Where those crystals end up depends on how viscous the magma is and the relative density of the crystals.”
“Crystals that are denser are likely to sink and thus change the composition of the remaining magma.”
“The rate at which magma solidifies depends on how viscous it is.”
“Less viscous magma will lead to faster cooling, whereas a magma ocean with thicker consistency will take a longer time to cool.”
Like this research, previous studies have used fundamental principles of physics and chemistry to simulate the high pressures and temperatures in the Earth’s deep interior.
Scientists also use experiments to simulate these extreme conditions.
But these experiments are limited to lower pressures, which exist at shallower depths within the Earth.
They don’t fully capture the scenario that existed in the planet’s early history, where the magma ocean extended to depths where pressure is likely to be three times higher than what experiments can reproduce.
To overcome those limitations, Dr. Mookherjee and colleagues ran their simulation for up to six months. This eliminated much of the statistical uncertainties in previous works.
“Earth is a big planet, so at depth, pressure is likely to be very high,” said Dr. Suraj Bajgain, a researcher at Lake Superior State University.
“Even if we know the viscosity of magma at the surface, that doesn’t tell us the viscosity hundreds of km below it. Finding that is very challenging.”
The research also helps explain the chemical diversity found within the Earth’s lower mantle.
Samples of lava from ridges at the bottom of the ocean floor and volcanic islands like Hawaii and Iceland crystallize into basaltic rock with similar appearances but distinct chemical compositions, a situation that has long perplexed Earth scientists.
“Why do they have distinct chemistry or chemical signals?” Dr. Mookherjee said.
“Since the magma originates from underneath the Earth’s surface, that means the source of the magma there has chemical diversity.”
“How did that chemical diversity begin in the first place, and how has it survived over geological time?”
“The starting point of chemical diversity in the mantle can be successfully explained by a magma ocean in the Earth’s early history with low viscosity.”
“Less viscous magma led to the rapid separation of the crystals suspended within it, a process often referred to as fractional crystallization.”
“That created a mix of different chemistry within the magma, rather than a uniform composition.”
The results were published in December 2022 in the journal Nature Communications.
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S.K. Bajgain et al. 2022. Insights into magma ocean dynamics from the transport properties of basaltic melt. Nat Commun 13, 7590; doi: 10.1038/s41467-022-35171-y
