Dense chunks of Earth's crust may have dripped into the mantle layer underneath it early in the planet's history, a new study suggests.
The study, detailed online Dec. 1 the journal Nature Geoscience, reveals the crust once behaved very differently than it does now, and helps shed light on how the cores of modern continents were born, scientists said.
During the Archean eon that began about 4 billion years ago, some 600 million years after Earth was born, the planet retained more of the heat of its creation and had more radioactive matter than it does now, making the world's innards much hotter than they are currently. This led more of the mantle layer to melt, and this molten rock would have risen upward and cooled to become part of an ancient, primitive crust that was much thicker than it is today.
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"At mid-ocean ridges on the modern Earth, temperatures are such that around 5 to 10 percent of the mantle melts to produce crust of around 5 to 10 kilometers [3 to 6 miles] thickness," said study lead author Tim Johnson, a petrologist at the University of Mainz in Germany. In contrast, under the much higher mantle temperatures during the Archean, "40 percent or more of the mantle may have melted, and would have produced a crust perhaps 40 kilometers [24 miles] or more in thickness," he said.
However, the amount of this ancient crust remaining today is low. This suggests that much of it was recycled back into the mantle, but scientists were uncertain how that happened.
Dripping crust
New computer models from Johnson and his colleagues suggest the rock at the base of this ancient thick crust was denser than the hot mantle underneath it. This would cause large portions of the crust to sink, dripping straight downward.
In contrast, the tectonic plates that make up Earth's crust nowadays mostly drift horizontally. Also, modern crust mostly gets recycled into the mantle at the border of tectonic plates, where one plate dives under another, not directly from the undersides of plates, Johnson said.
These findings could also help explain a conundrum over the nature of Archean crust uncovered after past studies of some of the oldest features of Earth's crust — the so-called tonalite–trondhjemite–granodiorite complexes found in areas such as Scotland and Greenland. These conglomerations of rocks are most commonly present in cratons, the oldest and most stable cores of modern continents.
These ancient complexes most likely originated from a source low in the element magnesium, but prior calculations suggested the ancient crust should have been high in magnesium. These new findings suggest that after the dense crust sunk into the mantle, it would generate a return flow of mantle rock that would melt to potentially generate the kind of magnesium-poor crust required for these ancient rocks.
"We have shown how the early Earth might have operated," Johnson told LiveScience.
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