Scientists have finally revealed how our planet got its crust.
The continents we see today all formed from supermassive pools of molten rock dozens of kilometers thick, and thousands of kilometers in diameter.
According to a recent study, more than 3.8 billion years ago, a time called the Hadean Eon, Earth’s continents were formed with the help of asteroids and comets.
During the Hadean, the surface of the Earth was unlike anything we see today. In fact, most of the surface rocks on Earth back then were basalts.
But with the help of extraterrestrial bombardment, asteroids that smashed into pour planet kickstarted the production of massive pools of molten rock that eventually resulted in the formation of continents.
“If you want to get an idea of what the surface of Earth looked like at that time, you can just look at the surface of the Moon which is covered by a vast amount of large impact craters,” explained Professor Rais Latypov from the School of Geosciences of the University of the Witwatersrand in South Africa.
But what exactly happened to these ancient, massive melt sheet remains after remains a highly debated question.
Some experts believe that upon cooling, they may have crystallized back into magmatic bodies of the same, broadly basaltic composition.
This theory suggests that asteroid impacts did not play a crucial role in the formation of the Earth’s early evolved crust.
However, a different model presupposes that these massive sheets underwent large-scale chemical changes to produce layered magmatic intrusions, such as the Bushveld Complex in South Africa.
This scenario involves asteroids playing an important role in the development of Earth’s early crust and may have helped contribute to this chemical change.
But none of these two theories can be tested since the ancient Hadean impact melts have long been obliterated by plate tectonics.
Nonetheless, by studying one of the largest, best exposed and accessible impact melt sheets on earth, the Sudbury Igneous Complex (SIC) in Canada, the group of scientists led by Latypov concluded that ancient asteroid impacts helped produce different rock types from the early Earth’s basaltic crust.
SIC resulted from a large asteroid impact believed to have occurred some 1.85 billion years ago.
It is believed that this massive collision produced a superheated melt sheet of up to 5 km thick.
Their new findings have been reported in a paper in Nature Communications.
“Our field and geochemical observations – especially the discovery of large discrete bodies of melanorites throughout the entire stratigraphy of the SIC – allowed us to reassess current models for the formation of the SIC and firmly conclude that its conspicuous magmatic stratigraphy is the result of large-scale fractional crystallization,” says Latypov.
“An important implication is that more ancient and primitive Hadean impact melt sheets on the early Earth and other terrestrial planets would also have undergone near-surface, large-volume differentiation to produce compositionally stratified bodies. The detachment of dense primitive layers from these bodies and their sinking into the mantle would leave behind substantial volumes of evolved rocks (buoyant crustal blocks) in the Hadean crust. This would make the crust compositionally layered and increasingly more evolved from its base towards the Earth’s surface.”