Johannesburg: Heavy bombardment of Earth by asteroids from space over 3.8 billion years ago contributed to the formation of the early evolved crust on our planet, that later gave rise to continents, a study has found.
In a time period called the Hadean eon, our planet Earth was constantly bombarded by asteroids, which caused the large-scale melting of its surface rocks, said researchers from the University of the Witwatersrand in South Africa said in a statement.
Most of these surface rocks were basalts, and the asteroid impacts produced large pools of superheated impact melt of such composition. These basaltic pools were tens of kilometres thick, and thousands of kilometres in diameter, according to the research published in the journal Nature Communications.
"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," said Professor Rais Latypov from the University of the Witwatersrand.
The subsequent fate of these ancient, giant melt sheet remains, however, highly debatable. By studying the younger impact melt sheet of the Sudbury Igneous Complex (SIC) in Canada, the team inferred that ancient asteroid impacts were capable of producing various rock types from the earlier Earth's basaltic crust.
Most importantly, these impacts may have made the crust compositionally more evolved, i e silica-rich in composition. The SIC is the largest, best exposed and accessible asteroid impact melt sheet on Earth, which has resulted from a large asteroid impact 1.85 billion years ago, researchers said.
This impact produced a superheated melt sheet of up to five kilometres thick. The SIC now shows a remarkable magmatic stratigraphy, with various layers of igneous rocks.
"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 crystallisation," said 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.
"These impacts made the crust compositionally more evolved ? in other words, silica-rich in composition," said Latypov. "Traditionally, researchers believe that such silica-rich evolved rocks -- which are essentially building buoyant blocks of our continents -- can only be generated deep in the Earth, but we now argue that such blocks can be produced at new-surface conditions within impact melt pools," Latypov said.