A new study has revealed that the heavy bombardment of the Earth by asteroids likely helped in forming the planet’s crust. During the Hadean eon over 3.8 billion years ago, Earth was constantly blasted by asteroids, which melted rocks into huge pools made up of basalts.
“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 in South Africa.
While some models suggest that these melt sheets crystallized back into their original composition, a new model shows that the sheets may have contributed to the chemical evolution of the Earth’s crust by producing igneous rocks, which form at the surface of the crust.
The current study was focused on the impact melt sheet of the Sudbury Igneous Complex (SIC) in Canada. The SIC is the largest and most accessible asteroid impact melt sheet on Earth and was formed by a large asteroid impact 1.85 billion years ago.
“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,” said Professor 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. 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.”
The study is published in the journal Nature Communications.