Earth’s oldest evolved rocks, found in part of the Acasta Gneiss Complex in northwestern Canada, have compositions different from those that make up the planet’s ancient continental crust, suggesting the former were created in a different process, possibly through asteroid impacts on early Earth’s crust.
During the eras known as Earth’s Hadean and early Archeon epochs, between 4.5 billion and 3.9 billion years ago, the solar system was a much more violent place. Many asteroids likely hit the Earth during this period, melting surface rocks and recycling them in the process, noted researchers at Australia’s Curtin University, who conducted a study on the subject.
Led by Tim Johnson of Curtin University’s School of Earth and Planetary Sciences, a research team used computer models to illustrate the partial melting of iron-rich basaltic rocks caused by impacting asteroids.
“The melting of these rocks at such shallow levels is most easily explained by meteorite impacts, which would have supplied the energy to attain the extreme temperatures required for melting,” Johnson said.
“Our computer simulations of asteroid impacts show that not only is this scenario physically plausible, but the region of shallow melting needed to form these ancient evolved rocks would have been widespread. Given the predicted high flux of meteorites about four billion years ago, impact melting may have been the predominant mechanism that generated granitic rocks at that time.”
Granite is an igeneous rock that forms from the slow crystallizing of subsurface magma. Created from the cooling and solidifying of molten material, igneous rock can form both on and/or below Earth’s surface.
Phil Bland, also of Curtin University’s School of Earth and Planetary Sciences, noted that almost no four-billion-year-old rocks are found on Earth because the numerous asteroid impacts at that time caused those rocks to melt and subsequently be recycled.
A paper on the study has been published in the journal Nature Geoscience.