did asteroids and comets help create our continents?

Asteroids and comet brought our planet water and the chemical precursors for life. But could they have also given rise to our continents and help set plate tectonics in motion?

comets bombarding planet

Plate tectonics is one of the most important discoveries in geology, allowing us to understand how our planet’s landmasses evolve without having to invent sunken continents and ponder the implications of lost civilizations that never existed. But one question it doesn’t answer is how the tectonic plates and continents were set in motion in the first place. We know that during Earth’s early days, as the surface was still trying to cool and settle, rocks that became our continents bubbled up from molten basalts tens of miles under the surface. How exactly that happened, however, is a still subject of debate because we’re still trying to account for the role played by comets and asteroids constantly slamming into the surface during our planet’s formation.

If we got into a time machine and went back to the Hadean Epoch, we’d find an alien world glowing red and orange as vast lakes of magma swirl and give off toxic fumes. Meteors and comets would steak across the sky, periodically slamming somewhere beyond the mountains with a heavy thud, some of them large enough to shake the ground under our feet. In a few hundred million years, these lakes will cool and out of them will arise the first building blocks of what we think was the first continent, Ur, which is believed to have slowly grown into the first supercontinent, Vaalbara. The big question is whether all those comets and asteroids might have helped make this possible, and if so, what role did they play and how.

According to a new paper, the answer is yes. By studying the oldest surviving asteroid melt sheet, the Sudbury Igneous Complex, geologists saw that impacts of extraterrestrial bodies helped separate different layers of rock, acting as agitators and helping along the evolution of silica-rich layers which make up the contents we live on today. This is actually very much in line with the evidence noted by aforementioned study which found that continents probably bubbled out of molten basalts. That paper’s main contribution was to show that instead of being formed 60-some miles underground, the buoyant rocks of our crust were created much closer to the surface.

If asteroids gave a helping hand, it means the rocks on which we live could’ve been formed in even shallower basalts, just a few miles under the molten lakes of the late Hadean and early Archean. And more importantly, it means that other rocky worlds in the universe go through a similar process since we know that constant bombardment in a new planet’s first few hundred million years is a given, and since we seem to understand what planets are made of and how they come together, it’s hardly a stretch to draw this conclusion. In other words, if this finding is correct, we can now say that not only do we know how our continents must have formed, we also know what to expect on other planets as we survey the cosmos.

See: Latypov, R., et. al,. Evidence for igneous differentiation in Sudbury Igneous Complex and impact-driven evolution of terrestrial planet proto-crusts. Nature Communications, 2019; 10 (1) DOI: 10.1038/s41467-019-08467-9

Nagel, T. J., et, al., Generation of Eoarchean tonalite-trondhjemite-granodiorite series from thickened mafic arc crust. Geology, 2012; DOI: 10.1130/G32729.1

# science // asteroids / comets / early earth / geology

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