Earth is unique among the known planets in having continents, and their formation fundamentally influenced the habitability of our planet. It is generally recognized that Earth’s continental crust formed by internal processes. However, new research by a team at Curtin University challenges this theory by ascribing incipient continent formation to high-energy comets bombarding early Earth.
Led by Chris Kirkland, the team studied the mineral zircon from Earth’s oldest continents, the North American craton in Greenland and the Pilbara craton in Western Australia. Crystals were taken from cratons because they are the oldest and most stable portions of a continent. Zircon crystals are ideal timekeepers because they contain trace amounts of uranium (U), isotopes that decay at a known rate to become lead (Pb). When zircon is analyzed, the amount of Pb produced by the decay of U corresponds to the amount of time that has passed since the zircon crystallized. Additionally, isotopes of hafnium (Hf) in zircon track the episodic influxes of magma that contribute to continent formation. Analyses of U decay reveal that both cratons formed between 2.8 and 3.8 billion years ago, while time spectrum analysis of Hf isotopes shows that they underwent periods of increased continental crust formation every ~170-200 million years. This periodicity matches the “galactic year,” which is the time required for the Sun to orbit around the Milky Way Galaxy. During this orbit, our solar system moves through the spiral arms of the Milky Way, where the density of stars is high, resulting in an increased frequency of comet bombardment on Earth. Kirkland and his team hypothesize that when these high-energy comets impact Earth, they initiate increased melting in the mantle, acting as nuclei for new continental crust formation. These findings oppose the leading theory that Earth formed in isolation and present an exciting link between terrestrial geologic processes and the movement of our solar system. READ MORE