? How was the land divided on the surface of the Earth into multiple continents
To this day, the Earth is the only planet divided into continents, as it is known. It's not clear exactly how it formed and evolved, but we know - because the edges of the continents that are thousands of miles apart - that once upon a time, Earth's land mass was concentrated into one large supercontinent.
Since that's not what the planet looks like today, something must have caused that giant continent to break up. Now, we have new evidence that giant meteorite impacts played an important role.
The hard evidence includes crystals of the mineral zircon, which is mined from a craton in Western Australia, a piece of Earth's crust that has been stable for more than a billion years. Known as the Pilbara Craton, it is the best preserved piece of crust on the planet. The zircon crystals within them contain evidence of an ancient meteorite impact before the breakup of the continents.
Geologist Tim Johnson of Curtin University in Australia explained that studying the oxygen isotope composition of these zircon crystals revealed a top-down process that begins with melting rocks near the surface and progresses deeper, consistent with the geological forcing of giant meteorite impacts.
He added that the research provides the first strong evidence that the processes that eventually formed the continents began with giant meteorite impacts, similar to those responsible for the extinction of the dinosaurs, which occurred billions of years ago.
The study was conducted on 26 rock samples containing parts of zircon dating back between 3.6 and 2.9 billion years.
The research team carefully analyzed the oxygen isotopes. Specifically, the ratios of oxygen-18 and oxygen-16, which contain 10 and 8 neutrons, respectively. These ratios are used in ancient geology to determine the temperature of formation of the rocks in which the isotopes are present.
Based on these lineages, the team was able to distinguish three distinct and essential stages in the formation and evolution of the Pilbara Craton.
The first stage is the formation of a large proportion of zircon consistent with partial dissolution of the crust. The researchers showed that this partial melting was likely the result of a bombardment of meteorites, which heated up the planetary crust upon impact.
The oldest set of these zircons, according to the team's explanation, were the result of a single giant impact that led to the formation of a kraton.
The second stage was a period of reframing and stabilization of the cortex core.
The third stage is the period of melting and formation of granite. Then, this stable core would evolve into today's continents, as cratons found on other continents around the world have done.
However, in the past, many meteorites have pelted Earth in numbers much higher than the number of continents. Only larger impacts could generate enough heat to form a craton, which appears to be twice the thickness of the surrounding lithosphere. These results are consistent with previously proposed models of craton formation around the world but constitute, the researchers said, the strongest evidence to date for this theory.
However, it is just one kraton, out of about 35 known. The team will need to compare their results with more samples from other cratons, to see if their model is globally consistent and thus hold the evidence stronger.
Johnson said that data on other regions of Earth's ancient continental crust appear to show patterns similar to those recognized in Western Australia, and that scientists are seeking to test their findings on these ancient rocks to see if our model is applicable more broadly.
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