This article shows how geological movements have been able to promote the emergence of life on Earth.

The rocks on the surface of the Earth are mainly divided into two types: felsic and mafic. The felsic rocks are generally of relatively low density - for a rock - and of light color, consisting of whitish minerals rich in silicon and aluminum. The Half Dome in California is made of granite, a felsic rock. The mafic rocks are denser and darker because they contain minerals rich in iron and magnesium. The Giant's Causeway in Northern Ireland is made of basalt, a mafic rock. The difference in density causes the felsic rocks to be more floating and at higher altitudes above the Earth’s mantle (the layer within the Earth between the crust and the core). Thus, the felsic rocks constitute the continents whereas the earth's crust located under the oceans is magnificent. The mechanisms that split the rocks on the Earth’s surface into two groups may also have created the environment for the development of life 4.3 billion years ago, early in Earth’s history. The separation into two types of rocks is the result of plate tectonics: where the plates separate and move away, the rocks below depressurize, melt and fill the space between them, as in the mid-atlantic ridge. The rock filling the space between the plates is magnificent. When one plate slides under another, the fluids released by the lower plate cause the coat to melt. The magma thus created must pass through the top plate to reach the surface. On its way to the surface, it undergoes a series of processes called fractional crystallization which can transform the mafic masses into felsic masses.

Chronologically

The time when this separation happened is a time of great debate among geoscientists, because knowing it could determine when our planet became habitable. Many researchers believe that the alteration of the continental crust may have provided the nutrients necessary for life. Thus, by discovering the moment of formation of the first continents, one would have an indication of the time when life might have been born. Geoscientists are also investigating whether the ancient processes of tectonic plates were the same as those of today and whether they allowed the formation of the continental crust. The first continental crust may have been created by the interaction between the oceanic crust and the mantle plumes from the terrestrial core. Another theory was that it was a meteorite bombardment that caused the formation of the continental crust. By knowing this mechanism, one can better understand the history and evolution of the Earth, as well as the processes that could occur on other planets.

From continent to continent

In our study, we compared all the chemical aspects of zircon crystals from Acasta rocks and Jack Hills zircons to see if they could have formed in a similar environment. We found that these zircon grains are all chemically identical, suggesting that they were created from the same rock types and probably in similar tectonic contexts. This means that the Earth may have begun to possess a continental crust very shortly after its formation. The chemical composition of these zircon crystals suggests that they have been created in magmas from deep Earth. Such magmas are a sign of subduction on contemporary Earth.