Geology and genesis | Clips resources


The rocks of the canadian and australian offer clues on the origin of life

by Joshua Davies, professor, Sciences of the Earth and of the atmosphere, University of Quebec at Montreal, and Jesse Reimink, assistant professor, geosciences, Pennsylvania State University | published with the permission of The Conversation | June 17, 2020

The rocks on the surface of the Earth modern are broadly divided into two types: felsic and mafic. In general, the rocks felsic have a relatively low density – for a rock – and a light color because they are made from minerals whitish rich in silicon and aluminum. Half Dome in California is made of granite, a rock felsic. The mafic rocks, in contrast, have a comparatively high density and a dark color because they contain minerals rich in iron and magnesium. The Giant’s Causeway in Northern Ireland is made of basalt, a rock, mafic.

The Giant’s Causeway in Northern Ireland is a
rock formation unusual, consisting of mafic rocks.
(Photo: Bruno Biancardi / Shutterstock)

The difference in density between the rocks felsic and mafic means that the rocks are felsic are more floating and are therefore at higher altitudes above the earth’s mantle (the layer inside the earth between the crust and the core). For this reason, the rocks felsic are the continents of the Earth, while the crust to lower altitude under the oceans is mafic.

The mechanisms that separated the rocks on the surface of the Earth in these two groups may also have created the environment necessary for life to thrive there has to be 4.3 million years ago, very early in the history of the Earth.

The separation in these two types of rocks is the result of plate tectonics: where the tectonic plates separate and move apart, the rocks below become depressurized, melt and fill the space between them, as the mid-atlantic ridge. The rock that fills the space between the plates is mafic.

When a plate slides under another, the fluid released from the lower plate causes melting in the mantle. These masses melted must pass through the upper plate to reach the surface. On their way to the surface, they undergo a series of processes called fractional crystallization, that can turn masses of mafic in masses felsic.

Establish time limits

The time when this separation occurred is the subject of a major debate in earth sciences because it can allow us to determine when the Earth became habitable for life. Many earth scientists believe that the alteration of the continental crust may have provided the nutrients necessary for life to thrive. Identify when the first continents were formed indicates when this could happen.

The Earth scientists also wonder whether the process of tectonic plates in the past were the same as those that occur today, and if they were even needed to form a continental crust in the past. The first continental crust may have been formed by the interaction of the oceanic crust and plumes in the mantle heat from the earth’s core. Another theory suggests that the continental crust was formed by the bombardment of meteorites.

The former iron ore mine in Jack Hills in western Australia
region with the most ancient rocks known to the world.
(Photo: SRK Consulting)

The exact mechanism is important for understanding the history and evolution of the Earth and can help to understand the processes that could occur on other planets.

Examination of the files

Our recent study has examined the oldest geological material from the Earth. The results suggest that the Earth is separated already in these two types of rocks there are 4.3 billion years, actually since the beginning of the reports of the geological of the Earth. Our data have also provided an overview intriguing of the tectonic processes that could occur at this time.

The origin of the continental crust is widely debated in part because the more you go back in time, the less there was of rocks to study. Samples of the complex Acasta Gneiss in the northwest Territories date back about four billion years – the oldest rocks known on Earth. These rocks of the Acasta Gneisses are felsic and composed of tonalite-trondhjémite-granodiorite.

There are very few samples former of the Earth, the most famous being the zircons of Jack Hills. Up to 4.3 billion years ago, 300 million years older than the Acasta Gneiss. These are tiny grains of zircon mineral that has been stripped of their rock-parental (the rock in which they originally crystallized).

These zircons are found in sediments, much more young people in Australia, which means that it is difficult to determine what type of rocks these minerals originally came, leaving open the question of whether there had been a continental crust during the first period of the history of the Earth.

Connections continental

In our recent study, we have compared all aspects of the chemistry of the zircon crystals of the rocks Acasta to the zircons of Jack Hills to see if they could have been formed in a similar environment.

The rocks of the canadian and australian offer clues on the origin of life
A satellite image in true-color contrast-enhanced
(Landsat 5) of the Jack Hills in western Australia.
(Image: Gretarsson / CC BY)

We found that the two sets of grains of zircon are chemically identical, suggesting that they are formed from the same types of rocks, and probably in the same types of settings, tectonic. This means that the earth may have begun to create a crust of continental type with very little time after its formation.

The chemical composition of the two suites of zircon crystals suggests that they are developed in magmas that originated at a great depth in the earth. The deepest origins of all magmas are a typical sign of subduction on earth modern.

We compared the amount of uranium in the crystals to the amount of ytterbium, a rare element. When a magma forms at great depth, the garnet mineral is often present, which collects the ytterbium. This means that less of ytterbium is absorbed by the crystals of zircon, which suggests that a relative lack of ytterbium indicates that these magmas were formed in deep environments.

The zircons of Jack Hills are known to have crystallized at relatively low temperatures. We found that the temperatures of zircons Acasta correspond exactly to the zircons of Jack Hills, which once again indicates their similarity.

Find the beginning

In the end, our results indicate that the tectonic processes that occur at the beginning of the recording geological may not have been so different from the process occurring thereafter. The evidence that things were not very different from the Land modern provides intriguing information on the potential origin of life and habitability of the early Earth, confirming perhaps that life was present early in the history of the Earth.

Joshua Davies receives funding from the research Council natural sciences and engineering of Canada and the Université du Québec à Montréal.

Jesse Reimink receives funding from the Pennsylvania State University and the U. S. National Science Foundation.
The conversation

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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