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Our planet is unique in the solar system. It is the only one with active plate tectonics, ocean basins, continents and, as far as we know, life. But the Earth in its present form needs 4.5 billion years; it is completely different from what it was in a much earlier era.
The details of how, when and why the planet’s early history unfolded as it did have largely eluded scientists, largely due to the scarcity of preserved rocks from this geological period.
Our research, published today in Nature, reveals that Earth’s first continents were ever-changing entities. They disappeared and reappeared over 1.5 billion years before finally taking shape.
Primeval land: a strange new world
The first 1.5 billion years of Earth’s history was a tumultuous period that set the stage for the rest of the planet’s journey. Several key events have occurred, including the formation of the first continents, the emergence of the earth, and the development of the first atmosphere and oceans.
All of these events were the result of the changing dynamics of the Earth’s interior. They were also catalysts of the first appearances of primitive life.
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The preserved record of the first 500 million years of the Earth is limited to a few tiny crystals of the mineral zircon. Over the next billion years, kilometer-long (and larger) fragments of rock were generated and preserved. These would continue to forge the nuclei of the main continents.
Scientists know the properties of rocks and the chemical reactions that must occur for their constituent minerals to be made. Based on this, we know that the early Earth boasted very high temperatures, hundreds of degrees warmer than today.
Read more: Earth’s solid connections between Canada and Australia contain clues to the origin of life
An epic metamorphosis
The earth’s crust today consists of a thick, floating continental crust that stands proudly above the sea. Meanwhile, there are thin but dense oceanic crusts beneath the oceans.
The planet is also divided into a series of plates which move in a process called “continental drift”. In some places, these plates recede and in others they converge to form mighty mountains.
This dynamic movement of the Earth’s tectonic plates is the mechanism by which heat from within is released into space. This results in volcanic activity concentrated mainly at the boundaries of the plates. A good example is the Ring of Fire, a path along the Pacific Ocean where volcanic eruptions and earthquakes are frequent.
To unravel the processes that operated on early Earth, we developed computer models to replicate its once much warmer conditions. These conditions were driven by large amounts of internal “primordial heat”. This is the residual heat from the first formation of the Earth.
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Our modeling shows that the release of primordial heat during the early stages of the Earth (which was three to four times warmer than today) caused extensive melting in the upper mantle. This is the mostly solid region under the crust, between 10km and 100km deep.
This internal fusion created magma which, through a hydraulic system, was pushed out like lava on the crust. The superficial mantle left behind, dry and rigid, bonded to the crust and formed the first continents.
The pulse of early life
Our research revealed a delay between the formation of the first earth’s crust and the development of the mantle keels at the base of the first continents.
The first crust formed, which was present between 4.5 billion and 4 billion years ago, was weak and subject to destruction. It got progressively stronger over the next billion years to form the core of modern continents.
This process was fundamental for the stability of the continents. As the magma was cleared from within the Earth, rigid rafts formed in the mantle beneath the new crust, protecting it from further destruction.
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Additionally, the rise of these rigid continents eventually led to weathering and erosion, which is when rocks and minerals break down or dissolve over long periods to eventually be transported and deposited as sediments.
Early erosion would have changed the composition of the Earth’s atmosphere. It would also provide the oceans with nutrients, sowing the development of life.
From our observations, we conclude that the rupture of the earth’s crust was necessary to make way for a more robust replacement. And if that didn’t happen, we wouldn’t have continents, nor life, as we know it.
Read more: The magnetism of Himalayan rocks reveals the complex tectonic history of the mountains
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