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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, to our knowledge, life. But the Earth in its present form is about 4.5 billion years old; it is quite different from what it was in a much older time.
Details on how, when and why the beginnings of the planet’s history unfolded, as they have largely escaped scientists, mainly due to the scarcity of preserved rocks from this geological period.
Our research, published today in Nature, reveals that Earth’s first continents were changing entities. They disappeared and reappeared over 1.5 billion years before finally taking shape.
Primitive Earth: A Strange New World
The first 1.5 billion years of Earth’s history were a tumultuous time that set the stage for the rest of the planet’s journey. Several key events took place, including the formation of the first continents, the emergence of land and the development of the early atmosphere and oceans.
All of these events were the result of the changing dynamics of the Earth’s interior. They were also the catalysts for the first appearances of primitive life.
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The preserved record of Earth’s first 500 million years is limited to a few tiny crystals of the mineral zircon. Over the next few billion years, fragments of rock several kilometers (and longer) were generated and preserved. These would continue to forge the nuclei of the great continents.
Scientists know the properties of rocks and the chemical reactions that must take place for their constituent minerals to be produced. Based on this, we know that early Earth had very high temperatures, hundreds of degrees warmer than today.
Read more: Earth’s strong ties between Canada and Australia contain clues to the origin of life
An epic metamorphosis
The earth’s crust today is a thick, floating continental crust that stands proudly above the sea. Meanwhile, beneath the oceans lie thin but dense oceanic crusts.
The planet is also divided into a series of plates which move in a process called “continental drift”. In some places these plates separate 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 its interior is released into space. This results in volcanic activity concentrated mainly at the edges of the plates. A good example is the Ring of Fire – a path along the Pacific Ocean where volcanic eruptions and earthquakes are common.
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”. It is the heat that remains since the formation of the Earth.
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Our modeling shows that the release of primordial heat during Earth’s early stages (which was three to four times hotter than today) caused significant melting in the upper mantle. It is the most solid region under the crust, between 10 km and 100 km deep.
This internal fusion created magma which, thanks to a plumbing system, was projected in the form of lava on the crust. The shallow mantle left behind, dry and rigid, fused to the crust and formed the first continents.
The pulse of the first life
Our research has revealed a time lag between the formation of the earth’s first crust and the development of 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 prone to destruction. It gradually strengthened over the next billion years to form the core of modern continents.
This process was crucial for the stability of the continents. When the magma was purged from the Earth’s interior, rigid rafts formed in the mantle beneath the new crust, protecting it from further destruction.
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In addition, the rise of these rigid continents eventually led to weathering and erosion, that is, when rocks and minerals break down or dissolve over long periods of time only to be washed away and deposited. in the form of sediment.
Early erosion would have changed the composition of the Earth’s atmosphere. It would also have provided nutrients to the oceans, sowing the development of life.
From our observations, we conclude that the rupture of the Earth’s first crust was necessary to make way for a more robust replacement. And if that hadn’t happened, we wouldn’t have the continents, or life as we know it.
Read more: Magnetism of Himalayan rocks reveals complex tectonic history of mountains
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