Convulsive origin of the continents

The origin of the continental crust, the solid soil in the shell of our planet, is still one of the greatest mysteries of the earth sciences. Where do continents come from? Where did the “mainland” come from? And how do they grow?

The first record of continental crust dates back a whopping 3.5 billion years, just 1.1 billion years after the formation of the Earth. Previously, according to our data, there was only water on the surface. Since its inception, numerous studies show that the continents have not stopped growing, although they disagree on the rate and mechanism of growth, and even more touchingly: they continue to do so.

Some models suggest that the growth of continents has been limited since their formation, arguing that the crust itself created is breaking down. Other models predict significant growth, especially concentrated over the last billion years.

Among the differences, everyone agrees that the continents are growing. But how do they do it? How does the ground we walk on grow? And, even deeper, what mechanism causes its growth?

Magmas as a starting point

Most studies use dating methods for zircon, a gemstone, as a starting point. Many civilizations have sought and treasured zircon without knowing that it is the most ancient gemstone ever. A study by scientists from the Universities of Wisconsin and Madison discovered the origin of zirconium crystallization exactly 4.4 billion years ago. Advanced methods and isotope systems are used to determine the age of the rock. However, despite the advances, the results are still subject to several sources of uncertainty.

But something is clear, namely that every process of continental crust growth begins with a cataclysm, with a geological convulsion, suggesting the formation of magma from the earth’s mantle, that this magma is embedded in the crust, and something basic that persists.

Thus, in order to find the origin, one must first decide what magmatism is responsible for the slow growth of the continents.

Arc magmatism and the birth of the Andes

According to the plate tectonics paradigm, crustal growth has historically been attributed to magmatism occurring in subduction zones, that is, where a plate, usually oceanic, is intruded under an adjacent, usually continental one. Thus, more or less, and in a process that shakes the Earth like a maraca, the mainland pushes the earth away from the seabed.

The subsidence of the crust in the mantle causes the magma to grow and be incorporated into the continents. This process is known as arc magmatism. Thus the Andes and their volcanic belt were born.

However, estimates of the development of arc magmatism contain a contradiction. The inclusion of magmas will unbalance the continental mass floating on the mantle. And the system tends to balance: the “excess” magma returns back to the mantle.

If the same mass that collapses is created, arc magmatism is useless in explaining the formation of continents.

New hypothesis: post-collision magmatism

When the subduction ends, the continents, originally separated by the ocean, collide with each other. This shock, while seemingly immediate, could last tens of millions of years. And its consequences can last even longer in the form of large mountain ranges, as happened in the case of the Himalayas.

The thermal and tectonic instability caused by the collision gives rise to magmatism. This is called post-collision magmatism. When this magma intrudes into the earth’s crust, it forms large granite formations such as Gredos (Central System, Spain).

Post-collisional magmatism has traditionally been associated with the melting of the earth’s crust and has been sidelined in discussions trying to explain the origin of the continents.

In a new study by the Higher Council for Scientific Research and the University of Salamanca, we link post-collision magmatism to continental growth.

We used experimental and isotope arguments to conclude that post-collision magmas originate from the Earth’s mantle and are not the result of crustal reworking. This means that after the collision of two continents, the continental mass increases, which models have not taken into account until now.

Where do continents come from?

Now that we have some tools for this, it’s time to answer the original question: Where do continents come from?

Continents are solidified magma. Therefore, any magmatism capable of causing continents to grow must come from the mantle.

According to our latest discovery, since the beginning of plate tectonics, two major tectonic events have been able to create a new continental mass: subduction zones and continental collision.

It can be said that the continents, the pillar on which man lives, were formed from magmas that emerged from the mantle, released after the movements of the cataclysmic volume, either by subduction or by the subsequent collision between the continents.

Daniel Gomez-Frutos, PhD Student, High Council for Scientific Research (CSIC)

This article was originally published on The Conversation. Read the original.

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