American geologists tested whether there was a connection between the collision of the proto-Earth with the hypothetical planet Theia and the first subduction of the Earth. Scientists have created two- and three-dimensional models of this mechanism to recreate the conditions of ancient geodynamics.
A big question in Earth geology is why tectonic plates collide and crawl over each other (geologists call this subduction). Our planet is unique because it is the only planet in the Solar System that has active plate tectonics. Venus, for example, is similar to Earth in mass, size, density, and even gravity (it is also called Earth’s sister), but the tectonics there are likely organized differently, with Venus’ magmatism being intra-plate. No plate motion is observed on Mars, another nearby Earth-type planet.
The causes of the Earth’s sinking are still controversial; researchers can evaluate geodynamics from ancient minerals. For example, analysis of zircon rocks in Western Australia showed that the formation of the Earth’s crust began 4.35 billion years ago. Interestingly, about 200 million years ago, the proto-Earth collided with the hypothetical planet Theia (but many consider this hypothesis obsolete), resulting in the formation of the Moon.
There is an assumption that both of these processes (the pattern of formation of the Moon and the beginning of the sinking of the Earth) are interconnected. In it, versions were expressed that plate tectonics was caused by a mantle plume or the impact of a large cosmic body.
This time, four American geologists from the University of California decided to check whether the collision with Theia triggered the first sinking of the Earth. To do this, scientists created a two-dimensional model of the early geodynamics of the entire mantle of the Catharchean Eon (4.567-4.031 billion years ago) and then tested the plate tectonics mechanism on three-dimensional models. The results of the scientific study are published in the journal. Geophysical Research Letters .
The models created relate to the period of 50 million years after the solidification of the liquid magma ocean. According to the study’s authors, the temperature of the core-mantle boundary was higher than now; Not 3473-4573 kelvins, but 5273 kelvins (the area was heated by iron deposits from hypothetical Theia). Such conditions can cause partial or complete melting of this layer. According to the models, the subduction process occurred as follows. After Theia crashed into Earth, part of the hypothetical planet was left with two large regions of low shear rates. They are located at the boundary between the lower mantle and the core.
A mantle plume is born in these giant structures 99 million years after the event. It rises towards the upper mantle (you can imagine drops for ease of understanding) and contracts due to low viscosity. As it approaches the already compressed lithospheric layer, the cloud weakens the lithosphere and, according to simulations, penetrates it after 109 million years. At high temperature, a wedge-shaped formation is formed that is not very viscous. 123 million years after the collision, this wedge separates the lithosphere and lowers its edges, creating subduction zones that the plates already support on their own.
However, a similar scenario is possible beyond the 80 megapascal fluidity of the lithosphere. At 100 megapascals subduction occurs only on one side, and at 150 megapascals plumes cannot penetrate the lithosphere. Furthermore, the process does not start at the low temperature of the mantle-core boundary: it continues until it cools to ≥ 3773 Kelvin, but does not occur at 3273 Kelvin.
Scientists tested this mechanism on several three-dimensional models; This was confirmed, except that subduction did not occur at the 100-megapascal yield point of the lithosphere. One way or another, researchers have proven that the temperature at the core-mantle boundary is an important condition for plate tectonics on Earth. And this region is heating up thanks to the accretion of Theia’s hypothetical core during the collision 4.5 billion years ago, part of the study’s authors’ modeling.
Source: Port Altele
