An investigation by the National Museum of Natural Sciences (MNCN), the Andaluz Institute of Earth Sciences (IACT) —both CSICs— and the University of Salamanca experimentally demonstrated that magma contributions originating in post-subduction events, when a tectonic plate is inserted under another, they come from the Earth’s mantle and not from the recycling of the crust, as was thought.
This discovery, published in Earth and Planetary Science Cardsit represents an important advance in the knowledge of the mechanisms that cause the growth of the continents and, therefore, a better understanding of the planet.
Discovery changes the scientific model that explains how new continental crust is created
The Earth’s crust has slowly but steadily increased in size since its formation some 3.5 billion years ago. Until now, the scientific paradigm attributed this contribution of new material to processes linked to the subsidence of the oceanic crust under the continental crust or tectonic processes of subduction, as occurs in the Andes.
“When that happens, new material is added to the crust, but the part that sinks into the mantle is also lost. This leads to a mass deficit because, in the end, approximately the same crust is gained in subduction zones as lost. So where does the new crust come from?” is the question asked by MNCN researcher Daniel Gómez.
In subduction zones, approximately the same crust is gained and lost.
On the other hand, there are large granite formations that grow from the magmatism produced millions of years after the occurrence of subduction tectonic movements, after the collision of two continents.
Contrary to classical hypotheses, they showed that large granite formations like Gredos come from the mantle.
This is post-collisional magmatism, in which magma formed at great depth is introduced into the crust and cools little by little, without coming to the surface, as occurs in volcanic eruptions. These events generate batholiths, large masses of granite that slowly cool and become part of the Earth’s crust.
The Serra dos Gredos is an example of these formations. “What we show through petrological experiments is that these magmas, which were previously attributed to the recycling of the crust, come from the melting of the mantle, which would imply continental growth during magmatic episodes and would explain the origin of the excess material that is generated and that does not it is attributable to subduction movements”, explains Antonio Castro, researcher at MNCN and IACT.
The material generated after a continental collision comes directly from the Earth’s mantle.
To prove their hypothesis, the research team reproduced the composition and mineralogy of post-collision batholiths without the need to include recycled crustal material. “Having reproduced the characteristics of batholiths in this way indicates that the material generated after a continental collision comes directly from the Earth’s mantle,” says Gómez.
“This discovery could bring us closer to understanding where continents come from and substantially change models of their origin that, until now, underestimated the role of post-collisional magmatism”, he concludes.
Gomes, D. et al. Post-collisional batholiths contribute to continental growth. Earth and Planetary Science Cards (2023).