The Earth works with the mechanics of plate tectonics, that is, tectonic plates move, collide and sink under each other. The question on the table of science is when it started to work like this and the answer is complicated, simply because no rock older than 4,030 million years is preserved that allows us to reconstruct that period (spoiler: It is the Acasta gneiss and is in Canada).
The only clue we have are zircons, crystals so resistant that they can almost withstand anything: they survive even when the rock that houses them disappears, so they function as a kind of time capsule. The oldest in the world They are in the hills of Jack Hillsin Western Australia and are up to 4.4 billion years old.
The discovery. An international research team led by John W. Valley of the University of Wisconsin-Madison has analyzed the chemical composition of these Australian zircons and compared it with other zircons of approximately the same age found in Barberton, South Africa. What they found was surprising: while the South Africans point to a still and immobile Earth’s crust, the Australians indicate that in that place, one layer was sinking on top of another (subduction).
The conclusion they reached is that 4.4 billion years ago different parts of the Earth operated with different tectonic mechanisms at the same time: in some places there was something similar to silver tectonics and in others, the crust remained stagnant, as if it were a rigid lid.
Why is it important. Until now, the official history of Earth’s geology tells that the planet went from having a stationary crust to having plate tectonics. around 3.8 billion years and that the change was more or less global and simultaneous. Well nothis study dismantles it: subduction was already happening in some parts 600 million years earlier, which means that the continents began to form much earlier than previously thought. And there were earthquakes back then.
This is also important for understanding the origin of life. Subduction produces granite and stable continental crust, which creates land, nourishes the oceans with minerals, and creates the environments where, according to the oldest records available, life began to develop 3,700 – 4,100 million years ago. If subduction dates back to before, those favorable conditions for life were also there before.
Context. This debate is not new and in fact, neither is the conclusion. There are studies that hold that plate tectonics began in the early Hadean, others that before the plates began to move, the Earth’s crust was a rigid, immobile layer, like a lid, and the heat from within was released through columns of molten rock rising from the mantle, not through the movement and collision of plates.
And be careful, because in both cases they used those same Jack Hills zircons to defend opposing positions, which gives an idea of how difficult it is to interpret them. In fact, already there are previous studies that use Barberton zircons to identify a tectonic regime change around 3.8 billion years ago. What this new work does is add a nuance in the form of complexity: the change was present in Barberton, but in Australia in Jack Hills the story was different and older.
How they have done it. With a technique called secondary ion mass spectrometry (SIMS), which makes it possible to measure with high precision some chemical elements present in zircon (scandium, ytterbium, niobium and uranium) because their proportions vary depending on the type of geological environment in which the mineral was formed. A zircon formed in a subduction zone has very different proportions than one formed in a rigid cap zone. In addition, they analyzed the age of the zircons and their hafnium and oxygen isotopes, which indicate both the origin of the mantle or whether water was involved in the process. The complete photo with these four data allows us to reconstruct the geological environment.
Yes, but. The big Achilles’ heel of the study is that these zircons are loose grains carried by erosion, not samples of rock in their original place. That is, they could travel thousands of kilometers from their origin. In short: it is not known where they come from. The second major problem is that the method used to identify tectonic environments is calibrated with modern rocksbecause there are no Hadean rocks. This implies assuming that the chemistry then was similar to that of today, something that no one can guarantee.
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