A new study led by geologists alec brenner Y Roger Fu, both from Harvard University, has just provided new and valuable clues about how a series of geological changes helped ‘prepare’ the Earth to host life.
In a recently published article in PNASIndeed, scientists explain how, by analyzing some of the oldest rocks on the planet, they found evidence that, 3.25 billion years ago, the earth’s crust already showed activity similar to modern plate tectonics. And at the same time they found the oldest known evidence of a polarity reversal, when the north and south magnetic poles switch positions.
Land
The work focused largely on the so-called ‘Pilbara craton’, in western Australia, one of the oldest ‘pieces’ of the earth’s crust. Using a whole series of new techniques, the researchers managed to show that at least that part of the ancient surface of the Earth was already moving at a speed of 6.1 cm per year and 0.55 degrees every million years, more than double that estimated by the same scientists in a previous work. The most logical explanation for the speed and direction of this drift is, according to the authors, plate tectonics.
“There is a lot of work that seems to suggest that, early in Earth’s history, plate tectonics was not the dominant way in which internal heat was released from the planet as it is today. Brenner explains. But this evidence allows us to rule out with much more confidence other explanations that do not involve plate tectonics.”.
For example, researchers can now rule out other phenomena that can also cause the Earth’s surface to shift but have nothing to do with plate tectonics, such as so-called ‘true polar wander’ and ‘plate tectonics’. stagnant lid’. Brenner and Fu’s results, in fact, clearly point to an early plate tectonics because the newly discovered velocity rate is incompatible with the other two processes.
Earth’s oldest magnetic reversal
In their study, the scientists also describe what they believe to be the oldest evidence of polarity reversal, when Earth’s north and south magnetic poles swapped positions more than 3.2 billion years ago. These types of magnetic reversals are common in the geological history of the planet, and up to 183 of them have been recorded in the last 83 million years and several hundred in the last 160 million years, according to NASA data.
Such an early pole reversal says a lot about what the Earth’s magnetic field was like at that distant time. But more importantly, the magnetic field was probably already stable and strong enough to prevent solar winds from eroding the atmosphere. This idea, combined with the results of plate tectonics, offers valuable clues about the conditions in which the first forms of life developed.
“These two facts paint a picture of an early Earth that was already geodynamically mature,” says Brenner. It had many of the same kinds of dynamical processes that result in an Earth with essentially stable environmental and surface conditions, making it more likely for life to evolve and develop.”.
Today, Earth’s outer shell consists of about 15 moving blocks of crust, or plates, that support the planet’s continents and oceans. Over eons, plates have pushed back and forth again and again, forming new continents and mountains and exposing new rock to the atmosphere, leading to chemical reactions that once stabilized Earth’s surface temperature for thousands of years. of millions of years.
A difficult investigation
It’s hard to find evidence of when plate tectonics started because the oldest pieces of crust are pushed deep into the inner mantle, never to surface again. In fact, only 5% of all rocks on Earth are older than 2.5 billion years, and none are older than 4 billion years.
Taken as a whole, the Brenner and Fu study adds to the idea that tectonic movement occurred relatively early in Earth’s history (which is about 4.5 billion years old) and that the first forms of life arose in a more ‘moderate’ environment than was long thought. In 2018, the researchers revisited the Pilbara Craton, which is around 480 km wide. They drilled into the thick primordial slab of crust to collect samples which were later analyzed in Cambridge for their ‘magnetic history’.
New techniques
To achieve this, the researchers had to create a set of new techniques with which to determine the age and the way in which the samples became magnetized. Which in turn allowed them to determine how, when and in what direction the crust moved, as well as the magnetic influence coming from the Earth’s magnetic poles. For future studies, Brenner and Fu plan to continue studying the Pilbara craton and other ancient crustal remnants around the world to find even older evidence of plate motion and magnetic reversal.
“In the end Fu says, being able to reliably read these very old rocks opens up a lot of possibilities for looking at a time period that is often known more through theory than hard data. Ultimately, we have a good chance of reconstructing not only when tectonic plates began to move, but also how their movements, and thus the processes deep inside the Earth that drive them, have changed over time.”.
Font: JOSE MANUEL NIEVES / ABC
Reference article: https://www.abc.es/ciencia/preparo-tierra-albergar-vida-20221024145632-nt.html
