Plate tectonics 4 billion years ago may have helped initiate life on Earth

The crust, the oldest layer of surface material that forms continents on Earth, is made up of 25–50 km-thick basalt volcanic rocks that date back around 4 billion years. In contrast to the various plates we see now, which were considered to have just started to emerge one billion years later, scientists formerly believed that one complete lithospheric crust covered the whole globe. Nonetheless, opinions on this theory are being contested.

It is still unclear how this continental crust formed, but scientists now believe plate tectonics—the movement of Earth's major surface plates across the planet over billions of years—may have had a role in creating the landmasses and topographic characteristics that we see today.

While another theory studies mechanisms occurring within the crust itself (at less than 50 km depth) that are entirely separate from plate boundaries but also cause partial melting, the first theory concentrates on the moment when the plates converge, which frequently causes one to subduct beneath the other, resulting in partial melting to change magma composition.

An analog of oceanic plateaus, which are vast, flat heights with steep edges and are indicative of the early basaltic crust that developed in the Eoarchean (3.6–4 billion years ago), is the subject of new study published in Nature Geoscience.

High-pressure-temperature melting tests were conducted on primitive oceanic plateau basalts from the southwestern Pacific Ontong Java Plateau by Dr. Alan Hastie, who is affiliated with the University of Edinburgh.

It was shown that pressures lower than 1.4 GigaPascals (GPa) could not arise in continental crust, even at depths of 50 km. This suggests that the formation of such magmas occurred during convergent subduction zones. As a result, they propose that plate tectonics occurred 4 billion years ago, although in a rudimentary form.

This information is crucial because plate tectonics causes mountain building, erosion, deposition, and volcanic activity—all of which contribute in different ways to the creation of continental crust. The study team hypothesizes that gases emitted during volcanism, particularly methane and carbon monoxide, may have aided in the beginning of life on Earth by providing primordial chemicals that eventually gave rise to the earliest microscopic creatures.

Smaller amounts of the silica-rich continental crust have also been discovered outside of Earth on Mars and Venus, providing information on the function of plate tectonics in the larger solar system.

Using projected mantle temperatures of 1,500–1,650°C, Dr. Hastie and colleagues examined the stability of many minerals at different pressures (1.2–1.4GPa, or ~40–50km deep) to ascertain the point at which they changed. Important minerals for the study included amphibole (controls dehydration melting processes), rutile (stable at 0.7-1.6GPa, ~25-55km depth), garnet (known to be stable at pressures >1GPa, equivalent to ~30km depth), and plagioclase feldspar (stable up to ~1.8GPa, ~60km depth).

Although this is higher than previous studies' findings, the experimental results showed that garnet and rutile were not stabilized at <1.4GPa (~45–50 km depth). The team attributes this to the higher magnesium content of their initial oceanic crust, which is more in line with the expected composition of Eoarchean mafic (iron and magnesium-rich) crust.

Additionally, scientists conducted a reversal experiment in which they cultivated garnet crystals at a greater pressure (2GPa) and then exposed them to a lower pressure (1.4GPa), discovering that the garnet crystals started to disintegrate. They then discovered that garnet was stable at pressures of around 1.6GPa, or at a depth of 50–55 km. This finding challenged the earlier theory that stability occurred at 1GPa, and consequently increased the formation's depth. Subduction is therefore a better strategy to account for this reaction.

Additionally, modeling indicates that early magmas underwent fractional crystallization as they rose through the crust. This process caused crystals to separate from the liquid magma, depleting certain elements in the remaining magma pool that were used to form the initial crystals. As a result, the composition of the magma pool changes continuously as more crystals form.

This allowed the study team to determine that amphibole crystallization, a hydrous mineral that may have been buried and overturned to become a component of the crust, was a significant factor in partial melting. This is consistent with the hallmarks of recognized tonalites and trondhjemites, two types of Eoarchean volcanic rocks.

It is believed that two relics of convergent plate boundaries above ancient subduction zones are the Isua Greenstone Belt in Greenland and the Archaean Slave Craton in Canada. Such locations would have seen the beginning of a cycle of continental destruction and rebirth that has created the globe we see today, as fluids from the melting subducting crust mingled with metabasic (metamorphosed basaltic and associated rocks) magmas to produce new silica-rich magmas.