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.
