“The timing and rate of early crustal growth on Earth remains contentious due to the scarcity of very ancient rocks,” are quite rare, but I think the history of the first crustal growth was much discussed by a ground breaking study, which has redefined the timeline of the formation of the first parts of our planet into continents. The scientists found 3.7-billion-year-old anorthosite rocks, which are some of the oldest surviving crustal rocks of the Earth, and whose isotopic characteristics have served to invalidate old geological theories in the remote Murchison region in Western Australia.
Anorthosites, which consist mostly of plagioclase feldspar, are geological rarities on the earth, but control the lunar highlands. They are only formed by slow cooling of molten magma that is deep underground to allow the feldspar crystals to grow large and entrap chemical fingerprints of their environment. The isolation of the freshest and least-alterred areas of these crystals provided Boyce and his colleagues with strontium and calcium isotopic data which revealed the depletion of the mantle, which became the material of the interior of the Earth and transformed itself into the continental crust. The findings indicate a major change: a 3.5 billion years ago age of continental growth, almost a billion years later than the age of the Earth, invalidates earlier models which had the continents appearing during the Hadean or early Archean eons.
This subsequent evolution of the continents is consistent with the larger geochemical evidence, including the vanadium isotope records of an mafic (basalt-rich) to felsic (granite-rich) crust after 3.0 Ga. It is believed that this shift is related to the beginning of global plate tectonics, the emergence of atmospheric oxygen and the increase or decrease in nutrient movement to the oceans which are key factors in the habitability of the earth. Dense iron-rich mafic crust inhibits the concentration of oxygen, and felsic crust provides stable continental shelves and increased nutrient supply, which may have contributed to the initial growth of biology.
The scope of the study is not restricted to the geology of the earth. The comparison between the isotopic data of these Australian anorthosites and lunar samples that NASA brought back via the Apollo missions was shocking as far as chemical similarities were concerned. The Earth and Moon also seem to have a similar starting composition of approximately 4.5 billion years ago. This strengthens the Giant Impact Hypothesis according to which a Mars-sized body named Theia crashed into proto-Earth in a cataclysmic collision. The collision would have been taking away most of both masses, hurling debris into space that would form the Moon, with the core of Theia combining with that of Earth, which is why our planet is so dense and why the Moon has a relatively low core.
Such studies of lunar anorthosites during the Apollo period found that they cooled out of an ocean of magma in the world with plagioclase being suspended to form the crust, with the heavier minerals falling to the bottom of the ocean to form the mantle. The depletion of volatiles on the Moon due to isotopic analyses of zinc and other elements has been found to be equal to the conditions of high energy as found under the Giant Impact scenario. The fact that the Moon has low iron compared to the earth, also support the fact that during the collision, heavy elements were retained on the earth.
However, according to planetary scientists, there are some challenges to Giant Impact Hypothesis. Isotopic measurements of titanium, hydrogen, and oxygen in lunar material indicate that the material of Theia was virtually identical to that of Earth, indicating that either Theia was composed together with improbable compositions or both were made of the same local material of the solar system. The Australian anorthosite data, which confirm common geochemical background of both Earth and Moon, give a hint that corroborates the latter possibility.
In the case of planetary science, these implications are far-reaching. These old rocks are not only the ruins of the crust of the Earth-they are the annals of a period when the fate of the Earth and Moon was mingled in molten anarchy. They are preserved and this enables scientists to recreate activities that were lost due to plate tectonics, erosion and metamorphism. Through the integration of precise civilisation isotopic analysis and comparative lunar geology, scholars are constructing a more detailed past of the crustal development, planetary impacts, and the environments that predetermined the existence of life.
With new missions planned under programs such as Artemis to bring back samples in areas of the moon that have never been visited such as the far side and polar areas; more opportunities arise to perfect these models. Every new set of data will shed light on the unanswered questions on the formation of the Moon and, consequently, the initial processes on the Earth. In the meantime, Murchison anorthosites remain the kind of testimony to the strength of antique minerals to clarify planetary pasts, which have taken billions of years to establish, and connecting worlds.
