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Unveiling the mystery of Earth's oldest crust: What is the composition and formation process of the primordial crust?
The evolution of the Earth's crust involves the formation, destruction, and renewal of its rocky outer shell. During these processes, the variability in the composition of the Earth's crust is much greater than that of other similar planets (such as Mars, Venus, and Mercury). Unlike the relatively single crust of other planets, the Earth's crust contains not only oceanic plates but also land plates. This unique property reflects the complex crustal processes that have occurred over Earth's history, especially the ongoing movement of plate tectonics. Scientists have proposed a series of theories about the mechanisms of the evolution of the Earth's crust and provided hypothetical solutions to the problems of the early Earth system based on fragmentary geological evidence and observations.
Early crust
Mechanism of early crust formation
Earth was initially molten globally, due to high temperatures generated and maintained by the compression of its early atmosphere, its rapid rotation, and frequent collisions with nearby asteroids. As planetary aggregation slowed, the Earth gradually cooled and heat from the lava ocean was lost to space via radiation. One theory about the solidification of lava is that when the temperature dropped to a certain level, the bottom of the lava ocean would first begin to crystallize, forming a thin layer of "cooling crust." This layer of cooled crust provided thermal insulation to the shallow subsurface, maintaining temperatures sufficient to promote crystallization processes in the deep molten ocean.
During the crystallization of the lava ocean, the composition of the resulting crystals varies with depth, with Mg-perovskite being the main component at greater depths and olivine being the main component in shallower areas.
Crustal dichotomy
The crustal dichotomy refers to the stark contrast between the composition and properties of oceanic and continental plates. Currently, oceanic and continental crust are generated and maintained by plate tectonic processes, but the dichotomy of early Earth's crust is unlikely to have been generated by these mechanisms. The study suggests that the dichotomy of the Earth's crust may have formed before the start of global plate movement, because thin, low-density land rocks covered the entire Earth at the time and failed to sink.
The impact of the collision
Many large impact craters are found on planets throughout the Solar System and are associated with a period of time known as the Late Heavy Bombardment, which ended about 4 billion years ago. Research in recent years has also pointed out that the Earth's erosion rate and continued plate movement mean that these impact craters are no longer visible today. If the impact craters observed on the Moon are scaled up, it is predicted that at least 50% of the Earth's initial crust is covered by impact basins. This estimate reveals the significant impact of impact indentations on the Earth's surface.
The main effects of impact craters on the early crust include the formation of large tunnels, adjustment of ground pressure, and increase in temperature of the bottom layer.
Crust Types
Original crust
Minerals crystallized from the molten ocean to form the original crust. A possible explanation for this process is that the solidification of the edge of the mantle about 4.43 billion years ago produced continents composed of magnesium-rich ultramafic rocks.
Secondary crust
The secondary crust is formed mainly by recycling the previous primitive crust to produce the basic secondary crust. Partial melting in this process increases the basic component in the melt, and most of the secondary crust is formed at the mid-ocean ridge to constitute the oceanic crust.
Third Crust
The current continental crust is the tertiary crust, the most differentiated type of crust, with a composition significantly different from the overall composition of the Earth. This crust contains more than 20% incompatible elements, which is caused by partial melting of the secondary crust.
The Beginning of Plate Tectonics
The formation and development of plates are caused by hot spots in the early mantle, which cause the crust to sink locally on the surface, promoting the subsequent sinking of plates. Numerical models show that only powerful hot spots can soften the crust and break its structure. According to the model, initial subduction began 3.6 billion years ago.
Modern analogies to the early crust
The geochemical characteristics of early Archean rocks from the Acasta metamorphic rock complex are very similar to some modern mesochistic rocks in Iceland, allowing us to explore the composition and formation processes of the Earth's crust at that time to some extent.
Behind the mysteries of the Earth, the study of these early crusts not only helps us understand the history of the Earth, but also inspires deep thinking about possible geological activities in the future. Faced with these complex crustal evolution mechanisms, we can't help but ask: So, are there similarities in the crustal evolution of other planets on Earth, or are they each unique?
