Continental crust, the foundation of our continents, formed through a complex process spanning billions of years. Early formation, during the Archean eon (starting 4 billion years ago), involved the partial melting of hydrated oceanic crust. This occurred in "vertical drips" of basaltic crust before plate tectonics became established around 3.2 billion years ago, or later in warm subduction zones.
Early Continental Crust Formation (Archean Eon):
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Partial Melting of Hydrated Oceanic Crust: High temperatures in the early Earth caused the partial melting of already existing hydrated oceanic crust. This magma, less dense than the surrounding mantle, rose and solidified to form the earliest continental crust.
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Vertical Drips and Subduction Zones: These early formations happened either in "vertical drips" of basaltic crust (before plate tectonics), or later, in warm subduction zones where oceanic plates dove beneath other plates.
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TTG Terranes: Large areas of juvenile crust with a tonalite-trondhjemite-granodiorite (TTG) composition were added to the continental crust from about 3.5 billion years ago onwards. These TTG terranes represent fragments of Earth's earliest continents.
Later Continental Crust Formation:
The process continued and diversified after the establishment of plate tectonics:
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Subduction Zones: Most continental crust forms today at subduction zones, where oceanic plates are forced under continental plates. Magma generated by the melting of the subducted oceanic plate rises and solidifies, adding to the continental crust. This is the process described in several references.
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Magmatism at Volcanic Arcs: Volcanic arcs, like the Aleutian Arc, are prime examples of this process. The magma created at subduction zones is often enriched with silica and other elements, giving rise to the characteristic composition of continental crust.
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Relamination: Another important process, relamination, starts at the edge of a continental plate, where an oceanic plate dives beneath it. The resulting interactions scrape off portions of the oceanic plate, adding to the existing continental crust.
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Magma Hybridization: The composition of the continental crust is also influenced by magma hybridization — the mixing of different magmas to create a unique composition.
Composition and Growth:
Continental crust is a mix of igneous, metamorphic, and sedimentary rocks. It's less dense than oceanic crust, which is why it sits higher on the Earth’s surface. The continental crust has grown significantly throughout Earth's history, with major pulses of growth occurring at different times. Each pulse was likely triggered by changes in the rate of subduction, magmatism, or other related tectonic processes.
