Smithsonian National Museum of Natural History Geologic Time The Story of a Changing Earth
Presented by the Department of Paleobiology.
The Hadean
Formation of the Earth
Earth's Atmosphere
Earth's Surface
Part of a meteorite that hit Arizona about 50,000 years ago.
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Augustine Volcano

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Earth’s Surface
Although we know very little about the Earth’s surface during the Hadean, the oldest rocks on Earth, such as the Acasta Gneiss from northern Canada, provide bits of evidence. The Acasta Gneiss is a metamorphic rock about 4 billion years old. (Metamorphic rocks form when other rocks are modified by heat and pressure.) The Acasta Gneiss is composed in part of quartz and feldspar, minerals that are probably derived from metamorphosed granite. Granite itself forms when igneous rocks such as basalt are melted in the presence of water. The composition of the Acasta Gneiss thus implies that granitic continents and surface water existed during the Hadean more than 4 billion years ago.

Although continents, oceans and an atmosphere must have existed before the Acasta Gneiss formed, the size of the continents is unclear. While Earth was still very hot, mantle convection must have been vigorous. It is not known if plate tectonics operated on the early Earth. One suggestion is that the earliest surface crust was thin and unstable, and made of minerals with an extremely high content of iron and magnesium, which are very dense. This early crust may have been disrupted by upwelling magma at spreading centers and consumed in subduction zones. Because of the high density of this earliest-formed crust, it would have descended deep into the melting zone of the mantle and been destroyed. This process would have led to recycling of the crust almost as soon as it formed. Other geologists think that early continents formed when plumes of molten mantle rose to the surface and cooled, but that the Earth was too hot for subduction of plates.

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