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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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