 |
|
|
|
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.
|
Eon Overview |
Formation of the Earth |
Earth’s Atmosphere
Earth’s Surface
Department of Paleobiology Home |
National Museum of Natural History Home
Smithsonian Institution Home |
HTML Version
| granite |
| Granite is a coarse-grained, usually light-colored, igneous rock that cools
deep under the Earth's surface. The major minerals are quartz, feldspar, and mica. |
| basalt |
| A hard, black volcanic rock with low silica content. |
| plate tectonics |
| The theory of plate tectonics proposes that the Earth's crust is composed
of a dozen or more large and small plates that move relative to one another as they ride on convection
currents in the hotter mantle that lies beneath. New sea floor is erupted along mid-ocean ridges, then
diverges from both sides in opposing directions toward continental margins, where it sinks back into
the mantle along subduction trenches. |
|
|
