The Earth is internally layered, with a basic structure consisting of crust, mantle, and inner and outer core.
The Earth's internal structure may be primary (formed initially as the Earth formed), or secondary due to later heating. The two hypotheses for the origin of the layered structure are:
Possible sources of heat for melting:
Parts of both models may have been in operation.
Once differentiation occurred, Earth's crust was dominated by iron and magnesium silicate minerals.
The first mafic, oceanic crust formed about 4.5 billion years ago by partial melting of rocks in the upper mantle.
Earth has two types of crust today:
Continental crust developed after the initial mafic to ultramafic crust.
Continental crust is sialic or felsic (such as granite). Dominated by light-colored minerals such as quartz and feldspar.
Felsic crust began forming around 4.4 billion years ago.
Felsic crust formed in subduction zones where descending slabs of crust partially melted. The early-melting, less dense components of the melt rose to the surface where they cooled to form continental crust.
One of the oldest dated felsic Earth rocks is the 4.04 billion year old Acasta Gneiss from northwestern Canada. Dates are from zircon grains in tonalite gneisses. (Tonalite gneiss is metamorphosed tonalite, a rock similar to diorite, with at least 10% quartz).
The Amitsoq Gneiss from Greenland is another old tonalite gneiss (3.8 b.y. old).
Patches of old felsic crust have also been found in Enderby Land, Antarctica (3.9 b.y. old).
A 3.46 by old fossil soil zone (or paleosol) associated with an unconformity in the Pilbara region of Australia indicates that Archean continents stood above sea level. This paleosol represents the oldest land surface known, and provides evidence that subaerial weathering, erosion, and soil formation processes were at work in the Archean.
The oldest zircon grains are 4.4 billion years old. They are found in quartzite
(metamorphosed sandstone) in western Australia.
Sedimentary structures in the quartzite resemble those in modern stream deposits. These rocks are interpreted as fluvial (river) deposits. They were derived from the weathering of granitic rocks (some of the earliest continental crust), and deposited above sea level, indicating the presence of both liquid water and continental crust by 4.4 billion years ago.
| ||Oceanic Crust||Continental Crust|
|First appearance||About 4.5 b.y. ago||About 4.4 b.y. ago|
|Where formed||Mid-ocean ridges||Subduction zones|
|Composition||Komatiite & basalt||Tonalite & granodiorite, and later, granites|
|Lateral extent||Widespread||Local (few hundred km or mi)|
|How formed||Partial melting of ultramafic rocks in upper mantle||Partial melting of wet, sediment-covered mafic rocks descending in subduction zones|
October 27-28, 2005