Eighth Edition
by Harold L. Levin

Chapter 8 - page 5

The Earliest Earth: 2,100,000,000 Years of the Archean Eon

Accretion and Differentiation of the Earth

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:

1. Solar Nebula Hypothesis or Cold Accretion Model (secondary differentiation):
   Planet Earth formed by accretion of dust and larger particles of metals and silicates.
   Earth was originally homogeneous throughout - a random mixture of space debris.
   But Earth is now internally layered.
   Origin of layering requires a process of differentiation.
   Differentiation is the result of heating and at least partial melting.
   Iron and nickel sink to form core.
   Less dense material (silicon and oxygen combined with remaining iron and other metals) forms mantle and lighter crust (dominated by silicon and oxygen).
   Presence of volatile gases on Earth today indicates that complete melting did not occur. The accreting Earth was repeatedly partly melted by great impacts, such as the Moon-forming impact.
   

Possible sources of heat for melting:

• Accretionary heat from bombardment (colliding meteorites)
• Heat from gravitational compression as material accumulated
• Radioactive decay

Diagram of the cold accretion model. Initial bombardment created homogeneous Earth. Later heating caused differentiation.
2. Hot Accretion Model - (primary differentiation):
   Internal zonation of planets is a result of hot heterogeneous accretion.
   Hot solar nebula (over 1000 ^oC).
   Initial crystallization of iron-rich materials forms planet cores.
   With continued cooling, lower density silicate materials crystallized.
   

Diagram of the hot accretion model. Different layers accreted at different times. The core was the first to form, and the crust was the last to form.

Parts of both models may have been in operation.

The Archean Crust

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's Crust Today

Earth has two types of crust today:

1. Dense, mafic (magnesium- and iron-rich) oceanic crust dominated by basalt.
2. Less dense, sialic (silicon- and aluminum-rich) continental crust dominated by granite.

When did the Earth's continental crust begin to form?

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.

Photomicrograph of 3.96 billion year old zircon grain from northwestern Canada.