Eighth Edition
by Harold L. Levin

Chapter 8 - page 7

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

The Precambrian

The Precambrian covers approximately 4 billion years (and 87%) of Earth history.

The Precambrian is divided into 2 eons:

1. Archean Eon 4.6 - 2.5 billion years ago (or 4600 - 2500 million years ago)
2. Proterozoic Eon 2.5 - 0.542 billion years ago (or 2500 - 542 million years ago)

Table of time divisions of the Precambrian.

The Precambrian is not well known or completely understood. Why?

• Precambrian rocks are poorly exposed.
• Many Precambrian rocks have been eroded or metamorphosed.
• Most Precambrian rocks are deeply buried beneath younger rocks.
• Many Precambrian rocks are exposed in fairly inaccessible or nearly uninhabited areas.
• Fossils are seldom found in Precambrian rocks; only way to correlate is by radiometric dating.

Map showing the distribution of exposures of Precambrian rocks around the world.
Areas where Precambrian rocks are exposed are shown in yellow, as well as in the red areas in orogenic belts.

Most information on the Precambrian is from cratons - large portions of continents which have not been deformed since Precambrian or Early Paleozoic time.

The most extensive exposures of Precambrian rocks are in geologically stable regions of continents called shields.

Example = Canadian shield in North America. Mostly igneous and metamorphic rocks; few sedimentary rocks. Overlying sedimentary rocks were scraped off by glaciers during last Ice Age.

North American craton, shield, platform, and orogenic belts.

Stable regions of the craton where shields are covered by sedimentary rocks are called platforms.

Precambrian rocks are often called basement rocks because they lie beneath a covering of fossil-bearing sedimentary strata.

Various Precambrian provinces can be delineated within the North American continent, based on radiometric ages of rocks, style of folding, and differences in trends of faults and folds.

Precambrian provinces in North America, with dates.
Oldest (Archean) rocks are shown in orange.
Younger (Proterozoic) rocks are shown in green.

Origin of Plate Tectonics

By about 4 b.y. ago, the Earth had probably cooled sufficiently for plate formation.

Once plate tectonics was in progress, it generated crustal rock that could be partially melted in subduction zones and added to the continental crust.

Continents also increased in size by addition of microcontinents along subduction zones.

Greater heat in Archean would have caused faster convection in mantle, more extensive volcanism, more midoceanic ridges, more hot spots, etc.

Growth of volcanic arcs next to subduction zones led to formation of greenstone belts.

Granulites and Greenstones

The major types of Archean rocks on the cratons are:

• Granulites - Highly metamorphosed gneisses (metamorphosed tonalites, granodiorites, and granites) and anorthosites (layered intrusive gabbroic rocks). Granulites formed from partially melted crust and sediments in subduction zones. Metamorphism altered the rocks to form granulites.
• Greenstones - Metamorphosed volcanic rocks and sediments derived from the weathering and erosion of the volcanic rocks. Greenstone volcanic rocks commonly have pillow structures, (called pillow basalts), indicating extrusion under water.
  The green color is the result of low-grade metamorphism, producing green minerals such as chlorite and hornblende.


Pillow basalts (look for large rounded structures). Also note glacial striations.

Greenstones are mostly found in trough-like or synclinal belts.

There is a specific sequence of rock types in greenstone belts. These include:

1. Ultramafic volcanic rocks near the bottom (komatiites)
2. Mafic volcanic rocks (basalts)
3. Felsic volcanic rocks (andesites and rhyolites)
4. Sedimentary rocks at the top (shales, graywackes, conglomerates, and sometimes BIF), deposited in deep water environments adjacent to mountainous coastlines.

Granulites are present between greenstone belts.


Generalized cross-section through two greenstone belts. Note sequence of rock types and relationships between granulites and the greenstones.

By 2.8 billion years ago, Earth had cooled sufficiently for glaciation to occur. Earth's earliest glaciation is recorded in 2.8 billion year-old sedimentary rocks in South Africa.