11.5 Precambrian Continents

What did the first crust look like?

These ancient greenstones are metamophosed pillow lavas from much earlier in Earth history. These rocks are found in eastern Canada and similar rocks are found in cratons around the world.

Early Continents

The first crust was made of basaltic rock, like the current ocean crust. Partial melting of the lower portion of the basaltic crust began more than 4 billion years ago. This created the silica-rich crust that became the felsic continents.

Craton

The earliest felsic continental crust is now found in the ancient cores of continents, called the cratons. Rapid plate motions meant that cratons experienced many continental collisions. Little is known about the paleogeography, or the ancient geography, of the early planet, although smaller continents could have come together and broken up.

Geologists can learn many things about the Pre-Archean by studying the rocks of the cratons.

• Cratons also contain felsic igneous rocks, which are remnants of the first continents.
• Cratonic rocks contain rounded sedimentary grains. Of what importance is this fact? Rounded grains indicate that the minerals eroded from an earlier rock type and that rivers or seas also existed.
• One common rock type in the cratons is greenstone, a metamorphosed volcanic rock (Figure below). Since greenstones are found today in oceanic trenches, what does the presence of greenstones mean? These ancient greenstones indicate the presence of subduction zones.

Shield

Places the craton crops out at the surface is known as a shield. Cratons date from the Precambrian and are called Precambrian shields. Many Precambrian shields are about 570 million years old (Figure below).

Platform

In most places the cratons were covered by younger rocks, which together are called a platform. Sometimes the younger rocks eroded away to expose the Precambrian craton (Figure below).

Early Convection

During the Pre-Archean and Archean, Earth’s interior was warmer than today. Mantle convection was faster and plate tectonics processes were more vigorous. Since subduction zones were more common, the early crustal plates were relatively small.

Since the time that it was completely molten, Earth has been cooling. Still, about half the internal heat that was generated when Earth formed remains in the planet and is the source of the heat in the core and mantle today.

Summary

• The ancient core of a continent, at and beneath the surface, is its craton.
• The cratonic rock that is seen at the surface is called the shield. Where the shield is covered by younger sediments is the platform.
• Convection on early Earth was faster and so plate tectonics was faster. Since then, Earth has been cooling.

Review

1. Why is it that the felsic continental crust could not be Earth's first crust?
2. What are greenstones and why are they important in understanding early Earth history?
3. Why was plate tectonics more vigorous in the early Earth? What would plate tectonics have been like?

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