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Precambrian Continents

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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.

Satellite image of ancient greenstone exposed by glaciers on the Canadian Shield

Ice age glaciers scraped the Canadian Shield down to the 4.28 billion year old greenstone in Northwestern Quebec.

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 ).

Satellite image of the Canadian Shield

The Canadian Shield is the ancient flat part of Canada that lies around Hudson Bay, the northern parts of Minnesota, Wisconsin and Michigan and much of Greenland.

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 ).

The Precambrian craton is exposed in the Grand Canyon where the Colorado River has cut through the younger sedimentary rocks

The Precambrian craton is exposed in the Grand Canyon where the Colorado River has cut through the younger sedimentary rocks.

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.

The presence of water on ancient Earth is revealed in a zircon crystal: http://www.youtube.com/watch?v=V21hFmZP5zM (3:13).

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.

Practice

Use this resource to answer the questions that follow.

http://essayweb.net/geology/quicknotes/continents.shtml

1. What was Yilgarn?

2. What was Yaalbara and when was it formed?

3. When was Ur formed?

4. How was Arctica formed?

5. What was Atlantica composed of?

6. How was Nena formed?

7. When was Columbia formed? When did it begin to break apart?

8. When was Rodinia formed?

Review

1. Why could the felsic continental crust 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?

Vocabulary

craton

craton

The ancient Precambrian felsic continental crust that forms the cores of continents.
greenstone

greenstone

A metamorphosed volcanic rock that forms at a subduction zone.
paleogeography

paleogeography

The arrangement of the continents; ancient geography.
platform

platform

A craton and its overlying younger sedimentary rocks.
shield

shield

The part of a craton that crops out at the surface.

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