precambrian 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 1.1). Since greenstones are found today in oceanic trenches, what does the presence of greenstones mean? These ancient greenstones indicate the presence of subduction zones. Ice age glaciers scraped the Canadian Shield down to the 4.28 billion year old greenstone in Northwestern Quebec.

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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 1.2). 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.

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early convection

During the Pre-Archean and Archean, Earths 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.

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 1.3).

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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.

instructional diagrams

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questions

the first crust was made of this rock.

a) granite

b) gneiss

c) limestone

-->  d) basalt

the first felsic continental crust came from

a) silica-rich minerals that floated on the magma ocean.

-->  b) partial melting of the lower portion of the basaltic crust.

c) asteroid and comet impacts.

d) none of these.

greenstones in cratons indicate that at the time they formed

a) there was felsic continental crust.

b) there was an atmosphere.

-->  c) there were subduction zones.

d) all of the above.

cratons

a) contain grains that were eroded from earlier rocks.

b) are found in the ancient interiors of some continents.

c) contain felsic and mafic igneous rocks.

-->  d) all of these.

precambrian shields are about

-->  a) 570 million years old

b) 670 million years old

c) 770 million years old

d) 870 million years old

at the grand canyon, the layered sedimentary rocks are on top of ancient precambrian craton. this is called a

a) greenstone

-->  b) platform

c) shield

d) cratonic layer

the presence of zircons in ancient rocks indicates that there was water in the environment.

-->  a) true

b) false

when earths interior was warmer than it is now

a) mantle convection was faster.

b) plate tectonics processes were faster.

c) subduction was more common so plates were relatively small.

-->  d) all of these.

earth has not cooled since right after the moon formed because of radioactivity.

c) true

d) false

diagram questions

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