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16.6: Precambrian Continents

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    When did Earth become a dynamic planet?

    Earth was always dynamic! Earth has changed many times over billions of years. Huge mountains have formed, been destroyed, and been replaced with new mountains. Continents have moved, split apart, and collided with each other. Ocean basins have opened up. Life on Earth evolved slowly for billions of years.

    Early Continents

    The earliest crust was probably basalt. It may have resembled the current seafloor. This crust formed before there were any oceans. More than four billion years ago, continental crust appeared. The first continents were very small compared with those today.

    Continents Grow

    Continents grow when microcontinents, or small continents, collide with each other or with a larger continent. Oceanic island arcs also collide with continents to make them grow.

    Cratons

    The earliest continental crust is now found in the ancient cores of continents, called cratons. Geologists can learn many things about the Precambrian by studying the rocks of the cratons.

    • Cratons contain felsic igneous rocks, which are remnants of the first continents.
    • Cratonic rocks contain rounded sedimentary grains. Rounded grains indicate that the minerals eroded from an earlier rock. It also means that rivers or seas 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.

    Supercontinents

    There are times in Earth history when all of the continents come together to form a supercontinent. Supercontinents come together and then break apart. Pangaea was the last supercontinent on Earth, but it was not the first. The supercontinent before Pangaea is called Rodinia. Rodinia contained about 75% of the continental landmass that is present today. The supercontinent came together about 1.1 billion years ago. Rodinia was not the first supercontinent either. Scientists think that three supercontinents came before Rodina, making five so far in Earth history.

    Early Plate Tectonics

    Since the early Earth was very hot, mantle convection was very rapid. Plate tectonics likely moved quickly. The early Earth was an active place with abundant volcanic eruptions and earthquakes. The remnants of these early rocks are now seen in the ancient cores of the continents.

    Further Reading

    Paleozoic Plate Tectonics

    Paleozoic and Mesozoic Seas

    Mesozoic Plate Tectonics

    Cenozoic Plate Tectonics

    Summary

    • The first crust was basalt. It resembled the modern seafloor.
    • Microcontinents come together to create continents and supercontinents.
    • Convection on early Earth was faster and so plate tectonics was faster. Since then, Earth has been cooling.

    Review

    1. What were the first continents like?
    2. Where do we find the oldest crust we can find? Is this what is left of the first crust? How do we know?
    3. What are supercontinents? How do we know of their existence (think back to the concepts in the Plate Tectonics chapter)?
    4. Why was plate tectonics faster in the early Earth?

    Explore More

    1. What did the early Earth look like?
    2. Why do some scientists think that life may have originated more than once?
    3. What landmasses existed 2.5 billion years ago?
    4. When did Arctica take shape?
    5. What was Rodinia?
    6. What came together to form Pangaea and when did this happen?
    7. What is driving the movement of the plates?
    8. What is convective flow?
    9. Where does the energy come from to drive plate movements?
    10. What material is found in the core that makes it hot?

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