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Unit 5-4: Continental Growth. The Craton ► The continents that we are familiar with today have not always been the same. ► The continents were actually.

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Presentation on theme: "Unit 5-4: Continental Growth. The Craton ► The continents that we are familiar with today have not always been the same. ► The continents were actually."— Presentation transcript:

1 Unit 5-4: Continental Growth

2 The Craton ► The continents that we are familiar with today have not always been the same. ► The continents were actually much smaller than they are now. ► Through tectonic processes, rock has been added to the ancient continental cores.  As this process continued over time, the continents we know today formed.

3 The Craton ► The ancient continental cores are known as cratons.  These continental cores, or cratons, have remained relatively the same for the past million years. ► The part of a craton that is exposed to the surface is called the shield. ► Observe the North American Craton:

4 Canadian Shield

5 Sources of Growth Material ► There are several sources from which land gets added to the cratons.  One example is deep-sea sediments. ► When an oceanic plate is subducted by a continental plate, ► Some of the uppermost part of the oceanic plate is scraped off and left behind as ocean sediment. ► As subduction continues, this sediment becomes permanently attached to the continent.

6 Sources of Growth Material ► A second source of material is volcanic rock.  Chains of volcanic islands are characteristic of subduction zones.  As these volcanic islands erupt, they add more rock from the mantle to the surface.  As millions of years pass, this volcanic rock adds up to a significant amount of land.

7 Sources of Growth Material ► Another source of material is sediments from rivers.  This material is not due to a plate boundary.  As a river cuts across a continent, it pulls small pieces of rock with it.  As the river reaches the ocean, the sediments get deposited at the edge of the continent and gradually build up.

8 Growth by Thin-Skinned Thrusting ► Thin-Skinned Thrusting:  The pushing of thin horizontal sheets of rock from continental margins over great distances along nearly level fault surfaces.  As these thin sheets stack up, it can add to continental growth.  An example of thin-skinned thrusting is likely to be what formed the Southern Appalachian Mountains.

9 Growth by Thin-Skinned Thrusting ► The Southern Appalachians have a complex history.  They formed during periods of spreading and colliding.  A brief history of time: ► Originally, the ancestor continents of North America and Africa were part of the same larger continent.

10 Growth by Thin-Skinned Thrusting ► A brief history of time:  About 650 million years ago, Africa and North America split apart ► This formed the basis of the Atlantic Ocean  About 400 million years ago, the two continents merged together again. ► The items created by the spreading of the continents like volcanic mountains and the ocean floor were pushed up onto land. ► This formed the Appalachian Mountains. ► The continents then split and moved to where they are today.

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12 Growth by Terranes ► Terrane:  A large block of lithospheric plate that has been moved (usually thousands of km) and attached to the edge of a continent.  This is the primary method of continental growth in western North America.

13 Growth by Terranes ► There are three characteristics an area must possess before it can be classified as a terrane.  It must be bound on all sides by major faults  Rocks and fossils found must not match with those of the neighboring continent  The magnetic polarity does not match the neighboring continent

14 Growth by Terranes ► An example is Cache Creek in Canada  The rocks in the terrane are limestone created by fusulinid fossils. ► The limestone found in North America is not made of the same fusulinid fossils. ► Fusulinid fossils like the ones found in the terrane are most commonly found in Japan and Asia.  It also is bounded by faults (the easiest thing to spot).  The magnetic polarity doesn’t match the surrounding area.  This evidence suggests that this terrane came from the Pacific ocean and travelled thousands of km.

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