Unit 5-4: Continental Growth

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 500-600 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.

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.