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 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.

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:

Canadian Shield

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.

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.

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.

Growth by 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.

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.

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.

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.

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

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.