Unit 5-4:Unit 5-4:Continental GrowthContinental Growth

The CratonThe Craton

►The continents that we are familiar with The continents that we are familiar with today have not always been the same.today have not always been the same.

►The continents were actually much The continents were actually much smaller than they are now.smaller than they are now.

►Through tectonic processes, rock has Through tectonic processes, rock has been added to the ancient continental been added to the ancient continental cores.cores. As this process continued over time, the As this process continued over time, the

continents we know today formed.continents we know today formed.

The CratonThe Craton

►The ancient continental cores are The ancient continental cores are known as cratons. known as cratons. These continental cores, or cratons, have These continental cores, or cratons, have

remained relatively the same for the past remained relatively the same for the past 500-600 million years.500-600 million years.

►The part of a craton that is exposed to The part of a craton that is exposed to the surface is called the shield.the surface is called the shield.

►Observe the North American Craton:Observe the North American Craton:

Canadian ShieldCanadian Shield

Sources of Growth MaterialSources of Growth Material

►There are several sources from which There are several sources from which land gets added to the cratons.land gets added to the cratons. One example is deep-sea sediments.One example is deep-sea sediments.

►When an oceanic plate is subducted by a When an oceanic plate is subducted by a continental plate,continental plate,

►Some of the uppermost part of the oceanic Some of the uppermost part of the oceanic plate is scraped off and left behind as ocean plate is scraped off and left behind as ocean sediment.sediment.

►As subduction continues, this sediment As subduction continues, this sediment becomes permanently attached to the becomes permanently attached to the continent.continent.

Sources of Growth MaterialSources of Growth Material

►A second source of material is volcanic A second source of material is volcanic rock.rock. Chains of volcanic islands are Chains of volcanic islands are

characteristic of subduction zones. characteristic of subduction zones. As these volcanic islands erupt, they add As these volcanic islands erupt, they add

more rock from the mantle to the surface.more rock from the mantle to the surface. As millions of years pass, this volcanic As millions of years pass, this volcanic

rock adds up to a significant amount of rock adds up to a significant amount of land. land.

Sources of Growth MaterialSources of Growth Material

►Another source of material is Another source of material is sediments from rivers.sediments from rivers. This material is not due to a plate This material is not due to a plate

boundary.boundary. As a river cuts across a continent, it pulls As a river cuts across a continent, it pulls

small pieces of rock with it.small pieces of rock with it. As the river reaches the ocean, the As the river reaches the ocean, the

sediments get deposited at the edge of sediments get deposited at the edge of the continent and gradually build up.the continent and gradually build up.

Growth by Thin-Skinned Growth by Thin-Skinned ThrustingThrusting

►Thin-Skinned Thrusting:Thin-Skinned Thrusting: The pushing of thin horizontal sheets of The pushing of thin horizontal sheets of

rock from continental margins over great rock from continental margins over great distances along nearly level fault surfaces.distances along nearly level fault surfaces.

As these thin sheets stack up, it can add As these thin sheets stack up, it can add to continental growth.to continental growth.

An example of thin-skinned thrusting is An example of thin-skinned thrusting is likely to be what formed the Southern likely to be what formed the Southern Appalachian Mountains.Appalachian Mountains.

Growth by Thin-Skinned Growth by Thin-Skinned ThrustingThrusting

►The Southern Appalachians have a The Southern Appalachians have a complex history.complex history. They formed during periods of spreading They formed during periods of spreading

and colliding.and colliding. A brief history of time:A brief history of time:

►Originally, the ancestor continents of North Originally, the ancestor continents of North America and Africa were part of the same America and Africa were part of the same larger continent.larger continent.

Growth by Thin-Skinned Growth by Thin-Skinned ThrustingThrusting

► A brief history of time:A brief history of time: About 650 million years ago, Africa and North About 650 million years ago, Africa and North

America split apartAmerica split apart►This formed the basis of the Atlantic OceanThis formed the basis of the Atlantic Ocean

About 400 million years ago, the two continents About 400 million years ago, the two continents merged together again.merged together again.►The items created by the spreading of the continents The items created by the spreading of the continents

like volcanic mountains and the ocean floor were like volcanic mountains and the ocean floor were pushed up onto land.pushed up onto land.

►This formed the Appalachian Mountains.This formed the Appalachian Mountains.►The continents then split and moved to where they are The continents then split and moved to where they are

today.today.

Growth by TerranesGrowth by Terranes

►Terrane:Terrane: A large block of lithospheric plate that has A large block of lithospheric plate that has

been moved (usually thousands of km) been moved (usually thousands of km) and attached to the edge of a continent.and attached to the edge of a continent.

This is the primary method of continental This is the primary method of continental growth in western North America.growth in western North America.

Growth by TerranesGrowth by Terranes

►There are three characteristics an area There are three characteristics an area must possess before it can be must possess before it can be classified as a terrane.classified as a terrane. It must be bound on all sides by major It must be bound on all sides by major

faultsfaults Rocks and fossils found must not match Rocks and fossils found must not match

with those of the neighboring continentwith those of the neighboring continent The magnetic polarity does not match the The magnetic polarity does not match the

neighboring continentneighboring continent

Growth by TerranesGrowth by Terranes

► An example is Cache Creek in CanadaAn example is Cache Creek in Canada The rocks in the terrane are limestone created by The rocks in the terrane are limestone created by fusulinidfusulinid fossils. fossils.►The limestone found in North America is not made of the The limestone found in North America is not made of the

same same fusulinidfusulinid fossils. fossils.►FusulinidFusulinid fossils like the ones found in the terrane are fossils like the ones found in the terrane are

most commonly found in Japan and Asia.most commonly found in Japan and Asia. It also is bounded by faults (the easiest thing to It also is bounded by faults (the easiest thing to

spot).spot). The magnetic polarity doesn’t match the The magnetic polarity doesn’t match the

surrounding area.surrounding area. This evidence suggests that this terrane came from This evidence suggests that this terrane came from

the Pacific ocean and travelled thousands of km.the Pacific ocean and travelled thousands of km.