The Crust of the Earth and Plate Tectonics. Convection in the Upper Mantle.

The Crust of the Earth and Plate TectonicsThe Crust of the Earth and Plate Tectonics

Convection in the Upper MantleConvection in the Upper Mantle

Plate BoundariesPlate Boundaries

There are 3 types of plate boundaries:

• DivergentDivergent boundaries: where new crust is generated as the plates pull away from each other

• ConvergentConvergent boundaries: where crust is destroyed as one plate dives under another

• TransformTransform boundaries: where crust is neither produced nor destroyed as the plates slide horizontally past each other

Plate BoundariesPlate Boundaries

DivergentDivergent

ConvergentConvergent

TransformTransform

Plate Boundaries

Divergent Plate BoundariesDivergent Plate Boundaries

• Perhaps the best known of the divergent boundaries is the Mid-Atlantic RidgeMid-Atlantic Ridge.

• The formation of oceanic crust involves The formation of oceanic crust involves meltingmelting of underlying of underlying uppermantle.uppermantle.

• This submerged mountain range, which extends from the Arctic Ocean to beyond the southern tip of Africa, is but one segment of the global mid-ocean ridge system that encircles the Earth.

• The rate of spreading along the Mid-Atlantic Ridge averages about 2.5 centimeters per year2.5 centimeters per year (cm/yr), or 25 km in a million years.

• This rate may seem slow by human standards, but because this process has been going on for millions of years, it has resulted in plate movement of thousands of kilometers.

•Seafloor spreading over the past 100 to 200 million years has caused the Atlantic Ocean to grow from a tiny inlet of water between the continents of Europe, Africa, and the Americas into the vast ocean that exists today.

Divergent BoundariesDivergent Boundaries• The volcanic country of IcelandIceland, which straddles the Mid-Atlantic RidgeMid-Atlantic Ridge, offers scientists a natural laboratory for studying on land the processes also occurring along the submerged parts of a spreading ridge. IcelandIceland is splitting along the spreading center between the North American and Eurasian Plates, as North America moves westward relative to Eurasia.

Krafla volcanoKrafla volcanolava fountainlava fountain

Iceland • Aerial view of the area around Thingvellir, Iceland, showing a fissure zone that is an on-land exposure of the Mid-Atlantic Ridge.

• Left of the fissure, the North American Plate is pulling westward away from the Eurasian Plate (right of fissure).

Iceland

Continental Spreading Centers (rifts)Continental Spreading Centers (rifts)

• East Africa shows some historically active volcanoesvolcanoes (red triangles) and the Afar TriangleAfar Triangle, a so-called triple junction (or triple point), where three plates are pulling away from one another: the Arabian Plate, and the two parts of the African Plate (the Nubian and the Somalian) splitting along the East African Rift ZoneEast African Rift Zone.


• Active lava lake within the summit crater of 'Erta 'Ale'Erta 'Ale (Ethiopia), one of the active volcanoes in the East African Rift Zone.

• Two helmeted, red-suited volcanologists, observing the activity from the crater rim provide scale.• 150 m diameter150 m diameter• 80 m deep80 m deep

• Red color within the crater shows where molten lava is breaking through the lava lake's solidified, black crust.


'Erta 'Ale (Ethiopia),

Convergent BoundariesConvergent Boundaries

Two plates come together, or converge, and the result is a collision.

The density of the two plates decide which will move on top of the other.

Oceanic crust is more dense than continental crust and sinks under the continental crust.

Continental crust that collide squeeze together and create mountains.

Convergent BoundariesConvergent Boundaries

Trenches at Conversion BoundariesTrenches at Conversion Boundaries

Transform BoundariesTransform Boundaries

Two plates slip past each other, moving in opposite directions.

Earthquakes often occur along transform boundaries.

Here no plate is created or destroyed.

Transform BoundariesTransform Boundaries

Review of BoundariesReview of Boundaries

The Crust of the Earth and Plate Tectonics. Convection in the Upper Mantle.

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