CH 6 Prentice Hallp.178-181

CH 6 Prentice Hallp.178-181

Volcanoes & Volcanoes & Plate Tectonics

Plate Tectonics

What is a Volcano?

• A weak spot in the crust where molten material, or magma comes to the surface.– Volcanic Activity is a constructive force that adds

new rock to existing land or forms new islands.

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Magma / Lava

• Magma- a mixture of rock forming substances, gases and water vapor from the mantle.

• Lava- Magma that reaches the surface.– Cools and forms solid

rock.

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Magma Rises

• Magma is a liquid- therefore it is less dense than surrounding solid material.– Flows upward into any

cracks in the rocks above.– Rises until it reaches the

surface or becomes trapped beneath layers or rocks.

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Volcano Erupts

• Dissolved gases trapped in magma are under tremendous pressure.

• The gasses dissolved in magma rush out, carrying magma with them.

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Crater

• Lava collects in the crater, the bowl-shaped area that forms around the volcano’s vent.

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Vent

• The point on the surface where magma and gas leave the volcanoes pipe.

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Pipe

• A narrow, almost vertical crack in the crust through which magma rises to the surface.

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Magma Chamber

• A large underground pocket of magma that forms from magma rising to the surface.

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Lava flow

• The river of lava that pours down a volcano over the land.

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Stages of a Volcano

• Active:– A live volcano that is erupting or shows signs that

it may erupt in the future.

• Dormant:– A sleeping volcano. It may awake in the future

and become active.

• Extinct:– A dead volcano. It is unlikely to erupt again.

Location of Volcanoes

• Most occur along diverging boundaries such as the mid-ocean ridge or in subduction zones.

Location of Volcanoes• About 600 active volcanoes on land, more lie

beneath the sea.• Occur in belts that extend across continents

and oceans.• Ring of Fire-formed by the many volcanoes that rim the

Pacific Ocean

Divergent Boundaries

• Form along the mid-ocean ridge.– Ridge a long underwater

rift valley.– Lava pours out of the

cracks in the ocean floor.

• Iceland- is a volcano on the mid-ocean ridge that has reached the surface.

Convergent Boundaries

• Plates collide– Denser plate subducts towards the mantle.• Forms a deep-ocean trench• The crust melts and forms magma that rises.• Rising magma erupts through a volcano on the surface.

Convergent Boundaries

• Ocean/Ocean– Denser ocean plate subducts• Forms a deep sea trench• Forms an island arc of

volcanoes.– Japan– New Zealand– Aleutians

Convergent Boundaries

• Continent to Ocean– Dense ocean plate

subducts• Forms a deep sea trench• Forms volcanoes on the

land. – Mt. St Helens– Andes Mountains-S.

America.

Hot Spot Volcanoes• An area where magma from deep

within the mantle melts through the crust like a blow torch.

• Lie in the middle of plates.– Hawaii- formed in the middle of the

Pacific Ocean plate. – Yellowstone- formed under the continent.

• The plate travels over the hot spot.– Created a chain of islands to form Hawaii

Hot Spot Volcanoes