8SCIENCE

Volcanoes and Eruptions

Review

• Where do volcanoes form?– Plate boundaries

• How do volcanoes form?– Heat and pressure from inside the Earth produces

magma and pushes up to the surface • What is a hotspot?– Very hot areas in the mantle that produce

volcanoes although they are not found on plate boundaries

Types of Volcanoes• Shield • Cinder cone• Composite

Shield volcanoes

• Quiet eruption that produces lava and spreads out in flat layers

• These layers build to form slightly sloping sides

• Ex: Hawaiian islands

Cinder cone volcano

• Explosive eruptions that throw lava and rocks high into the air

• These bits of rock and hardened lava are called tephra– Tephra layers build up to form steep sided

volcanoes

Composite volcano

• Produces eruptions that are quiet then violent• Cycles of lava and tephra form different layers• Usually found where one plate is forced under

another

What causes an eruption to be quiet or violent?

• Amount of water vapor and other gases trapped in magma– High gas content = more violent

eruption– Gases are trapped in magma and pressure builds up– As magma moves closer to the surface, there is less

pressure and the gas can escape

• Amount of silica (silicon and oxygen) present in magma– Low in silica = more quiet eruption

Other volcanic structures• Not all magma flows to the surface• Some may rise from the volcano and cools

underground to form other rock structures• Erosion of the rock layers make these visible

to us today

COLOMBIA

DEATH VALLEY, CALIFORNIA

SHIPROCK, NEW MEXICO

CALDERAS IN OREGON (US) AND PHILLIPINES