8SCIENCE Volcanoes and Eruptions
Jan 28, 2015
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