Review<br />Where do volcanoes form?<br />Plate boundaries<br />How do volcanoes form?<br />Heat and pressure from inside the Earth produces magma and pushes up to the surface <br />What is a hotspot?<br />Very hot areas in the mantle that produce volcanoes although they are not found on plate boundaries<br />
Shield volcanoes<br />Quiet eruption that produces lava and spreads out in flat layers<br />These layers build to form slightly sloping sides<br />Ex: Hawaiian islands<br />
Cinder cone volcano<br />Explosive eruptions that throw lava and rocks high into the air <br />These bits of rock and hardened lava are called tephra<br />Tephra layers build up to form steep sided volcanoes<br />
Composite volcano<br />Produces eruptions that are quiet then violent<br />Cycles of lava and tephra form different layers<br />Usually found where one plate is forced under another<br />
What causes an eruption to be quiet or violent?<br />Amount of water vapor and other gases trapped in magma<br />High gas content = more violent eruption<br />Gases are trapped in magma and pressure builds up<br />As magma moves closer to the surface, there is less pressure and the gas can escape <br />Amount of silica (silicon and oxygen) present in magma<br />Low in silica = more quiet eruption<br />
Other volcanic structures<br />Not all magma flows to the surface<br />Some may rise from the volcano and cools underground to form other rock structures<br />Erosion of the rock layers make these visible to us today<br />
CALDERAS IN OREGON (US) AND PHILLIPINES<br />DEATH VALLEY, CALIFORNIA<br />SHIPROCK, NEW MEXICO<br />COLOMBIA<br />
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