2. 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
4. Shield volcanoes Quiet eruption that produces lava and spreads out in flat layers These layers build to form slightly sloping sides Ex: Hawaiian islands
5. 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
6. 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
7. 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
8. 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
9. CALDERAS IN OREGON (US) AND PHILLIPINES DEATH VALLEY, CALIFORNIA SHIPROCK, NEW MEXICO COLOMBIA
