Volcanoes form at plate boundaries or in hotspots in the mantle not associated with plate boundaries. There are three main types of volcanoes: shield volcanoes which have gentle slopes built up from fluid lava flows; cinder cone volcanoes which are steep sided due to explosive eruptions throwing tephra into the air; and composite volcanoes which have layers of both lava and tephra from eruptions that alternate between explosive and nonexplosive. The amount of gas and silica in magma determines the explosiveness of an eruption, with more gas and less silica leading to more violent eruptions as the gas escapes quickly closer to the surface.

1. Volcanoes and Eruptions 8SCIENCE

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

3. Types of Volcanoes Shield Cinder cone Composite

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