Volcanoes

http://www.youtube.com/watch?v=BcFtpWjZwlE Volcano – opening in Earth’s surface that often forms a mountain when layers of lava and volcanic ash erupt and build up.

Dormant volcano - volcano that is not “active”

Where volcanoes occur 1. Divergent Boundaries- location where two plates are moving apart Long deep cracks are present and lava flows up and quickly cools

Where volcanoes occur 2. Convergent Boundaries - location where plates are moving together One plate may slide under the other and partially melt = subduction zone http://www.youtube.com/ watch?v=kQES0ALjUoI The magma may then get forced upward and form a volcano Ex. Pacific Ring of Fire (Mt. Saint Helens)

3. Hot Spots Occur because mantle is hotter in some areas than others and the magma is closer to the crust and can sometimes melt the Earth’s crust. Ex. Hawaiian Islands

Two factors that determine if an eruption will be explosive or quiet: 1. Amount of water vapor and other gases that are trapped in the magma. 2. If the magma is basaltic or granitic Basaltic (dark) magma contains less silica and is very fluid Granitic (light) magma contains higher levels of silica and is very thick

Forms of Volcanoes The type of volcano is determined by the type of eruption The type of eruption is determined by the type of magma There are three types of volcanoes.

Composite/Stratovolcanoes Formed by eruptions that vary between quiet and explosive. Made from andesitic lava Composed of layers of tephra and lava. Tall, steep-sided cones Ex: Mt. Saint Helens & Mt. Vesuvius

Cinder Cone Explosive eruptions Composed of tephra – volcanic material (ash, cinders, rocks, lava bombs) Small, steep-sided cone Ex: Paricutin, Mexico

Shield Volcano Gently sloping sides Basaltic lava Quiet eruptions Ex. Hawaiian Islands