1. Volcanoes

3. Volcanoes and Plate tectonics  Volcano is a mountain formed when layers of lava and volcanic ash erupt and build up  Most are dormant  About 600 are active

4. What cause Volcanoes MMagma is less dense then solid rock IIt slowly rises to the earths surface FFlows out through an opening called a vent LLava cools quickly and forms igneous rock OOpening at the top of the vent is called a crater PPyroclastic flow – massive avalanches of hot glowing rock and gases (complete destruction)

7. Where do Volcanoes Form  1. Divergent Boundaries  Mid Atlantic Ridge  Forms long deep cracks called rifts where magma flows from  2. Convergent boundaries  Cascade Mt. Range in Oregon and Washington  Mt. Saint Helen  Pacific Ring of Fire

8.  3. Hot Spots  Hawaiian Islands  Far from the edge of any plate  Some area of the mantle are hotter then others  These hot spots melt rock which rise to the surface to form volcanoes  Once the volcano rises above the water it becomes an island

12. What determines how a volcano Erupts  Composition of the magma plays a big role in determining the manner in how a volcano erupts  Lave – more silica (silicon and oxygen) tend to be thicker and resists flow  Lava containing more iron and magnesium and less silica tend to flow easier

17.  Amount of water vapor in lava influences how lava erupts  More water (steam) the more pressure builds up and the more violent the eruption is

18. Forms of Volcanoes  Three basic volcanoes  Shield volcanoes  Cinder volcanoes  Composite volcanoes

19. 1. Shield  Quiet eruption  Broad volcanoes  Gently sloping sides  Ex. Hawaiian, rift zones like Iceland

23. Cinder Cone  Explosive Volcano  Throws lava into the air, lava cools and hardens  Tephra – small- cinders, large – bombs  Forms steep sides  Sunset crater

27. Composite Volcano  Can vary between quiet or violent  Alternating layers of lava and tephra  Convergent boundaries above subduction zone  Mt. St. Helens

32.  Fissure Eruption  Magma that is highly fluid can ooze from cracks or fissures in earths surface