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Slide 2 Frequency and Quantity There are ~ 550 historically active volcanoes About 60 erupt annually The majority are 1) at the edge of continents and 2) underwater Dormant volcanic cone in Japan Slide 3 What causes volcanoes? Slide 4 Underwater Volcanoes Rift valley magma plate Slide 5 Mid ocean ridges Slide 6 Mid ocean ridge Slide 7 Mid ocean ridges Are continuous underwater mountain ranges Are formed by rising magma Cause sea floor spreading Form a rift valley between spreading plates Pillow lava at mid ocean ridge Slide 8 Hot Spots Hot spots create chains of islands like the Hawaiian islands or the Aleutian chain. Slide 9 Slide 10 Hawaiian Islands hot spot Slide 11 Slide 12 Land Based Volcanoes Slide 13 A volcano at the edge of a continent Sinking plate Mid ocean ridge Melted rock forming magma ? Slide 14 Three Types of Land Based Volcanoes Volcanoes take different shapes depending on the amount of SILICA and DISSOLVED GASSES in the magma High Silica content More likely to be explosive!!! Why? Silica has a thick, stiff consistency Flows slowly Tends to Harden in the volcanos vent High Dissolved Gas content More gas comes out, an eruption can launch ash and debris great distances However, if lava is viscous and flows quickly, these eruptions may not be that dramatic Slide 15 What Erupts from a Volcano? Lava can be thick or thin. Blocky lava Pahoehoe Aa Pillow lava Slide 16 What Erupts from a Volcano? Pyroclastic material Rock fragments created by eruptions magma explodes from volcano and solidifies in the air existing rock is shattered by powerful eruptions Volcanic blocks Volcanic bombs Lapilli Volcanic ash Slide 17 Lets see it in action http://environment.nationalgeographic.com/environ ment/natural-disasters/forces-of- nature.html?section=t http://environment.nationalgeographic.com/environ ment/natural-disasters/forces-of- nature.html?section=t Slide 18 Shield Volcanoes Volcanoes that build up from many slow, steady, flows of hot lava, are called shield volcanoes. This kind of volcano is low and broad with gently sloping sides. They look like a warriors shield. They have LOW silica content and LOW dissolved gases. Unlikely to yield massive explostions Slide 19 Shield Volcanoes Slide 20 Cinder Cone Volcano Cinder cone volcanoes form when solid rock and ash shoot up into the air and fall back around the volcano opening. The cinder cone volcano has steeply sloped sides. Cinder cone volcanoes have LOW silica content and HIGH dissolved gas content Cinder cones can have violent eruptions, but are unlikely to send ash or lava too far Slide 21 Cinder Cone Slide 22 Slide 23 Composite Volcano Composite volcanoes, erupt with molten lava, solid rock, and ash. The layers pile up much like layers of cake and frosting. The layers form into symmetrical cones, and the slopes are steep. Composite volcanoes have HIGH silica content and HIGH dissolved gas. Composite volcanoes erupt violently and can send ash and lava great distances Slide 24 Composite Volcano Slide 25 Slide 26 Mt St Helens one day before eruption Slide 27 Mt St Helens 4 months after eruption Slide 28 Another before Slide 29 and 2 years later Slide 30 Now, it is your turn. Pick a type of volcano and build it out of clay. Then, we are going to make TOPOGRAPHIC MAPS.