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All About Volcanoes..... What is a volcano? - A volcano is an opening in Earth that erupts gases, ash & lava - FYI – There are over 600 active volcanoes on Earth right now! Especially in Hawaii and Iceland How do volcanoes form? A. Deep inside Earth, heat & pressure changes cause rock to melt B. Melted rock is less dense than solid rock so it is slowly forced up C. After a long period of time, the magma reaches the surface & flows out of a vent D. When the lava flows out, it cools, forming layers of igneous rock around the vent E. The steep walled depression around the vent becomes a crater. Where do volcanoes occur? Volcanoes usually occur : A. Where tectonic plates are moving apart (divergent plate boundaries) example: Mid-Atlantic Ridge B. Where plates move together (convergent plate boundaries) example: Andes Mountains (these type of volcanoes tend to be more violent) C. Hot Spots – Thin spots where magma comes to surface in middle of plates ex. Hawaii The Hawaiian Islands form as they move over a Hot Spot (or thin area of crust where magma rises) Volcano Parts Volcanoes have an opening at the summit and depending on its size, it can be: A. a crater (steep walled depression at summit) or B. a caldera ( summit depression > 1 km diameter) They also have a Vent ( the tunnel that connects the crater to the magma chamber Crater Lake – with remnants of Mt Mazama (OR) 2 Calderas (in Galapagos Islands) 3 Main Volcano Types 1. Shield Volcano - broad, slightly domed - primarily made of basaltic (less viscous) lava - generally very large (& flat) & produce a lot of lava example: Mauna Loa in Hawaii Shield Volcano - Hawaii 3 Main Volcano Types 2. Cinder Cone - built from ejected lava fragments (pyroclastics) - steep slope with loosely packed fragments - usually small & often occur in groups - Can form quickly ex. Paricutin in Mexico In 1943 a a farmer noticed a hole in his field spewing smoke. A few days later, a cinder cone several hundred meters covered his field! Cinder cone – Mojave Desert California 3 Main Volcano Types 3. Composite cone (Stratovolcano) - most are in Ring of Fire (encircle Pacific Ocean) - large & most violent - produce granitic (more viscous) lava - erupt a mix of lava & pyroclastics ex. Mt. St. Helens in Washington Kanaga stratovolcano - Alaska Relative sizes of the 3 volcano types. Cinder cones are tiny, shield volcanoes are huge Remember not all magma reaches the surface and erupts