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What is a volcano? Melted rock from subduction Zones and mantle plumes called Magma, rises to the surface through an opening (becoming Lava). Igneous rocks “Born of Fire” form from cooled magma. Distribution of Volcanoes Pacific Plate forms the “Ring of Fire” Cascade Range Volcanoes Where do Volcanoes Occur? Divergent boundaries: rift valleys and midocean ridges. Convergent Oceanic-Continental Convergent Oceanic-Oceanic. Intraplate Volcanism: occurring in areas within the plate due to hot spots found under the continents. Ex. Yellowstone and Hawaii. Types of Magma Mafic • Non Violent Eruption • Low Amounts of Silicates (SiO2) • Less Viscous (warm honey) • Oceanic crust origin. • Ex. Hawaii Hot Spot • Less dissolved gas Felsic • Violent Eruption • High Amounts of Silicates (SiO2) • Viscous (cold honey) • Continental crust origin • Ex. Mt. St. Helens • More dissolved gas Types of Magma • Basaltic magmas (MAFIC)– Divergent Boundaries and Convergent oceanic-oceanic • Andesitic magmas (INTERMEDIATE) Convergent oceanic-continental • Rhyolitic magmas (FELSIC) Varies in origin. http://hvo. wr.usgs.g ov/multi media/?di splay=def ault Volcano Characteristics Classifying Volcanoes • An active volcano is one that is currently erupting or has erupted recently (in geological terms). • A dormant volcano is one that has not erupted lately but is considered likely to do so in the future. • An extinct volcano is one that has not erupted for a very long time and is considered unlikely to do so in the future. Volcano Types •Shield volcanoes - very large •Composite (or Stratovolcano) – mid sized •Cinder cones - small Composite volcanoes (or stratovolcanoes) •FELSIC- Erupt violently ejecting igneous rocks called pyroclastic material. •Alternating violent eruptions and non violent eruptions. Cinder cones • Tend to be smaller than other types of volcanoes. • Made up of cinders. Cinder Cone Shield volcanoes • MAFIC - Non violent eruptions • have broad bases and gently sloping sides. Example: Mt. Loa, Hawaii Lava Flows Lava flows on land: 1. Pahoehoe lava less viscous (runny) lava that cools as smooth, ropelike surfaces. 2. Aa lava more viscous (thick) lava that cools as rough, jagged surfaces. Lava that cools underwater: Pillow lava has a rounded, “pillow-like” form with a hard crust. Pahoehoe (Ropy) Lava Flow Slow-Moving Aa Flow Mount Kilauea Pyroclastic Materials (ejected into the air). • Ash -the smallest pieces with a diameter or less than 2 mm. • Lapilli -Intermediate sized pieces with a diameter from 2 – 64 mm. • Bombs -the largest sized pieces with a diameter greater than 64 mm. Pyroclastic Flow The eruption of Mount Vesuvius in A.D. 79 produced a pyroclastic flow (Pyroclastic material and gas) that buried the Roman city of Pompeii. Mt. Vesuvius & Pompeii Hazards: 1. Lava Flow 2. Lahar (debris flow) 3. Pyroclastic Flow (gas & ash) 4. Landslides 5. Lava Bombs 6. Acid Rain Volcanoes & the Atmosphere Volcanic eruptions are thought to be responsible for • global cooling; and • Mass extinction. Igneous Rock Plutonic Igneous Rock that cools underground. Larger crystals due to slower cooling rate. Volcanic Igneous Rock that cools above ground. Smaller crystals due to faster cooling rate. Types of Igneous Plutons Batholiths Sills Sill