V Volcanoes and Volcanism A Mafic Volcanism and

V. Volcanoes and Volcanism A. Mafic Volcanism and Volcanic Rocks B. Felsic Volcanism and Volcanic Rocks

Mafic Volcanism n 6. Characteristics of formations n 5. Types of Rocks n 4. Types of eruptions which controls n 3. Type of lava which controls n 2. Source of lava which controls n 1. Plate tectonics

1. 2. Source of Lava n Partial melting of mantle n n n Partial melting of dry mantle At mantle plumes or Divergent plate boundaries 3. Types of Lava n Types of lava Basaltic n n n Hot (>1000 o. C) Non-Viscous (runny, flows easily) “Dry” (no H 2 O or C 02)

4. Types of Eruptions n Non-explosive n Lava flows, streams, ponds, floods n n n Fountains Spatter cones Pyroclastic eruptions

5. Types of Rocks Composition and Texture n Basalt n n n ‘A’a Pahoehoe Vesicular Basalt Pillow Basalt Columnar Jointing Obsidian n Volcanic Glass Figs. 4. 9, 4. 11, 4. 17, 4. 18

6. Characteristic Formations Flood basalts and basalt plateaus n Shield volcanoes n Cinder cones Fissures Through Crust n Calderas atop of Kilauea Shield Hawaii Cinder Cone

Columnar Jointing in basalt floods n n n As lava floods cool and solidify The basalt contracts and Splits into hexagonal columns Devil’s Post Pile, California

Columnar Jointing n Columnar jointing may occur in volcanic stocks (large cylindrical core of volcanoes) Devils Tower, Wyoming

Pillow Basalts Evidence of submarine eruptions

Volcanic Hazards n n Map out rift zones Use topography to determine flow direction

Rift Zones and Lava Flows n Kilauea, Hawaii

Hawaii Hazards

B. Felsic Volcanism n 6. Characteristics of volcanoes n 5. Types of Rocks n 4. Types of eruptions n 3. Type of lava n 2. Source of lava n 1. Plate tectonics

1, 2. Plate tectonics and Sources of Lava Fig. 3. 24 n Partial melting at Subduction Zone n Partial melting of continental crust n Convergent plate boundaries

3. Types of Lava n Felsic n n n Cool (<800 o. C) Viscous Gaseous (H 2 O, CO 2)

4. Types of Eruptions Fig. 4. 1 Mt. St. Helen’s Cascade Range n Explosive n n Pyroclastic Flows and surges Lahars (saturated pyroclastics) Fig. 4. 20

5. Types of Rocks Composition and Texture n Andesite (and Rhyolite) n n Pumice (quenched glass froth) Porphyritic Texture (partial crystallization and extrusion) n Welded Tuffs (welded pyroclastics) n Breccias (welded, coarse, angular pyroclatics) 1 mm

6. Characteristics of Volcanism n Lava Domes Composite Volcanoes n n Layers of Pyroclastics (or tephra), ash and Lava flows Pyroclastic Layers

Mt. St. Helen’s Before and After 1 mile

Vocanic Hazards n n n n Volcanic Earthquakes Directed Blast Tephra Volcanic Gases Lava Flows Pyroclastic Flows and Surges Lahars Debris Avalanches, Landslides, and Tsunamis

Locating Volcanoes and Volcanism • • Convergent Plate Boundaries Composite Volcanoes of Andesite Mid-Ocean Ridges Pillow basalts forming new oceanic crust Hot spots in oceans Forming shield volcanoes Hot spots and divergent boundaries on continents Forming flood basalts, shield volcanoes (maybe some composite volcanoes)