1	Extrusive/Volcanic Igneous Rocks
2	Where do volcanoes form? Convergent boundaries-subduction zone Oceanic crust is pushed downward into the upper mantle, where it undergoes partial melting. Magmas form and work their way upward Example: here in the NW, the oceanic Juan de Fuca plate is subducted beneath the continental North American plate
3	Where do volcanoes form? Divergent boundary volcanism As plates move apart, underlying material rises and spreads laterally As plates move, pressure is reduced which lowers the melting temperature of rocks Basaltic lava rises and fills the fissures of the mid-oceanic ridges
4	Where do volcanoes form? Intraplate volcanoes occur on oceanic crust as well as continental crust Example of oceanic crust: Hawaiian Islands Example of continental: Yellowstone Intraplate volcanoes develop over “hot spots” (rising mantle material) As material rises and melts, fractional crystallization and assimilation forms silica-rich melts
5	Explosive Nature of Volcanoes Explosive power of a volcano depends on the magma, specifically: The amount of gas in the magma Water vapor, CO₂, SO₂, H₂S (poisonous gas-smells like rotten eggs), and HCl (acidic) The viscosity of lava (resistance to flow) Viscosity affected by temperature, silica and gas High dissolved gas content = less viscosity Temperature high = less viscosity Low Si = less viscosity
6	Explosive Power by Composition Felsic/Intermediate volcanoes Continental, subduction zones Silica Rich High viscosity More explosive Rhyolitic or Andesitic or Dacitic-composition flows Mafic volcanoes Oceanic, hot spots Quiet, low viscosity Basaltic-composition flows Surface water affects power of eruption!
7	Types of Lava and Lava Flows Eruptions take 1 of 2 forms: Lava flow (4 types) pyroclastic
8	Lava Flow 4 types of Lava Pillow Erupt under water or ice Most abundant lava type Pahoehoe Smooth, ropy appearance 2nd most abundant lava Aa Rough jagged surface of broken lava blocks Thick flows Flood Lava Pours from fissures Mafic, Very low viscosity Forms Plateaus
9	Flood Lava
10	Eruption form 2: Pyroclastics Pyroclasts: fragments of lava explosively thrown from a volcano Pyroclastic material includes: Dust: <1/8 mm Ash: 1/8-2 mm Cinder: 2-64 mm Bombs: >64 mm Pyroclastic flow most deadly! Example: Mt. St. Helens!
11	Volcano types Classified by shape, size, and ultimately ruled by magma type 3 main types: Cinder Composite Shield General facts (not always true) Cone shape Single vent eruptions Crater in middle
12	Cinder Cones Simplest volcano Formed from pyroclastic materials, i.e. cinders Intermediate size Examples of cinder cone: Paricutin, Mexico Sunset Crater, Arizona Brown Mountain, Oregon
13	Composite Cones Also called Strato-Volcanoes ~60% of earth’s volcanoes are composite Large, steep, symmetrical cones Built of alternating layers of lava flow and pyroclastic material Lava: Mostly Intermediate composition Aa lava type, or lava domes Source of most human casualties Abundance Proximity Lahars**** Buried town of Armero, Columbia, 28,000 fatalities
14	Aftermath of a Lahar
15	Famous Composite Volcanoes Cascade Volcanoes Ie, Mt. St. Helens, Mt. Rainier, Mt. Adams Mt. Fuji, Japan Mt. Etna, Italy
16	Shield Volcanoes Largest volcanoes on earth Broad, gently sloping cone Low viscosity, mafic flow after flow Mafic = Basaltic composition High magma supply Gentle flow Associated spatter cones Common in hotspots Hawaiian Islands
17	Famous Shield Volcanoes Mauna Loa, Hawaii World’s largest & most active 28,000 feet above ocean floor! Only about a million years old
18	Caldera Forming Eruptions “Supervolcano” eruptions Greater than 8 on the volcano explosivity index Empty magma chamber, cause collapse Forms Caldera Example: Yellowstone, Mt. Mazama
19	Caldera Forming Eruptions
20	Hydrothermal Features Features Associated w/Volcanic Terrains: Hotspring Fumarole Mud pot Geyser Yellowstone- largest geyser field in world