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10/6/2015 The Nature of Volcanic Eruptions GEOL 110: GEOLOGY Mt. St. Helens GEOL110WSP2015 MGEOLROUGHLEY52385 http://8bitcomics.com/wp-content/uploads/2011/06/Venting_Volcano.png Chapter 5: VOLCANOES AND VOLCANIC HAZARDS •https://www.youtube.com/watch?v=-H_HZVY1tT4 •David Johnston 6 miles away Not for Distribution. For study purposes only for related class. © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Volcanoes The Nature of Volcanic Eruptions The Nature of Volcanic Eruptions • All volcanic eruptions involve magma • Mobility of magma is called viscosity • The more viscous the material, the greater its resistance to flow • Ex: honey is more viscous than oil 3 Types of Volcanoes • Cinder Cones: Cinder Cone • Shield: Prospect Peak • Stratovolcano (composite): Mt. Tehama © 2014 Pearson Education, Inc. Photo reference cited in notes Photo reference cited in notes The Nature of Volcanic Eruptions Viscosity is dependent upon the magma’s: • Temperature: hotter =less viscous; cooler =more viscous. • Chemical composition: more Si =more viscous • Dissolved gases: Dissolved water in magma reduces viscosity; © 2014 Pearson Education, Inc. The Nature of Volcanic Eruptions Explosivity of Volcanoes •Si-rich magmas create explosive eruptions as gases become “stuck” in the viscous fluid of the magma. •Fluid basaltic lavas create quieter eruptions due to gases readily escaping the magma at eruption. © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. 1 10/6/2015 The Nature of Volcanic Eruptions The Nature of Volcanic Eruptions Different compositions result in volcanic landforms Quiescent vs. Explosive Eruptions • Quiescent Hawaiian‐Type Eruptions Pyroclastic flows; lava domes Stratovolcano • Involves fluid basaltic lavas • Eruptions are characterized by outpourings of lava that can last weeks, months, or even years Shield volcanoes, basalt plateaus, cinder cones • Explosive Eruptions • Associated with highly viscous magmas • Volatiles released; not major explosives • Eruptions expel particles of fragmented lava and gases at high speeds that evolve into eruption columns. © 2014 Pearson Education, Inc. Photo reference cited in notes Materials Extruded During an Eruption What materials are extruded during eruptions? © 2014 Pearson Education, Inc. Materials Extruded During an Eruption Lava on Earth • ~1 percent of lava is rhyolitic lava • <10 percent of lava is andesitic lava • ~90 percent of lava is basaltic lava • Lava Flows • Gases • Pyroclastic Materials (pulverized rock and lava fragments) © 2014 Pearson Education, Inc. Materials Extruded During an Eruption Basaltic Lava Flows Very fluid; flow in thin, broad sheets or stream-like ribbons; move quickly. • Aa flows have surfaces of rough jagged blocks • Pahoehoe flows have smooth surfaces and resemble twisted braids of rope • Pillow Lavas are composed of basaltic lavas extruded underwater • © 2014 Pearson Education, Inc. Materials Extruded During an Eruption Andesitic and Rhyolitic Lava Flows •Thick flows •Andesitic eruptions are intermediate between basaltic and rhyolitic. • Block Lava Flows are composed of andesitic and rhyolitic lava; upper surface consists of massive, detached blocks https://www.youtube.com/watch?v=0yWYlv3x G-c © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. 2 10/6/2015 Materials Extruded During an Eruption Gases • Gases make up 1%‒6% of the total weight of a magma • Mostly water vapor with smaller amounts of CO2, S, Cl, and other gases • Gases are held in molten rock by confining pressures • Gases are released as the magma reaches the surface and the pressure is reduced • Relative proportion of gases varies by volcanic region © 2014 Pearson Education, Inc. Materials Extruded During an Eruption Materials Extruded During an Eruption Pyroclastic Materials: Pulverized rock and lava fragments ejected from a volcano 5 Types by Size: • Volcanic Ash – fine, glassy fragments; welded tuff is created when hot ash falls and glassy fragments fuse together. • Lapilli – pyroclasts 2-64mm in size; walnut size • Cinders – pyroclasts 2-64mm in size; pea size © 2014 Pearson Education, Inc. Materials Extruded During an Eruption Pyroclastic Materials Pyroclastic Materials 5 Types by Size (con’t): 2 Types by Composition: • Scoria – reddish-brown porous rock from frothy basaltic and andesitic lava; many voids (vesicles); more dense than pumice • Blocks hardened or cooled lava fragments >64mm (2.5”) • Bombs ejected hot lava blobs that harden tin streamlined shapes mid-air >64mm (2.5”) © 2014 Pearson Education, Inc. Anatomy of a Volcano General Features of a Volcano • Pumice – vesicular rock made from “frothy” lava; many very small vesicles; less dense than scoria; andesitic to rhyolitic © 2014 Pearson Education, Inc. Anatomy of a Volcano General features at the summit Classified by eruptive patterns and landform General features at the summit • Conduit – a fissure that magma moves through to reach the surface • Vent –surface opening of a conduit • Volcanic cone – cone shaped structure created by eruptions of lava and pyroclastic material at vent © 2014 Pearson Education, Inc. • Crater – a funnel-shaped depression at the summit of most volcanic cones; generally less than 1 km in diameter; formed by accumulated materials • Caldera – a volcanic crater >1 km in diameter; produced by a collapse following a massive eruption © 2014 Pearson Education, Inc. 3 10/6/2015 Anatomy of a Volcano Volcanoes General features on the sides 3 Types of Volcanoes • Parasitic cones – a flank vent that emits lava and pyroclastic material • Shield • Cinder Cones • Composite (Stratovolcano) • Fumaroles – a flank vent that emits gases (Lassen) © 2014 Pearson Education, Inc. Shield Volcanoes © 2014 Pearson Education, Inc. Shield Volcanoes Shield Volcanoes • Mauna Loa is the largest shield volcano on Earth • Produced by mild eruptions of large volumes of basaltic lava • Broad, slightly dome-shaped • Most begin on the seafloor as seamounts; only a few grow large enough to form a volcanic island • Gentle slopes formed from very hot, fluid lava that travels far from the vent • Often have a large, steepwalled caldera at summit from magma chamber collapse • Examples include the Hawaiian Islands, the Canary Islands, the Galapagos, and Easter Island © 2014 Pearson Education, Inc. Cinder Cones Cinder Cones • Lassen NP • Built from basaltic ejected lava that hardens in flight (scoria) • Material is commonly pea- to walnut-sized • Can produce large lava flows in final eruptive stages • Steep slope angle: 30-40° • Frequently occur in groups; sometimes associated with extensive lava fields • Can be produced as parasitic cones, or in calderas of large volcanoes © 2014 Pearson Education, Inc. Shield Volcanoes Mauna Loa •Earth’s largest shield volcano •9km (6mi) tall •1 of 5 overlapping shield volcanoes of Hawaiian Islands © 2014 Pearson Education, Inc. Cinder Cones Paricutin in Mexico City • Grew over short period (9 years) • Produced ash and volcanic fragments • Ended with aa flows 30 meters thick that covered nearby villages © 2014 Pearson Education, Inc. 4 10/6/2015 Composite Volcanoes Composite Volcanoes Composite Volcanoes (Stratovolcanoes) Composite Volcanoes (Stratovolcanoes) • Large, classic-shaped volcano (symmetrical cone, thousands of feet high and several miles wide at the base) • Composed of alternating layers of cinders and ash interbedded with lava flows • Viscous nature of eruptive materials • Made from silica-rich, andesitic magmas • Many will also emit basalt lava and rhyolitic pyroclasts © 2014 Pearson Education, Inc. • Most erupt explosively with huge quantities of pyroclasts • Fine ash Is deposited in a thin layer over large area • Course fragments accumulate near the vent and produce steep slopes of summit • Long lava flows common in early stages of growth; become shorter with time • Many have secondary vents that create ciner cones or other volcanic structures on flanks • Many are located adjacent to the Pacific Ocean in the Ring of Fire © 2014 Pearson Education, Inc. Composite Volcanoes Volcanic Hazards Volcanic Hazards Mount St. Helens, WA Fujiyama, Japan Mt. Shasta, CA Mount Etna, Italy • Pyroclastic Flows • Lahars • Other • • • • • Fujiyama Ash and pyroclasts deposits Tsunami Airborne Ash Volcanic Gases Ash and gases in atmosphere Mt. Shasta © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Volcanic Hazards Volcanic Hazards Pyroclastic Flows • Mixture of hot gases infused with ash and lava fragments that flows down a volcanic slope • Cloud of hot gas and ash with ground-hugging mixture of pumice and pyroclasts • Moves very quickly >60mph • Driven by gravity, mobilized by expanding gases, follows valleys and other topographic lows. Photo reference cited in notes Lahars https://www.youtube.com/watch ?v=Cvjwt9nnwXY • A mudflow created as volcanic debris becomes saturated with water • Follows stream valleys downslope • Common on glacier and snow-covered volcanoes and heavy rainfall areas • Some lahars are triggered when magma nears the surface of a volcano covered in ice and snow and causes it to melt https://www.youtube.com/watch?v=kznwnpNTB6k © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. 5 10/6/2015 Volcanic Hazards Other hazards • Volcano-related tsunamis due to sudden collapse of volcanic flanks into the ocean • Volcanic ash – a hazard to airplanes; destroy plant life • Volcanic gases – a respiratory health hazard © 2014 Pearson Education, Inc. Other Volcanic Landforms Other Volcanic Landforms Other Volcanic Landforms • • • • • Calderas Fissure Eruptions Lava Domes Volcanic Necks Volcanic Pipes © 2014 Pearson Education, Inc. Formation of Crater Lake-Type Calderas Calderas are circular, steep-sided depressions with a diameter >1 km Crater Lake-type calderas: Three different types • Crater Lake-type calderas: • Hawaiian-type calderas: • Yellowstone-type calderas: Form from the collapse of the summit of a large composite volcano following an eruption; these calderas eventually fill with rainwater © 2014 Pearson Education, Inc. Hawaiian-Type Calderas Hawaiian-type calderas: Form gradually from the collapse of the summit of a shield volcano following the subterranean drainage of the central magma chamber © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Formation of Yellowstone-Type Calderas Yellowstone-type calderas: Form from the collapse of a large area after the discharge of large volumes of silica-rich pumice and ash; these calderas tend to exhibit a complex history © 2014 Pearson Education, Inc. 6 10/6/2015 Other Volcanic Landforms Fissure Eruptions • Long, narrow cracks in Earth that emit fluid basaltic lava • Creates large, flat, broad igneous provinces (basalt plateaus) • Also called flood basalts • The Colombia Plateau and the Deccan Traps are two examples © 2014 Pearson Education, Inc. Other Volcanic Landforms Lava Domes • Viscous silica-rich, ryholitic lavas “squeezed” out of a vent, creating a dome-shaped mass • Usually only a few 10’s of meters high • Can collapse and produce pyroclastic flows © 2014 Pearson Education, Inc. Other Volcanic Landforms Volcanic Necks • The remains of magma that solidified in a volcanic conduit • Weathering erodes surrounding volcanic structure, leaving resistant conduit • Ex: Devil’s Tower, WY © 2014 Pearson Education, Inc. Other Volcanic Landforms Volcanic Pipes • Conduits of magma from deep within the mantle that traveled upward through the crust © 2014 Pearson Education, Inc. Plate Tectonics and Volcanic Activity Plate Tectonics and Volcanic Activity Volcanic activity not random, most commonly associated with plate boundaries • Convergent Boundary • Divergent Boundary • Intraplate Plate Tectonics and Volcanic Activity Convergent Plate Boundaries • Partial melting of subducting plates creates buoyant magma that rises in crust • Most common along Pacific Ocean Ring of Fire • Eruptions tend to be explosive and associated with volatile-rich, andesitic magma http://earth.rice.edu/mtpe/geo/geosphere/topics/plates2.gif © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. https://engwell.wikispaces.com/file/view/WorldVolcanoes%5B1%5D.JPG/59838698/WorldVolcanoes%5B1%5D.JPG 7 10/6/2015 Plate Tectonics and Volcanic Activity Convergent Plate Boundaries • Volcanic arcs develop parallel to the associated subduction zone trench • The Aleutians, the Tongas, and the Marianas are examples of volcanic island arcs • The Cascade Range is an example of a continental volcanic arc © 2014 Pearson Education, Inc. Plate Tectonics and Volcanic Activity Divergent plate boundaries • Mid-Atlantic Ridge • ~60% of Earth’s yearly output of magma is created at oceanic ridge systems • Mantle material melts through decompression melting as it rises to fill rift • Basaltic magma creates oceanic crust, pillow basalts © 2014 Pearson Education, Inc. Plate Tectonics and Volcanic Activity Intraplate volcanism • Occurs when a mantle plume ascends towards the surface • Occur thousands of kilometers from plate boundaries • Ex. include the Hawaiian Islands, the Columbia River Basalts, and the Galapagos Islands © 2014 Pearson Education, Inc. End © 2014 Pearson Education, Inc. 8