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Volcanic Activity

Volcanic Activity. Nature and Results. Magma. The primary factor determining the nature of volcanic eruptions is in the magma Differing composition, temperature, and dissolved gas content affect activity

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Volcanic Activity

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  1. Volcanic Activity Nature and Results

  2. Magma • The primary factor determining the nature of volcanic eruptions is in the magma • Differing composition, temperature, and dissolved gas content affect activity • All three factors affect magma’s viscosity, which in turn determines the characteristics of an eruption

  3. Magma Temperature • Viscosity is obviously affected by temperature • The hotter the lava, the easier it can flow, making the results of the cooling of lava flows different depending on initial temperature • As a lava flow cools and begins to congeal, its mobility decreases and eventually the flowing halts, creating igneous rocks of various types • Temperature can affect texture, size, and cleavage of igneous rocks

  4. Magma Composition • The major difference between igneous rocks (and therefore their precursor magmas) is silica content • Magma viscosity is directly related to silica content- the higher the silica, the greater the viscosity • Silica molecules link into long chains even before crystallization begins, impeding flow • There are three major types of magma: basaltic, andesitic, and granitic • Basaltic: 50% silica, low viscosity • Andesitic: 60% silica, intermediate viscosity • Granitic: 70% silica, high viscosity

  5. Magma Gas Content • Dissolved gases tend to increase the fluidity of magma • Gases also have the ability to propel molten rock from a volcanic vent (more important) • As magma moves into a near-surface environment, confining pressure near the top is reduced, allowing previously dissolved gases to be released suddenly • Low-viscosity magmas (basaltic) tend to be quiescent because they allow the gas bubbles to pass through relatively easily • End up making lava fountains by carrying incandescent lava hundreds of meters into the air with escaping gas • High-viscosity magmas (granitic) tend to be more explosive because they impede upward migration of gases • The gases collect as bubbles and pockets that increase in size and pressure until they are ejected explosively • Once upper magma is ejected, reduced pressure on molten rock below causes it to be blown at as well, resulting in a series of explosions rather than just one • Could theoretically continue endlessly; however, since gas bubbles move slowly, they only get enough pressure to be explosive near the top of the thermal vent

  6. Property Variations of Magma Due to Composition

  7. Volcanic Extrusion: Gas • Think of gases in magma like CO2 in soda • As soon as pressure is reduced, gases escape • Gases compose 1-5% of total magma weight, but can still exceed thousands of tons of emission per day • Composition: • ~70% water vapor • ~15% CO2 • ~5% nitrogen compounds • ~5% sulfur compounds • ~5% other chlorine, hydrogen, and argon compounds • Besides propelling magma, gases shape the conduit • Intense heat from magma body cracks rock above • Hot streams of high-pressure gases expand cracks and develop passageway to surface • Hot gases armed with rock fragments erode walls of passageway to enlarge conduit • Magma moves upward to produce surface activity • Volcanic pipe becomes choked with debris and must be cleared before erupting again

  8. Volcanic Extrusion: Basalt • Basaltic lavas  low silica content  fluid • Basalt lavas flow in thin, broad sheets or tongues • Two types of basaltic lava flows: • Pahoehoe- occurs when fluid lavas form smooth skin that sometimes wrinkles when sub-surface lava continues to advance • Aa- lava has a surface of rough, jagged blocks with dangerously sharp edges and spiny projections • Active flows are cool and thick, resulting in jagged texture • Escaping gases fragment the cool surface and produce voids and spines in lava • As molten interior advances, outer crust gets broken further • Basically looks like a pile of rubble by the end of it • Hawaiian lava flows are pahoehoe

  9. Basalt cont. • As lava flows harden, tunnels form that were once used to transport molten lava • As lava flows occur, the outer lava congeals faster than interior lava • The rocks around the interior lava also insulate it • Therefore, interior lava cools much more slowly, allowing it to travel much further before congealing • As it flows, it leaves behind tunnels where the outer lava cooled, but the inner didn’t • Oceanic Flows • When lava flows enter the ocean, the outer zones quickly congeal, but the lava within usually moves forward and breaks the hard surface • This occurs repeatedly, creating pillow lava

  10. Volcanic Extrusion: Pyroclasts • Granitic and andesitic lavas don’t flow as easily as basaltic flows, so they generally explode • Any particle produced by the processes of superheated gases blowing pulverized rock and lava from the volcanic vent is called a pyroclastic material • Vary in size as ash and dust, lapilli and cinders, and blocks and bombs

  11. Ash and Dust • Finest size of all particles • Produced when extruded lava contains so many gas bubbles that it resembles froth • Think froth from a bottle of champagne • As hot gases expand explosively, lava disseminates into very fine fragments • When it falls, glassy shard often fuse to form welded tuff • Sometimes froth-like lava is ejected in larger pieces as pumice

  12. Lapilli and Cinders • Cinders • Pea-sized • Form when blobs of lava get pulverized by escaping gas • Contain numerous voids • Lapilli • Walnut-sized • Simply a size classification; any particle within size range is a lapilli

  13. Blocks and Bombs • Both are considered any particle larger than lapilli, but they differ in extrusive form • Blocks are large pyroclasts made of hardened lava • Bombs are large pyroclasts made of semimolten or incandescent lava • As bombs get ejected, they become “streamlined” as they fly through the air • Both usually end up on the slope of the volcano, but they can sometimes be ejected like rockets by escaping gases

  14. Volcanoes • A volcano is a mountainous accumulation of material formed from successive eruptions from a central vent • At the summit of many volcanoes is a crater which is connected to a magma chamber by a pipe-like conduit • If a volcano has a summit depression (crater) that is larger than 1 kilometer, it is known as a caldera • Magma doesn’t always flow out the central vent; if easier, it may push through fissures located on flanks of volcano • Known as a parasitic cone • If they only extrude gases, the secondary vents are called fumaroles

  15. Shield Volcanoes • Broad, slightly domed structure formed from fluid lava extrusions • Shield volcanoes are formed by frequent eruptions of thin flows of very fluid basaltic lavas • As the structure enlarges, flank eruptions occur along with the summit eruptions • Collapse of the summit area frequently follows each eruptive phase • Lavas continually increase in viscosity over time, resulting in thicker, shorter flows • This explains why older volcanoes have steeper summits than younger ones

  16. Cinder Cone Volcanoes • Built from ejected lava fragments • Usually have very steep slopes, but are rather small (300 meters high) • Often form as parasitic cones on or near larger volcanoes; also form in groups • Lava rarely issues from the top except as a fountain because the walls are generally too weak to support the pressurized magma moving upward through the conduit

  17. Stratovolcanoes • A.k.a. composite cones • Produced when relatively viscous lavas of andesitic composition are extruded • Composite cone extrudes viscous lava for a long time, then suddenly violently ejects pyroclastic material and deposits it near the summit • Creates alternating lava/pyroclast layering • Vesuvius was a stratovolcano • NueeArdente • Occurs when hot gases infused with incandescent ash are ejected • These “glowing avalanches” are black in daylight and red at night • Although very dense, they are supported by expanding gas emitted from hot lava particles, and thus flow nearly frictionlessly down slopes • Most devastating type of volcanic eruption

  18. Stratovolcanoes cont. • Lava Domes • When highly viscous lava is extruded by a volcano, it can sometimes form a lava dome • This occurs when the thick lava is squeezed out of the vent and create a bulbous mass at the opening of the vent • They’re like giant volcano buttplugs

  19. Volcanic Necks and Pipes • Volcanoes are eroded by natural erosion forces • Cinder cones are the easiest to erode because they are composed of unconsolidated materials • Rock occupying the vent is often more resistant and remains standing above the terrain after the cone has vanished • Called volcanic necks • Most volcanic conduits extend to the upper mantle; others can reach the asthenosphere • Ferromagnesian-rich pipes extend a distance of 200 kilometers into the asthenosphere, allowing observation of the largely unknown characteristics of the layer

  20. Fissure Eruptions • Fissures are cracks or fractures in the crust through which large amounts of volcanic material extrude • Lava is usually extruded from several vents along fissures, resulting in a wide distribution of volcanic material • Fissure eruptions can extrude very fluid basaltic lava, creating a lava plain • The general name for this type of flow is flood basalts • This is most common in the ocean near the mid-ocean ridge

  21. Pyroclastic Flows • Almost identical to fissure eruptions, but involves high-silica magmas rather than basaltic magmas • Ash and pumice fragments are the most common constituents of pyroclastic flows • When ejected, they move away from the vent at high speeds and blanket extensive areas before coming to rest • After deposition, they closely resemble lava flows

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