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 • All three factors affect magma’s viscosity, which in turn determines the characteristics of an eruption

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

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

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

Property Variations of Magma Due to Composition

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Volcanic Activity