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Volcanic Rocks: Eruption Style & Lava Flows Lava Fountains
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The Causes of Volcanic Eruptions How do magmas rise? If only due to bouyancy most basaltic lavas could not erupt through continental crust
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The Causes of Volcanic Eruptions: Volatiles Decrease in Pressure H 2 O is the most important volatile in controlling volcanic eruptions Dissolved H 2 O Increase in Temperature Crystallisation (decrease in temp) Depolymerisation + confining pressure Depolymerisation Fractionation
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The Closed Magma Chamber Magma rises until it is trapped within a closed magma chamber
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The Closed Magma Chamber Cooling and crystallisation causes the exsolution of volatiles Magma chamber expands due to an over-pressure
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The Closed Magma Chamber Expansion of the magma chamber causes tensile failure of country rocks Magma chamber expands due to an over-pressure Rise of less dense vesicular magma Conduit formed due to fracturing
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An Open Conduit Exsolution of volatiles due to decreasing pressure Increasing acceleration
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An Open Conduit Exsolution of volatiles due to decreasing pressure Break up of magma Foam reaches 77 vol% vesicles Magma/gas aerosol 600 m/s
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Interactions with Water
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Hydrovolcanic Eruptions
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Hyaloclastite
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Hydrovolcanic Eruptions Efficient cooling of magma Phreatic EruptionsPhreatomagmatic 0.3
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Hydrovolcanic Eruptions Efficient cooling of magma Phreatic EruptionsPhreatomagmatic Phreatic Eruptions: Maars Mainly fragmented country rock, sediments can deform in-situ
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Hydrovolcanic Eruptions Efficient cooling of magma Phreatic EruptionsPhreatomagmatic Phreatic Eruptions: Maars Mainly fragmented country rock, sediments can deform in-situ Contact between magma and an acquifer High abundance of lithic materials
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Hydrovolcanic Eruptions Surtseyan Phreatomagmatic Eruptions
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Hydrovolcanic Eruptions Kilauea Explains the presence of ashes on kilauea
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Magma-Mixing Acid Magma Basaltic Magma Extra volatiles Extra heat Expansion due to vesicle formation, temp increase and/or volume of the injected magma
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Extrusive Activity Loss of volatiles allows extrusive activity Low viscosity magmas Low H 2 O magmas Basaltic magmas Intermediate/acid magmas after explosive activity (i.e. loss of volatiles)
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Size and Shape of Lava Flows Andesite lava flow Rhyolite Dome Basaltic pahoehoe flows Rhyolites Andesites, Dacites Basalts
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Size and Shape of Lava Flows Rhyolites Andesites, Dacites Basalts Gradient Viscosity Low viscosity High viscosity High temperature Low crystal content High water content Low-Si (basaltic) Low temperature High crystal content Low water content High-Si Effusion Rate High Low
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Shield Volcanoes Mauna Kea Mauna Loa Rift Zone Low viscosity basaltic magmas usually lead to shield volcanoes. Eruptions occur mainly from vents.
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Fissure Eruptions Splatter Ramparts and Splatter Cones Lava Fountains Mauna Loa
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Fissure Eruptions Splatter Ramparts and Splatter Cones Lava Fountains Mauna Loa Splatter forms agglutinate scoria Low viscosity, high extrusion rate lavas expand as single flow units
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Basaltic Flows: Pahoehoe & Aa Pahoehoe (ropey) surface formed by folding of the crust Low viscosity
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Basaltic Flows: Pahoehoe & Aa Flow expands by extrusion of tongues and toes of lava Higher viscosity/lower extrusion rates
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Basaltic Flows: Pahoehoe & Aa Lava blisters and hornitos can occur on the upper surface of the flow
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Basaltic Flows: Pahoehoe & Aa Insulated flow
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Basaltic Flows: Pahoehoe & Aa Aa flows roll forwards over their breccia Higher viscosity (lower T)
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Basaltic Flows: Channels Flows tend to form channels
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Basaltic Flows: Channels
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Flood Basalts Snake River Flood Basalts
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Flood Basalts
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Giant’s Causeway Shrinkage cracks due to contraction on cooling. Intermediate cooling rates.
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Pillow Lavas
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Submarine Volcanics
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Intermediate Lavas Channelled by block levees Ridges due to compression of magma Extensive basal and flow top breccias. Associated with stratovolcanoes or calderas
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Dacite & Rhyolite Lavas Mono Coulee, California Can form lava flows where viscosity and extrusion rate are sufficiently high
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Dacite & Rhyolite Lavas Mono Coulee, California Obsidian Banding
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Dacite & Rhyolite Domes More commonly form lava domes, often within summit crater. Collapse of lava domes is a major cause of pyroclastic eruptions.
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Extrusive Igneous Rocks: Ultrabasic Komatiite Mg-rich Ultrabasic lavas Pyroxenite lavas are also known. Archean lavas
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Extrusive Igneous Rocks: Basic Rocks Intermediate Rocks Dacite, latite and Andesite often contain hornblende
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Extrusive Igneous Rocks: Basic Rocks Intermediate Rocks Dacite, latite and Andesite often contain hornblende Basalt Anorthite-rich plag + px Often contains olivine
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Extrusive Igneous Rocks: Basic Rocks Intermediate Rocks Dacite, latite and Andesite often contain hornblende Basalt Anorthite-rich plag + px Often contains olivine Andesite More Na-rich plag Hornblende
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Extrusive Igneous Rocks: Basic Rocks Intermediate Rocks Dacite, latite and Andesite often contain hornblende Basalt Anorthite-rich plag + px Often contains olivine Gabbro Anorthite-rich plag + px Often with olivine Dolerite = medium-grained equivalent
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Extrusive Igneous Rocks: Basic Rocks Basalts are split into Tholeiites – these have no olivine in their groundmass and can contain quartz. Alkali Basalts – these have olivine in their groundmass. Olivine-rich basalts (i.e. Mg-rich) are known as picrites. Tholeiite – can contain phenocrysts of olivine but not in the groundmass
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Extrusive Igneous Rocks: Basic Rocks Basalts are split into Tholeiites – these have no olivine in their groundmass and can contain quartz. Alkali Basalts – these have olivine in their groundmass. Alkaline basalts are the trachybasalts and include varieties such as hawaiite, mugearite and benmoreite. Trachyte – alkali feldspar + px, often has aligned phenocrysts = trachytic texture.
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Extrusive Igneous Rocks: Alkaline Rocks Rocks rich in alkali metals (Na and K) within feldspathoids Phonolites: sanidine, anorthoclase (alkali feld) phenocrysts, nepheline, px in groundmass. Tephrite: plagioclase, px and nepheline. If olivine is present it is a basanite. Nephelinite: nepheline and cpx. Leucitite: leucite and cpx. If olivine and pyroxene dominate it is a ugandite. Feldspathoids
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