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IGNEOUS ROCKS EXTRUSIVE Volcanic- Fine-grained INTRUSIVE Plutonic- Coarse-grained MAGMA • Molten Rock – Usually with dissolved gasses • Generated at depth • Eruptions if magma (lava) reaches surface • If doesn’t reach surface, Solidifies underground – Intrudes country or host rock – Intrusive contact – Chill zone – Xenolith Igneous Rocks • Names based on mineral composition reflects chemical composition of the magma and... Grain size – Very coarse-grained Pegmatitic – Coarse-grained: Phaneritic > 1 mm. – Fine-grained: Aphanitic < 1 mm. – Porphyritic- 2 crystal sizes Igneous Rocks • Names based on mineral composition reflects chemical composition of the magma and... Grain size – Very coarse-grained Pegmatitic – Coarse-grained: Phaneritic > 1 mm. – Fine-grained: Aphanitic < 1 mm. – Porphyritic- 2 crystal sizes Igneous Rocks • Names based on mineral composition reflects chemical composition of the magma and... Grain size – Very coarse-grained Pegmatitic – Coarse-grained: Phaneritic > 1 mm. – Fine-grained: Aphanitic < 1 mm. – Porphyritic- 2 crystal sizes Igneous Rocks • Names based on mineral composition reflects chemical composition of the magma and... Grain size – Very coarse-grained Pegmatitic – Coarse-grained: Phaneritic > 1 mm. – Fine-grained: Aphanitic < 1 mm. – Porphyritic- 2 crystal sizes Igneous RocksClassification • Coarse-grained• Plutonic (Intrusive) – Granite – Diorite – Gabbro • Fine-Grained • Volcanic (Extrusive) – Rhyolite – Andesite – Basalt Igneous Rock Identification • Granite (& Rhyolite) – High in Si + O – Low in Fe + Mg – Mostly feldspar & quartz – Light-colored • Basalt (& Gabbro) – “Low” in Si + O – High in Fe + Mg – no quartz, abundant ferromagnesian minerals – Dark colored • Andesite (& Diorite- intermediate) Igneous Rock Identification • Granite (& Rhyolite) – High in Si + O – Low in Fe + Mg – Mostly feldspar & quartz – Light-colored • Basalt (& Gabbro) – “Low” in Si + O – High in Fe + Mg – no quartz, abundant ferromagnesian minerals – Dark colored • Andesite (& Diorite- intermediate) Igneous Rock Identification • Granite (& Rhyolite) – High in Si + O – Low in Fe + Mg – Mostly feldspar & quartz – Light-colored • Basalt (& Gabbro) – “Low” in Si + O – High in Fe + Mg – no quartz, abundant ferromagnesian minerals – Dark colored • Andesite (& Diorite- intermediate) Chemistry of Igneous Rocks • • • • Mafic rocks or magma Silicic (or felsic) rocks or magma Intermediate rocks or magma Ultramafic rocks(no extrusive equivalent) – Dunite – Peridotite INTRUSIVE BODIES (STRUCTURES) • Bodies that solidified underground-Plutons • Volcanic neck- shallow intrusion • Fills cracks (joints)- tabular bodies – DIKE• • – If no layering in country rock If country rock is layered- Discordant SILL- less common • Concordant- parallel to layering in country rock INTRUSIVE BODIES (STRUCTURES) • Bodies that solidified underground-Plutons • Volcanic neck- shallow intrusion • Fills cracks- tabular bodies – DIKE• • – If no layering in country rock If country rock is layered- Discordant SILL- less common • Concordant- parallel to layering in country rock INTRUSIVE STRUCTURES Plutons • BATHOLITH– Large intrusive body – Exposed in an area greater than 100 square Km. – Coalesced smaller plutons • smaller bodies are called STOCKS • Batholiths a gathering of smaller blobs • Magma moves upward from depth as diapirs INTRUSIVE STRUCTURES Plutons • BATHOLITH– Large intrusive body – Exposed in an area greater than 100 square Km. – Coalesced smaller plutons • smaller bodies are called STOCKS • Batholiths a gathering of smaller blobs • Magma moves upward from depth as diapirs INTRUSIVE STRUCTURES Plutons • BATHOLITH– Large intrusive body – Exposed in an area greater than 100 square Km. – Coalesced smaller plutons • smaller bodies are called STOCKS • Batholiths a gathering of smaller blobs • Magma moves upward from depth as diapirs DISTRIBUTION OF PLUTONIC- Coarse-grained ROCK – Granite most abundant • • – Common in mountain ranges In ancient rock that were mountain ranges that are now plains Ultramafic rock in the mantle VOLCANISM & Igneous Activity VOLCANISM • Lava = Magma at earth surface – Silica content controls “explosiveness” • Pyroclasts = Fragments of rock due to explosion- Ash falls (pumice); Volcanic Bombs (scoria) • Lava flows • Extrusive rocks • Volcano VOLCANISM • Lava = Magma at earth surface – Silica content controls “explosiveness” • Pyroclasts = Fragments of rock due to explosion- Ash falls (pumice); Volcanic Bombs (scoria) • Lava flows • Extrusive rocks • Volcano VOLCANISM • Lava = Magma at earth surface – Silica content controls “explosiveness” • Pyroclasts = Fragments of rock due to explosion- Ash falls (pumice); Volcanic Bombs (scoria) • Lava flows • Extrusive rocks • Volcano Effects on Humans • Growth of Hawaii – 1980’s & 90’s 1.5 billion cubic meters • Geothermal energy- New Zealand; California • Effect on climate- 1816 “year without summer” • Volcanic catastrophies – Mt. St. Helens 1980 – Vesuvius 79 AD – Krakatoa 1883 – Crater Lake 6,600 y.b.p. Effects on Humans • Growth of Hawaii – 1980’s & 90’s 1.5 billion cubic meters • Geothermal energy- New Zealand; California • Effect on climate- 1816 “year without summer” • Volcanic catastrophies – Mt. St. Helens 1980 – Vesuvius 79 AD – Krakatoa 1883 – Crater Lake 6,600 y.b.p. Mount St. Helens • • • • • Northern flank bulged at 1.5m/day All vegetation stripped for 10km w/in seconds $100s millions in damage 63 people died Damage minimized due to prior planning by USGS and governor Effects on Humans • Growth of Hawaii – 1980’s & 90’s 1.5 billion cubic meters • Geothermal energy- New Zealand; California • Effect on climate- 1816 “year without summer” • Volcanic catastrophies – Mt. St. Helens 1980 – Vesuvius 79 AD – Krakatoa 1883 – Crater Lake 6,600 y.b.p. Eruptive Violence & Characteristics of Lava • Gas in lava • Viscosity – Temperature – Silica content • Silicic lavas- most viscous • Mafic lavas- least viscous Extrusive Rocks & Gases • Scientific study of volcanism • Gases – Primarily H2O – Also CO2 , SO2 , H2S, HCl • Gases & pyroclastics – Ashfall – Pyroclastic flow Extrusive Rocks • Importance of silica content • Rhyolite- silicic – Predominantly feldspar and quartz • Andesite- intermediate – Plagioclase feldspar & ferromagnesian minerals • Basalt- mafic – Ferromagnesian minerals & plagioclase feldspar Extrusive Rocks • Textures – Fine-grained (smaller than 1 mm) – Glassy- Obsidian – Due to • rapid cooling (mainly) • high viscosity – Porphyritic • Phenocrysts – Due to trapped gas • Vesicles • Scoria • Pumice Extrusive Rocks • Textures – Due to trapped gas • Vesicles • Scoria • Pumice – Fragmental • • • • • Pyroclasts Dust, ash, cinders Blocks & bombs Tuff Volcanic Breccia VOLCANOES • • • • • • Volcanoes are cone-shaped Vent Crater Flank eruption Caldera Types: – Cinder Cone, Shield, Composite VOLCANOES • • • • • • Volcanoes are cone-shaped Vent Crater Flank eruption Caldera Types: – Cinder Cone, Shield, Composite CINDER CONES • • • • Formed of pyroclastics only Steep sides- ~30 degrees Relatively small Short duration of activity VOLCANOES • • • • • • Volcanoes are cone-shaped Vent Crater Flank eruption Caldera Types: – Cinder Cone, Shield, Composite SHIELD VOLCANOES • Low viscosity lava flows – Low silica magma- mafic – Basalt • Pahoehoe • Aa • Gently sloping flanks- between 2 and 10 degrees • Tend to be very large • Spatter Cone- minor feature VOLCANOES • • • • • • Volcanoes are cone-shaped Vent Crater Flank eruption Caldera Types: – Cinder Cone, Shield, Composite COMPOSITE VOLCANO • • • • • Alternating pyroclastic layers & lava flows Slopes intermediate in steepness Intermittent eruptions over long time span Mostly Andesite Distribution – Circum-Pacific Belt (“Ring of Fire”) – Mediterranean Belt COMPOSITE VOLCANO • • • • • Alternating pyroclastic layers & lava flows Slopes intermediate in steepness Intermittent eruptions over long time span Mostly Andesite Distribution – Circum-Pacific Belt (“Ring of Fire”) – Mediterranean Belt VOLCANIC DOMES • Forms above a volcanic vent • Viscous lava – Usually silica-rich (or cooler magma) • Associated with violent eruptions Lava Flows • AA – rubbly surface, broken, jagged • Pahoehoe – ropy surface LAVA FLOODS • Mafic lava- solidifies to basalt • Fissure flows – Plateau basalts • Columnar structure or jointing SUBMARINE ERUPTIONS • Pillow basalt