geol test 2 notes

geol test 2 notes - Geol 1001 Test 2 VOLCANISM Magma after...

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Geol 1001 Test 2 2/12/07 VOLCANISM - Magma after it erupts at the earths surface is called lava - Volcano – mountain formed from erupted lavas and other material - Volcanism 1. Fundamental process in building the earths crust 2. A major natural hazard in places 3. Analysis of lavas allows geologists to infer properties of the earth’s interior 4. Gases escape into the atmosphere - Types of lava 1. Mafic – Basalt 2. Intermediate – Andesite 3. Felsic – Rhyolite - Basaltic Lavas - Dark in color, hot (~ 1200C), low viscosity; can flow downhill at ~10 km/hr 1. Flood basalts – erupts on flat terrain, spreads out in sheets. Successive flows build up “plateaus” 2. Other lavas on land called Pahoehoe and Aa 3. Pillow lavas – formed by underwater eruption. Ellipsoidal, pillow like blocks ~ 1m wide - Formed at/near mid-ocean ridges – spreading centers. Can later be scraped up on land at/near subduction zone - Rhyolitic Lavas – felsic, light in color, erupted at < 900C. High viscosity, lavas tend to pile up into thick, bulbous deposits - Andesitic Lavas – properties in between basaltic and rhyoitic - Textures of Lava – Generally fine grained or glassy. - Vesicles – holes in top of lava formed by gases escaping before solidification. - Rhyolitic lava with lots of holes, is so light it floats in water – pumice - Pyroclastic deposits – water and gases in magma can be released explosively shattering lava and overlying rocks forming pyroclasts. If very fine (<2mm size) called volcanic ash. Other pieces may be much larger. When pyroclasts fall to earth and cool they form tuffs/volcanic breccias. - Pyroclastic Flows – hot ash, dust, gases ejected from volcano as glowing cloud. Rolls downhill at ~200km/hr. Little warning, can lead to great loss of life/property. These did most of the damage at Pompeii in 79 AD. - Eruptive Styles - Central eruptions 1. Shield volcano, broad, gently sloping, made up of many thin basaltic lava eg. Hawaii 2. Volcanic dome – bulbous mass of felsic lavas piled up at/near vent. Plugs top of volcano, eg. Mt. St. Helens
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3. Cinder cone – ejected material ( mainly pyroclasts) dip away from the summit, eg. Cerro Negro 4. Composite (strato) volcano – made up of “alternating” layers of pyroclasts and lavas ( generally andesites) build up classical volcano shape (quite steep), eg. Mts. Hood/St. Helens/ Vesuvius/Etna/Fujiyama. 5. Crater – pit at the summit of many volcanoes. May be much wider than “vent” from which eruptions occur. Can be >100km wide and deep 6. Caldera – after violent eruption, empty “magma chamber” no longer able to support “roof.” Volcanic structure collapses, forming basin like depression, often miles wide. Lake forms. 7. Phreatic Explosions – hot magma meets sea or ground water, forming superheated steam and causing violent explosions, eg Krakatoa (Indonesia) – 1883, explosion heard in Australia – thousands of km away. Killed ~30,000 people (mainly due to a tsunami) - Fissure eruptions 1.
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