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Notes for Test 2

Notes for Test 2 - Geology Test 2 Ch 12 16 5 6 8 Chapter 12...

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Geology Test 2: Ch. 12, 16, 5, 6, 8 Chapter 12: Volcanism Lava - magmaafter it erupts at the Earth’s surface Volcano - mountain formed from erupted lavas and other material Volcanism: o Is a fundamental process in building the Earth’s crust o A major naturalhazard in places o Analysis of the lavas allows geologists to infer properties about the Earth’s interior o Gases escape into the atmosphere Types of lavas: o Mafic – forms basalt rock (BASALTIC LAVA) o Intermediate- forms andesite rock (ANDESITIC LAVA) o Felsic- forms rhyolite rock (RHYOLITIC LAVA) Basaltic Lavas Dark in color Hot ( 1200 ° C) Low viscosity Can flow downhill at approx 100 km/hr 1. Flood basalts - erupts on flat terrain, spreads out in sheets;successive flows build up “plateaus” (Fig. 12.16) 2. Other lavas on land called Pahoehoe and Aa (Fig. 12.3) 3. Pillow lavas - formed by underwater eruption; ellipsoidal, pillow like blocks 1 m wide (Fig. 4.12) Ex: formed at/nearmid-ocean ridges- spreading centers; can later be scraped up on land at/nearsubduction zone Rhyolitic Lavas Felsic Light in color Erupted at < 900 ° C High viscosity Lavas tend to pile up into thick, bilbous deposits (Fig. 12.11) Andesite Lavas Properties in betweenbasaltic and rhyolitic Texture of Lavas Generally fine-grained or glassy Vesicles - holes in top of lava formed by gases escaping before solidification (Fig. 12.6) Pumice - rhyolitic lava with lots of “holes”; is so light it floats in water Pyroclastic deposits Water and gases in magma can be releasedexplosively shattering lava and
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If very fine (<2mm size) called volcanic ash Other pieces may be much larger (Fig 12.8) 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 approx 200km/hr o Little warning, can lead to great loss of life/property o Fig. 12.10 o These did most of the damage at Pompeii in 79 AD Eruptive Styles Central eruptions (Fig. 12.11) 1. Shield volcano - broad, gently sloping made up of many thin basaltic lavas (ex: Hawaii) 2. Volcanic dome - bulbous mass of felsic lavas piled up at/near vent (ex: top of Mt. St. Helens) 3. Cinder cone - ejected material (mainly pyroclasts) dip away from the summit (ex: Cerro Negro) 4. Composite (strato) volcano - made up of “alternating” layers of pyroclasts and lavas (generally andesites) build up classical volcano shape (quite steep) (ex: Mt. Hoods, 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 >100m 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 (Fig. 12.11-12) 7. Phreatic explosions - hot magma meets sea or ground water, forming superheated steam and causing violent explosions (Fig. 12.5) (ex: Krakatoa (Indonesia)- in 1883, explosion heard in Australia- thousands
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