Portrait of a Planet ch. 9 - CHAPTER 9 1 Volcanoes erupt in...

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CHAPTER 9 1. Volcanoes erupt in three forms: lava flows, pyroclastic debris and gas. 2. Lava flows= characteristics of lava are primarily determined by viscosity, viscosity depends on chem. composition, temp., gas content, and crystal content. 3. Basaltic lava flow= low viscosity (b/c of low silica content) is very hot. Most don’t travel more than a few km but can go as much 600 km. Flow begins very fast but top of it cools with distance creating new hard crust, the inside still flows under crust expanding thickness of flow, as interior also solidifies molten lava within moves through lava tubes. Tubes can be up to tens of meters in diameter. These flows may have smooth rope like ridges at the surface and are then called pahoehoe, but if the surface breaks up due to inner movement it becomes a jumble of sharp angular fragment calle a”a. in final stage of cooling may result in columnar jointing. Underwater basaltic flows result in pillow lava. 4. Andesitic = higher silica so higher viscosity, forms mound at vent and advances 1-5 m a day. Due to slowness outside solidifies and surface breaks up into angular blocks called blocky lava. 5. Rhyolitic= most viscous and coolest temp. there. creates lava dome at mouth of volcano. May form in vent itself and push upward as lava spine. 6. Pyroclastic debris includes fragments ejected from lava blasted apart from eruption, debris on flank of volcano from landslide. a) In basaltic eruption, volatiles such as water bubble up to surface of lava bursting and ejecting clots and drops of molten lava upward. Lapilli are little stones, pele tears and hair, apple-refrigerator sized blocks- if chunky blocks they are blocks, if streamlined and polished they are called bombs. b) Andesitic and rhyolitic can also explode producing immense debris, much more than basaltic flows. Debris from these explosive eru[tions are ash (2 mm) when frothy lava or recently formed pumice explode. Pumice lapilli (angular pumice fragments) and accretionary lapilli (snowball like lumps of ash from ash mixed with water in air).
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