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Volcano

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Introduction Of Lava

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  • 1. Volcano

2. The word volcano, from the Roman god of fire, hastwo meanings:1) The opening in the ground out of which hot moltenrock (lava or magma) and/or volcanic gas issues fromthe earth's interior onto the surface.2) The hill or mountain built up around such anopening by the accumulation of cooled magma. 3. Volcanic openings may be:1) Circular- Lava that is poured or thrown outof circular pipes (vents) forms tall,steep-sided mountains (like Mt. Agua).These are common above subduction zones.Circular openings vary in size from manholecovers to those that are larger than theLBS Ground of IIT Roorkee. 4. 2) Long, skinny, snakelike cracks called rifts or fissures.Lava that is poured or thrown out of long,skinny cracks (rifts) forms plateus(like the Columbia or Snake River) or lowangled hills or mountains (like Mona Loa). 5. These are common along mid-ocean ridges and hot spots. Crack-like openings range in size from pencil width to those thatare far larger than the width of a football field. 6. Overall there are three major kinds of lava: Mafic- called basalt. Composed of lots of iron, magnesium, and calciumalong with aluminum, silica and oxygen to make minerals like olivine,pyroxene, and calcic plagioclase). Felsic- called dacite or rhyolite. Composed of lots of potassium, sodium,aluminum, silica, and oxygen to form such minerals as quartz, potassiumfeldspar, sodic-rich plagioclase. Mixed or intermediate- called andesite. This lava is in between the othertwo, containing some iron and magnesium-rich minerals as well assodium and potassium-rich minerals. 7. BASALTIC LAVA Basaltic lava flows erupt primarily from shield volcanoes, fissuresystems, scoria cones, and spatter cones. These fluid lava flowscan be subdivided into two end-member structural types, basedprimarily on the nature of lava flow surfaces: 8. Pahoehoe Lava -- Surfaces are smooth, billowy, or ropy. It istypically the first to erupt from a vent. 9. As the smooth lava surface cools to turns to a dark gray color andbecomes less fluid and more viscous, behaving more like a plasticsubstance than a truly liquid substance. As lava continues to flow underneath this plastic skin, the surfacecan bunch up or wrinkle into a form that resembles coiled rope.Such a surface is called ropy pahoehoe. Similarly solidified basalt flows can also display shelly surfacescalled Shelly pahoehoe. 10. A'a lava -- Surfaces are fragmented, rough,and spiny, with a "cindery" appearance 11. The a'a flows are advancing over older pahoehoe surfaces. These are more viscous, and typically thicker, than pahoehoelavas. Pahoehoe is often converted to a'a as lava advances downslope,away from the volcano. caused by either an increase in flow viscosity, or an increase in therate of shear. 12. Andestic Lava andesite generally produces blocky lava the surface contains smooth-sided, angular fragments (blocks) 13. FELSIC LAVADACITIC TO RHYOLITIC LAVA With increasing silica (SiO2) content and polymerization, the viscositiesof these felsic lavas increases. 14. Unusual Lava Type STRONGLY ALKALINE LAVAS-alkali content,reflected in their weight percent ofNa2O + K2O 15. They are typically found in(1) continental or oceanic intraplate settings, where there is often a lackof significant tectonic control,(2) continental rift zones, and(3) the back-arc setting of subduction zones. 16. CARBONATITES - They are defined, when crystalline, by having morethan 50% carbonate (CO3-bearing) minerals, and typically they arecomposed of less than 10% SiO2. 17. KOMATIITES - komatiites are ultramafic volcanic rocks, having verylow silica contents (~40-45%) and very high MgO contents (~18%). A spectauclar identifyingtrait of komatiites is theirspinifex texture