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Chapter 7 summary

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Volcanic activity occurred in North America on May 18, 1980 caused much loss and turnedbeautiful place into decapitated remnant. Volcanic activity is a process by which molten materialcomes on the surface. Different igneous rocks have different magma composition like basaltcontains high percentage of silicate minerals. Mostly magma generated in upper mantle. Magmacan be explosive or non-explosive. Temperature and silica content changed magma viscosity.Carbon dioxide causes pressure in magma. Confining pressure decreases below the magma whenmagma chamber is forcefully ejected and this causes a series of violent explosions. Mostly lavais basaltic lava almost 90% and this erupt by a process called submarine volcanism. Basaltic lavais usually fluid and can flow and is of two types, Aa and Pahoehoe. Pahoehoe is much hotter andmore fluid then Aa lava. Tube like structure lava is called pillow lava. Gases are 1 to 8 % of totalvolume in lava. Pyroclastic material is ejected from energetic lava. Conical mountains areformed by the lava’s that occurred for thousands of year. Volcanic landform are present in avariety of sizes and shapes. Fissure is the first step of volcanic activity and lava comes on surfacefrom a conduit and it end at surface opening called volcanic cone. The funnel type depressionformed called crater and when magma chamber empties then depression is called caldera. The largest type of volcano is called shield volcano. They are found in Hawaiian Islands, Icelandand in some area of Pacific Northwest. The examples of shield volcanos are Mauna Loa which isearth’s biggest shield volcano, Kilauea which is most active volcano. Cinder cones lava are talland steep sharp volcano and form from very thick lava. They have high angle of repose. Theyoccur in groups. For example near Flagstaff, Arizona consists of 600 cones. The most dangeroustype of volcano is composite volcano. Alternating layers of lava and ash are present in compositevolcanoes. They have symmetrical form and are very complex in nature. Example of compositevolcanoes are Mount St. Helens and Mt Rainier. Volcanoes erupt very less as 1500 earth’s knownvolcanoes have erupt only once or very few times in 10,000 years. People live near volcanoesface hazards, pyroclastic flows, volcanic bombs. Example of hazards include pyroclastic flowwhich consists of hot gases, Lahars which contains water, volcanic ash and aviation that effectclimate badly.Some volcanoes do not erupt and solidifies under surface. Magma enters into pre-existing rock iscalled host rock and structures form called intrusions or plutons. Some structures formed bymagma intrusion include Dikes which are discordant, sills which are horizontal concordant,columnar joints which often have six sides, batholiths which occur as mammoth linear structuresand laccolith which are injected between sedimentary rocks. Magma comes from the partialmelting of rocks which is an incomplete melting. Melting begins from lowest melting pointminerals and then to higher melting point minerals. Temperature is not only the reason ofmelting, pressure increases with depth also plays an important role. Reducing pressure also playsan important role and it also lowers melting temperature. Decompression melting is triggeredwhen confining pressure drops sufficiently. Addition of water causes rocks to melt at lowertemperature. Most active volcanoes are along margins within circum-pacific belt is called Ringof fire. Due to sea floor spreading, magma also erupt along divergent plate boundary. Alongconvergent plate boundaries, there occurs curved chain of volcanoes is called volcanic arc.Intraplate volcanism consists of large outpourings of basaltic lavas.I have learned that lavas causes much destructions and people live near have to face muchtroubles due to lava. Lava also causes environmental issues. Composite volcanoes are mostdestructive of all volcanoes. Lava burns all the things in their way but lava also carries usefulminerals like silica. These minerals can be used to boost up country economy. If we couldcontrol lavas then we can get useful benefits from them.I have a question for you that what are the other benefits of


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