Cross cracks-How long has plate tectonics been active on the Earth? oWhat about earthquakes that are not at plate boundaries?
Eg. Mississippi river valley Intraplate earthquakes Earthquakes that occur away from plate margins Represent only about 1% of all earthquakes And most are along former plate margins Where plate margins failed to open up and create oceans (creates rivers) o 20,000 years ago Laurentide ice sheet caused crust to subside and once melted the crust is still gradually rebounding (causes earthquakes) Volcanic risk - Igneous rock o Start out as molten rock (magma) underground o Forms when melted rock (magma) cools down and crystallizes - Where does magma originate? o Almost all igneous activity happens along constructive and destructive plate margins o Most (but not all) magma is generated at plate boundaries/margins o Mainly forms in the upper mantle (down to about 200 km) o Sometimes in the crust o But some igneous activity happens away from plate margins (eg. Hawaii) Hotter zone beneath the lithosphere creates a string of volcanic islands - Magmas contain dissolved gases o Water, carbon dioxide, sulphur dioxide, fluorine, chlorine, etc. o We can see this in rocks from trapped gas bubbles (eg. Pumice) - Igneous rocks can form (crystallize) either: o A) inside the Earth (intrusive) Batholith – big pool of igneous rock that crystallizes inside the earth (huge) o B) on the surface of the Earth (extrusive) Dyke – crack that cuts up (vertical) across layers of rock on the surface to release magma Sill – crack that cuts horizontal with the layers of rock to release magma - The shape and features of an igneous rock body tell us something about the environment of formation - The size of crystals in any igneous rock depends mainly on the rate of cooling of the magma o Rapid – tiny crystals Eg. extrusive o Slow – medium to big crystals Eg. intrusive - Note: there is a range of chemical composition of magmas (igneous rocks) o Seen most readily by their different colours o Magmas differ in the proportion of elements o Results in rocks of different colours and magmas with different properties o Rocks rich in iron and magnesium with be black (mafic) o Rocks rich in silicon and feldspar will be pinkish whitish (felsic) - Basalt – an extrusive mafic rock (black) - Rhyolite – an extrusive felsic rock (light) - Andesite – extrusive igneous rock that is intermediate between felsic and mafic (grey)
Volcanic Risk Analysis and Risk Management - A: Risk analysis o 1. Understand the hazard in general
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