Cross cracks How long has plate tectonics been active on the Earth o Probably

Cross cracks how long has plate tectonics been active

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Cross cracks-How long has plate tectonics been active on the Earth? oWhat about earthquakes that are not at plate boundaries?
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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)
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Volcanic Risk Analysis and Risk Management - A: Risk analysis o 1. Understand the hazard in general
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