disasters10.21notes - 1 8.1: Ch. 8 Volcanic Eruptions:...

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8.1: Ch. 8 – Volcanic Eruptions: Plate Tectonics and Magmas Date: ____________ Plate Tectonic Setting of Volcanoes : (Fig. 8.6) Over 90% of volcanism is associated with the edges of tectonic plates Most other volcanism occurs above hot spots Over 80% of Earth’s magma extruded through volcanism takes place at the oceanic spreading centers, as pure basalt magma (below sea level), and creates 20 km 3 (5 mi 3 ) of new oceanic crust each year ● Subduction zones account for 7-13% of all magma, which is more of a composite mixture , as the presence of water lowers the melting point of the continental crust; this adds new melt of different compositions to rising plumes of magma Transform faults and continent-continent collisions have little or no associated volcanism ♦♦ Spreading center volcanoes are relatively peaceful , but subduction-zone volcanoes are explosive and dangerous ! 1
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Chemical Composition of Magmas Of the 92 naturally occurring elements, only eight (8) make up more than 98% of Earth’s crust, with the next four (4) raising the total to 99.23% The most abundant element in the Earth’s crust is oxygen at ~45% , while the second most abundant element is silicon (Si) at ~27% (Fig. 8.7) As magma begins cooling, some silicon and oxygen atoms will form a silicon-oxygen Tetrahedron (SiO4) covalent bond . The central Si atom (+4 charge) shares
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disasters10.21notes - 1 8.1: Ch. 8 Volcanic Eruptions:...

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