ESS 101 Chapter Notes - Chapter 6: Intermediate Composition, Igneous Rock, Dacite

• School: UW
• Department: Earth and Space Sciences
• Course: ESS 101
• Professor: Swanson Terry

Document Summary

Intermediate composition magma can crystallize below the surface beneath subduction zones and create large plutonic bodies composed of coarse grained igneous rock. Compositions can range from granite to diorite. Andesitic magma is produced from the partial melt of the oceanic crust along subduction zones. Introduction of water forced out of the subducting plate lowers the melting temperature of the upper mantle, which rises and partially melts the overlying crust. St. helens is composed of intermediate composition dacitic flows: dacite is slightly more felsic (greater silica content) than andesite, but more mafic (higher in fe and mg content) than rhyolite. Because minerals crystallize at specific temperatures, certain minerals will be compatible to form together in igneous rocks (i. e. olivine, pyroxene, and ca-rich plagioclase) The crystallization temperature is highest for olivine and becomes progressively lower until quartz forms last from the residual silicate melt.