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F13 quiz 1 condensed notes

Lecture 5 weathering sedimentary rocks sedimentary

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Unformatted text preview: alley and the granite domes of Enchanted Rock in central Texas are good examples of batholiths. A fracture that was once a conduit for granitic or basaltic magma, now filled with solid igneous rock, is called a dike. If the magma squeezes out laterally, for example between adjacent strata, it crystallizes as a sill. A dike cuts across the structure of the host rock, whereas a sill lies parallel to the structure. Intrusive igneous rocks are exposed at Earth’s surface only after uplift has occurred and much erosion has removed the material that once lay overhead. Dikes, sills and large columns of igneous rock called volcanic necks (ex. Shiprock) are sometimes left after erosion has exposed the “plumbing system” of an inactive volcano. Different types of lava build different types of volcanoes with various eruptive styles. Basaltic eruptions are easily observable in the mid ­Pacific Hawaiian Islands where we see gigantic volcanoes with gentle topographic profiles, called shield volcanoes, that are the world’s largest mountains (including their underwater bulk). Basalt lava is a runny, relatively low ­viscosity liquid that flows laterally rather than piling up into a steep slope. Shield volcanoes are composed of numerous thin basalt flows. Eruptions may be from the summit or from the flank of a shield volcano. Not all volcanic centers produce volcanic mountains, however, and basaltic lava also erupts from fissures on the seafloor and continental volcanic centers to form broad, relatively flat ­lying layers. A contrasting style was illustrated by the explosive eruptions of Mount St Helens in May, 1980 and Mt Unzen in Japan in June, 1991. Intermediate and felsic composition lavas create pyroclastic eruptions during which pyroclastic material – hot volcanic rock, shards of volcanic glass, steam and other volatiles, and pieces of rock broken from the mountain—are violently ejected high into the air. In case of Mount St Helens, the 1980 cataclysm blew the head off this once ­symmetrical, once ­beautiful volcano. Volcanoes constructed of alternating pyroclastic deposits and intermediate/felsic lava flows are called stratovolcanoes (or composite volcanoes). Most volcanoes have relatively small, bowl ­shaped pits, or craters, that encircle their volcanic vents where lava exits onto the land surface. A caldera is a much larger, steep ­walled depression created when the magma chamber beneath a volcano empties, and the land surface collapses into it. Examples of calderas are the Yellowstone caldera, which encompasses the entire northwestern corner of Wyoming, and the incorrectly named Crater L...
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