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Rocks - GEOL 114 The Earth's Dynamic Interior Lecture...

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GEOL 114 The Earth's Dynamic Interior Lecture Notes (Copyright © 2006 by Jeffrey S. Barker) 13. Basic Geology There is obviously a great deal we could cover on the subject of Geology; it is an entire field of study. In this course, we have been concentrating on the Earth's interior and its processes. The rocks found at the Earth's surface provide our only direct evidence of the processes taking place below. Conversely, understanding the surficial processes that produced and altered these rocks requires an understanding of the sub-surface processes (heat flow, mantle rheology and composition, the geomagnetic field, plate tectonics, etc.). So, this is an appropriate time to introduce some of the terminology and concepts of Geology, with the realization that we are leaving out a lot of interesting details. Rocks are simply combinations of minerals (the study of how minerals combine to form rocks is called petrology. There are many ways of categorizing and naming rocks, but the primary distinction is based on the process in which the rock formed. The three primary categories of rocks are Igneous, Sedimentary, and Metamorphic. Igneous Rocks crystallize from a molten state (from magma ). We can draw a distinction between those that cool and crystallize rapidly at the surface, volcanic or extrusive rocks, and those that cool and crystallize slowly at depth, plutonic or intrusive rocks. Slow cooling allows large crystals to grow, whereas rapid cooling allows only small crystals. So, we can distinguish volcanic and plutonic rocks based on their crystal texture. The formation of volcanic rocks has an obvious impact on man, and we will consider volcanic eruptions and hazards later in the course. Volcanic rocks can form by flows of liquid lava ("lava" is just magma that has made its way to the GEOL 114 1 13. Basic Geology
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surface) or by the fall of ash, glassy particles, or pyroclastic "bombs". These airborne volcanic materials can cover broad areas and generate quite thick layers of rock. For example, the eruption of Long Valley caldera in eastern California 700,000 years ago generated ash which covered more than 1000 km 2 with an average thickness of 600 meters! Plutonic rocks form the cores of nearly all of the great mountain ranges, such as the Sierra Nevada (shown below), the Rocky Mountains, Andes, Alps, etc. Magma can flow into fractures or between layers in the overlying rock, forming geologic structures which are given names such as dikes (the American spelling), sills and batholiths.
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