Page 392 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

356 part III The Earth–Atmosphere Interface
volcanic eruptions and flows, and are discussed in ad- ditional detail in Chapter 13.
The location and rate of cooling determine the crys- talline texture of a rock, that is, whether it is made of coarser (larger) or finer (smaller) materials. Thus, the texture indicates the environment in which the rock formed. The slower cooling of magma beneath the sur- face allows more time for crystals to form, resulting in coarse-grained rocks such as granite. Even though this rock cooled below Earth’s surface, subsequent uplift of the landscape has exposed granitic rocks (Figure 12.5a), some of which form the world’s most famous cliff faces and rock climbing destinations—El Capitan and Half Dome in Yosemite Valley, California, and the Great Trango Tower in Pakistan are examples. The faster cooling of lava at the surface forms finer-grained rocks, such as basalt, the most common extrusive ig- neous rock. As discussed later in the chapter, basalt makes up the bulk of the ocean floor, accounting for
71% of Earth’s surface. Basalt is actively forming on the Big Island of Hawai‘i, where lava flows are a major tour- ist destination (Figure 12.5b).
If cooling is so rapid that crystals cannot form, the result is a glassy rock such as obsidian, or volcanic glass. Pumice is another glassy rock (one that does not have a crystal structure), which forms when the bubbles from escaping gases create a frothy texture in the lava. Pumice is full of small holes, is light in weight, and is low enough in density to float in water.
Igneous Rock Classification Scientists classify the many types of igneous rocks according to their texture and composition (Table 12.2). The same magma that pro- duces coarse-grained granite when it cools beneath the surface can form fine-grained basalt when it cools above the surface. The mineral composition of a rock, especially the relative amount of silica (SiO2), provides information about the source of the magma that formed it and affects
KILOMETRES
▲Figure 12.6 bedrock geology and physiography of Canada. Distribution of intrusive igneous, sedimentary, and metamorphic rocks within the various physiographic regions of Canada. Intrusive igneous rocks, volcanic rocks, and metamorphic rocks dominate the geology of the Canadian Shield. The geology of the Appalachian Orogen and Cordilleran Orogen represents a mixture of intrusive igneous rocks, volcanic rocks, metamorphic rocks, and deformed sedimentary rocks. Deformed sedimentary rocks with minor intrusive igneous and volcanic rocks dominate the geology of the Innuitian Orogen. Clastic and chemical sedimentary rocks dominate the geology of the various sedimentary platforms (great Lakes–St. Lawrence, Hudson Bay, Arctic, Interior). The geology of the Arctic Coastal Plain consists largely of unlithified clastic sediments. [Map by Keith Bigelow, University of Saskatchewan. Data used by permission of the Minister of Public Works and government Services Canada; natural resources Canada, geological Survey of Canada.]