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<br /> <br /> <br /> <br />18 <br /> <br />Figure 3 - QAP Diagram for Intrusive Rock <br /> <br />Figure 4 - QAP Diagram for Extrusive Rock <br />Igneous rocks are an assemblage of crystalline <br />minerals. Igneous rocks started as molten masses of <br />minerals and elements. As these masses moved away <br />from their sources and cooled, the minerals in the mix <br />cooled and solidified into rock. Igneous rocks are <br />classified by whether they cooled below or above the <br />earth’s surface and by the texture and type of <br />minerals. Igneous intrusive rocks form below the <br />earth’s surface. Igneous extrusive rocks formed as the <br />molten magma cooled at or above the ground surface. <br />Texture of igneous rocks can be fine-grained <br />(aphanitic) or coarse-grained (phaneritic) depending <br />on how quickly the magma cooled. Intrusive rocks, <br />such as granite, diorite, and gabbro, cooled slowly at <br />depth, are medium- to coarse-grained and usually dark <br />colored. Figure 5 shows an example of a coarse- <br />grained granite and a fine-grained basalt. <br /> <br />Sample of Granite Sample of Basalt <br />Figure 5 - Examples of Igneous Rocks <br />A pegmatite is a granitic rock that cooled slowly and <br />allowed feldspar, quartz, and mica crystals to grow <br />large. Although these rocks are beautiful, from an <br />engineering standpoint, they present more problems <br />than rocks with smaller and tighter crystal structures. <br />Figure 6 shows an example of granitic igneous intrusive <br />rock at Half Dome in Yosemite Valley. <br /> <br />Figure 6 - Igneous Rock at Half Dome in Yosemite <br />Extrusive rocks such as basalt and andesite lava flows <br />cooled fast at the surface, are fine-grained, and are <br />usually dark colored like the basalt flow shown on <br />Figure 7.