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Andesite is a fine-grained extruded igneous rock composed mainly of plagioclase with other minerals such as hornblende, pyroxene and biotite. The specimen shown is approximately five centimeters in diameter. Igneous rocks are formed by the solidification of molten rock material. Some form below the surface of the earth. Some form on or above the ground. We describe these two main types: Intrusive igneous rocks crystallize beneath the Earths surface, and the slow cooling that occurs there allows large crystals to form. Examples of intrusive igneous rocks include diabase, diorite, gabbro, granite, pegmatite, and peridotite. Extruded igneous rock is thrown to the surface where it cools quickly and forms small crystals. Some cool down so quickly that they form amorphous glass. These rocks include andesite, basalt, dacite, obsidian, pumice, rhyolite, scoria, and tuff. This page has photos and brief descriptions of some common types of igneous rock. Gabbro is a coarse, intrusive, dark colored igneous rock containing feldspar, pyroxene and sometimes peridot. The specimen shown above is approximately five centimeters in diameter. A rhyolite is a fine-grained, light igneous rock that mainly contains quartz and feldspar minerals. The specimen shown above is approximately five centimeters in diameter. Scoria is a gurgling, extruded igneous rock with a dark color. Bubbles form as a result of gas pockets in the foot during solidification. It often forms as a foamed crust on a lava flow or as a material ejected from a volcanic chimney and solidified in the air. The specimen shown above is approximately five centimeters in diameter. Unakite is a colored rock composed of a green epidote and pink orthoclase. It is formed when granite, an igneous rock, is transformed by a hydrothermal action. Cabochons, loose stones, small carvings and other stone carvings are often made of attractive pieces of unakit. It is namedUnaka Mountains in East Tennessee. intrusive igneous rocks Volcanic erosion immediately exposes shallow intrusive bodies such as volcanic necks and diatremes (see Figure 6). The volcanic neck is the neck of the volcano and consists of a tubular channel filled with hypobasal rocks. Ship Rock in New Mexico and Devils Tower in Wyoming are the remains of volcanic necks exposed by erosion of the surrounding sedimentary rocks. Many crater-like basins may be filled with angular breccia fragments and juvenile pyroclastic fragments. After erosion, such a depression exposes a funnel-shaped vertical pipe resembling the neck of a volcano, except for filling with breccia. These tubes are called diatremes. Many diatremes form as a result of an explosion resulting from the rapid expansion of a gas - carbon dioxide and water vapor. These gases are released by the pressure drop of the rising magma as it approaches the surface. Some diatremes contain kimberlite, a peridotite containing a hydrated mineral called phlogopite. Kimberlite may contain diamonds. Dikes are usually plate-like bodies that may emanate from the central vent of a volcano or from its vent (see Fig. 6). Not all dikes are associated with volcanoes, but they can be distinguished by the contrasting interaction with the rock structure they intersect. Many dams are only a few meters wide, but large dams, such as those that fueled the Muscovite invasion of Canadas Northwest Territories, are over 150 meters wide. Dikes are associated with features that maintain a constant relationship with the rock structure in the country. Magmas can penetrate between bedrock layers and solidify parallel to them, forming sills (see Fig. 6). On the west bank of the Hudson River, across from New York City, the 300-meter Palisades Rapid is exposed, which can be traced for 80 kilometers. Laccolith is also compatible withRock, but differs from a parapet in that it has a flat bottom with a domed (mushroom-shaped) roof (see Figure 6). Laccoliths were first described in Utahs Henry Mountains, where they can be up to 200 meters thick and have a base diameter of more than three kilometers. These domed intrusions are usually composed of intermediate siliceous rocks. In contrast, lopolites are saucer-shaped bodies with a concave roof and floor, usually made of mafic rocks. Lopolitans are huge; For example, the Bushveld intrusive complex in South Africa covers an area of about 66,000 square kilometers and has an exposed thickness of 8 kilometers. The Musk intrusion mentioned above is another large lopolite, estimated to be about 80 kilometers long and 11 kilometers wide (the roof rocks covering part of the intrusion make an exact measurement impossible). These lopolites are usually covered by layers of igneous minerals and rocks; In the case of an intrusion, in the Bushveld, a layer about 1 meter thick, consisting of almost pure chromite (chromium ore), extends for tens of kilometers. Large, irregularly shaped conduits are called stocks or batholiths, depending on their size (see Figure 6). Plutons larger than 100 square kilometers are called batholiths, and smaller ones are called stocks. However, it is possible that some reserves show fragments of batholiths that have not been subject to erosion. Batholiths (from the Greek bathos meaning depth) are deep indentations in the earths crust, while shallow deposits can form only a few kilometers below the surface. Rocks ranging from quartz diorite to granite are common in the batholiths. North Americas largest batholiths include the Sierra Nevada, Idaho, and the Coastal Range, which is about 600 kilometers long and 200 kilometers wide and stretches from the Alaskan border through British Columbia to Washington State. Their formation is facilitated by many invading impulses

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