0000003498 00000 n Metamorphic rocks tend to have more luster than igneous or sedimentary rock. Individual crystals have time and space to grow large. Igneous rocks are made from the rapid or slow cooling of magma/lava. Specimens will scratch glass, although it will be crumbly if it has been out in the weather for a long time. Sedimentary rocks may have visible fossils such as whole or partial leaves, shells, footprints etc. Metamorphic rocks have layers made up crystals that are the same size. 0000019027 00000 n You want to look for any type of metallic appearance or streamlined flow structures. Formed by sudden cooling with no time to form even tiny crystals. Scoria: Basalt with over 50% vesicles. Mineral composition can only be identified with a microscope unless phenocrysts are present (see below). Classify the grain type in your rock. This also formed with very rapid cooling. They cool below the earth’s surface, generally deep. Example: Porphyritic basalt, porphyritic andesite. Physical Geology, Adapted by Anne Huth, Pima Community College. 0000145176 00000 n Biotite: Small black flakes with perfect cleavage in 1 direction (layers), reflects light. The presence and distribution of layering can help you identify the main type of rock you have. They may have a glassy appearance. Intrusive rocks form from the cooling of magma deep beneath the earths surface. These rocks have greater than 85% mafic mineral crystals. 0000004048 00000 n When you hold an igneous rock, the rock you hold in your hand is one of the oldest things in the world. 0 0000002614 00000 n Metamorphic rocks will contain only highly fragmented fossils. Rocks can be a mixture of different kinds of minerals, a mixture of many grains of the same kind of mineral, or a mixture of different grains of rocks. Metamorphic rocks change form based on the influence of heat, pressure, or chemical activity. If the rock has been weathered by the elements, it may be crumbly. Very hard! Field identification of hand specimens is based on light or dark color, unless phenocrysts are present. Igneous rocks are formed by cooling magma. Mafic minerals are olivine, pyroxene, amphibole, and biotite. The name of the igneous rock depends on what minerals are present. 0000002129 00000 n Agglomerate: Contains volcanic fragments larger than 2 centimeters (about 1 inch in diameter) that were blown out of a volcanic vent. If there are mostly dark-colored minerals and the rock is fine grained, it is basalt. Look at the composition of your rock. In the light, you should be able to tell whether the rock has an overall shimmery quality to it. Sedimentary rocks with no grains also tend to be soft, as they can usually be scratched easily with a fingernail. If you break a big chunk of quartz into smaller pieces, you still have pieces of quartz. These are often long and arranged in a parallel pattern. Metamorphic rocks may also have a glassy appearance. Example: Vesicular basalt. Remember, not all rocks will have visible grains. Extrusive, or volcanic, igneous rocks look dull and do not sparkle much because they are fine grained. Lets begin with the definition of the different types of rocks ,viz. Dunite is an example of an ultramafic rock. Extrusive rocks have very small, almost microscopic crystals. Sedimentary rocks can also have crystals. Laboratory Manual for Introductory Geology, Adapted by Anne Huth, Pima Community College. Igneous Rock Identification. For example, when you break granite apart, you get small pieces of quartz (clear), feldspar (pink or white), and mica (black). Quartz: Occurs as irregular, glassy grains, commonly clear to smoky with no cleavage. 0000000876 00000 n Porphyritic igneous rocks have crystals of two different sizes, often with large crystals set in areas of smaller crystals. Vitrophyre is a glassy rock containing crystals (phenocrysts). Felsic igneous rocks are light in color. Ultramafic igneous rocks are also dark in color and contain higher amounts of the minerals found in mafic rocks. Sedimentary rocks are essentially formed by pieces of smaller rocks, fossils, and sediments. Obsidian is a black glass with conchoidal (circular) fracture. Some volcanic rocks have vesicles, which are holes caused by gas bubbles when the lava was molten. Some are welded tuffs and look like rhyolite. 0000001656 00000 n

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