Identifying Volcanic Rocks Carefully observe each rock sample (use a hand lens if you wish). Use the information and table below to identify each unknown sample. Types of Volcanic Rocks Texture General Color Composition Rock Name Number & Reason Fine Grained Light Felsic Rhyolite Intermediate Intermediate Andesite Dark Mafic Basalt Medium Light Felsic ------- Grained Intermediate Intermediate Dacite Dark Mafic Diabase Coarse Grained Light Felsic Granite Intermediate Intermediate Diorite Dark Mafic Gabbro Glassy Dark to Black Felsic (does not follow rule) Obsidian Frothy Light Felsic Pumice Dark Mafic Scoria Coarse Any Rounded Conglomerate Fragments Any Angular Breccia How do composition and texture relate to igneous rocks? Igneous rocks are crystalline solids which cool from magma: the liquid phase of solid rock. Magmas occur at depth in the crust, and are said to exist in "magma chambers," a rather loose term indicating an area where the temperature is great enough to melt the rock, and the pressure is low enough to allow the material to expand and exist in the liquid state. Many different types of igneous rocks can be produced. The key factors to use in determining which rock you have are the rock's texture and composition. Texture Texture relates to how large the individual mineral grains are in the final, solid rock. In most cases, the resulting grain size depends on how quickly the magma cooled. In general, the slower the cooling, the larger the crystals in the final rock. Because of this, we assume that coarse grained igneous rocks are "intrusive," in that they cooled at depth in the crust where they were insulated by layers of rock and sediment. Fine grained rocks are called "extrusive" and are generally produced through volcanic eruptions. Grain size can vary greatly, from extremely coarse grained rocks with crystals the size of your fist, down to glassy material which cooled so quickly that there are no mineral grains at all. Obviously, there are innumerable intermediate stages to confuse the issue. What do the terms mafic and felsic mean? Mafic is used for silicate minerals, magmas, and rocks which are relatively high in the heavier elements. The term is derived from using the MA from magnesium and the FIC from the Latin word for iron, but mafic magmas also are relatively enriched in calcium and sodium. Mafic minerals are usually dark in color and have relatively high densities (greater than 3.0). Minerals the make up mafic rocks include olivine, pyroxene, amphibole, biotite mica, and the plagioclase feldspars. Mafic magmas are usually produced at spreading centers, and represent material which is newly altered from the upper mantle. Felsic, on the other hand, is used for silicate minerals, magmas, and rocks which have a lower percentage of the heavier elements, and are correspondingly enriched in the lighter elements, such as silica and oxygen, aluminum, and potassium. The term comes from FEL for feldspar (in this case the potassium-rich variety) and SIC, which indicates the higher percentage of silica. Felsic minerals are usually light in color and have densities less than 3.0. Common felsic minerals include quartz, muscovite mica, and the orthoclase feldspars. It is important to note that there are many intermediate steps in the purification process, and many intermediate magmas which are produced during the conversion from mafic to felsic. We call the magmas associated with these intermediate stages "intermediate."
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