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unit 7: igneous Rocks Student information Igneous rock, known as"fire rock,"is rock formed by the cooling of melted material, such as magma inside the earth and lava above the ground. Igneous rocks are formed as a result of activity at plate boundaries: volcanoes and sea-floor spreading. Obsidian is an example of igneous rock. Obsidian forms when lava cools quickly above ground. Obsidian Volcanoes are the result of converging boundaries crashing together. The heat and pressure becomes so great that rock melts within the upper mantle, or asthenosphere, layer of the earth. The molten rock is forced to the surface, forming mountains of rock. If enough pressure builds up, the molten rock may be forced out of the earth in an explosive eruption. Molten rock is known as magma and is converted into lava as it is extruded (forced through an opening in the earth) onto the earth's surface to cool. This process forms extrusive igneous rocks. Basalt is an example of an extrusive igneous rock. Magma that cools inside the earth forms intrusive igneous rocks. Granite is an example of an intrusive igneous rock. Physical properties of igneous rocks are largely determined by the rate at which the rocks cool and the manner in which they are extruded. Slowly cooling lava results in larger, more definitive crystals, whereas rapidly cooling crystals are often so small they can't be seen with the naked eye. Geologists use crystal size and arrangement as clues to identify the conditions under which igneous rocks were formed. The mineral composition of igneous rocks depends on minerals in the magma. Igneous rocks are divided into three families depending on the minerals they contain. felics—high in silica content (+65%); lightweight, main minerals include orthoclastic feldspar, mica, and quartz; rock examples include rhyolite and granites. Rhyolite Granite mafics—high in iron and magnesium; lower in silica content (45-55%); usually darker in color; main minerals include plagioclastic feldspar, olivine, and pyroxene; rock examples include basalts and gabbros. Basalt Gabbro intermediates—contain 55-65% silica; medium color; main minerals include hornblende, biotite mica, and pyroxene; rock examples include andesite and diorite. Andesite Diorite Magma is composed of silicates, or combinations of other chemical elements with silicon ancfoxygen. The amount of silica controls the viscosity (the resistance of a substance to flow). Magmas with high viscosity tend to contain higher amounts of silica and commonly form lighter-colored igneous rocks. The highly viscous magma often builds up under intense pressure and is associated with spectacular volcanic eruptions. Low viscosity basalts, which contain less silica, are commonly associated with sea-floor spreading as thin magma oozes through the earth's surface.