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1 Study Guide 4-2 Igneous Rocks Ms. Grady How is an igneous rock formed? When lava or magma cools and hardens, it forms igneous rocks. The two main categories are extrusive and intrusive. Because Magma is less dense than the surrounding rock, it is forced upward to the surface. When magma reaches the Earth’s surface, and flows from volcanoes, it is called lava. Intrusive Rocks Magma is melted rock material made of common elements and fluids. As magma cools, atoms and compounds (in the liquid) rearrange themselves into new crystals called mineral grains. Rocks form as these mineral grains grow together. Rocks that form from magma below the surface are called intrusive igneous rocks. Intrusive rocks are found at the surface only after the layers of rock and soil have been removed by erosion. Extrusive Rocks Extrusive igneous rocks are formed as lava cools on the surface of Earth. When lava flows on the surface, it is exposed (open) to air and water. Erosion happens when the rocks are pushed up by forces within Earth. Mineral grains can’t grow large because the atoms in the liquid don’t have time to arrange into large crystals. It has a quick cooling rate. Extrusive igneous rocks are fine grained. Intrusive rocks form at depth below the surface and they are surrounded by other rocks so it takes a long time for them to cool. Lava such as the basaltic lava cools quickly under these conditions. See picture below. 2 Slowly cooled magma produces or makes mineral grains that are large enough to be observed by the eye. 3 An igneous rock can be from basaltic, andesitic or granitic magma. There are 3 types of magma: Rhyolite Rock (Felsic) (also known as Granitic Rocks) Light colored Lots of silica if it contains quartz and feldspar. Low density Thick – flows slowly Ex. rhyolite Andesitic Rock Intermediate in color and silica amounts. It contains feldspar and amphiboles. Ex. Andesite Basaltic Rock (Mafic) Dark colored minerals Poor in silica Rich in iron and magnesium High density Thin - watery Ex. Basalt Igneous Rock Classification Igneous rocks are classified according to their composition and texture. Composition refers to the minerals of which rocks are formed (how they are made up). Texture means the shape, size, arrangement (order) and distribution of the minerals inside. 4 Igneous Rock Composition There are 3 basic igneous rock composition which produce 3 colors of igneous rocks. Dark (Mafic) Intermediate Light (Felsic) Formed from basaltic magma Formed from Formed from granitic or andesitic magma rhyolitic magma Igneous Texture There are 4 textures of igneous rock: Fine Coarse Glassy These grained rocks have crystals too small to be seen without the help of a microscope. These extrusive rocks grains cool quickly on the surface of the earth. The fine grained rock is rhyolite. Basaltic rocks are found mainly in the crust under the ocean and volcanic islands like Hawaii. Consist of interlocking mineral crystals which are all roughly the same size and visible to the eye. Ex. Granite Are shiny and look like glass. They cool so quickly, the crystals do not have a chance to grow. Ex. obsidian Vesicular (full of holes or frothy) Are formed when lava is full of gas bubbles cools and produces a rock with open holes. Examples: Pumice and Scoria 5