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Igneous Rocks the Earth Science Journal Issue 9 November 2013

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Igneous Rocks the Earth Science Journal Issue 9 November 2013 IGNEOUS ROCKS THE EARTH SCIENCE JOURNAL ISSUE 9 NOVEMBER 2013 IN THIS ISSUE Overview of Igneous Rocks P.1 Intrusive Rocks P.1 Igneous Rock Characteristics P.2 Extrusive Rocks P.2 Overview of Igneous Rocks The word igneous is derived from the Latin word ignis (meaning fire), and it is a very appropri- ate name for this particular rock type. All igneous rocks were at one time molten (melted) and Intrusive Rocks then solidified into their present form. For the rocks to be molten, they needed to be very hot, Intrusive igneous rocks form inside so thinking of fire when thinking of igneous rocks is appropriate. (below the surface) of the Earth. The cooling rate of intrusive rocks Although all igneous rocks were molten, not all came from the same source minerals or cooled tends to be very slow because they in the same area, or even cooled at the same rate—all of these factors play a role in how igne- are insulated by surrounding rocks. ous rocks appear and the characteristics that they have. This slow rate of cooling leads to larger crystal grains because the Igneous rocks formed from lava or magma. Lava is molten rock that has reached the Earth’s crystals have time to grow. The surface and been erupted onto it. Magma is molten rock beneath the Earth’s surface. If a rock granite sample below cooled deep is formed from magma that cools and hardens beneath the surface of the Earth, it is consid- beneath the Earth’s surface and is ered an intrusive igneous rock. If a rock is formed from lava that cools and hardens on the composed of several different min- Earth’s surface, it is considered an extrusive igneous rock. It is helpful to think of paints when erals with coarse crystal grains. trying to remember the difference between the two: interior paint is put on the walls inside of the house, and exterior paint is applied to the outside of the house. Both have the same pre- fixes in– and ex– meaning inside and outside just like that of intrusive and extrusive igneous rocks. This distinct difference between the two results in variable cooling rates, however the generalization can be made that extrusive rocks cool much faster than intrusive rocks. Extrusive Rocks Extrusive igneous rocks formed from lava at the Earth’s surface. The tex- tures of extrusive igneous rocks vary greatly based on their cooling rate and composition. Some common intrusive igneous rock textures are: glassy (such as in obsidian), vesicular (such as in scoria), and fine-grained (such as in basalt). Obsidian is easy to spot and usually looks like a black glass while scoria appears to have bubbles trapped inside and basalt has crystals that are barely visible. Igneous Rock Characteristics Density Igneous rocks vary in density but in general mafic rocks (rocks containing elements such as iron and magnesium) are more dense and felsic rocks (rocks containing elements such as silicon and aluminum) are less dense. Color Igneous rocks can contain a number of different colored minerals in a single sample but mafic rocks typically are darker while felsic rocks are lighter in color. Texture A rocks texture is very important to its identification, and texture can tell a scientist many things about the environment the rock formed in: Glassy Texture → Cooled Quickly → Formed from Lava → Extrusive Vesicular Texture (Gas Pockets) → Cooled Quickly → Formed from Lava → Extrusive Fine Intergrown Crystals → Cooled Quickly → Formed from Lava → Extrusive Coarse Intergrown Crystals → Cooled Slowly → Formed from Magma → Intrusive THE EARTH SCIENCE JOURNAL Created By: Dave Burgess Rochester STEM High School I do not own any rights to the images in this journal. All images used for educational purposes only. .
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