igneous - Rocks Monomineralic and polymineralic rocks...

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Rocks Monomineralic and polymineralic rocks Identification of rocks: Rocks can be identified and classified based on two criteria: (a) Mineralogical composition, i.e. the types of minerals in each rock, and the relative abundance of these minerals. Only the most abundant or essential minerals in a rock are used in its classification, and not the rare or accessory minerals. (b) Texture or fabric: which describes the shapes and sizes of, and relationships between the different minerals, grains or crystals in the rock. A third criterion which is sometimes useful in identifying rock types in the field is primary structures, which are large - scale features that occur within the rock, or between that rock and surrounding ones. Primary structures develop at the time of formation of the rock , and reflect the conditions and environment under which it formed. Examples of the use of these three criteria for classifying rocks: 1- Limestone and marble are two different rocks, yet both are made predominantly of the mineral calcite. Limestone is a sedimentary rock, whereas marble is metamorphic. These two rock types can be distinguished by their textures. Limestones may have rounded grains or particles with pores in between, whereas marbles consist of interlocking crystals of calcite that may have a preferred orientation. 2- Gabbro and basalt are two igneous rocks having the same chemical composition, and the same minerals (plagioclase feldspar + pyroxene), but are of different origins. Gabbros form at depths and cool slowly, whereas basalts are volcanic rocks that crystallize rapidly at the surface of the earth. The two rocks can be differentiated by their textures: basalts are fine-grained, whereas gabbros are coarse-grained (Fig. 1). 3- Basalts may form in different environments. Some may form on the ocean floor, whereas others result from eruptions on land. Although both types will have similar mineralogical compositions and possibly textures, they will have different structures: basalts forming on the ocean floor occur as bulbous masses that resemble " pillows " as a result of their interaction with cold sea water, whereas on-land basalts may have " ropy " or " blocky " structures. Differences between Igneous, Sedimenatry and metamorphic rocks: Mineral types Layering Intrusive structures Porosity Fossils Preferred orientation of minerals in planes
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2 A- Magmas 1- What is a magma? A melt (usually of silicates) + crystals + gases that forms and occurs beneath the surface of the earth. When a magma reaches the surface and begins to flow, it loses its gases and becomes a lava . 2- What is the chemical composition of a magma? Magmas do not always have the same chemical composition. This is evidenced by the variety of igneous rocks that occur at the surface of the earth or which formed at depth, and the different types of volcanic eruptions. By carefully studying the chemistry of the different types of igneous rocks, and their associations with each other, petrologists were
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This note was uploaded on 02/27/2012 for the course GEOLOGY 110 taught by Professor Staff during the Fall '08 term at Marshall.

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igneous - Rocks Monomineralic and polymineralic rocks...

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