notes-3 (1) - C1 Rocks and Magmas A rock is defined as a...

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C1 - Rocks and Magmas A rock is defined as a consolidated mixture of minerals. By consolidated we mean hard and solid. A mixture of minerals implies the presence of more than one mineral grain, but not necessarily more than one type of mineral. A rock can be composed of only one type of mineral (e.g., limestone is commonly made up of just calcite), but most rocks are composed of several different types of minerals. It is very important to understand the difference between rocks and minerals. A rock can also include non-minerals, such as the organic matter within a coal bed, or within some shales. Rocks are grouped into three main categories: IGNEOUS formed from the cooling of a magma (i.e., from molten rock) SEDIMENTARY formed when weathered fragments of other rocks are compressed and cemented together METAMORPHIC formed by alteration (due to heat, pressure and/or chemical action) of a pre-existing igneous or sedimentary rock The materials that make up the rocks of the crust are slowly but constantly being changed from one form to another. The inter-relationships between rock types can be summarized on what is known as the rock cycle diagram [see p. 28, and the figure to the left]. Magma can either cool slowly (over centuries to millions of years) within the crust forming intrusive igneous rock, or erupt onto the surface and cool quickly (within seconds to years) forming extrusive igneous rock . Through the various processes of mountain building, all types of rocks are uplifted and exposed at surface. They are weathered, both physically and chemically, and the weathering products are eroded, transported and then deposited as sediments. The sediments are buried and compressed and become hardened and cemented into sedimentary rock . Again through various means, largely resulting from plate tectonic forces, different kinds of rocks are buried deep within the crust where they are heated
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Vancouver Island University Geology 111 Discovering Planet Earth Steven Earle 2010 2 up, squeezed and chemically changed into metamorphic rock . If the heat is sufficient, part or all of the rock may melt into magma. Magmas can have quite widely varying compositions, but they are all made up largely of only eight elements, in order of importance: oxygen, silicon, aluminum, iron, calcium, sodium, magnesium and potassium (see the figure to the right). Magmas derived from recycled crustal material are dominated by oxygen, silicon and aluminum, sodium and potassium. Magmas derived from the mantle material beneath the crust have higher levels of iron, magnesium and calcium, but they are still likely to be dominated by oxygen and silicon. All magmas also have varying proportions of dissolved water as well as gases such as carbon dioxide and hydrogen sulphide. At temperatures of well over 1000º C
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This note was uploaded on 01/13/2012 for the course GLG 111 taught by Professor Staff during the Fall '11 term at Virginia Intermont.

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notes-3 (1) - C1 Rocks and Magmas A rock is defined as a...

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