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• Joyce Pamela Junsay

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Petrology (from the Greek πέτρα, pétra, "rock" and λόγος, logos, "study") is the branch of geology that studies the origin, composition, distribution and structure ofrocks. 1. INTRODUCTION There are many definitions of rocks, according to (Ehlers and Blatt, 1997). He defines rocks as stuff which the earth is made off. A more acceptable scientific definition of rocks is that; a rock is a natural occurring solid cohesive aggregate of one or more mineral or mineral materials. Rocks are broadly classified into three groups based on their process of formation. The three major rock types are: 1. Igneous rocks 2. Sedimentary rocks 3. Metarmoprphic rocks Sedimentary rocks makes up to 66% of the earth’s crust, with 34 % being the igneous and the metamorphics. Igneous rocks however, forms the majority of 34 % (Ehlers and Blatt, 1997).The reason why sedimentary rocks accounts for most of the rocks on the earth’s surface is because they are mainly found ocean floor basins which accounts to 70% of total area of the earth. The three rock types are further classified based on chemistry, environment of formation and how they are formed. What are Igneous Rocks? Igneous rocks are formed from the solidification of molten rock material. There are two basic types. Intrusive/Plutonic igneous rocks crystallize below Earth's surface and the slow cooling that occurs there allows large crystals to form. Examples of intrusive igneous rocks are diorite,gabbro, granite, pegmatite, and peridotite. Extrusive/Volcanic igneous rocks erupt onto the surface where they cool quickly to form small crystals. Some cool so quickly that they form an amorphous glass. These rocks includeandesite, basalt, obsidian, pumice, rhyolite, scoria, andtuff. What are Metamorphic Rocks? Metamorphic rocks have been modified by heat, pressure and chemical processes, usually while buried deep below Earth's surface. Exposure to these extreme conditions has altered the mineralogy, texture and chemical composition of the rocks. There are two basic types of metamorphic rocks. Foliated metamorphic rocks such as gneiss, phyllite, schist, andslate have a layered or banded appearance that is produced by exposure to heat and directed pressure. Non-foliated metamorphic rocks such as hornfels, marble, quartzite, and novaculite do not have a layered or banded appearance. Pictures and brief descriptions of some common types of metamorphic rocks are shown on this page. What Are Sedimentary Rocks? Sedimentary rocks are formed by the accumulation of sediments. There are three basic types of sedimentary rocks. Clastic sedimentary rocks such as breccia, conglomerate,sandstone, siltstone, and shale are formed from mechanical weathering debris.

Chemical sedimentary rocks, such as rock salt, iron ore,chert, flint, some dolomites, and some limestones, form when dissolved materials precipitate from solution. Organic sedimentary rocks such as coal, some dolomites, and some limestones, form from the accumulation of plant or animal debris. Differentiate: 1.Igneous rocks are formed when magma (or molten rocks) have cooled down and solidified. Sedimentary rocks are formed by the accumulation of other eroded substances, while Metamorphic rocks are formed when rocks change their original shape and form due to intense heat or pressure. 2.Igneous rocks are commonly found inside the Earth’s crust or mantle, while Sedimentary rocks are usually found in water bodies (sea, oceans etc.). Metamorphic rocks are found on the Earth’s surface. 3.Igneous rocks can be an important source of minerals, and Sedimentary rocks, or their bedding structure, is mostly used in civil engineering; for the construction of housing, roads, tunnels, canals etc. Geologists study the geological properties of Metamorphic rocks, as their crystalline nature provides valuable information about the temperatures and pressures within the Earth’s crust. 4.Examples of Igneous rocks include granite and basalt, while examples of Sedimentary rocks include shale, limestone and sandstone. Common examples of Metamorphic rocks are marble, slate and quartzite. Students can also develop nicknames for the rocks, and they can call the individual types of rocks: IGNEOUS - fire or hot; SEDIMENTARY - cool, wet, fossil; METAMORPHIC - flat, squished, changed

Rock Cycle

Terms: Cementation- The process by which clastic sediment is lithified by precipitation of mineral cement, such as calcite cement, among the grains of the sediment. Compaction- Tighter packing of sedimentary grains causing weak lithification and a decrease in porosity, usually from the weight of overlying sediment. Erosion- The processes that loosen sediment and move it from one place to another on Earth's surface. Agents of erosion include water, ice, wind, and gravity. Lithification- The processes by which sediment is converted into sedimentary rock. These processes include cementation and compaction. Magma- Molten rock, generally a silicate melt with suspended crystals and dissolved gases. Melting- To go from a solid state to a liquid state. Metamorphism- Alteration of the minerals and textures of a rock by changes in temperature and pressure, and/or by a gain or loss of chemical components. Sediment- Material (such as gravel, sand, mud, and lime) that is transported and deposited by wind, water, ice, or gravity; material that is precipitated from solution; deposits of organic origin (such as coal and coral reefs). Crystallization- The formation of mineral grains from cooling magma

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