Igneous rocks, types, uses and their composition

In this presentation we have described igneous rocks, their composition, their types and their uses in various fields of engineering.

1. ENGINEERING GEOLOGY PRESENTATION NAME: MOHAMMAD ASRAR SHAH CLASS: CIVIL B SEMEMSTER: II RRN:170011601049 IGNEOUS ROCKS

2. DEFINITION : Igneous rock or magmatic rock (derived from Latin word ‘IGNIS’ meaning fire) is one of the three main rock type, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava. COMPOSITION: The great majority of the igneous rocks are composed of silicate minerals (meaning that the basic building blocks for the magmas that formed them are made of silicon [Si] and oxygen [O]), but minor occurrences of carbonate rich igneous rocks are found as well.

3. TEXTURES : The main factor that determines the texture of an igneous rock is the cooling rate ( dT / dt ). Other factors involved are : The diffusion rate, The rate of nucleation of new crystals, The rate of growth of crystals. Depending upon the rate at which temperature is lowered below the crystallization temperature. Three types of textures exist: For small degrees of undercooling the crystals will have a coarse grained texture called phaneritic texture. For larger degrees of undercooling, the crystals will have a fine grained texture. the texture is said to be  aphanitic. At high degrees of undercooling, the crystals will have a glassy texture called   holohyaline texture .

4. STRUCTURE : Igneous structures are mostly classified into three major groups, as follows: 1. Mega-structures : These are usually formed in the flow stage of the magma. 2. Minor structures : These structures are formed in the fluid stage of the magma. 3. Micro-structures : These are formed due to reaction between already solidified crystals and the rest of the magma.

5. CLASSIFICATION : Igneous rocks can be classified according to chemical or mineralogical parameters. CHEMICAL: Felsic igneous rocks containing a high silica content (examples granite ) Intermediate igneous rocks containing between 52–63% SiO2 (example andesite) Mafic igneous rocks have low silica 45–52% (example gabbro and basalt) Ultramafic rock igneous rocks with less than 45% silica (examples picrate and peridotite) MINERALOGIC: 1. Felsic  rock , highest content of silicon, with predominance of quartz and alkali feldspar (E.g., granite) 2.Mafic  rock , lesser content of silicon relative to felsic rocks, with predominance of mafic minerals ( E.g , basalt and gabbro) 3.Ultramafic  rock , lowest content of silicon, with more than 90% of mafic minerals (e.g.,  dunite ).

6. TYPES : Igneous rocks are of two types, intrusive (plutonic rocks) and extrusive (volcanic rocks). Intrusive Igneous Rocks Intrusive igneous rocks are formed when the magma cools off slowly under the earth’s crust and hardens into rocks. Gabbro and granite are examples of intrusive igneous rocks. Intrusive rocks are very hard in nature and are often coarse-grained. Extrusive Igneous Rocks Extrusive igneous rocks are formed when molten magma spill over to the surface as a result of volcanic eruption. The magma on the surface (lava) cools faster on the surface to form igneous rocks that are fine grained. Examples of such kind of rocks include pumice, basalt, or obsidian.

7. Examples of Igneous Rocks 1. GRANITE 2.BASALT 3.DIORITE

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