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Rocks • Rocks are composed of two or more minerals • Rocks are very helpful in dating the age of the earth Classification of Rocks Igneous rocks Sedimentary rocks Metamorphic rocks Igneous Rocks The word ‘igneous’ has been derived from a latin word ‘ignis’ which means fire. Igneous rocks are formed due to cooling, solidification and crystallization of hot and molten materials known as magma and lava. These rocks are also known as primary or parent rocks. Characteristics of igneous rocks 1. Hard rocks 2. Non porous rocks 3. Granular rocks but there are variations in form, size & texture of grains 4. Do not have any layers 5. Less affected by chemical weathering 6. Do not contain fossils 7. Mainly associated with volcanic activities that’s why also known as volcanic rocks Classification of igneous rocks Based on mode of occurence Based on amount of silica Acidic- having more silica Example- Granites Intrusive Batholiths Laccoliths Basic- having lower amount of silica Example- gabbro Phacoliths Lapoliths Sills Dykes extrusive Intrusive igneous rocks • When the rising magma cooled and solidifies below the surface of the earth, the resultant rocks are intrusive igneous rocks. 1. Batholiths- long irregular & undulating form of solidified magma • Dome shaped • Almost vertical side walls 2. laccoliths- formed due to injection of magma along the bedding planes of horizontally bedded sedimentary rocks. • Mushroom shaped • Convex summital form. 3. Phacoliths- formed due to injection of magma along the anticlines and synclines. 4. Lapoliths- magma solidified in concave shallow basin whose central part is sagged downward 5. Sills- syl means ledge • Formed due to injection & solidification of magmas between the bedding planes of sedimentry rocks. 6. Dykes- wall like formation of solidified magma. • Perpendicular to the beds of sedimentary rocks. Extrusive igneous rocks • Igneous rocks formed due to cooling and solidification of hot and molten lavas at the surface of the earth are extrusive igneous rocks. • These rocks are also known as volcanic rocks. • Example – Basalt Difference between intrusive and extrusive igneous rocks Intrusive igneous rocks Extrusive igneous rocks Magma cooled and solidified below Lava cooled and solidified at the the surface of the earth surface of the earth Sedimentary rocks These rocks are formed due to aggregation and compaction of sediments. Sedimentary rocks are also called stratified or layered rocks. The sediments and debris derived through the disintegration and decomposition of rocks by the agents of weathering and erosion are gradually deposited in water bodies. Thus, layer after layer of sediments are regularly deposited after consolidating the layers form sedimentary rocks. Characteristics of sedimentary rocks 1. Contains fossils of plants and animals 2. 75% of surface area of the globe is covered with sedimentary rocks 3. Contains several layers 4. They are not found in massive form like igneous rocks 5. Most of the rocks are permeable and porous Classification of sedimentary rocks Mechanically formed Sandstones Clay and shale conglomerates Chemically formed When chemically active water comes in contact with rocks soluble materials are removed from the rocks . Such materials are chemically derived rocks. Organically formed Calcareous Carbonaceous Siliceous Metamorphic rocks These rocks include rocks that have been changed either in form or composition without disintegration They generally form due to changes in form of sedimentary and igneous rocks. Types of metamorphism Based on nature of agents Thermal (due to heat) Dynamic (due to pressure) Hydro (due to hydro static ressure) Hydro thermal (due to water & heat) Based on place Contact Regional Composite classification Contact or thermal Dynamic or regional Hydro Hydro- thermal Types of metamorphism • Contact- It takes place when the mineral composition of the surrounding rocks known as aureoles is changed due to intense heat of the intruding magmas. • Regional- when the rocks are altered in their forms in extensive area the process is called regional metamorphism. • Hydro- When chemically active water passes through the countryside rocks, there occur several chemical changes in the rocks due to varied chemical reactions. • Hydro- Thermo- The minor alteration in the physical & chemical composition of the rocks caused by the weight and pressure of water mass & chemically active hot gases and water vapour.