Igneous And Sedimentary Rocks example essay topic

Knowledge of structure of earth is essential - Why? 1.) Earth- Structure- 1. Surface features 2. Internal structure. Changes in the Landforms and surface features of earth take place due to internal forces which operate in the interior of the earth.

2. Structure of earth- Three concentric layers A). Crust- Lithosphere, State- solid, A layer of rocks- Thickness 60 km Subdivided in two layers- a) Sial layer / continental layer b) Sima layer / Oceanic crust Sial layer Sima layer Topmost layer lies below sial layer Lighter, density 2.7 Denser, density 3.0 Thick over continents, forms the ocean floor Thin or absent on ocean floor - - Composition-Silicate, Aluminium Composition-Silicate, Magnesium B) Mantle - Mesosphere, State-solid, a layer of mixed silicates & metals, Thickness 2850 kms, density between 3.0- 5.5, high temp. C) Core- Barysphere, radius of 3500 kms, Density between 12 to 15 Temp 50000 C, Consists of metals Nickel, & Iron called - Nice 3. Materials of the Earth's crust- Rocks, sand, silt, clay Rocks are made up of minerals in different combinations.

Silicate minerals- Quartz, feldspar, mica, ; other minerals- calcite, dolomite, gypsum, rock salt. Chemical elements- The most abundant element -- -- Oxygen Second most abundant element -- -- Silicon Other elements Aluminum, Iron, Calcium, Sodium (Application- As Oxygen and Silicon are the most abundant elements; silicates are the most common minerals in the earth's crust.) 4. Rocks - Major types Based on the mode of their formation- o Igneous rocks o Sedimentary rocks o Metamorphic rocks 5. Igneous Rocks- How are they formed? During the volcanic eruptions the hot molten magma, originating from the interior of the earth, erupts to the surface of the earth. This magma may get solidified either below the surface of the earth or on the earth's surface.

Thus igneous rocks are formed by the cooling and solidifying processes of magma. Sub types of Igneous rocks- based on the mode of formation Intrusive rocks Extrusive rocks Solidification of magma on the earth's surface solidification below earth's surface Sudden cooling of lava slow cooling of lava Consists crystals of small size Consists crystals of large size Volcanic rocks- as found in volcanic regions Plutonic rocks- e.g. Fine grained volcanic rock-Basalt e.g. Plutonic rock Granite, Dolomite 6. Classification of Igneous rocks based on Chemical composition- Acidic igneous rock Basic igneous rocks Contain higher percentage of Silica, 65% Contain lower percentage of Silica Light coloured darker in colour Less dense More Dense e.g. Granite - made up of large crystals e.g. Basalt Of quartz, feldspar, and mica 7. Sedimentary Rocks- What are sedimentary rocks? Sedimentary rocks are those which are formed by the process of consolidation of sediments deposited commonly in water bodies like sea or lakes.

How are they formed? In the process of denudation, primary rocks or parent rocks break down due to exposure on the land to rivers, glaciers, winds and waves, into loose particles called sediments. The sediments consist of gravel, sand, silt and clay in various proportions. In course of time, these sediments get consolidated or compacted into hard rocks because of the pressure of overlying deposits and the presence of cementing substances like Lime.

Sedimentary rocks are called Secondary rocks or Derived rocks because they are derived by the denudation of other rocks. Sediments, while being deposited in water bodies, get sorted out according to their size and get accumulated in different layers or strata arranged one above the other. Hence sedimentary rocks are also called Stratified Rocks. The organic remains of plants and animals or the skeletal impressions get preserved between the strata of sedimentary rocks and are called Fossils. 8.

Characteristics of Sedimentary rocks- o They are secondary rocks that are formed by the denudation of primary rocks. o They are Stratified rocks. o They contain fossils that help in fixing their age. 9. Sub types of sedimentary rocks- based on the nature of sediments. A) Mechanical sediments or detrital sediments-are the loose particles or broken fragments of various sizes, such as gravel, sand, silt and clay denuded from the parent rock.

Conglomerate- is the mechanical sedimentary rock formed by the consolidation of rounded pebbles at the river bed and is cemented by materials like lime. Breccia- is the mechanical sedimentary rock formed by the consolidation of angular fragments of gravel that accumulate at the bottom of the cliff. Sandstone -Consolidation of deposits loose sand. Shale or mudstone- Consolidation of deposits of Clay, Shale is a Soft rock which breaks into layers. Siltstone -- -Deposits of Silt. Classification based on the basis of the agents which deposit them-Alluvial Sediments - deposits are laid down by Rivers.

Moraines deposits are laid down by Glaciers Sand dunes -- - Coarse deposits are laid down by the Winds. Loess deposits -- - Fine deposits Gravel, sand and Silt- -- deposits laid down by waves. B). Chemical Sediments- Formed by evaporation of water containing salts in solution. Shallow lakes evaporate in summer leaving sediments of salt on the surface which gets consolidated to form rocks. e.g. rock salt, gypsum, potash, nitrates. Calcite, and Dolomite are hard rocks that are formed by precipitation of certain substances owing to chemical changes C).

Organic sediments- Derived by the accumulation of remains of organisms such as shells of marine organisms. e.g. limestone, chalk, coal. The pressure of overlying sediments converts them hard rocks. Also called carbonaceous rocks. Igneous Rocks Sedimentary Rocks Formed by the solidification of hot molten magma Formed by the sediments derived from primary rocks. Un stratified in nature Stratified in nature Two types - intrusive & extrusive Three types - mechanical, chemical, organic e.g. granite and basalt e.g. conglomerate, sandstone, gypsum, limestone and chalk. 10.

Metamorphic Rocks What are metamorphic rocks? Sometimes, because of tremendous pressure and high temperatures, both igneous and sedimentary rocks may be altered so greatly in appearance and composition that they entirely lose their original character. These rocks are called metamorphic rocks. 11. How are they formed? Metamorphism occurs when rocks are subjected to high temperature, pressure or both.

During this process original minerals are converted into new minerals which combine to form rocks that are harder and more resistant than the original rocks. 12. Characteristics of Metamorphic rocks- Harder than the original rocks They have their minerals arranged in a series of bands. -called Foliation 13. Original Metamorphic Limestone Marble (construction) Shale Slate (roofing) Sandstone Quartzite Coal Graphite Granite Gneiss Basalt Schist 14.

Thermal Metamorphism- The process of transformation of the original rock takes place because of the influence of high temperature as a result of intrusion of hot magma or friction of moving rock layers is known as Thermal Metamorphism. e.g. Graphite from Coal, and Slate from Clay 15. Dynamic Metamorphism. - The process of transformation of original rock due to influence of pressure at a greater depth within the earth's crust is known as Dynamic metamorphism. 16. Regional Metamorphism-Metamorphism of bed rocks over a large area. 17.

Local Metamorphism -- Metamorphism over a small area 18. Why are rocks important to us? a) Soil formation occurs by denudation of rocks and transformation occurs by chemical and biological processes. b) Storehouse of minerals- Igneous and metamorphic rocks are rich in metallic minerals. Rocks with high concentration of minerals -- -are called Ores. c) Source of Fuel minerals- Rocks are valuable deposits of coal, oil, and natural gas. d) Provide building stones for road construction, dam construction. e.g. Granite, Marble, sandstone, Cement- Limestone and gypsum. e) Provide Raw material for mineral based industries like Iron, steel, Aluminium etc. f) They are a Source of precious metals like Gold, silver, platinum etc.