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Rocks ii
This document discusses different types of sedimentary and metamorphic rocks. It describes mechanically and chemically formed sedimentary rocks such as sandstones, conglomerates, shales, and limestones. It also discusses organically formed sedimentary rocks including coal. Metamorphic rocks are described as being formed from the alteration of pre-existing rocks under heat, pressure, and chemical processes without melting. Contact and regional metamorphism are summarized as localized and broader alterations caused by heat or pressure, respectively.
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Rocks ii
- 1. ROCKS II TAMILNADU IASACADEMY
- 2. ON THE BASISOF NATURE OF SEDIMENTS: MECHANICALLY FORMED SEDIMENTARY ROCKS Previously formed rocks - subjected to physical/mechanical disintegration Broken into fragments – becomes source material for the formation Fragments – transported and deposited ( exogenous processes) Further broken – mutual collision The fragments- deposited and consolidated in different water bodies – clastic sedimentary rocks 1. SANDSTONES When sand grins are deposited in water bodies and are aggregated and consolidated by cementing elements ( silica, calcium, clay etc..) sandstones are formed Color – nature and amount of cementing elements Sandstones – RED- when cemented by iron oxides Sandstones –WHITE – when cemented by calcium carbonate Becomes hard and resistant when cemented by silica Naturally porous CLASSIFICATION : 1.QUARTZ ARENTIES 2. ARKOSE SANDSTONE 3. LITHIC SANDSTONE 4. GRAYWACKES SANDSTONE
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- 4. 2. CONGLOMERATES Cementation and consolidation of pebbles of various sizes together with sands Cemented fragmental rocks containing rounded fragments such as pebbles and boulders If the fragments are angular or sub-angular the rock is called breccia Rounded polished fragments 4-64mm – pebbles Fragments diameter – 256mm – boulders Rocks after being cemented – gravels Rounded fragments are cemented by quartz – conglomerates
- 6. 3. CLAY ROCKAND SHALE: Deposition and sedimentation of fine sediments Grain size: 0.0312mm to 0.004 – silts 0.004mm – 0.00012 – clays Silt and clay- weak and rocks - insoluble Least affected by chemical weathering Easily eroded away Pure clay – white color Color changes on the addition of impurities Consolidation of silt and clay = Shale Impermeable rocks and therefore hold mineral oils above them
- 8. CHEMICALLY FORMED SEDIMENTARY ROCKS Running water contains chemicals in suspension Chemically active water + country rocks soluble materials removed form the rock chemically formed settlements Chemical materials settle down --- rocks E.g.. Gypsum and salt rocks
- 9. ORGANICALLY FORMED SEDIMENTARYROCS Disintegration or decomposition of plants and animals being deposited and consolidated to form organic sedimentary rocks. Divided into 3 a) calcareous rocks b) carbonaceous rocks c) siliceous rocks a) Calcareous Rocks: Consolidation of sediments derived from the skeletons and remains of those animals and plants which contains larger portion of lime The calcareous rocks collectively called as carbonate rocks Limestone and dolomite are important calcareous rocks Most susceptible to chemical weathering Carbonate rocks after erosion give birth to karst topography Chalk is another example of calcareous rock but its much softer and porous than limestone.
- 13. b) Carbonaceousrocks: Dominated by carbonic materials which represent vegetation remains Transformation of vegetation because of their burial during earth movements and consequent weight and pressure of overlying deposits Initial form being PEAT and the other subsequent forms being LIGNITE, BITUMINOUS & ANTHRACITE coals Coal layers are known as coal seams ECONOMICALLY IMPORTANT
- 15. c) SiliceousRock Formed due to the dominance of silica content Aggregation and compaction of wastes derived from sponge and radiolarian organisms and diatom plants E.g. geyserite – deposits of silica around glaciers
- 16. ON THE BASISOF TRANSPORTING AGENTS 1. ARGILLACEOUS ROCKS Also called as aqueous rocks as they are formed in water areas They are called argillaceous because of the dominance of clay Characterized by general softness 3 subtypes a) marine argillaceous rocks b) riverine argillaceous rocks c) lacustrine argillaceous rocks Marine argillaceous rocks Deposition and consolidation of sediments in the oceans and the seas mainly in the littoral areas Sedimentation is well ordered and sequential Size of the particle decreases progressively from the coastal lands towards the oceans Boulders > cobbles >pebbles > granules > sands > silts > clay > lime E.g. limestone, sandstones, dolomite & chalk
- 17. Lacustrine argillaceoussedimentary rocks Deposition and consolidation of sediments in the lake The lacustrine rocks is visible under 3 conditions i) when the lake becomes dry ii) if the floor of the lake is raised due to earth movements iii) if the whole lake is filled up with sediments Riverine argillaceous sedimentary rocks Deposition of sediments in the river Deposition – beds of the rivers flood plains Deposition includes alluvia which are dominated by clay Alluvial deposits are renewed almost every year
- 18. 2. AEOLIAN SEDIMENTARYROCKS Deposition of sand brought down by the winds Continuous deposition of sands results in the formation of different layers but these layers are not well consolidated Sometimes there is complete absence of layers LOESS is the most important member of this group They are heaps of unconsolidated matter They may stand like a vertical cliff or wall Extensive loessic deposits are found in north China – yellow river of china carries largest amounts of deposits in the world – yellow color of the river is due to the deposition of yellow colored Chinese loess
- 19. 3. GLACIAL SEDIMENTARYROCKS The materials deposited by glaciers are called glacial drifts which are deposited in four conditions i) lateral moraines – When glacial deposits are deposited on the either side of the glacier ii) medial moraines – When glacial sedimentary deposits are deposited along the joining glaciers iii) ground moraines – when the glacial materials are deposited at the bed of the glaciers iv) terminal moraines - when the glacier is ablated and materials are deposited
- 20. METAMORPHIC ROCKS METAMORPHICROCKS include rocks that have been changed either in form or composition without disintegration CHRACTERISTICS: METAMORPHOSE – change in form A complete alteration of pre-existing rocks due to change in mineral composition and texture through temperature and pressure Change in the form of sedimentary rocks , igneous rocks and even previously formed metamorphic rocks No disintegration or decomposition Metamorphic rock – again metamorphosed – re-metamorphism When the rocks are metamorphosed at great intensity it is said to be intense metamorphism i) physical metamorphism – changes in the textural composition of rocks ii) chemical metamorphism – change in the chemical composition of the rocks Sometimes both become operative together
- 21. The textureof metamorphic rocks is the result of recrystallization or conversion of one mineral to another in the solid state Foliation, defined as streaking or parallel arrangement of the constituent crystals which generally cut the rocs at an angle to the bedding planes of the original sediments of the parent rocks The coarse grained metamorphic rocks are imperfectly foliated (e.g. gneisses from granites) while fine grained metamorphic rocks are perfectly foliated The property of metamorphic rocks to part or split along the bedding planes is known as fissility The structure of the presence of numerous closely spaced parallel planes of splitting is known as cleavage Schistocity refers to the growth of large crystals and segregation of some minerals into lighter and darker bands
- 22. Agents ofmetamorphism: Heat : mineral composition is entirely changed due to intense heat but the rocks are seldom melted Heat for metamorphism – vulcanicity Compression: convergent horizontal movement caused by endogenic forces causes folding in rock beds Folding changes the form and composition of parent rock Operative during orogeny Solution: chemically active hot gases and water while passing through the rocks change their chemical composition Magmatic water and water confined in the beds of sedimentary rocks also helps in introducing chemical changes in the rocks
- 23. Types ofmetamorphism: 1. on the basis of the nature of agents: i) thermal metamorphism (due to heat) ii) dynamic metamorphism ( due to pressure) iii) hydro-metamorphism ( due to hydrostatic pressure) iv) hydro-thermal metamorphism (due to water and heat) 2. on the basis of place or area i) contact metamorphism (localized in area) ii) regional metamorphism ( involving larger area) 3. composite classification i) contact or thermal metamorphism ii) dynamic & regional metamorphism iii) hydro-metamorphism iv) hydro- thermal metamorphism
- 24. 1. CONTACTMETAMORPHISM: Occurs when the Mineral composition of the surrounding rock known as aureoles is changed due to intense heat of the intruding magmas Metamorphism occurs when the rocks come in contact with the intruding magmas Also called thermal metamorphism because the rocks are changed due to intense heat rising from magmas Limestone are changed to marbles due to contact metamorphism The margins of the altered rock around igneous intrusions are called aureoles the width of which depends upon mainly two factors i) the temperature of intruding magma ii) the depth of magma intrusions in the crust
- 28. 2. REGIONAL METAMORPHISM When the rocks are altered in their forms in extensive area the process is called regional metamorphism Also known as dynamic metamorphism because pressure plays dominant role in the alteration of the form of the rocks through temperature is also an important factor Sedimentary rocks are folded due to compressional forces during the period of mountain building Crystallization and recrystallization occurs Regional metamorphism is a characteristic feature of the mountain areas Two types i) dynamic regional metamorphism ii) static regional metamorphism Dynamic – metamorphosed due to compressive forces and resultant high pressure caused by convergent horizontal movements Static- metamorphosed at greater depth due to intense pressure and weight overlying rocks
- 30. HYDRO METAMORPHISM The alteration in the composition of the rocks due to hydrological factor takes place in a number of ways i) when the chemically active water passes through the country rocks – changes occurs ii) the storage of immense volume of water in big reservoir exerts high pressure on the underlying rocks and thus rocks are altered in their forms HYDRO-THERMO-METAMORPHISM The minor alteration in the physical and chemical composition of the rocks caused by the weight and pressure of water mass and chemically active hot gases and water vapor
- 32. CLASSIFICATON OF METAMORPHICROCKS 1. META-SEDIMENTARY OR PARA METAMORPHIC ROCKS: Rocks formed due to the alteration of sedimentary rocks E.g. marbles from limestone Quartzite from sandstone Slates from shale and clay 2. META IGNEOUS OR ORTHO METAMORPHIC ROCKS: Rocks formed due to the alteration of igneous rocks E.g. gneisses from granite Serpentine from gabbro Basic granulite from amphibolite Eclogite from basaltic rock
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