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Notes of Rocks of Geography Section for UPSC Exams


SSBInsights Team
11 Nov 2017
exams

Rocks

There are 6 main kinds of sedimentary rocks depending on the appearance of the rock, namely

Conglomerate, Sandstone, Shale, Limestone, Gypsum and Porphyry rock

Metamorphic Rocks . Marble is formed from Limestone, Quartzite from Sandstone, and Slate from Shale and Gneiss from Granite. Metamorphosed limestone becomes calcite marble; metamorphosed dolomite becomes dolomitic marble. Schist is a medium grade metamorphic rock. This means that it has been subjected to more heat and pressure than slate, which is a low grade metamorphic rock. It is formed on metamorphism of granite.

Physical (mechanical) weathering can be divided into following types.

Thermal expansion also known as onion-skin weathering, exfoliation, insolation weathering or thermal shock, often occurs in areas, like deserts, where there is a large diurnal temperature range. Though this is caused mainly by temperature changes, thermal expansion is enhanced by the presence of moisture.

 

Freeze thaw weathering is common in mountain areas where the temperature is around freezing point and is attributed to the expansion of freezing water captured in cracks.

 

Pressure release In this case overlying materials are removed which causes underlying rocks to expand and fracture. Often the overlying material is heavy, and the underlying rocks experience high pressure under them, for example, a moving glacierPressure release may also cause exfoliation to occur.

 

Hydraulic action This is when water rushes into cracks in the rock face rapidly. This traps a layer of air at the bottom of the crack, compressing it and weakening the rock. When the wave retreats, the trapped air is suddenly released with explosive force.

 

Salt-crystal growth (haloclasty) Salt crystallisation or otherwise known as Haloclasty causes disintegration of rocks when saline solutions seep into cracks and joints in the rocks and evaporate, leaving salt crystals behind. It is also known as Granule disintegration. It is normally associated with arid climates. An example of salt weathering can be seen in the honeycombed stones in sea wall.

 

The splitting of rocks along the joints into blocks is called block disintegration. The roots of plants especially trees can force joints and cracks apart in rocks. Some animals by borrowing also help to break up rocks.

 

Chemical weathering

 

Dissolution/ Carbonation Rainfall is naturally slightly acidic because atmospheric carbon dioxide dissolves in the rainwater producing weak carbonic acid. One of the most well-known solution weathering processes is carbonation, which occurs on rocks which contain calcium carbonate such as limestone and chalk. weak carbonic acid reacts with calcium carbonate (the limestone) and forms calcium bicarbonate. This process speeds up with a decrease in temperature and therefore is a large feature of glacial weathering.

 

Hydrolysis is a chemical weathering process affecting Silicate minerals. In such reactions, pure water ionizes slightly and reacts with silicate minerals. This reaction results in complete dissolution of the original mineral.

 

Mineral hydration, is a reaction in which water is combined into the crystalline structure of a mineral

 

Oxidation. The most commonly observed is the oxidation of iron and combination with oxygen and water to form hydroxides and oxides. This gives the affected rocks a reddish-brown coloration on the surface which crumbles easily and weakens the rock. This process is better known as rusting.

Glacial Deposition Like running water, glaciers transport and deposit sediments.  Most sediment is suspended within the ice. They have a greater ability to carry sediment in suspension as compared to water. When glaciers melt, they deposit debris that they have picked up. The deposition of sediment is usually unsorted that is of all shapes and sizes. Collectively they are called Glacial Till.

Karst topography is a landscape formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum. Stalactites and Stalagmite are commonly seen in this topography.

Constant deposition of water like dripping, gives rise to stalactites. They are tapering hanging structures formed of calcium salts deposited by dripping water.

Stalagmite a mound or tapering column rising from the floor of a cave, more or less resembling an inverted stalactite, formed of calcium salts deposited by dripping water.

For Better Understanding we have attached the pictures


1 Cross section of earth 732001.jpg2 igneous 316075.jpg3 granite 568381.jpg4 basalt 717354.jpg5 sedimentary rock 925153.jpg6 sedimentary-rocks1 927186.jpg7 Rock Gypsum 412052.jpg8 porphyry 285991.jpg9 metamorphic_rocks 106750.jpg10 Thermal expansion 292768.JPG11 Pressure release 122371.jpg12 Salt Crystalisation 508029.jpg13 Limestone comparison 481176.jpgLabeled_speleothems 839981.jpgBlock disintegration - Buddha Palm 897425.jpgexfoliation 676361.jpgGabbroRock 35166.jpgGlacial Till 993402.jpgstalactites 871545.jpg

Pictures Taken from Internet for illustration purepose only

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