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
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 glacier. Pressure release may also cause exfoliation
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.
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.
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
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
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.