Rocks and its types


Meaning of rocks

  • Rocks are an aggregate of one or more minerals held together by chemical bonds.
  • Feldspar and quartz are the most common minerals found in rocks.
  • The scientific study of rocks is called petrology.
  • Based on the mode of formation three major groups of rocks are defined igneous, sedimentary, and metamorphic.
  • Igneous Rocks — solidified from magma and lava.
  • Sedimentary Rocks — the result of deposition of fragments of rocks.
  • Metamorphic Rocks — formed out of existing rocks undergoing recrystallization.

Types of rocks

  1. Igneous rocks
  2. Sedimentary rocks
  3. Metamorphic rocks

Igneous Rocks or Primary Rocks

  1. By analyzing the rate of change in temperature and pressure from the surface towards the interior.
  2. Meteors, as they belong to the same type of materials earth is made of.
  3. Gravitation is greater near the poles and less at the equator.
  4. Gravity anomaly, which is the change in gravity value according to the mass of material, gives us information about the materials in the earth’s interior.
  5. Magnetic sources.

Acidic Rocks

  1. Acidic rocks are characterised by a high content of silica (quartz and feldspar) up to 80 per cent.
  2. The rest is divided among aluminium, alkalis, magnesium, iron oxide, lime etc.
  3. These rocks have a lesser content of heavier minerals like iron and magnesium. Hence, they are less dense and are lighter in colour than basic rocks.
  4. These rocks constitute the Sial portion of the crust.
  5. Due to the excess of silicon, acidic magma cools fast and does not flow and spread far away.
  6. High mountains are formed of this type of rock.
  7. Add rocks that are hard, compact, massive and resistant to weathering.
  8. Granite, quartz and feldspar are typical examples.

Basic Rocks

These rocks are poor in silica (about 40 per cent); magnesia content is up to 40 per cent, and the remaining is spread over iron oxide, lime, aluminium, alkalis, potassium etc.
Due to low silica content, the parent material of such rocks cools slowly and thus, flows and spreads far away. This flow and cooling give rise to plateaus.
The presence of heavy elements imparts to these rocks a dark colour. Not being very hard, these rocks are weathered relatively easily.
Basalt, gabbro and dolerite are typical examples.

Intrusive igneous rocks

  1. If magma cools slowly at great depths, mineral grains formed in the rocks may be very large.
  2. Such rocks are called intrusive rocks or plutonic rocks (e.g. Granite).
  3. These rocks appear on the surface only after being uplifted and denuded.

Extrusive igneous rocks (Lava or Volcanic rocks)

  1. Sudden cooling of magma just below the surface or lava above the surface results in small and smooth grains in rocks as rapid cooling prevents crystallization, as a result, such rocks are fine-grained.
  2. Such rocks are called extrusive rocks or volcanic rocks (e.g. Basalt).
  3. The Deccan traps in the Indian peninsular region are of basaltic origin.
  4. Basic rocks contain a greater proportion of basic oxides, e.g. of iron, aluminium, or magnesium, and are thus denser and darker in colour.

Sedimentary Rocks

Sedimentary rocks are formed by lithification, consolidation and compaction of sediments.
Hence, they are layered or stratified of varying thicknesses. Example: sandstone, shale etc.
Sediments are a result of denudation (weathering and erosion) of all types of rocks.
These types of rocks cover 75 per cent of the earth’s crust but volumetrically occupy only 5 per cent (because they are available only in the upper part of the crust).
Ice deposited sedimentary rocks are called till or tillite. Wind-deposited sediments are called loess.
Depending upon the mode of formation, sedimentary rocks are classified into:
  1. mechanically formed — sandstone, conglomerate, limestone, shale, loess.
  2. organically formed — geyserite, chalk, limestone, coal.
  3. chemically formed — limestone, halite, potash.

I. Mechanically Formed Sedimentary Rocks

They are formed by mechanical agents like running water, wind, ocean currents, ice, etc.
Arenaceous sedimentary rocks have more sand and bigger sized particles and are hard and porous. They form the best reservoirs for liquids like groundwater and petroleum. E.g. sandstone.
Argillaceous rocks have more clay and are fine-grained, softer, mostly impermeable (mostly non-porous or have very tiny pores). For E.g. claystone and shales are predominantly argillaceous.

II. Organically Formed Sedimentary Rocks

The remains of plants and animals are buried under sediments, and due to heat and pressure from overlying layers, their composition changes. Coal and limestone are well-known examples.

III. Chemically Formed Sedimentary Rocks

Water containing minerals evaporates at the mouth of springs or salt lakes and give rise to Stalactites and stalagmites (deposits of lime left over by the lime-mixed water as it evaporates in the underground caves).
Depending on the predominance of calcium content or carbon content, sedimentary rocks may be calcareous (limestone, chalk, dolomite) or carbonaceous (coal).

Chief Characteristics of Sedimentary Rocks

  1. They are stratified and consist of many layers or strata.
  2. They hold the most informative geological records due to the marks left behind by various geophysical (weather patterns, wind and water flow) and biological activities (fossils).
  3. They are fossiliferous and have fossils of plants and animals.
  4. These rocks are generally porous and allow water to percolate through them.

Metamorphic Rocks

  1. The word metamorphic means ‘change of form’.
  2. The igneous and sedimentary rocks undergo physical and chemical changes due to the influence of temperature and pressure resulting in the formation of metamorphic rocks.
  3. When rocks are forced down to lower levels by tectonic processes or when molten magma rising through the crust comes in contact with the crustal rocks, metamorphosis occurs.
  4. In the process of metamorphism in some rocks grains or minerals get arranged in layers or lines. Such an arrangement is called foliation or lineation.
  5. Sometimes minerals or materials of different groups are arranged into alternating thin to thick layers. Such a structure is called banding.
  6. slate, schist, marble, quartzite etc. are some examples of metamorphic rocks.
Metamorphic Rocks

Causes of Metamorphism

  1. Orogenic (Mountain Building) Movements: Such movements often take place with an interplay of folding, warping and high temperatures. These processes give existing rocks a new appearance.
  2. Lava Inflow: The molten magmatic material inside the earth’s crust brings the surrounding rocks under the influence of intense temperature pressure and causes changes in them.
  3. Geodynamic Forces: The omnipresent geodynamic forces such as plate tectonics also play an important role in metamorphism.

On the basis of the agency of metamorphism, metamorphic rocks can be of two types

Thermal Metamorphism

  1. The change of form or re-crystallisation of minerals of sedimentary and igneous rocks under the influence of high temperatures is known as thermal metamorphism.
  2. A magmatic intrusion causing thermal metamorphism is responsible for the peak of Mount Everest consisting of metamorphosed limestone.
  3. As a result of thermal metamorphism, sandstone changes into quartzite and limestone into marble.

Dynamic Metamorphism

  1. This refers to the formation of metamorphic rocks under high pressure.
  2. Sometimes high pressure is accompanied by high temperatures and the action of chemically charged water.
  3. The combination of directed pressure and heat is very powerful in producing metamorphism because it leads to more or less complete recrystallisation of rocks and the production of new structures. This is known as dynamo thermal metamorphism.
  4. Under high pressure, granite is converted into gneiss; clay and shale are transformed into schist.

Some examples of Metamorphosis

  1. Igneous or Sedimentary rock transforms into metamorphic rocks.
  2. Sandstone changes to quarts.
  3. Shale changes into slate
  4. Limestone changes to marble
  5. Coal changes to graphite
  6. Graphite changes to the diamond

Rock cycle

  1. The Rock cycle is a continuous process through which old rocks are transformed into new ones.
  2. Igneous rocks are primary rocks, and other rocks form from these rocks.
  3. Igneous rocks can be changed into sedimentary or metamorphic rocks.
  4. The fragments derived from igneous and metamorphic rocks form sedimentary rocks.
  5. Sedimentary and igneous rocks themselves can turn into metamorphic rocks.
  6. The crustal rocks (igneous, metamorphic and sedimentary) may be carried down into the mantle (interior of the earth) through the subduction process and the same meltdown and turn into molten magma, the source for igneous rocks
Rock cycle