Type of Rocks | Famous Books for UPSC Exam (Summary & Tests) PDF Download

Rock System

The Earth's crust is composed of a variety of rocks. The scientific study of rocks is called petrology. Rocks are distinguished from one another by several observable and measurable characteristics:

• Texture - the degree to which a rock is rough or smooth, and the size and arrangement of its constituent grains or crystals.
• Structure - the spatial arrangement of parts within the rock (for example layering, foliation, banding).
• Mode of occurrence - how and where a rock is found (for example in beds, as intrusions, as lava flows).
• Colour - often indicative of mineral composition (for example light-coloured felsic rocks and dark-coloured mafic rocks).
• Permeability - the capacity of a rock to allow fluids (water, gas) to pass through it.
• Degree of resistance to denudation - how readily the rock weathers or erodes under surface conditions.

On the basis of origin and appearance, rocks are grouped into three major classes:

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

Igneous Rocks

Igneous rocks form by the cooling and solidification of molten rock. Molten rock beneath the surface is called magma; when magma reaches the surface and flows out, it is called lava. Rocks formed from the solidification of magma or lava are called igneous (also called primary) rocks.

• The internal structure of most igneous rocks is crystalline, composed of interlocking minerals.
• Igneous rocks do not typically occur in strata (layers) and usually do not contain fossils.
• Classification by silica content: Acid (felsic) igneous rocks - high silica content, generally lighter in colour and less dense; common minerals include quartz and feldspar; examples: granite. Basic (mafic) igneous rocks - lower silica content, darker colour and denser; examples: basalt, gabbro, dolerite.
• Texture varies with cooling rate: slow cooling produces coarse crystals, rapid cooling produces fine crystals or glassy texture.
• Igneous rocks may be intrusive (plutonic) or extrusive (volcanic) depending on where they solidified.

Classification by origin

1. Plutonic (Intrusive) rocks
2. Volcanic (Extrusive) rocks

Plutonic (Intrusive) Rocks

• Formed at considerable depth within the Earth's crust from slowly cooled magma.
• Usually have a medium to coarse-grained (phaneritic) texture because slow cooling allows large crystals to grow.
• Often lighter in colour when rich in silica (for example granite); other examples include diorite and gabbro.
• Large intrusive bodies include batholiths, laccoliths, sills and dykes. Exposure at the surface occurs by uplift and denudation (erosion).

Volcanic (Extrusive) Rocks

• Formed by the eruption of magma as lava at or near the Earth's surface and by rapid cooling.
• Have fine-grained (aphanitic) or glassy textures because crystals had little time to grow.
• Often denser and darker in colour (for example basalt).
• Volcanic textures and features include vesicular (gas-bubble holes), porphyritic (large crystals in a fine matrix), glassy, and pyroclastic (fragmental) textures.
• Notable occurrences:
  • Giant's Causeway (Antrim, Northern Ireland) - columnar basalt showing polygonal columns.
  • Deccan Traps (Deccan Plateau, India) - extensive basaltic lava flows.
  • Columbia Plateau (north-west USA) - large flood basalt province.

Sedimentary Rocks

Sedimentary rocks form by the accumulation, compaction and cementation of sediments - a process collectively called lithification. Sediments are derived from the weathering and erosion of pre-existing rocks and are transported and deposited by water, wind, ice or gravity.

• Sedimentary rocks are commonly layered or stratified with beds of varying thickness; examples include sandstone and shale.
• Although sedimentary rocks cover a large area of the Earth's surface (about 75% of the surface), they represent only a small volumetric proportion of the crust (roughly 5%) because they form the uppermost layers.
• Sediments deposited by ice are called till or when consolidated tillite. Wind-deposited silt is called loess.

Depending on their mode of formation, sedimentary rocks are classed as:

1. Mechanically formed (Clastic) - rocks formed from fragments of other rocks (for example sandstone, conglomerate, shale, loess).
2. Organically formed (Biochemical) - rocks formed from the accumulation of biological material (for example coal, chalk, some limestones).
3. Chemically formed (Chemical precipitates) - rocks formed by precipitation from solution (for example rock salt, gypsum, potash deposits).

Mechanically formed (Clastic) sedimentary rocks

• Formed by physical agents such as running water, wind, ocean currents and glaciers which erode, transport and deposit rock fragments.
• Sandstone is composed of sand-sized grains, commonly quartz; it is widely used as a building stone and for grindstones. Large deposits of sandstone are found in regions such as Rajasthan (India).
• Variants:
  • Grit - a coarser sandstone.
  • Conglomerate - contains rounded pebbles cemented together.
  • Breccia - contains angular rock fragments.
• Clay and very fine sediments form shale or mudstone; clay is used in brick making and pottery.

Organically formed sedimentary rocks

• Formed from the accumulation and chemical alteration of plant and animal remains under sediments.
• Calcareous rocks (for example limestone and chalk) are made mainly of calcium carbonate derived from shells, corals and other organisms.
• Carbonaceous rocks, such as coal, form from compacted plant material and represent an important fossil fuel resource. The transformation begins with peat and progresses to lignite and then to higher grades of coal with increasing compaction and burial.

Chemically formed sedimentary rocks

• Produced by precipitation from solution when water evaporates or when the chemistry of a body of water changes.
• Examples:
  • Rock salt - formed by evaporation of saline water in basins or dried-up seas and lakes.
  • Gypsum - formed by evaporation of shallow saline waters.
  • Potash and other evaporite minerals are similarly precipitated.

Metamorphic Rocks

The term metamorphic means "change of form". Metamorphism is the process by which pre-existing rocks (igneous, sedimentary or older metamorphic rocks) undergo mineralogical, textural and structural changes in response to changes in temperature, pressure and chemically active fluids without the rock passing through a liquid state.

• Metamorphism causes recrystallisation and reorganisation of minerals within a rock.
• Under directed pressure and high temperature, minerals may align to form foliation or lineation. Alternating light and dark mineral layers are referred to as banding.
• Examples of metamorphic rocks include gneiss, schist, slate, marble and quartzite.

Major causes of metamorphism:

• Orogenic (regional) movements - mountain-building processes produce folding, deep burial, elevated pressures and temperatures that metamorphose large volumes of rock.
• Contact (thermal) metamorphism - heat from an intruding magma body 'bakes' the surrounding country rock, causing recrystallisation over a limited zone around the intrusion.
• Geodynamic forces - plate tectonic stresses, subduction, and crustal movements change pressure-temperature conditions and cause widespread metamorphism.

Types of metamorphism based on agency

1. Thermal (Contact) Metamorphism
   • Results mainly from the high temperatures associated with igneous intrusions; changes are greatest close to the heat source and decrease with distance.
   • Common transformations: sandstone → quartzite, limestone → marble.
   • Contact metamorphism typically produces non-foliated rocks where pressure is not the dominant agent.
2. Dynamic (Regional / Dynamothermal) Metamorphism
   • Occurs under high pressures (often with elevated temperatures) across broad regions during mountain building and crustal deformation.
   • Directed pressure and heat, sometimes aided by chemically active fluids, cause extensive recrystallisation and new structures; this is known as dynamothermal metamorphism.
   • Common transformations: granite → gneiss, clay/shale → schist, clay → slate.

Some examples of metamorphic transformations

• Clay → Slate
• Limestone → Marble
• Sandstone → Quartzite
• Granite → Gneiss
• Shale → Schist
• Coal → Graphite (with extreme pressure and temperature)

Metamorphic rocks in India

• Gneisses and schists are widely found in the Himalayan region and in parts of Assam, West Bengal, Bihar, Odisha, Madhya Pradesh and Rajasthan.
• Quartzite occurs in Rajasthan, Bihar, Madhya Pradesh, Tamil Nadu and around Delhi.
• Marble is present near Alwar, Ajmer, Jaipur and Jodhpur in Rajasthan and in parts of the Narmada valley in Madhya Pradesh.
• Slate (used for roofing and historically for writing slates) is found around Rewari (Haryana), Kangra (Himachal Pradesh) and parts of Bihar.
• Graphite is found in Odisha and Andhra Pradesh.
  Question for Type of Rocks

  Try yourself:Which of the following is not a metamorphic rock?

  • A.

    Gneiss
  • B.

    Conglomerate
  • C.

    Quartzite
  • D.

    Schist

  Explanation

  A conglomerate is not a metamorphic rock. A conglomerate is a rock consisting of individual clasts within a finer-grained matrix that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts.

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Rock Cycle

The rock cycle is a continuous, dynamic set of processes by which rocks are transformed from one type to another over geological time. The major steps and processes are:

• Weathering and erosion break down rocks at the surface into sediments.
• Transportation and deposition move and lay down sediments in basins by rivers, wind, ice or marine processes.
• Lithification (compaction and cementation) converts sediments into sedimentary rocks.
• Burial, pressure and heat can convert sedimentary or igneous rocks into metamorphic rocks.
• Melting of rocks in the deep crust or mantle produces magma; cooling of magma forms igneous rocks (intrusive or extrusive).
• Uplift and deformation expose deeper rocks to the surface where they may again weather and complete the cycle.
• Subduction carries crustal rocks into the mantle where they can melt and contribute to new magmas.

Common textbook examples of transformations:

• Clay → Slate
• Limestone → Marble
• Sandstone → Quartzite
• Granite → Gneiss
• Shale → Schist
• Coal → Graphite

Economic and practical importance of rocks

• Construction and building materials: granite, sandstone, limestone and marble are used for buildings, monuments and decorative work.
• Industrial minerals: clay for bricks and ceramics; gypsum for cement and plaster; rock salt for chemical industries.
• Energy resources: coal is a metamorphosed/organic sedimentary resource; many hydrocarbon reservoirs occur in sedimentary basins.
• Ornamental stones: marble, onyx and polished granites are used in sculpture and interior decoration.
• Groundwater and aquifers: permeable sedimentary rocks such as sandstone commonly store and transmit groundwater; impermeable rocks act as aquicludes.
• Mineral deposits: many ore deposits (for example of iron, copper, lead, zinc) are hosted in igneous, metamorphic and sedimentary environments.

Summary

Rocks are classified as igneous, sedimentary and metamorphic based on their origin and characteristics. Understanding the processes of formation, distinguishing features, typical examples and common transformations between these classes is essential for geology and for many applied fields such as engineering, mineral exploration and environmental science. The rock cycle connects all rock types through a set of geological processes driven by heat, pressure, weathering and plate tectonics.