Rocks
- Earth’s crust is made up of various types of rocks.
- The scientific study of rocks is called petrology.
- A rock is a naturally occurring aggregate of minerals and certain non-mineral materials.
- Rocks are the natural building blocks of the Earth’s lithosphere, asthenosphere, mesosphere, and even part of the core.
- Most rocks now exposed at the surface of the Earth formed in or on continental or oceanic crust.
- Many rocks formed beneath the surface and now exposed at the surface were delivered from great depths in the crust or the underlying mantle.
- Rocks come to be exposed at the surface through formation at the surface or formation below the surface, followed by tectonic uplift and removal of overlying material by erosion.
- According to origin and appearance rocks can be divided into 3 groups:
- Igneous rocks
- Sedimentary rocks
- Metamorphic rocks
Igneous Rocks
- The first rocks on Earth formed through solidification of magma.
- Rocks formed through solidification of magma and lava are known as igneous or primary rocks.
- Igneous rocks have their origin under conditions of high temperatures and are unfossiliferous.
- Examples of igneous rocks include granite, gabbro, and basalt.
- Classification:
- Based on cooling time: plutonic rocks, volcanic rocks, and intermediate rocks.
- Based on the presence of silicon: acidic rocks and basic rocks.
- Based on origin: intrusive (plutonic) and extrusive (volcanic) igneous rocks.
- Plutonic Rocks:
- If magma cools slowly at great depths, mineral grains formed may be very large.
- Rocks formed this way are called intrusive or plutonic rocks.
- These rocks appear on the surface only after being uplifted and denuded.
- Intrusive rocks usually have a medium to coarse-grained texture due to slow cooling.
- Examples: granite, diorite, and gabbro.
- These rocks are exposed at the surface by the process of denudation and erosion.
- Volcanic Rocks:
- Sudden cooling of magma just below the surface or lava above the surface results in small and smooth grains in rocks.
- Rocks formed this way are called extrusive or volcanic rocks (e.g. Basalt).
- The Deccan traps in the Indian peninsular region are of basaltic origin.
- Extrusive rocks are fine-grained due to rapid cooling preventing crystallization.
- Basic rocks contain a greater proportion of basic oxides and are denser and darker in color.
- Examples of places where basic rocks are found include Antrim in Northern Ireland, Deccan plateau in India, and Snake plateau in Columbia.
Sedimentary Rocks
- Sedimentary rocks are formed by lithification, which is the consolidation and compaction of sediments
- Hence they are layered or stratified of varying thickness and examples include sandstone and shale.
- Sediments are a result of denudation, which is weathering and erosion of all types of rocks.
- Sedimentary rocks cover 75% of the Earth’s crust but volumetrically occupy only 5% 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:
- mechanically formed —
- 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. Eg: sandstone.
- Argillaceous rocks have more clay and are fine-grained, softer, and mostly impermeable. They are mostly non-porous or have very tiny pores. Eg: claystone and shales.
- organically formed —
- The remains of plants and animals are buried under sediments, and their composition changes due to heat and pressure from overlying layers. Eg: Coal and limestone.
- Sedimentary rocks may be calcareous (limestone, chalk, and dolomite) or carbonaceous (coal), depending on the predominance of calcium or carbon content.
- chemically formed —
- Water containing minerals evaporate 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).
- Example –
- Rock salt – derived from old sea bed or lakes
- Gypsum – evaporation of Salt Lake.
- Potash and nitrate are also obtained in a similar way.
- mechanically formed —
Metamorphic Rocks
- The term metamorphic means ‘change of form’.
- Metamorphism is the process of recrystallisation and reorganization of minerals in rocks due to pressure, volume, and temperature changes.
- It occurs when rocks are forced down to lower levels by tectonic processes or when molten magma comes in contact with crustal rocks.
- Foliation or lineation: It occurs when minerals or grains are arranged in layers or lines during metamorphism.
- Banded structure: It occurs when materials of different groups are arranged in alternating thin to thick layers during metamorphism.
- Gneissoid, slate, schist, marble, and quartzite are examples of metamorphic rocks.
- Causes of metamorphism:
- Orogenic movements –
- Such movements often take place with an interplay of folding, warping and high temperatures.
- These processes give existing rocks a new appearance.
- 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.
- Geodynamic forces –
- The omnipresent geodynamic forces such as plate tectonics also play an important role in metamorphism.
- Orogenic movements –
- On the basis of the agency of metamorphism, metamorphic rocks can be of two types:
- Thermal Metamorphism –
- It is the change of form or re-crystallisation of minerals of sedimentary and igneous rocks under high temperatures.
- A magmatic intrusion causing thermal metamorphism is responsible for the peak of Mount Everest consisting of metamorphosed limestone.
- It results in sandstone changing into quartzite and limestone into marble.
- Dynamic Metamorphism –
- It refers to formation of rocks under high pressure.
- High pressure is sometimes accompanied by high temperatures and chemically charged water.
- The combination of directed pressure and heat can lead to the complete recrystallization of rocks and the production of new structures, which is called dynamo thermal metamorphism.
- Under high pressure, granite can turn into gneiss, and clay and shale can transform into schist.
- Thermal Metamorphism –
- Examples of metamorphosis:
| Igneous or Sedimentary rock | Influence | Metamorphosed rock |
| Granite | Pressure | Gneiss |
| Clay, Shale | Pressure | Schist |
| Sandstone | Heat | Quartzite |
| Clay, Shale | Heat | Slate => Phyllite |
| Coal | Heat | Anthracite => Graphite |
| Limestone | Heat | Marble |
Rock Cycle
- The rock cycle is a continuous process of transformation from old rocks to new ones.
- Igneous rocks are the primary rocks and other rocks form from these.
- Fragments from igneous and metamorphic rocks can form sedimentary rocks.
- Sedimentary and igneous rocks themselves can become metamorphic rocks.
- Crustal rocks may be carried down into the mantle through subduction and can turn into molten magma, which is the source for igneous rocks.
- Example –
- Clay → Slate
- Limestone → Marble
- Sandstone → Quartzite
- Granite → Gneiss
- Shale → Schist
- Coal → Graphite
Rocks
- Earth’s crust is made up of various types of rocks.
- The scientific study of rocks is called petrology.
- A rock is a naturally occurring aggregate of minerals and certain non-mineral materials.
- Rocks are the natural building blocks of the Earth’s lithosphere, asthenosphere, mesosphere, and even part of the core.
- Most rocks now exposed at the surface of the Earth formed in or on continental or oceanic crust.
- Many rocks formed beneath the surface and now exposed at the surface were delivered from great depths in the crust or the underlying mantle.
- Rocks come to be exposed at the surface through formation at the surface or formation below the surface, followed by tectonic uplift and removal of overlying material by erosion.
- According to origin and appearance rocks can be divided into 3 groups:
- Igneous rocks
- Sedimentary rocks
- Metamorphic rocks
Igneous Rocks
- The first rocks on Earth formed through solidification of magma.
- Rocks formed through solidification of magma and lava are known as igneous or primary rocks.
- Igneous rocks have their origin under conditions of high temperatures and are unfossiliferous.
- Examples of igneous rocks include granite, gabbro, and basalt.
- Classification:
- Based on cooling time: plutonic rocks, volcanic rocks, and intermediate rocks.
- Based on the presence of silicon: acidic rocks and basic rocks.
- Based on origin: intrusive (plutonic) and extrusive (volcanic) igneous rocks.
- Plutonic Rocks:
- If magma cools slowly at great depths, mineral grains formed may be very large.
- Rocks formed this way are called intrusive or plutonic rocks.
- These rocks appear on the surface only after being uplifted and denuded.
- Intrusive rocks usually have a medium to coarse-grained texture due to slow cooling.
- Examples: granite, diorite, and gabbro.
- These rocks are exposed at the surface by the process of denudation and erosion.
- Volcanic Rocks:
- Sudden cooling of magma just below the surface or lava above the surface results in small and smooth grains in rocks.
- Rocks formed this way are called extrusive or volcanic rocks (e.g. Basalt).
- The Deccan traps in the Indian peninsular region are of basaltic origin.
- Extrusive rocks are fine-grained due to rapid cooling preventing crystallization.
- Basic rocks contain a greater proportion of basic oxides and are denser and darker in color.
- Examples of places where basic rocks are found include Antrim in Northern Ireland, Deccan plateau in India, and Snake plateau in Columbia.
Sedimentary Rocks
- Sedimentary rocks are formed by lithification, which is the consolidation and compaction of sediments
- Hence they are layered or stratified of varying thickness and examples include sandstone and shale.
- Sediments are a result of denudation, which is weathering and erosion of all types of rocks.
- Sedimentary rocks cover 75% of the Earth’s crust but volumetrically occupy only 5% 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:
- mechanically formed —
- 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. Eg: sandstone.
- Argillaceous rocks have more clay and are fine-grained, softer, and mostly impermeable. They are mostly non-porous or have very tiny pores. Eg: claystone and shales.
- organically formed —
- The remains of plants and animals are buried under sediments, and their composition changes due to heat and pressure from overlying layers. Eg: Coal and limestone.
- Sedimentary rocks may be calcareous (limestone, chalk, and dolomite) or carbonaceous (coal), depending on the predominance of calcium or carbon content.
- chemically formed —
- Water containing minerals evaporate 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).
- Example –
- Rock salt – derived from old sea bed or lakes
- Gypsum – evaporation of Salt Lake.
- Potash and nitrate are also obtained in a similar way.
- mechanically formed —
Metamorphic Rocks
- The term metamorphic means ‘change of form’.
- Metamorphism is the process of recrystallisation and reorganization of minerals in rocks due to pressure, volume, and temperature changes.
- It occurs when rocks are forced down to lower levels by tectonic processes or when molten magma comes in contact with crustal rocks.
- Foliation or lineation: It occurs when minerals or grains are arranged in layers or lines during metamorphism.
- Banded structure: It occurs when materials of different groups are arranged in alternating thin to thick layers during metamorphism.
- Gneissoid, slate, schist, marble, and quartzite are examples of metamorphic rocks.
- Causes of metamorphism:
- Orogenic movements –
- Such movements often take place with an interplay of folding, warping and high temperatures.
- These processes give existing rocks a new appearance.
- 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.
- Geodynamic forces –
- The omnipresent geodynamic forces such as plate tectonics also play an important role in metamorphism.
- Orogenic movements –
- On the basis of the agency of metamorphism, metamorphic rocks can be of two types:
- Thermal Metamorphism –
- It is the change of form or re-crystallisation of minerals of sedimentary and igneous rocks under high temperatures.
- A magmatic intrusion causing thermal metamorphism is responsible for the peak of Mount Everest consisting of metamorphosed limestone.
- It results in sandstone changing into quartzite and limestone into marble.
- Dynamic Metamorphism –
- It refers to formation of rocks under high pressure.
- High pressure is sometimes accompanied by high temperatures and chemically charged water.
- The combination of directed pressure and heat can lead to the complete recrystallization of rocks and the production of new structures, which is called dynamo thermal metamorphism.
- Under high pressure, granite can turn into gneiss, and clay and shale can transform into schist.
- Thermal Metamorphism –
- Examples of metamorphosis:
| Igneous or Sedimentary rock | Influence | Metamorphosed rock |
| Granite | Pressure | Gneiss |
| Clay, Shale | Pressure | Schist |
| Sandstone | Heat | Quartzite |
| Clay, Shale | Heat | Slate => Phyllite |
| Coal | Heat | Anthracite => Graphite |
| Limestone | Heat | Marble |
Rock Cycle
- The rock cycle is a continuous process of transformation from old rocks to new ones.
- Igneous rocks are the primary rocks and other rocks form from these.
- Fragments from igneous and metamorphic rocks can form sedimentary rocks.
- Sedimentary and igneous rocks themselves can become metamorphic rocks.
- Crustal rocks may be carried down into the mantle through subduction and can turn into molten magma, which is the source for igneous rocks.
- Example –
- Clay → Slate
- Limestone → Marble
- Sandstone → Quartzite
- Granite → Gneiss
- Shale → Schist
- Coal → Graphite
Rocks
- Earth’s crust is made up of various types of rocks.
- The scientific study of rocks is called petrology.
- A rock is a naturally occurring aggregate of minerals and certain non-mineral materials.
- Rocks are the natural building blocks of the Earth’s lithosphere, asthenosphere, mesosphere, and even part of the core.
- Most rocks now exposed at the surface of the Earth formed in or on continental or oceanic crust.
- Many rocks formed beneath the surface and now exposed at the surface were delivered from great depths in the crust or the underlying mantle.
- Rocks come to be exposed at the surface through formation at the surface or formation below the surface, followed by tectonic uplift and removal of overlying material by erosion.
- According to origin and appearance rocks can be divided into 3 groups:
- Igneous rocks
- Sedimentary rocks
- Metamorphic rocks
Igneous Rocks
- The first rocks on Earth formed through solidification of magma.
- Rocks formed through solidification of magma and lava are known as igneous or primary rocks.
- Igneous rocks have their origin under conditions of high temperatures and are unfossiliferous.
- Examples of igneous rocks include granite, gabbro, and basalt.
- Classification:
- Based on cooling time: plutonic rocks, volcanic rocks, and intermediate rocks.
- Based on the presence of silicon: acidic rocks and basic rocks.
- Based on origin: intrusive (plutonic) and extrusive (volcanic) igneous rocks.
- Plutonic Rocks:
- If magma cools slowly at great depths, mineral grains formed may be very large.
- Rocks formed this way are called intrusive or plutonic rocks.
- These rocks appear on the surface only after being uplifted and denuded.
- Intrusive rocks usually have a medium to coarse-grained texture due to slow cooling.
- Examples: granite, diorite, and gabbro.
- These rocks are exposed at the surface by the process of denudation and erosion.
- Volcanic Rocks:
- Sudden cooling of magma just below the surface or lava above the surface results in small and smooth grains in rocks.
- Rocks formed this way are called extrusive or volcanic rocks (e.g. Basalt).
- The Deccan traps in the Indian peninsular region are of basaltic origin.
- Extrusive rocks are fine-grained due to rapid cooling preventing crystallization.
- Basic rocks contain a greater proportion of basic oxides and are denser and darker in color.
- Examples of places where basic rocks are found include Antrim in Northern Ireland, Deccan plateau in India, and Snake plateau in Columbia.
Sedimentary Rocks
- Sedimentary rocks are formed by lithification, which is the consolidation and compaction of sediments
- Hence they are layered or stratified of varying thickness and examples include sandstone and shale.
- Sediments are a result of denudation, which is weathering and erosion of all types of rocks.
- Sedimentary rocks cover 75% of the Earth’s crust but volumetrically occupy only 5% 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:
- mechanically formed —
- 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. Eg: sandstone.
- Argillaceous rocks have more clay and are fine-grained, softer, and mostly impermeable. They are mostly non-porous or have very tiny pores. Eg: claystone and shales.
- organically formed —
- The remains of plants and animals are buried under sediments, and their composition changes due to heat and pressure from overlying layers. Eg: Coal and limestone.
- Sedimentary rocks may be calcareous (limestone, chalk, and dolomite) or carbonaceous (coal), depending on the predominance of calcium or carbon content.
- chemically formed —
- Water containing minerals evaporate 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).
- Example –
- Rock salt – derived from old sea bed or lakes
- Gypsum – evaporation of Salt Lake.
- Potash and nitrate are also obtained in a similar way.
- mechanically formed —
Metamorphic Rocks
- The term metamorphic means ‘change of form’.
- Metamorphism is the process of recrystallisation and reorganization of minerals in rocks due to pressure, volume, and temperature changes.
- It occurs when rocks are forced down to lower levels by tectonic processes or when molten magma comes in contact with crustal rocks.
- Foliation or lineation: It occurs when minerals or grains are arranged in layers or lines during metamorphism.
- Banded structure: It occurs when materials of different groups are arranged in alternating thin to thick layers during metamorphism.
- Gneissoid, slate, schist, marble, and quartzite are examples of metamorphic rocks.
- Causes of metamorphism:
- Orogenic movements –
- Such movements often take place with an interplay of folding, warping and high temperatures.
- These processes give existing rocks a new appearance.
- 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.
- Geodynamic forces –
- The omnipresent geodynamic forces such as plate tectonics also play an important role in metamorphism.
- Orogenic movements –
- On the basis of the agency of metamorphism, metamorphic rocks can be of two types:
- Thermal Metamorphism –
- It is the change of form or re-crystallisation of minerals of sedimentary and igneous rocks under high temperatures.
- A magmatic intrusion causing thermal metamorphism is responsible for the peak of Mount Everest consisting of metamorphosed limestone.
- It results in sandstone changing into quartzite and limestone into marble.
- Dynamic Metamorphism –
- It refers to formation of rocks under high pressure.
- High pressure is sometimes accompanied by high temperatures and chemically charged water.
- The combination of directed pressure and heat can lead to the complete recrystallization of rocks and the production of new structures, which is called dynamo thermal metamorphism.
- Under high pressure, granite can turn into gneiss, and clay and shale can transform into schist.
- Thermal Metamorphism –
- Examples of metamorphosis:
| Igneous or Sedimentary rock | Influence | Metamorphosed rock |
| Granite | Pressure | Gneiss |
| Clay, Shale | Pressure | Schist |
| Sandstone | Heat | Quartzite |
| Clay, Shale | Heat | Slate => Phyllite |
| Coal | Heat | Anthracite => Graphite |
| Limestone | Heat | Marble |
Rock Cycle
- The rock cycle is a continuous process of transformation from old rocks to new ones.
- Igneous rocks are the primary rocks and other rocks form from these.
- Fragments from igneous and metamorphic rocks can form sedimentary rocks.
- Sedimentary and igneous rocks themselves can become metamorphic rocks.
- Crustal rocks may be carried down into the mantle through subduction and can turn into molten magma, which is the source for igneous rocks.
- Example –
- Clay → Slate
- Limestone → Marble
- Sandstone → Quartzite
- Granite → Gneiss
- Shale → Schist
- Coal → Graphite
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