The rock cycle

Detail

What the rock cycle is

• Rocks do not keep their first form for ever. Over geological time, old rocks are broken down, buried, heated or melted, and they become new rocks.
• Rock cycle: the process, which never ends, by which rocks change from one type into another. Each of the three rock types can turn into the other two.
• The rock cycle is a chain of processes. These processes make the three main rock types: igneous, sedimentary and metamorphic [2].
• Igneous rock: rock formed when magma (molten rock inside the earth) or lava (magma that comes out on the surface) cools and becomes solid. Igneous rocks are called primary rocks because the other rock types are formed from them.
• Sedimentary rock: rock formed when sediments (loose pieces of older rock, or plant and animal remains) are laid down in layers and harden.
• Metamorphic rock: an older rock changed in form by great heat and pressure, without melting. ("Metamorphic" means "changed in form".)

The rock cycle as a labelled flow diagram

                 ┌────────────── cooling / solidification ──────────────┐
                 │                                                      ▼
              MAGMA                                                 IGNEOUS ROCK
                 ▲                                                   │       │
                 │ melting                     weathering + erosion  │       │ heat + pressure
                 │ (deep burial,                                     ▼       │
                 │  subduction zone)                            SEDIMENTS    │
                 │                                                   │       │
                 │                       compaction + cementation    │       │
                 │                       (lithification)             ▼       ▼
                 │                                          SEDIMENTARY ──heat + pressure──► METAMORPHIC ROCK
                 │                                                                              │
                 └─────────────────────────────── melting ◄─────────────────────────────────────┘

   SHORTCUTS:  METAMORPHIC ──weathering + erosion──► SEDIMENTS
               IGNEOUS ──remelting──► MAGMA
               SEDIMENTARY ──weathering + erosion──► new SEDIMENTS
   UPLIFT + EROSION bring any rock back up to the surface, so the cycle can start again.

From → To	Process that links them	Force behind it
Magma → Igneous	Cooling and solidification (inside the crust or on the surface)	Endogenic
Igneous → Sediments	Weathering and erosion	Exogenic
Sediments → Sedimentary	Deposition, then compaction and cementation (lithification)	Exogenic (plus the weight of layers above)
Igneous / Sedimentary → Metamorphic	Heat and pressure	Endogenic
Any rock → Magma	Melting (deep burial, subduction)	Endogenic
Metamorphic → Sediments (shortcut)	Weathering and erosion	Exogenic
Igneous → Magma (shortcut)	Remelting	Endogenic
Any deep rock → surface	Uplift, then erosion of the rock lying above	Endogenic + exogenic

Step 1: Igneous rock breaks into sediments

• Weathering: rock breaking down where it lies, by sun, rain, frost, plants and chemical action. It does not move the rock.
• Erosion: wearing away of rock and carrying the pieces away by running water, wind, ice (glaciers) and waves.
• Cause → process → result:
• Igneous rock such as granite or basalt is exposed at the surface.
• Heat by day, cold by night, rain and chemical action break it into fragments.

• Rivers, wind and glaciers carry the fragments away as sediments.

• Weathering also makes soil. Broken rock mixes with humus (decayed plant and animal matter) over a long time.

• Indian example: the Deccan Trap basalt (an igneous rock) of Maharashtra, Gujarat and Madhya Pradesh weathers into black soil (regur). This soil is good for cotton.

Step 2: Sediments become sedimentary rock

• To form sedimentary rock, existing rock or organic matter has to be weathered, eroded, transported and deposited [2].
• Lithification: the hardening of loose sediments into solid rock. It has two parts:
• Compaction: new layers pile up on top, and their weight squeezes the lower layers tightly together.

• Cementation: minerals carried in water (for example silica or calcite) fill the gaps and glue the grains together.

• Sediment that is buried under enough new sediment is compacted, and then cemented under very high pressure [2].

• The result is layered rock: sandstone (from sand), shale (from clay or mud), limestone (from shells and lime), and coal (from buried plant matter).
• Indian examples:
• Cuddapah (Andhra Pradesh) and Vindhyan (Madhya Pradesh–Rajasthan) systems: thick sandstone, shale and limestone.
• Gondwana system: coal-bearing sedimentary rocks of the Damodar, Mahanadi, Son and Godavari valleys. This coal is about 200 million years old (NCERT).
• Tertiary rocks: younger sediments. They include the Tertiary coal of north-east India, about 55 million years old (NCERT), and the rocks that were folded up to make the Himalaya.

Step 3: Igneous or sedimentary rock becomes metamorphic

• Metamorphism: the change of a rock's minerals and texture by heat and pressure while it stays solid.
• Cause → process → result:
• Tectonic plates move towards each other (converge). Rock is buried deep or squeezed in a mountain belt.
• Temperature and pressure rise.

• The rock changes physically and chemically into metamorphic rock [2].

• Common pairs (often tested):

Parent rock	Type	Metamorphic rock
Granite	Igneous	Gneiss
Limestone	Sedimentary	Marble
Sandstone	Sedimentary	Quartzite
Shale / clay	Sedimentary	Slate → schist (with more heat and pressure)
Coal	Sedimentary	Graphite

• Indian examples:
• Archaean gneisses and schists: India's oldest rocks, found in Karnataka, Tamil Nadu, Odisha, Jharkhand and Rajasthan. They are mostly metamorphic.
• Dharwar system: old sediments and lavas changed into schists. They hold iron ore (Karnataka, Odisha–Jharkhand) and gold (Kolar and Hutti in Karnataka).
• Makrana (Rajasthan) marble formed from limestone.

Step 4: Any rock melts into magma, which becomes new igneous rock

• Subduction zone: a place where one tectonic plate slides under another and sinks into the mantle (the hot layer below the crust). The deep ocean trench marks where this happens. An example is the Mariana Trench in the Pacific Ocean.
• Cause → process → result:
• The sinking rock goes deeper, where it is hotter.
• With more heat, the rock melts into magma [2].

• Magma rises. It either cools slowly inside the crust as intrusive igneous rock (granite), or erupts as lava and ash and cools fast as extrusive igneous rock (basalt) [2].

• Indian example: the Deccan Traps. Huge basalt lava flows poured out through fissures (long cracks) at the end of the Cretaceous period, about 65 million years ago, and covered a large part of the north-western Peninsula.

Step 5: Uplift brings rocks back to the surface

• Uplift: the raising of crust by earth movements, for example during mountain building.
• Erosion then removes the rock lying above. Deep igneous and metamorphic rocks become exposed.
• Once exposed, they can be weathered, and the cycle starts again.
• Example: marine (sea-formed) sedimentary rocks and old metamorphic rocks are now found high up in the Himalaya. They were lifted up when the Indian Plate collided with the Eurasian Plate.

Shortcuts in the cycle

• Rocks do not have to follow the full loop in order.
• Metamorphic → sediments: a metamorphic rock that is uplifted can be weathered directly. It does not have to melt first.
• Igneous → magma: an igneous rock can be pulled down again and remelted.
• Sedimentary → new sediments: a sedimentary rock can be weathered and re-deposited as a new sedimentary rock.
• Igneous → metamorphic: heat and pressure act on igneous rock directly, as when granite becomes gneiss.

Energy that drives the cycle

• Endogenic forces: forces that come from inside the earth.
• Source of energy: the earth's internal heat. This heat comes from radioactivity (the decay of elements such as uranium, thorium and potassium) and primordial heat (heat left over from when the earth formed).

• What they do: melting, metamorphism, volcanic activity, and uplift and folding of the crust.

• Exogenic forces: forces that act on the earth's surface.

• Source of energy: the sun (it drives weather, wind and the water cycle) and gravity (it pulls water, ice and rock debris downhill).

• What they do: weathering, erosion, transport and deposition.

• Balance: endogenic forces build relief up, for example by raising mountains. Exogenic forces wear relief down. The rock cycle runs because of the contest between the two.

Feature	Endogenic	Exogenic
Energy source	Internal heat (radioactivity, primordial heat)	Solar energy, gravity
Rock-cycle steps	Melting, metamorphism, uplift, volcanism	Weathering, erosion, deposition, lithification
Effect on relief	Builds up (constructive)	Wears down (levelling)

Why the ocean floor is young and continents are old

• Mid-ocean ridge: an underwater mountain chain where plates move apart and new crust forms from rising magma.
• How the ocean floor is recycled:
• New ocean crust forms at mid-ocean ridges.
• It spreads away on both sides, so the seafloor gets older as you move away from the ridge [4].

• At trenches it sinks back into the mantle by subduction, and melts.

• Result: the world's seafloor is nowhere older than about 180 million years [3] (NCERT: ocean-floor rocks are younger than 200 million years).

• The oldest seafloor lies in the Atlantic, close to the coasts of North America and north-west Africa [3].
• Continental crust is thicker and lighter. It does not sink easily, so it escapes being recycled. Continental rocks can be up to 3,200 million years old (NCERT).
• In India: the Archaean gneisses of the Peninsular plateau, for example in the Dharwar region of Karnataka, are among these very old continental rocks. They are why the Peninsula is called an ancient, stable block (a craton: a very old and stable core of a continent).

Prelims Hooks

• Sedimentary → metamorphic: heat and pressure. Any rock → magma: melting. Sediments → sedimentary rock: compaction and cementation (lithification).
• Igneous rocks are "primary rocks". The other two types are ultimately formed from them.
• Shortcut trap: metamorphic rock can weather directly into sediment. It does not have to melt first. So "metamorphic rock must become magma before re-entering the cycle" is wrong.
• Rock pairs: limestone → marble (Makrana, Rajasthan); granite → gneiss; sandstone → quartzite; shale → slate; coal → graphite.
• Energy match: internal heat (radioactivity, primordial heat) drives endogenic processes. Solar energy and gravity drive exogenic processes.
• Seafloor age: the oldest seafloor is about 180 million years old [3]. Continental rocks are up to 3,200 million years old (NCERT).
• The ocean floor is youngest at mid-ocean ridges and oldest near continental margins and trenches [4].
• Indian rock systems by type: Archaean gneiss = metamorphic; Cuddapah, Vindhyan and Gondwana = sedimentary; Deccan Traps = igneous (basalt, about 65 million years old).
• Gondwana coal is about 200 million years old; Tertiary coal is about 55 million years old. Gondwana coal is the main coal of India (Damodar valley).

Mains Points

• Endogenic–exogenic interaction shapes India's relief (GS-I):
• The Himalaya: uplift raised marine sediments to great heights.
• The Indo-Gangetic plain: eroded Himalayan sediment is being deposited here, and is the raw material for future sedimentary rock.

• The Peninsula: an old, eroded Archaean–Dharwar block.

• Rock cycle and mineral distribution (GS-I/III):

• Each stage of the cycle concentrates different resources.
• Sedimentary stage: coal (Gondwana, Tertiary), limestone and petroleum.
• Metamorphic and igneous stages: iron ore, gold and manganese (Dharwar), and bauxite and black soil from weathered basalt (Deccan).

• So India's mining belts follow its rock systems.

• Young seafloor vs old continents (GS-I):

• Subduction recycles ocean crust, so the seafloor is no older than about 180 million years [3].
• Continental cratons survive for billions of years.

• This difference is strong evidence for plate tectonics and seafloor spreading.

• Rock cycle and hazards (GS-III):

• The endogenic stages, such as subduction and uplift, bring earthquakes and volcanoes. In India these are found in the Himalaya and the Andaman–Nicobar arc (Barren Island).
• The exogenic stages, such as weathering and erosion, cause landslides and soil loss in the Western Ghats and the Himalaya.