Rock cycle

Topic: Minerals, Rocks and the Rock Cycle · NCERT: Beyond NCERT

Meaning

The rock cycle is the process, which never ends, by which rocks change from one type into another. Any of the three main rock types (igneous, sedimentary and metamorphic) can turn into either of the other two over geological time [1]. It shows that rocks are recycled rather than made once and kept for ever. It also explains why India's coal, iron ore, marble and black soil are found where they are.

Explanation

The three rock types in the cycle

• Igneous rock: forms when magma (molten rock inside the earth) or lava (magma that comes out onto the surface) cools and becomes solid.
• Igneous rocks are called primary rocks, because the other two types are formed from them in the end.

• Intrusive igneous rock cools slowly inside the crust, for example granite. Extrusive igneous rock cools fast on the surface, for example basalt [1].

• Sedimentary rock: forms when sediments (loose pieces of older rock, or plant and animal remains) are laid down in layers and harden. Examples are sandstone, shale, limestone and coal.

• Metamorphic rock: an older rock whose minerals and texture are changed by great heat and pressure while it stays solid. "Metamorphic" means "changed in form".

How the cycle works, step by step (cause → process → result)

• Step 1: Igneous rock breaks into sediments
• Igneous rock such as granite or basalt is exposed at the surface.
• Weathering (rock breaking down where it lies, through sun, rain, frost, plants and chemical action) breaks it into pieces.

• Erosion (wearing away rock and carrying the pieces off by running water, wind, glaciers and waves) moves these pieces away as sediments.

• Step 2: Sediments become sedimentary rock

• The sediments are weathered, eroded, transported and deposited [1].

• Lithification (loose sediment hardening into solid rock) happens in two parts:

  • Compaction: the weight of new layers above squeezes the lower layers tightly together.
  • Cementation: minerals carried in water, such as silica or calcite, fill the gaps and glue the grains together [1].

• Step 3: Igneous or sedimentary rock becomes metamorphic

• Tectonic plates move towards each other (converge). Rock gets buried deep or squeezed in a mountain belt.
• Temperature and pressure rise.

• The rock changes physically and chemically into metamorphic rock [1].

• Step 4: Any rock melts into magma

• At a subduction zone (where one plate slides under another and sinks into the mantle, the hot layer below the crust), the sinking rock goes deeper and gets hotter.
• It melts into magma [1].

• The magma rises. It cools as new intrusive or extrusive igneous rock [1].

• Step 5: Uplift brings rocks back to the surface

• Uplift (the crust being raised by earth movements, for example during mountain building) lifts deep rocks up.
• Erosion removes the rock lying on top. Deep igneous and metamorphic rocks are now exposed.
• Once exposed, they can be weathered, and the cycle starts again.

Shortcuts: the cycle has no fixed order

• Metamorphic → sediments: uplifted metamorphic rock can be weathered directly. It does not have to melt first.
• Igneous → magma: igneous rock can be pulled down again and remelted.
• Sedimentary → new sediments: sedimentary rock can be weathered and laid down again as new sedimentary rock.
• Igneous → metamorphic: heat and pressure can change igneous rock directly, as when granite becomes gneiss.

Energy that drives the cycle

• Endogenic forces (forces from inside the earth):
• Their energy comes from the earth's internal heat. This heat comes from radioactivity (decay of uranium, thorium and potassium) and primordial heat (heat left over from when the earth formed).
• They cause melting, metamorphism, volcanic activity, and the uplift and folding of the crust.

• They build relief up, for example by raising mountains.

• Exogenic forces (forces acting on the earth's surface):

• Their energy comes from the sun (which drives weather, wind and the water cycle) and gravity (which pulls water, ice and rock debris downhill).
• They cause weathering, erosion, transport, deposition and lithification.

• They wear relief down.

• The cycle runs because of the contest between the two: one side builds up and the other wears down.

From → To	Process	Force
Magma → Igneous	Cooling and solidification	Endogenic
Igneous → Sediments	Weathering and erosion	Exogenic
Sediments → Sedimentary	Deposition, compaction, cementation	Exogenic (plus weight of the layers above)
Igneous / Sedimentary → Metamorphic	Heat and pressure	Endogenic
Any rock → Magma	Melting (deep burial, subduction)	Endogenic
Deep rock → Surface	Uplift, then erosion	Endogenic + exogenic

Where it occurs

• Igneous stage, India: the Deccan Traps. Basalt lava 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.
• Weathering stage, India: Deccan basalt in Maharashtra, Gujarat and Madhya Pradesh weathers into black soil (regur), which is good for cotton.
• Sedimentary stage, India:
• Cuddapah (Andhra Pradesh) and Vindhyan (Madhya Pradesh–Rajasthan) systems: thick sandstone, shale and limestone.
• Gondwana system: coal-bearing rocks of the Damodar, Mahanadi, Son and Godavari valleys. The coal is about 200 million years old (NCERT).

• Tertiary rocks: the coal of north-east India, about 55 million years old (NCERT), and the rocks that were folded up to form the Himalaya.

• Metamorphic stage, India:

• Archaean gneisses and schists: India's oldest rocks, found in Karnataka, Tamil Nadu, Odisha, Jharkhand and Rajasthan.
• Dharwar system: old sediments and lavas changed into schists. They hold iron ore (Karnataka, Odisha–Jharkhand) and gold (Kolar and Hutti, Karnataka).

• Makrana (Rajasthan): marble formed from limestone.

• Uplift stage, India: marine (sea-formed) sedimentary rocks and old metamorphic rocks now lie high in the Himalaya. They were lifted when the Indian Plate collided with the Eurasian Plate.

• Deposition stage, India: the Indo-Gangetic plain is receiving eroded Himalayan sediment. This is the raw material for future sedimentary rock.
• Melting stage, world: subduction zones are marked by deep ocean trenches, such as the Mariana Trench in the Pacific Ocean.
• Ocean-floor recycling, world:
• New crust forms at mid-ocean ridges (underwater mountain chains where plates move apart). The seafloor gets older as you move away from the ridge [3].
• Old crust sinks and melts again at trenches. So the seafloor is nowhere older than about 180 million years [2]. NCERT says 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 [2].
• Continental crust is thicker and lighter, so it does not sink easily and escapes recycling. Continental rocks can be up to 3,200 million years old (NCERT). The Peninsular plateau of India is one such craton (a very old, stable core of a continent).

Don't confuse with

• Metamorphism vs melting: in metamorphism, the rock changes while still solid. Once it melts, it becomes magma, and when that cools it is igneous rock, not metamorphic.
• Weathering vs erosion: weathering breaks rock down where it lies and does not move it. Erosion wears away and carries the pieces off by water, wind, ice and waves.
• Rock cycle vs water cycle: both are cycles with no end, but the rock cycle takes place over geological time (millions of years) and is driven by both internal heat and the sun. The water cycle is short and driven mainly by solar energy.
• Intrusive vs extrusive igneous rock: intrusive rock cools slowly inside the crust (granite). Extrusive rock cools fast on the surface (basalt, as in the Deccan Traps).

Prelims Hooks

• Process matches: sediments → sedimentary rock = compaction and cementation (lithification). Sedimentary → metamorphic = heat and pressure. Any rock → magma = melting.
• Trap: "Metamorphic rock must melt into magma before it can re-enter the cycle." This is wrong. It can weather directly into sediment.
• Rock pairs: limestone → marble (Makrana, Rajasthan); granite → gneiss; sandstone → quartzite; shale → slate (→ schist with more heat and pressure); coal → graphite.
• Indian rock systems by type: Archaean gneiss = metamorphic; Cuddapah, Vindhyan and Gondwana = sedimentary; Deccan Traps = igneous (basalt, about 65 million years old).
• Energy match: internal heat (radioactivity, primordial heat) drives endogenic processes. Solar energy and gravity drive exogenic processes.
• Ages: the oldest seafloor is about 180 million years old [2]. Continental rocks are up to 3,200 million years old (NCERT). The seafloor is youngest at mid-ocean ridges [3].

Mains Points

• Endogenic and exogenic forces together shape India's relief (GS-I):
• Himalaya: uplift raised marine sediments to great heights.
• Indo-Gangetic plain: eroded Himalayan sediment is being laid down here as the base for future sedimentary rock.

• Peninsula: an old, worn-down Archaean–Dharwar block, a stable craton.

• India's mining belts follow its rock systems (GS-I/III):

• Sedimentary stage: coal (Gondwana in the Damodar valley, Tertiary in the north-east), limestone and petroleum.

• Metamorphic and igneous stages: iron ore, gold and manganese (Dharwar). Weathered Deccan basalt gives bauxite and black soil.

• Rock-cycle stages bring hazards (GS-III):

• Endogenic stages (subduction, uplift) cause earthquakes and volcanoes. In India these occur in the Himalaya and the Andaman–Nicobar arc (Barren Island).
• Exogenic stages (weathering, erosion) cause landslides and soil loss in the Western Ghats and the Himalaya.
• Seafloor recycled within about 180 million years [2] while cratons survive for billions of years is strong evidence for plate tectonics and seafloor spreading.

Read more

• The rock cycle