Maarg Manthan · Topic 2.9

The Rock Cycle

Geography › Geology – Minerals and Rocks · Topic 2.9

The rock cycle is the continuous process by which rocks are changed from one family to another. Magma cools into igneous rock. Weathering and erosion break rocks into sediment, which is deposited and hardened into sedimentary rock. Heat and pressure change any rock into metamorphic rock. If metamorphic rock melts it becomes magma again. The cycle is driven by the Earth's internal heat and by the Sun's energy acting through water, wind and ice, and it takes millions of years to complete.

5 min read · UPSC Prelims · Geography · Geology – Minerals and Rocks

At a Glance

Engine Internal heat and solar energy
Key processes Cooling, weathering, deposition, lithification, metamorphism, melting
Shortcuts Any rock can become any other
Time Millions of years
Link Plate tectonics
Exam link UPSC Prelims, Geography: Geology – Minerals and Rocks

Where Does It Fit?

This is the last topic of the macro on minerals and rocks. It joins the three families into one system and shows that every rock is temporary, which prepares the study of weathering, erosion and plate movement in the next macro. Previous topic: Three Families of Rocks: Igneous, Sedimentary, Metamorphic. Next topic: epicentre above it Main cause Sudden slip along faults at plate boundaries Scales Magnitude…">Earthquakes: Types, Causes, Effects and Frequency.

What Is the Rock Cycle?

The rock cycle describes how the three rock families are changed into each other by natural processes over millions of years. There is no fixed starting or ending point, and many shortcuts are possible.

Magma cools and crystallises to form igneous rock, as in Igneous Rocks.
Exposed igneous rock is weathered and eroded, and the sediments are transported and deposited.
Deposited sediments are buried and lithified to become sedimentary rock, as in Sedimentary Rocks.
Deep burial, heat and pressure change sedimentary or igneous rock into metamorphic rock.
Further heating melts metamorphic rock into magma, and the cycle begins again.

Shortcuts include igneous rock being metamorphosed without weathering, sedimentary rock being weathered again to form new sediment, and metamorphic rock being uplifted and eroded. The cycle is driven by internal heat and plate movement, which provide melting, burial and uplift, and by the Sun, which drives weathering, erosion and deposition. As an analytical point, the rock cycle is a model of the continuity of change in the crust, so that no rock is permanent and landforms are the surface record of these changes.

What Are the Possible Pathways?

From	Process	To
Magma or lava	Cooling and crystallisation	Igneous rock
Any rock at the surface	Weathering, erosion and transport	Sediment
Sediment	Deposition, burial, compaction and cementation (lithification)	Sedimentary rock
Sedimentary or igneous rock	Heat and pressure (metamorphism)	Metamorphic rock
Any rock deep in the crust	Melting	Magma
Deeply buried rock	Uplift and exposure	Back to the surface for weathering

The cycle is not a fixed loop. Igneous rock can be metamorphosed without ever being weathered, sedimentary rock can be melted directly, and a metamorphic rock can be metamorphosed again to a higher grade. Any family can turn into any other.

What Drives the Rock Cycle?

Internal heat: radioactive decay and leftover heat keep the interior hot, melt rock and fuel volcanoes, and drive the metamorphism of buried rocks.
Solar energy: powers the water cycle, wind and ice, which weather and erode rocks and carry the sediments to the sea.
Plate tectonics: moves rocks between the surface and the interior. At subduction zones, sediments and crust are pushed down, metamorphosed and melted. At ridges, new igneous crust forms. Colliding plates raise mountains and expose deep rocks to weathering, as in Plate Tectonics: Major and Minor Lithospheric Plates.
Gravity: pulls sediments downhill and compacts them under their own weight.

How Long Does the Rock Cycle Take and What Are Some Examples?

A complete turn of the rock cycle takes tens to hundreds of millions of years, though parts of it, such as a lava flow cooling or a landslide, take only moments or days. The oldest rocks on the Earth survived because they were buried deeply in stable continental interiors.

Granite to sandstone to quartzite: an igneous rock weathers to sand, which is cemented into sandstone and then turned into quartzite by heat and pressure. The Aravalli quartzite is a case in India.
Limestone to marble to magma: limestone formed from shells is turned into marble, and then melted if subducted.
Basalt to soil: the Deccan Traps weather into black soil, and the soils are carried to river deltas, as in Relationship Between Rocks and Soils.

As an analytical point, the rock cycle shows that the minerals of the crust are used again and again, and that landforms, soils and resources are all stages in this long cycle. It is a base for the study of weathering and erosion in Exogenic Processes: Denudation, Weathering, Mass Movements, Erosion, Transportation, Deposition.

Why Is This Topic Important?

System view Links all rocks
Pathways Conversions are asked
Tectonics Shows recycling of crust

What Came Next?

The next macro begins the study of the processes that shape the land, with earthquakes and volcanoes. Next topic: Earthquakes: Types, Causes, Effects and Frequency.

Key People and Terms

Lithification Hardening of sediment into rock
Metamorphism Change of rock by heat and pressure
Recrystallisation New crystals forming in solid rock
Subduction Plate sinking into the mantle
Uplift Raising of rock to the surface
Magma Molten rock below the surface

Exam Corner

Points to Remember

Heat and pressure change any rock into metamorphic rock.
Melting any rock gives magma.
Sediment becomes rock by lithification.
Any rock can change into any other.
Plate subduction recycles crustal rocks.

Do Not Confuse With

Metamorphism and melting Change in solid state versus change to liquid
Lithification and metamorphism Low-temperature hardening versus change by heat and pressure
Weathering and erosion Breakdown versus removal

Memory Hook

Cool to igneous, break to sediment, bury to rock, bake to metamorphic, melt to magma again.

Mains Angle

Questions ask for the cycle and its link to plates. These points add depth.

System Closed cycle of materials
Energy Internal and solar
Link Plate tectonics

A Question You May Face

An original practice question, not a past paper question.

“Explain the rock cycle and show how plate tectonics keeps it going.”

How to Answer

Define Rock cycle.
Pathways Cooling, weathering, lithification, metamorphism, melting.
Energy Heat and sun.
Link Subduction and uplift.

GS Relevance

Prelims: stages and pathways of the rock cycle. GS Paper 1: geomorphology, plate tectonics and rock processes.

Frequently Asked Questions

What is the rock cycle?

The rock cycle is the continuous process by which rocks change from one type to another over geological time. Magma cools into igneous rock, weathering turns rocks into sediments which harden into sedimentary rock, heat and pressure turn rocks into metamorphic rock, and melting returns them to magma.

What drives the rock cycle?

The rock cycle is driven by two energy sources. The Earth's internal heat drives melting, metamorphism, volcanic activity and plate movement, while the Sun's energy drives the water cycle, wind and ice that weather and erode rocks. Gravity moves and compacts the sediments.

What is lithification?

Lithification is the process by which loose sediments are turned into solid sedimentary rock. It involves compaction, in which the weight of overlying layers squeezes out water and air, and cementation, in which minerals such as silica or calcite precipitate between grains and bind them together.

Can an igneous rock become a sedimentary rock directly?

Yes. An igneous rock exposed at the surface is weathered and eroded, and the fragments are transported and deposited as sediment. When the sediment is buried and hardened by compaction and cementation, it forms a sedimentary rock. For instance, granite breaks down to form sandstone.

How does plate tectonics affect the rock cycle?

Plate tectonics moves rocks between the surface and the interior. At subduction zones, crust and sediments are pushed down, metamorphosed and melted, while at ridges new igneous crust forms. Colliding plates raise mountains, exposing deep rocks to weathering and starting the cycle again.

PYQ Practice — Statement Analysis

1 The rock cycle shows that igneous rocks can never become sedimentary rocks.

False

Weathering of igneous rock produces sediments.

2 Lithification turns sediment into sedimentary rock.

True

By compaction and cementation.

3 Metamorphic rocks form by melting of existing rocks.