Metamorphic Rocks and the Rock Cycle
Geography › Rocks, Erosion and Landforms · Topic 5.3
Metamorphic rocks are formed when existing rocks are changed in the solid state by heat, pressure or chemically active fluids without melting, as granite becomes gneiss, limestone becomes marble and sandstone becomes quartzite. They may be foliated, with banded layers, or non-foliated. The rock cycle is the continuous transformation of rocks among the igneous, sedimentary and metamorphic types.
5 min read · UPSC Prelims · Geography · Rocks, Erosion and Landforms
At a Glance
- Meaning Rocks changed by heat, pressure or fluids
- Agents Temperature, pressure, chemical fluids
- Foliated Slate, phyllite, schist, gneiss
- Non-foliated Marble, quartzite
- Rock cycle Continuous recycling of all rock types
- Exam link UPSC Prelims, Geography: Rocks, Erosion and Landforms
Where Does It Fit?
This is the third topic of the category on rocks, erosion and landforms. The previous topics described igneous and sedimentary rocks, and this one completes the three families and joins them in the rock cycle. The next topic covers soil erosion. Previous topic: Sedimentary Rocks. Next topic: Soil Erosion – Water and Wind Erosion.
What Are Metamorphic Rocks?
In short: metamorphic rocks are old rocks that have been transformed in the solid state by heat and pressure.
The word metamorphic means changed in form. Metamorphism is the process by which existing rocks of any type, igneous, sedimentary or metamorphic, are altered in structure and mineral composition by heat, pressure and chemically active fluids, without melting. If the rock melts, it turns into magma, and the new rock would be igneous. The change takes place deep in the crust or at the margins of plates and intrusions. Metamorphic rocks often have larger crystals and a harder, denser texture than the parent rock. They make up a large share of the crust, along with igneous rocks, as in Crust, Mantle and Core – Internal Structure of the Earth.
What Are the Types of Metamorphism?
| Type | Cause | Where it occurs | Examples |
|---|---|---|---|
| Contact (thermal) | Heat from a nearby magma body | Narrow zone around an intrusion | Limestone to marble; sandstone to quartzite |
| Regional (dynamothermal) | High pressure and heat over a wide area | Mountain-building zones at convergent boundaries | Shale to slate, schist and gneiss |
| Dynamic (cataclastic) | Intense directed pressure and shearing | Along fault zones | Crushed and sheared rocks |
| Hydrothermal | Hot chemically active fluids | Around vents and cracks | Mineral veins |
Regional metamorphism is the most widespread and is associated with the mountain-building zones described in Plate Tectonics and Types of Plate Boundaries.
What Are Foliated and Non-Foliated Rocks?
When metamorphism takes place under directed pressure, mineral grains are flattened and aligned in parallel bands or layers, which is called foliation. Foliated rocks split along these planes. A sequence of increasing metamorphism from shale is: slate, then phyllite, then schist, then gneiss. Non-foliated rocks are made of equal-sized crystals with no banding and generally form from a single mineral.
| Parent rock | Metamorphic rock | Notes |
|---|---|---|
| Shale or clay | Slate, phyllite, schist | Slate splits into thin sheets and is used for roofing and blackboards |
| Granite | Gneiss | Banded rock of coarse grain |
| Limestone | Marble | Used for sculpture and buildings; Makrana marble of Rajasthan was used in the Taj Mahal |
| Sandstone | Quartzite | Very hard and resistant |
| Coal | Anthracite, graphite | Increasing carbon content |
| Basalt | Greenschist, amphibolite | Occurs in old mountain belts |
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 Uses and Indian Examples?
Metamorphic rocks supply building and decorative stones such as marble and slate, and rich ore deposits, including iron, gold, copper and graphite, are found in metamorphic belts. In India, much of the Peninsular shield consists of metamorphic rocks of the Archaean and Dharwar systems, including gneisses and schists, and the Dharwar rocks hold gold, iron and manganese, as in Archaean Rock System of India. Marble comes from Makrana in Rajasthan, and slates from parts of Himachal Pradesh and the Aravalli region.
Why Is This Topic Important?
- Transformations Parent-product pairs asked
- Rock cycle Standard concept
- India Peninsular shield and marble
What Came Next?
The next topic describes soil erosion by water and wind. Next topic: Soil Erosion – Water and Wind Erosion.
Key People and Terms
- Metamorphism Change of rocks by heat and pressure
- Foliation Parallel banding of minerals
- Gneiss Banded metamorphic rock from granite
- Marble Metamorphosed limestone
- Quartzite Metamorphosed sandstone
- Rock cycle Continuous recycling of rocks
Exam Corner
Points to Remember
- Limestone becomes marble; sandstone becomes quartzite.
- Granite becomes gneiss.
- Shale becomes slate, then phyllite, schist and gneiss.
- Metamorphism occurs without melting.
- The rock cycle has no fixed start or end.
Do Not Confuse With
- Contact and regional metamorphism Local heat versus wide pressure and heat
- Foliated and non-foliated Banded versus uniform grains
- Metamorphic and igneous Changed without melting versus formed from melt
Memory Hook
Heat and squeeze, no melt: limestone to marble, sandstone to quartzite, shale to slate.
Mains Angle
Questions ask for transformations and the cycle. These points add depth.
- Agents Heat, pressure and fluids
- Cycle Continuous recycling
- Link Plate tectonics drives it
A Question You May Face
An original practice question, not a past paper question.
“Explain the rock cycle and describe the processes by which one type of rock changes into another.”
How to Answer
- Three families Igneous, sedimentary, metamorphic.
- Processes Cooling, lithification, metamorphism.
- Pathways Shortcuts.
- Drivers Heat and weathering.
GS Relevance
Frequently Asked Questions
What are metamorphic rocks?
Metamorphic rocks are rocks that have been changed in structure and mineral composition by heat, pressure or chemically active fluids without melting. The parent rock may be igneous, sedimentary or already metamorphic, and examples include marble, quartzite, slate and gneiss.
What are the main types of metamorphism?
The main types are contact metamorphism, caused by heat from nearby magma; regional metamorphism, caused by heat and pressure over a wide area in mountain belts; dynamic metamorphism, caused by shearing along faults; and hydrothermal alteration by hot fluids.
What is foliation in rocks?
Foliation is the parallel arrangement of flattened mineral grains in bands or layers, produced when rocks are metamorphosed under directed pressure. Foliated rocks such as slate, schist and gneiss tend to split along the planes, while marble and quartzite are non-foliated.
What is the rock cycle?
The rock cycle is the continuous process by which the three rock families change into one another. Igneous rock is weathered into sediments, which form sedimentary rock, and heat and pressure make metamorphic rock, which may melt into magma and again cool into igneous rock.
Which rocks form marble and quartzite?
Marble is formed when limestone is metamorphosed by heat and pressure, and quartzite is formed when sandstone is metamorphosed. Both are non-foliated and hard, and marble, as from Makrana in Rajasthan, is widely used for buildings and sculpture.
PYQ Practice — Statement Analysis
1 Marble is formed from limestone by metamorphism.
It is a non-foliated rock.
2 Metamorphic rocks form by the melting and cooling of magma.
That is igneous rock.
3 Gneiss is formed from granite.
It is a banded foliated rock.
4 The rock cycle has a fixed starting point at the igneous rocks and cannot be shortened.
It has no fixed start and has many shortcuts.
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