Last reviewed 16 Sept 2026 · 8 min read
Rock cycle
Rocks continuously change from one type to another:
- Magma cools → igneous rocks.
- Igneous (or any) rocks weather and erode → sediments → deposition and lithification → sedimentary rocks.
- Rocks subjected to heat, pressure and fluids → metamorphic rocks.
- Deep burial and melting → magma again.
Igneous rocks
Formed by cooling and solidification of magma (below the surface) or lava (at the surface).
Intrusive and extrusive rocks
| Type | Cooling | Texture | Examples |
|---|---|---|---|
| Plutonic (intrusive, deep-seated) | Slow, at great depth | Coarse-grained (phaneritic) | Granite, diorite, gabbro, syenite, peridotite |
| Hypabyssal (intrusive, shallow) | Moderate | Medium-grained, often porphyritic | Dolerite, porphyries, pegmatite (very coarse) |
| Volcanic (extrusive) | Rapid, at surface | Fine-grained (aphanitic) or glassy | Basalt, rhyolite, andesite, obsidian (glass), pumice |
Textures
- Phaneritic — crystals visible to the naked eye (slow cooling).
- Aphanitic — crystals too small to see (fast cooling).
- Porphyritic — large crystals (phenocrysts) in a finer groundmass (two-stage cooling).
- Glassy — no crystals (very rapid cooling) — obsidian.
- Vesicular — gas cavities (pumice, scoria, vesicular basalt); amygdaloidal — vesicles filled with secondary minerals.
- Pegmatitic — very coarse crystals.
Classification by silica content
| Group | SiO₂ (%) | Colour | Plutonic | Volcanic |
|---|---|---|---|---|
| Acidic (felsic) | > 65 | Light | Granite | Rhyolite |
| Intermediate | 55–65 | Medium | Diorite (syenite) | Andesite (trachyte) |
| Basic (mafic) | 45–55 | Dark | Gabbro | Basalt |
| Ultrabasic (ultramafic) | < 45 | Very dark/green | Peridotite, dunite | (rare) |
- Acidic rocks are rich in quartz and orthoclase; basic rocks in plagioclase, pyroxene and olivine.
- Dolerite is the hypabyssal equivalent of gabbro/basalt.
Forms of igneous intrusions
| Form | Description | Concordant/discordant |
|---|---|---|
| Batholith | Very large, deep-seated body (often granite), no visible floor | Discordant |
| Stock | Smaller than a batholith | Discordant |
| Laccolith | Mushroom/dome-shaped body that arches overlying strata | Concordant |
| Lopolith | Saucer-shaped body sagging downward | Concordant |
| Phacolith | Lens-shaped body in the crests/troughs of folds | Concordant |
| Sill | Sheet injected parallel to bedding | Concordant |
| Dyke | Wall-like sheet cutting across bedding | Discordant |
| Volcanic neck | Solidified conduit of a volcano | Discordant |
Extrusive forms: lava flows, volcanic cones; flood basalts such as the Deccan Traps (western and central India) — layered basalt flows.
Sedimentary rocks
Formed by weathering → erosion and transportation → deposition → compaction and cementation (lithification) of sediments. They cover most of the earth's land surface but form only a small fraction of the crust by volume.
Classification
| Group | Formation | Examples |
|---|---|---|
| Clastic (mechanical/detrital) | Fragments of pre-existing rocks cemented together | Conglomerate (rounded gravel), breccia (angular fragments), sandstone (sand), siltstone, shale (clay, fissile), mudstone |
| Chemical | Precipitation from solution | Limestone (some), rock salt, gypsum, travertine, chert, dolomite |
| Organic (biogenic) | Accumulation of organic remains | Coal (plant matter), chalk and fossiliferous limestone (shells), coquina |
- Cementing materials in sandstones: siliceous (strongest, most durable), calcareous, ferruginous, argillaceous (weakest).
- Laterite — residual deposit rich in iron and aluminium oxides formed by intense weathering in tropical climates (widely used locally as building blocks in India).
Sedimentary structures
- Bedding (stratification) — layers; the most characteristic feature.
- Cross-bedding — inclined layers (wind/water currents).
- Graded bedding — coarse at bottom to fine at top.
- Ripple marks, mud cracks, rain prints.
- Fossils — remains of organisms (almost exclusively in sedimentary rocks).
- Concretions and nodules.
Metamorphic rocks
Formed by transformation of existing rocks in the solid state due to heat, pressure and chemically active fluids, without melting.
Types of metamorphism
| Type | Agent | Features |
|---|---|---|
| Contact (thermal) | Heat from intrusions | Local aureole; non-foliated rocks — hornfels, marble, quartzite |
| Regional (dynamothermal) | Heat + directed pressure over large areas (mountain building) | Foliated rocks — slate, schist, gneiss |
| Dynamic (cataclastic) | Shearing stress along faults | Crushed rocks — mylonite, fault breccia |
| Hydrothermal/metasomatic | Hot fluids | Chemical change |
Foliated and non-foliated rocks
| Parent rock | Metamorphic rock | Features |
|---|---|---|
| Shale / mudstone | Slate (low grade) → phyllite → schist → gneiss (high grade) | Slate — slaty cleavage (splits into thin sheets); phyllite — silky sheen; schist — schistosity with visible micas; gneiss — gneissic banding of light and dark minerals |
| Granite | Gneiss (granite gneiss) | Banded |
| Basalt / dolerite | Greenschist, amphibolite | Foliated/lineated |
| Limestone / dolomite | Marble | Non-foliated, crystalline, effervesces with acid |
| Sandstone (quartz) | Quartzite | Non-foliated, very hard; breaks through grains |
| Shale (contact) | Hornfels | Fine, hard, non-foliated |
| Coal | Anthracite / graphite |
Increasing metamorphic grade: slate → phyllite → schist → gneiss.
Weathering of rocks
| Type | Processes |
|---|---|
| Physical (mechanical) | Frost wedging (freeze–thaw), exfoliation (sheet jointing of granite from unloading/temperature changes), thermal expansion, salt crystallisation, abrasion, plant roots |
| Chemical | Hydrolysis (feldspar → clay), oxidation (iron minerals), carbonation/solution (limestone dissolution → karst), hydration |
| Biological | Roots, burrowing organisms, lichens (organic acids) |
- Hot humid climates favour chemical weathering (deep residual soils, laterites); cold or arid climates favour physical weathering.
- Spheroidal weathering — rounded boulders in jointed rocks such as basalt and granite.
Engineering properties and uses of rocks
| Rock | Typical engineering character (general) | Uses |
|---|---|---|
| Granite | Very strong, durable, low porosity; takes polish; high quartz makes cutting harder; fire resistance lower than some rocks (quartz expansion) | Foundations, bridges, dams, facing, kerbs, aggregates |
| Basalt / dolerite | Very strong, dense, tough | Aggregates, road metal, rubble masonry, ballast |
| Sandstone | Strength depends on cementing material; porous | Building stone, masonry (e.g. red sandstone monuments) |
| Limestone | Moderate strength; soluble — cavities; varies widely | Cement manufacture, lime, building stone, aggregates (if sound) |
| Shale | Weak, slakes on wetting and drying, low permeability, poor foundation/tunnel rock | Brick/cement raw material |
| Marble | Moderate strength, attractive, soluble in acids | Flooring, decoration |
| Quartzite | Very hard and strong, abrasive | Aggregates, ballast (hard to crush) |
| Slate | Splits into thin durable sheets, impervious | Roofing, flooring, DPC |
| Gneiss / schist | Gneiss strong; schist weak along foliation | Gneiss — building stone; schist — often unsuitable where planes are unfavourable |
| Laterite | Soft when quarried, hardens on exposure | Local building blocks, road material |
Requirements of good building stone
High crushing strength, durability (resistance to weathering), low water absorption (commonly less than about 5% by weight for good stones), hardness and toughness (for road and paving use), appearance, workability, fire resistance, freedom from defects (cracks, cavities, soft patches) and availability.
Worked examples
Worked ExampleExample 1 — classification
A coarse-grained, light-coloured igneous rock contains about 72% silica with abundant quartz and orthoclase. Name it and its volcanic equivalent.
Solution. Acidic (> 65% SiO₂), plutonic → granite; volcanic equivalent → rhyolite.
Worked ExampleExample 2 — water absorption
A dry stone specimen weighs 2.50 kg; after 24 h immersion it weighs 2.56 kg. Find water absorption.
Solution. 2.4% — low absorption, suitable as building stone.
Worked ExampleExample 3 — parent rocks
Identify the parent rocks of marble, quartzite, slate and gneiss.
Solution. Marble — limestone; quartzite — sandstone; slate — shale; gneiss — granite (or other feldspathic rocks).
Frequently tested points
- Igneous: plutonic (coarse — granite, gabbro), hypabyssal (dolerite), volcanic (fine — basalt, rhyolite); obsidian glassy, pumice vesicular.
- Silica: acidic > 65% (granite/rhyolite), intermediate 55–65% (diorite/andesite), basic 45–55% (gabbro/basalt), ultrabasic < 45% (peridotite).
- Sill concordant; dyke discordant; laccolith dome-shaped; lopolith saucer-shaped; batholith large deep body.
- Deccan Traps — flood basalts.
- Sedimentary: clastic (conglomerate rounded, breccia angular, sandstone, shale), chemical (rock salt, gypsum, limestone), organic (coal, chalk); bedding and fossils characteristic.
- Siliceous cement strongest in sandstones.
- Metamorphic: shale → slate → phyllite → schist → gneiss; limestone → marble; sandstone → quartzite; contact metamorphism → hornfels.
- Weathering: physical (frost, exfoliation), chemical (hydrolysis, oxidation, carbonation), biological.
- Shale slakes and is weak; limestone solution cavities; granite and basalt strong.
Common MistakeCommon mistakes
- Calling a sill discordant or a dyke concordant.
- Treating all sandstones as equally strong regardless of cement.
- Confusing quartzite (metamorphic) with sandstone (sedimentary) — quartzite breaks through grains.
Revision SummaryChapter summary
- The rock cycle links igneous, sedimentary and metamorphic rocks through melting, weathering, deposition and metamorphism.
- Igneous rocks are classified by mode of occurrence, texture and silica content, forming bodies such as batholiths, sills and dykes.
- Sedimentary rocks — clastic, chemical and organic — show bedding, fossils and other structures.
- Metamorphism by heat, pressure and fluids produces foliated (slate, schist, gneiss) and non-foliated (marble, quartzite) rocks.
- Weathering and rock properties govern suitability for foundations, building stones and aggregates.