Rocks & Rock Cycle
Notes
A rock is a natural aggregate of one or more minerals. The crust consists of three major rock types.
1. Igneous rocks (primary rocks)
- Formed by the cooling and solidification of magma or lava. Non-stratified, no fossils, generally hard and crystalline.
- Intrusive (plutonic): cool slowly beneath the surface, large crystals: granite, gabbro, diorite, pegmatite.
- Extrusive (volcanic): cool quickly on the surface, fine-grained or glassy: basalt, rhyolite, andesite, obsidian. Hypabyssal rocks (e.g., dolerite) form at intermediate depths.
- By silica content: acidic (more than about 65% silica: granite, rhyolite) and basic (less than about 55%: basalt, gabbro).
2. Sedimentary rocks
- Formed by the deposition, compaction and cementation of sediments; stratified (layered) and the only type rich in fossils. They cover about 75% of the Earth's surface area but make up only about 5% of the crust by volume.
- Mechanically formed (clastic): sandstone, conglomerate, shale, loess.
- Organically formed: limestone, coal, peat, chalk.
- Chemically formed: gypsum, rock salt, potash, dolomite.
3. Metamorphic rocks
- Existing rocks changed by heat, pressure and chemically active fluids without melting. Contact (thermal) and regional (dynamic) metamorphism.
- Examples: granite to gneiss; sandstone to quartzite; limestone to marble; shale to slate, then phyllite and schist; coal to graphite (via anthracite).
Rock cycle: a continuous process in which rocks change from one type to another: igneous rocks are weathered into sediments, which form sedimentary rocks; heat and pressure turn them into metamorphic rocks; melting returns them to magma, which cools to igneous rocks. Any rock can enter the cycle at any stage.
Basis | Igneous | Sedimentary | Metamorphic |
|---|---|---|---|
Origin | Cooling of magma | Deposition of sediments | Alteration by heat and pressure |
Structure | Crystalline, non-layered | Layered (strata) | Foliated or non-foliated |
Fossils | Absent | Present | Usually destroyed |
Examples | Granite, basalt | Sandstone, limestone | Marble, slate, gneiss |
Indian context: the Archaean (gneiss and granite) is the oldest; the Dharwar system holds iron, manganese and gold; the Gondwana system holds coal; the Deccan Trap (basalt) gives black soil; the Vindhyan system gives sandstone and limestone.
Key Points
- Igneous: primary; sedimentary: layered with fossils; metamorphic: altered.
- Granite (intrusive) and basalt (extrusive).
- Limestone to marble; sandstone to quartzite; shale to slate; granite to gneiss.
- Sedimentary rocks cover about 75% of the surface.
- Rock cycle is continuous.
MCQs
Q1. Fossils are found mostly in:
(a) Igneous rocks
(b) Sedimentary rocks
(c) Metamorphic rocks
(d) Volcanic rocks
Ans: (b).
Q2. Limestone is metamorphosed into:
(a) Slate
(b) Quartzite
(c) Marble
(d) Gneiss
Ans: (c).
Q3. Granite is an example of:
(a) Extrusive igneous rock
(b) Intrusive igneous rock
(c) Sedimentary rock
(d) Metamorphic rock
Ans: (b).