Every rock on Earth’s surface belongs to one of just three families — and each family reveals a different chapter of how that rock came to exist.
MCQ Questionsâś… Must Know
The three rock families
- All rocks fall into three families based on how they form: igneous, sedimentary, and metamorphic. Each forms through a distinct process.
Igneous rock: from melted material
- Igneous rock forms when magma or lava cools and solidifies. Granite forms slowly underground; basalt forms quickly at the surface.
Sedimentary rock: from accumulated material
- Sedimentary rock forms when sediment, mineral grains, or organic matter accumulate in layers and get compacted and cemented together. Sandstone and limestone are common examples.
Metamorphic rock: from transformed material
- Metamorphic rock forms when heat and pressure transform existing rock without melting it. Marble forms this way from limestone; slate forms from shale.
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Intrusive vs extrusive igneous rock
- Igneous rock that cools slowly underground is called intrusive (or plutonic) and has large, visible crystals, as in granite. Igneous rock that cools quickly at the surface is extrusive (or volcanic) and has small, fine crystals, as in basalt.
Clastic vs chemical/organic sedimentary rock
- Clastic sedimentary rock forms from broken rock fragments, like sandstone from sand grains. Chemical or organic sedimentary rock forms from precipitated minerals or accumulated organic matter, like limestone from shell debris or coal from plant material.
Layering and fossils
- Sedimentary rock typically shows visible layering, called stratification, since each layer marks a separate depositional event. It is also the only rock type that commonly preserves fossils.
Foliated vs non-foliated metamorphic rock
- Foliated metamorphic rock, like schist and gneiss, develops banded or layered texture under directed pressure. Non-foliated metamorphic rock, like marble and quartzite, recrystallizes without developing bands.
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The rock cycle links all three
- The three rock types are not fixed categories — they connect through the rock cycle. Weathering and erosion break any rock into sediment, which can become sedimentary rock. Heat and pressure can turn any rock into metamorphic rock. Enough heat melts any rock into magma, which cools into new igneous rock.
Igneous rock and mineral wealth
- Igneous activity concentrates many metallic ores as magma cools and crystallizes. India’s copper and associated metal belts, and the Deccan Trap basalts themselves, both trace back to igneous processes.
Sedimentary rock and fossil fuels
- Sedimentary basins host most of the world’s coal, oil, and natural gas, since these fuels form from buried organic matter under sedimentary layers. Sedimentary rock also hosts major groundwater aquifers.
Metamorphic rock and building stone
- Metamorphic rocks supply valued building and dimension stone. Marble is prized for monuments and flooring; slate is used for roofing tiles; both owe their properties to the metamorphic process itself.
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