Types of Rocks
All rocks fall into three categories based on how they form. Each type tells a geological story.
Igneous Rocks
"Fire rocks" — formed from cooled magma or lava.
- Cool slowly underground
- Large crystals
- Examples: granite, diorite, gabbro
- Cool quickly on surface
- Small crystals or glassy
- Examples: basalt, obsidian, pumice
Sedimentary Rocks
Formed from accumulated sediments — weathered rock fragments, organic material, or chemical precipitates.
- Sandstone, shale, conglomerate
- Limestone (often from shells), rock salt
- Coal, some limestone
Sedimentary rocks often contain fossils and form in layers (strata).
Metamorphic Rocks
"Changed rocks" — formed when existing rocks are transformed by heat and/or pressure.
- Limestone → Marble
- Shale → Slate
- Granite → Gneiss
Metamorphism can occur near magma chambers (contact metamorphism) or in mountain-building zones (regional metamorphism).
The Rock Cycle
Rocks continuously transform:
1. Igneous rock weathers into sediments
2. Sediments lithify into sedimentary rock
3. Heat/pressure transforms rock into metamorphic rock
4. Melting creates magma, which cools into igneous rock
No rock is permanent. Given enough time, any rock can become any other type.
Related Reading
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