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Rocks & the Rock Cycle

Rocks & the Rock Cycle Rocks are aggregates of one or more minerals. Every rock on Earth belongs to one of three families, and any rock can transform into any other type through the rock cycle . The Three Rock Types Igneous Rocks "Fire Rocks" Formed from the cooling and crystallization of molten rock magma or lava . - Intrusive plutonic : Cooled slowly underground → large crystals. Examples: granite, diorite, gabbro - Extrusive volcanic : Cooled quickly at surface → small or no crystals. Examples: basalt, rhyolite, obsidian Sedimentary Rocks "Layered Rocks" Formed from accumulated sediment that is compacted and cemented over time. - Clastic : Made of rock fragments. Examples: sandstone, shale, conglomerate - Chemical : Precipitated from solution. Examples: limestone some , rock salt, chert - Biochemical/Organic : Made from biological material. Examples: coal, coquina, chalk Sedimentary rocks cover about 75% of Earth's land surface but make up only 5% of the crust by volume. Metamorphic Rocks "Changed Rocks" Formed when existing rocks are transformed by heat, pressure, or chemically active fluids — without melting. - Foliated : Minerals aligned in layers. Examples: slate, schist, gneiss - Non-foliated : No layering. Examples: marble, quartzite, hornfels The Rock Cycle The rock cycle describes how rocks transform from one type to another: 1. Igneous rocks weather and erode into sediment 2. Sediment compacts and cements into sedimentary rock 3. Sedimentary rock buried deep enough undergoes metamorphism → metamorphic rock 4. Metamorphic rock melts → magma → cools into new igneous rock 5. Any rock type can weather back into sediment or melt into magma The cycle has no beginning or end — it's been running for over 4 billion years. Why This Matters Understanding rock types is the foundation for: - Geological mapping - Mineral exploration - Engineering and construction - Understanding Earth's history recorded in rock layers

Verification resources

Cross-check terminology, classification, methods, and safety with these authoritative external resources.

  • Geology education — U.S. Geological Survey. Government educational material on Earth structure, geologic time, rocks, minerals, and plate tectonics.
  • The life cycle of a mineral deposit — U.S. Geological Survey. A scientific teaching guide connecting geologic processes, mineral deposits, extraction, use, and reclamation.

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