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The Rock Cycle
Igneous, metamorphic, and sedimentary rocks
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The rock cycle explains how Earth continually recycles solid materials into igneous, sedimentary, and metamorphic rocks. It matters because rocks store clues about volcanoes, oceans, mountains, climate, and ancient life. The cycle connects surface processes like weathering and erosion with deep Earth processes like heat, pressure, melting, and cooling. Instead of moving in one perfect circle, rock material can follow many different paths over millions of years.
Igneous rocks form when melted rock cools and crystallizes, either underground as magma or above ground as lava. Sedimentary rocks form when weathered pieces of rock are eroded, deposited in layers, compacted, and cemented. Metamorphic rocks form when existing rocks are changed by heat and pressure without fully melting. Plate movement, mountain building, volcanoes, rivers, and oceans all help move rock material through the cycle.
Key Facts
- Igneous rock forms when magma or lava cools and solidifies.
- Sedimentary rock forms by weathering, erosion, deposition, compaction, and cementation.
- Metamorphic rock forms when heat and pressure change existing rock without melting it.
- Weathering breaks rock into sediment, while erosion moves sediment by water, wind, ice, or gravity.
- Melting turns rock into magma, and cooling turns magma or lava into igneous rock.
- The rock cycle has many possible paths, so any rock type can eventually become another rock type.
Vocabulary
- Igneous rock
- Rock that forms when melted rock cools and hardens into solid crystals.
- Sedimentary rock
- Rock that forms from layers of sediment that are pressed and cemented together over time.
- Metamorphic rock
- Rock that forms when an existing rock is changed by heat and pressure without completely melting.
- Weathering
- The breakdown of rock into smaller pieces by water, wind, ice, temperature changes, plants, or chemicals.
- Deposition
- The process in which sediment settles out of water, wind, ice, or gravity and builds up in a new location.
Common Mistakes to Avoid
- Thinking the rock cycle always moves in one fixed order is wrong because rock material can skip steps, repeat steps, or take many different paths.
- Confusing weathering with erosion is wrong because weathering breaks rock apart, while erosion transports the broken pieces.
- Saying metamorphic rock forms by melting is wrong because full melting makes magma, which cools into igneous rock instead.
- Assuming all rocks change quickly is wrong because many rock cycle processes take thousands to millions of years, although events like volcanic eruptions can happen quickly.
Practice Questions
- 1 A lava flow cools on Earth’s surface after a volcanic eruption. What type of rock forms, and what process caused it to form?
- 2 A river carries 500 kg of sediment toward an ocean basin each day. How much sediment does it carry in 30 days if the rate stays constant?
- 3 A sandstone layer is buried deep underground and exposed to high heat and pressure, but it does not melt. Explain what type of rock it may become and why.