Earth Science · Lesson 08

The Rock Cycle

Rocks are not permanent objects. They are snapshots in a slow planetary recycling system driven by heat, pressure, water, weathering, erosion, melting, cooling, and plate tectonics.

Igneous Sedimentary Metamorphic Weathering Plate Tectonics

Cycle Model Click each rock type or process to trace the pathway.

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What Is the Rock Cycle?

The rock cycle is the set of processes that transform rocks from one type into another. It does not move in only one direction. Igneous rock can weather into sediment, sedimentary rock can metamorphose, metamorphic rock can melt, and any exposed rock can be broken down by weathering and erosion.

The cycle is powered by two main engines: Earth's internal heat, which drives melting, metamorphism, and plate tectonics; and solar energy plus gravity, which drive weathering, erosion, transport, and deposition at the surface.

Cooling Magma or lava solidifies into igneous rock.

Weathering Rock breaks into sediment through water, ice, wind, roots, and chemistry.

Compaction Buried sediments squeeze together and cement into sedimentary rock.

Metamorphism Heat and pressure change rock without fully melting it.

Igneous Rocks

Igneous rocks form when molten rock cools. If magma cools slowly underground, large crystals can grow, as in granite. If lava cools quickly at the surface, crystals are small or glassy, as in basalt or obsidian.

Sedimentary Rocks

Sedimentary rocks form from particles, minerals, shells, or dissolved material deposited in layers. Over time, pressure compacts the layers and minerals cement the grains together. Sandstone, limestone, and shale are common examples.

Metamorphic Rocks

Metamorphic rocks form when existing rock is changed by heat, pressure, or chemically active fluids. Limestone can become marble; shale can become slate; granite can become gneiss.

Why Plate Tectonics Matters

Plate boundaries act like rock-cycle machines. At divergent boundaries, magma rises and forms new igneous crust. At convergent boundaries, rocks are buried, squeezed, metamorphosed, melted, and recycled into volcanoes. Mountain building lifts rocks high enough for weathering to attack them.

Big idea: A rock's type tells a story about the environment where it formed.

Field Clues

Crystals: often point to igneous or metamorphic formation.
Layers: often indicate sedimentary deposition.
Foliation: bands or sheets from pressure in metamorphic rock.
Fossils: usually found in sedimentary rock.

Practice Check

1. Which rock type forms from cooled magma or lava?

2. What process changes rock with heat and pressure without fully melting it?

Class Challenge

Choose a rock sample or photo. Identify three clues: texture, layering or foliation, and possible minerals. Then explain the rock-cycle path that could have produced it.