Rock Cycle

Igneous, Sedimentary, and Metamorphic Rocks

Related Labs

The rock cycle describes the continuous transformation of rocks from one type to another over geological time. It begins with magma — molten rock in Earth's interior. When magma cools and solidifies, it forms igneous rock (e.g., granite if slow cooling underground, basalt if fast cooling at the surface). Surface igneous rock is broken down by weathering and erosion into sediments (gravel, sand, silt, clay) that are transported and deposited in layers.

Over millions of years, accumulated sediment layers are compacted and cemented into sedimentary rock (sandstone, shale, limestone). When any rock type is subjected to high heat and pressure deep in the crust — without melting — it transforms into metamorphic rock (marble from limestone, slate from shale, quartzite from sandstone). If temperatures are high enough, any rock type can melt, completing the cycle. The cycle has no fixed starting point; each rock type can form from any other.

Key Facts

Three rock types: igneous (cooled magma/lava), sedimentary (compacted sediments), metamorphic (heat + pressure without melting)
Intrusive igneous: slow cooling underground → large crystals (granite); extrusive: fast cooling at surface → small crystals (basalt)
Weathering → erosion → deposition → compaction/cementation → sedimentary rock
Metamorphism: existing rock recrystallizes under heat and pressure; parent rock is called protolith
Foliation: parallel mineral alignment in metamorphic rock produced by directed pressure (e.g., slate, schist)
Melting any rock type produces magma; cooling produces new igneous rock

Vocabulary

Igneous rock: Rock formed by the cooling and solidification of magma or lava; classified as intrusive (underground) or extrusive (surface).
Sedimentary rock: Rock formed from compressed and cemented layers of sediment (fragments of other rocks, minerals, or organic material).
Metamorphic rock: Rock that has been transformed by heat, pressure, or chemical activity without melting, changing its mineral composition or texture.
Weathering: The physical or chemical breakdown of rock at Earth's surface into smaller particles (sediment).
Lithification: The process by which loose sediment is converted into sedimentary rock through compaction and cementation.

Common Mistakes to Avoid

Thinking the rock cycle has a fixed starting point. Any rock type can transform into any other — the cycle is truly circular. Magma is not the mandatory starting point.
Assuming metamorphic rock forms from melting. Metamorphism occurs below the melting point — the rock recrystallizes in the solid state. Melting would produce magma, leading to igneous rock instead.
Confusing intrusive and extrusive igneous rocks and their crystal sizes. Slow underground cooling gives large crystals (granite); fast surface cooling gives small crystals (basalt) or glassy texture (obsidian).
Thinking sedimentary rocks only form in the ocean. Sedimentary rocks form wherever sediments accumulate: rivers, lakes, deserts (windblown sand), and marine environments.

Practice Questions

1 Trace the path of a granite crystal from its formation as magma through at least two complete transformations in the rock cycle.
2 Why does granite have large visible crystals while basalt (same chemical composition, different origin) has tiny crystals?
3 Limestone is a sedimentary rock. What metamorphic rock does it become under heat and pressure, and what characteristic texture does it gain?