What Is the Rock Cycle?
How rocks transform over millions of years — a cycle of melting, cooling, pressure, and erosion.
Rocks Are Always Changing
Rocks seem permanent — they're literally the symbol of things that don't change. But on a geological timescale (millions of years), rocks are constantly transforming. They melt underground, cool into new forms, get crushed by mountain-building forces, weather into sand, compress into layers, and melt again. This endless cycle of transformation is called the rock cycle.
There are three main types of rock, and each one can transform into either of the other two. Understanding the rock cycle means understanding how and why these transformations happen.
Igneous Rocks: Born from Fire
Igneous rocks form when molten rock (magma underground, lava on the surface) cools and solidifies. The word "igneous" comes from the Latin word for fire. When magma cools slowly underground, large crystals have time to form — creating rocks like granite with visible specks and patterns. When lava cools quickly on the surface, crystals don't have time to form — creating smooth rocks like obsidian (volcanic glass) or basalt.
Examples: granite, basalt, obsidian, pumice.
Sedimentary Rocks: Built in Layers
When wind, water, and ice break down existing rocks into tiny pieces (sediment), those pieces eventually settle in layers — often at the bottom of lakes, rivers, and oceans. Over millions of years, the weight of layer upon layer compresses the sediment together, and minerals in the water cement the grains. This process, called lithification, creates sedimentary rocks.
Sedimentary rocks are special because they're the only type that can contain fossils. When ancient organisms died and were buried in sediment, their remains were preserved as the sediment turned to rock. Every fossil you've ever seen was found in sedimentary rock.
Examples: sandstone, limestone, shale, conglomerate.
Metamorphic Rocks: Changed by Heat and Pressure
When any rock — igneous, sedimentary, or even another metamorphic rock — is exposed to intense heat and/or pressure deep underground, its minerals rearrange without melting. The rock transforms into a new type with different properties. This is metamorphism (meta = change, morph = form). The original rock is called the "parent rock."
Limestone becomes marble. Shale becomes slate. Granite becomes gneiss. Each transformation happens because extreme conditions force minerals to reorganize into more stable arrangements.
Examples: marble, slate, quartzite, gneiss.
The Cycle Never Stops
Here's the key insight: any rock type can become any other rock type. Igneous rock can weather into sediment and become sedimentary. Sedimentary rock can get buried deep enough to become metamorphic. Metamorphic rock can melt and cool into igneous. The arrows in the rock cycle go in every direction — it's not a one-way conveyor belt but a web of possible transformations.
The oldest rocks ever found on Earth are about 4 billion years old — but Earth itself is 4.5 billion years old. The missing 500 million years of rock? It was recycled by the rock cycle. Those ancient rocks were melted, weathered, and transformed into the rocks we see today. The rock cycle has been erasing and rewriting Earth's geological history since the planet formed.
Last reviewed: April 2026