The Rock Cycle Quiz
Three kinds of rock
Igneous rock forms when molten rock cools and hardens. Magma that cools slowly underground grows large crystals, as in granite. Lava that cools quickly at the surface makes fine-grained basalt, or even glassy obsidian.
Sedimentary rock forms from layers of sand, mud, pebbles or shells that pile up, usually under water, and are pressed and cemented together. Sandstone, shale and limestone are examples, and most fossils are found in these rocks.
Metamorphic rock is rock that has been changed by heat and pressure without fully melting. Limestone becomes marble, shale becomes slate, and granite can become gneiss.
How rocks change
Rock at the surface is slowly broken down where it stands by rain, frost, plant roots and chemicals. That's weathering. Wind, rivers, glaciers and waves then carry the pieces away, which is erosion, and drop them somewhere else as sediment.
Buried under new layers, the sediment is squeezed and glued together by minerals, a process called compaction and cementation. Pushed deeper still, any rock can be heated and squeezed into metamorphic rock, and if it gets hot enough it melts into magma.
The cycle has no fixed order. An igneous rock can be weathered into sediment, a metamorphic rock can melt, and a sedimentary rock can be lifted and worn away again.
What drives the cycle
Two engines keep rocks moving. Heat from inside Earth melts rock and drives plate tectonics, which pushes rock down into the mantle and lifts it up into mountains. Energy from the Sun drives the weather and the water that wear rock down.
The changes are very slow. A single trip around the cycle can take hundreds of millions of years.
The Scottish geologist James Hutton was among the first to see this in the 1700s. Looking at layers of rock, he realized the Earth must be extremely old, with 'no vestige of a beginning, no prospect of an end'.