Every change is real: granite weathers into sand, sand settles and cements into sandstone, sandstone becomes quartzite under heat and pressure, and melting gives magma that cools again into granite or basalt. Tap the rock to inspect its crystals or layers. Densities and temperatures are typical values, and depths are approximate.
- What are the stages of the rock cycle?
- Magma cools to form igneous rock. Weathering and erosion break any rock into sediment, which is carried away and deposited in layers. Compaction and cementation turn the sediment into sedimentary rock. Heat and pressure deep underground turn rock into metamorphic rock, and if it melts it becomes magma again.
- What is the difference between igneous, sedimentary and metamorphic rock?
- Igneous rock forms when magma or lava cools and crystallises. Sedimentary rock forms from layers of sediment that are compacted and cemented together. Metamorphic rock forms when existing rock is changed by heat and pressure without melting, so it becomes harder and often banded.
- How does sedimentary rock form?
- Rock is weathered into fragments, the fragments are carried by water or wind and settle in layers, and new layers bury them. The weight squeezes out water, and minerals such as calcite or silica grow between the grains and cement them. Sand becomes sandstone and shell fragments become limestone.
- How does metamorphic rock form?
- Heat and pressure, usually 10 to 20 km down, recrystallise the minerals in a rock without melting it. Granite becomes gneiss, limestone becomes marble, shale becomes slate and sandstone becomes quartzite.
- Why does granite have large crystals and basalt small ones?
- Granite cools slowly, several km underground, so its crystals have centuries to grow. Basalt is lava that cools within days at the surface, so its crystals are too small to see without a lens, and gas escapes to form holes.
- Does rock really go round the cycle?
- Yes, over millions of years. A rock may skip stages: a granite can be uplifted and weathered without ever melting, while another can be buried and melted in a single cycle. Matter is never created or destroyed, only changed.