Rocks change their form over very long time periods in a process described by the rock cycle. Here's one possible journey through the rock cycle:
Mountains are weathered and eroded by rain, wind and ice. Some of the resulting broken down rock, in the form of sand, is transported by rivers to the ocean and deposited in layers that build up over time. The loose sand can be transformed into sedimentary rock, sandstone, in part by the increased pressure that results from burial.
If the sedimentary rock is buried further, the temperature and pressure rises, and the minerals in rock can start to change form; this process is called metamorphism and these new rocks are called metamorphic rocks. At very high temperatures rocks can melt and become magma. When the magma cools, either in the Earth's crust or at the surface, it forms igneous rock.
Image: Sandstone in the Paria Wilderness Area, Arizona (credit: Adam Jones/SPL)
Ray Mears explains how the Malvern Hills provide pure water.
Ray Mears visits the Malvern Hills and explains their geological history and why the region has such pure springwater.
Kate Humble visits Dolaucothi mine in Wales.
Kate Humble visits Dolaucothi mine in Wales to see rocks that are rich in gold. Expert Alwyn Annals explains the history of the rocks that now form the mine. Mineral-rich hot water concentrated the gold in the rocks of Dolaucothi mine when they were crushed and folded by a collision between continents millions of years ago.
The rock cycle is a basic concept in geology that describes the dynamic transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous. As the diagram to the right illustrates, each of the types of rocks is altered or destroyed when it is forced out of its equilibrium conditions. An igneous rock such as basalt may break down and dissolve when exposed to the atmosphere, or melt as it is subducted under a continent. Due to the driving forces of the rock cycle, plate tectonics and the water cycle, rocks do not remain in equilibrium and are forced to change as they encounter new environments. The rock cycle is an illustration that explains how the three rock types are related to each other, and how processes change from one type to another over time.