Formation of rocks

Igneous rocks are formed when molten rock cools down and solidifies. This molten rock is called magma when it's beneath the Earth's surface and lava when it erupts onto the surface. The size of the crystals in an igneous rock tells a story about how it was formed. If the magma cools slowly deep underground (intrusive), large crystals have time to grow, like in granite. If lava cools quickly on the surface (extrusive), only small crystals can form, like in basalt. Because they are formed from molten material, igneous rocks are crystalline and do not contain fossils.

Sedimentary rocks are formed from pieces of other rocks, or from the remains of living things. The process happens in stages. First, existing rocks are broken down into small particles called sediment by weathering (e.g., by rain, wind, frost). These sediments are then moved by erosion and transport (e.g., by rivers, glaciers). When the water or wind slows down, the sediments are dropped in a process called deposition, often forming layers. Over millions of years, the weight of overlying layers causes compaction, squeezing the water out. Finally, dissolved minerals act like glue and cement the particles together, forming a solid rock. Because they form in layers and at low temperatures, sedimentary rocks are the only type of rock that can contain fossils.

Metamorphic rocks are rocks that have been 'changed' (the word 'metamorphosis' means to change form). They start out as either igneous, sedimentary, or even other metamorphic rocks. When these parent rocks are subjected to intense heat (over 150°C) and/or high pressure, deep within the Earth, they change. Importantly, the rock does not melt. Instead, the minerals within the rock recrystallise and realign, often forming layers or bands. This makes the rock harder and more dense than the original. For example, the sedimentary rock limestone changes into the metamorphic rock marble, and the sedimentary rock shale changes into slate.

The rock cycle is a model that describes how rocks are continuously created, changed, and destroyed. It shows that the three main rock types—igneous, sedimentary, and metamorphic—are not permanent and can transform into one another through various geological processes. For example, any rock on the surface can be weathered and eroded to form sediments, which then become sedimentary rock. Any rock can be buried deep in the Earth and changed by heat and pressure to become a metamorphic rock. If the heat is great enough, any rock can melt to form magma, which will then cool to become an igneous rock. The cycle has no beginning or end and shows how materials are recycled over geological time.