Rocks and minerals

A mineral is a naturally occurring solid with a definite chemical make-up and, usually, a crystal structure. Quartz, feldspar, mica and calcite are common minerals. A rock is a naturally occurring solid made of one or more minerals. Granite, for example, is made mostly of quartz, feldspar and mica.

Geologists group rocks into three types according to how they formed: igneous, sedimentary and metamorphic.

Igneous rocks

Igneous rocks form when molten rock cools and becomes solid. Molten rock underground is called magma; when it reaches the surface, for example in a volcanic eruption, it is called lava. Igneous rocks are made of interlocking crystals and do not contain fossils.

Sedimentary rocks

Sedimentary rocks form from sediments: small pieces of rock, sand, mud, and the shells and remains of living things. They form in a series of steps:

1. Weathering breaks rocks down into smaller pieces (by water, ice, temperature changes, plant roots or chemical reactions).
2. Erosion and transport carry the pieces away, by water, wind, ice or gravity. Pieces become more rounded as they are carried.
3. Deposition: the sediments settle in layers, often at the bottom of lakes and seas.
4. Compaction: the weight of the layers above squeezes the lower layers and pushes out water.
5. Cementation: dissolved minerals crystallise between the grains and glue them together into solid rock.

Sedimentary rocks often show layers (strata) and are the rocks most likely to contain fossils. In undisturbed layers, the oldest layer is at the bottom and the youngest is at the top.

Metamorphic rocks

Metamorphic rocks form when an existing rock (igneous, sedimentary or even another metamorphic rock) is changed by great heat and/or pressure deep underground, without melting. The minerals change and grow into new crystals, and the rock often becomes harder. High pressure can line up the minerals in bands or flat layers.

The rock cycle

The rock cycle describes how rocks slowly change from one type to another over very long periods of time. There is no fixed order: any type of rock can become any other type.

• Any rock at the surface can be weathered and eroded into sediments, which can form sedimentary rock.
• Any rock buried deep underground can be changed by heat and pressure into metamorphic rock.
• Any rock that melts becomes magma; when the magma cools and solidifies it forms igneous rock.
• Uplift (from movements of Earth's crust) can bring deeply buried rocks back to the surface, where the cycle continues.

Check yourself

1. What is the difference between magma and lava?
2. Granite has large crystals and basalt has small crystals. Explain why.
3. List the five steps that turn rock fragments into sedimentary rock.
4. Which type of rock is most likely to contain fossils, and why are fossils not found in igneous rock?
5. What metamorphic rock forms from limestone? From shale?
6. A rock is made of rounded pebbles stuck together in finer material. Name the rock and its type, and explain what the rounded pebbles tell you.
7. Does a rock melt when it becomes a metamorphic rock? Explain.
8. Describe how an igneous rock could become a sedimentary rock.

Answers: 1. Both are molten rock; magma is underground and lava is molten rock that has reached the surface. 2. Granite cooled slowly underground, giving crystals time to grow large; basalt cooled quickly at the surface, so only small crystals formed. 3. Weathering, erosion (and transport), deposition, compaction, cementation. 4. Sedimentary rock. Remains of living things are buried in the sediment and preserved as the rock forms; igneous rock forms from molten rock, which would destroy any remains. 5. Marble; slate. 6. Conglomerate, a sedimentary rock. Rounded pebbles show the pieces were worn smooth while being carried, usually by flowing water. 7. No. It is changed by heat and pressure while it stays solid; if it melted, it would become magma and then igneous rock. 8. It is exposed at the surface, weathered into small pieces, eroded and carried away, deposited in layers, then compacted and cemented into sedimentary rock.