Summary

• A rock is a naturally occurring solid object composed of one or more minerals.

• Rocks are grouped into three main categories based on how they are formed:

• Igneous rocks are formed when magma or lava cools and solidifies.

• They are generally hard and strong as they are composed of interlocking crystals.

• Sedimentary rocks are formed by the compaction and cementation of sediments or the precipitation of dissolved minerals.

• They are generally soft and easily broken apart as they are composed of non-interlocking grains.

• Metamorphic rocks are formed when existing rocks undergo a transformation under high pressure and/or high temperature.

• They are generally very hard and strong as they are composed of compressed, interlocking crystals.

• Rocks can also be classified based on their physical structure or their chemical composition.

• A mineral is a naturally occurring solid substance with a defined chemical composition and crystalline structure.

• Minerals may can be elements or compounds. Minerals that are compounds are named differently to the chemical compounds they are composed of.

• The rock cycle is a series of geological processes that represent transitions between igneous, sedimentary and metamorphic rocks.

• The rock cycle includes processes that occur on Earth’s surface and processes that occur beneath Earth’s surface.

• Most of the processes that are part of the rock cycle take place over very long periods of time.

• The key processes involved in the rock cycle are:

• Melting – the formation of magma.

• Crystallisation – the formation of igneous rock.

• Weathering and erosion – the formation of sediments.

• Deposition and lithification – the formation of sedimentary rock.

• Metamorphism – the formation of metamorphic rock.

• Rocks have been used by humans for thousands of years, for their physical properties and for the minerals that they contain.

• The study of rock strata reveals information about past geological events, such as volcanoes, earthquakes and other tectonic activity.

• The study of fossils found in rocks reveals information the evolution of life.

• The geological time scale is a timeline that divides Earth history into intervals that correspond with major geological and biological events.

(Image: Dieter Mueller, Wikimedia Commons)