Photographs and text describe how to recognise each rock in the field.
Igneous rocks – formed by cooling of magma. In this case, texture, composition and acidity are important. Acid igneous rocks, such as granite and rhyolite, have over 65% total silica. Often they are pale in colour and have low density. By contrast basic igenous rocks, such as basalt and doleriate, have 45-55% total silica.
Sedimentary rocks – formed from grains worn off older rocks, by weathering and erosion. They split into detrital, organic and chemical sedimentary rocks. Therefore grain size and shape, colour and composition are useful clues.
Metamorphic rocks – formed by heat or pressure on older rocks. Rocks produced by contact metamorphism, such as marble and hornfels, are crystalline. On the other hand, rocks produced by regional metamorphism may be less strongly changed.
In essence a rock is an aggregate of mineral particles. These minerals are chemical compounds or single elements, often with crystalline shapes. Silicon, oxygen, aluminium, iron, calcium and sodium are some of the commonest rock-forming elements in the Earth’s crust. In turn the minerals making up rocks include silicates (like feldspars and micas), quartz and calcite.