Rocks are natural solids made of one or more minerals. They are grouped into three main types by how they form.
The three rock types
- Igneous rock forms when molten magma or lava cools and hardens. If it cools slowly deep underground it forms coarse intrusive rock such as granite; if lava cools quickly at the surface it forms fine extrusive rock such as basalt.
- Sedimentary rock forms when sediments are deposited in layers, then compacted and cemented over long periods. Examples are limestone, sandstone and shale. It is often layered and may contain fossils.
- Metamorphic rock forms when existing rock is changed by great heat and pressure without melting. Limestone becomes marble, shale becomes slate, and sandstone becomes quartzite.
Key idea
Igneous = cooled magma/lava; sedimentary = compacted layers of sediment; metamorphic = rock changed by heat and pressure.
Importance of rocks
Example
In Malaysia, granite is quarried for construction, sedimentary layers yield tin and petroleum, and limestone hills such as those at Ipoh and Langkawi form caves that attract tourists.
Rocks are also important because they weather to form soil for agriculture, supply minerals and building stone, store groundwater, and create scenic landscapes that support tourism and recreation. Different rocks give different resources: igneous granite is crushed for roads and buildings, sedimentary rocks hold coal, petroleum and tin ore, and metamorphic marble is prized as a decorative stone. Studying rocks therefore helps us plan quarrying, farming and construction wisely.
Remember
- Granite is igneous; limestone is sedimentary; marble is metamorphic.
- Sedimentary rocks are layered and may hold fossils.
- Rocks provide minerals, building material, soil and tourism sites.