Three Types of Rock
A rock is a naturally formed solid that makes up the Earth's crust and is made of one or more minerals. Based on how they form, rocks are classified into three main types.
Igneous rock forms when magma or lava cools and solidifies. Granite cools slowly deep underground and has coarse grains, while basalt cools quickly at the surface and has fine grains. Igneous rock is hard, dense and unlayered.
Sedimentary rock forms from sediments that are deposited, compacted and cemented in layers (strata) over thousands of years. Examples include limestone, sandstone and shale; these rocks are often soft and may contain fossils.
Metamorphic rock forms when igneous or sedimentary rock is changed by great heat and pressure. Limestone becomes marble, shale becomes slate, and sandstone becomes the harder quartzite.
Uses of Rocks
- Granite — building material, roads and tombstones.
- Limestone — the main raw material for cement and fertiliser.
- Marble — decorative floors, walls and sculptures.
- Slate — roofing tiles and writing boards.
In Malaysia, limestone is common in areas such as the Kinta Valley and Gua Tempurung, where it forms striking karst hills.
The Rock Cycle
The three rock types are linked through the rock cycle. Rock may weather into sediment and form sedimentary rock, or be melted again into magma that solidifies as igneous rock.
Key idea
Igneous rock comes from cooling magma, sedimentary rock from layered sediments, and metamorphic rock from heat and pressure acting on existing rock.