A rock is a naturally occurring, solid aggregate of one or more minerals (or mineraloids) that forms part of the Earth’s crust. Rocks may be composed of a single mineral, such as limestone (calcite), or a mixture of several minerals, such as granite, which contains quartz, feldspar, and mica.
Rocks differ from minerals because minerals have a specific chemical composition and crystal structure, while rocks are usually mixtures and can vary widely in texture, composition, and origin.
How Rocks Form
Rocks form through geological processes that operate over thousands to millions of years, including:
- Cooling and crystallization of magma or lava
- Accumulation and compaction of sediments
- Heat and pressure deep within the Earth
These processes are interconnected through the rock cycle, where one rock type can transform into another over time.
The Three Main Types of Rocks
1. Igneous Rocks
Igneous rocks form when molten material solidifies.
- Intrusive igneous rocks form inside the Earth (slow cooling, coarse grains).
Example: Granite - Extrusive igneous rocks form on the surface after volcanic eruptions (fast cooling, fine grains).
Example: Basalt
2. Sedimentary Rocks
Sedimentary rocks form from the accumulation of sediments such as sand, clay, organic remains, or dissolved minerals.
They typically show layers and may contain fossils.
Examples:
- Limestone
- Sandstone
- Shale
3. Metamorphic Rocks
Metamorphic rocks form when existing rocks are altered by heat, pressure, or chemical fluids, without melting.
Examples:
- Slate (from shale)
- Marble (from limestone)
- Gneiss (from granite)
