Pick up a pebble. It may have been a mountain, the floor of a sea or melted rock deep underground.
Key idea
Minerals are natural solids with their own pattern of particles, like quartz, mica or salt. Rocks are made of one or more minerals. There are three kinds of rock, named for how they form.
| Kind | How it forms | Examples |
|---|---|---|
| Igneous | melted rock (magma or lava) cools and hardens | granite, basalt, obsidian, pumice |
| Sedimentary | layers of sand, mud, shells or pebbles are pressed and cemented together | sandstone, limestone, shale |
| Metamorphic | heat and pressure deep underground change a rock without melting it | marble (from limestone), slate (from shale) |
Fossils are found mostly in sedimentary rock, because plants and animals were buried in the soft layers before they hardened.
Worked example: Which kind?
A rock has layers like a stack of pancakes and a fossil shell in it. Which kind is it most likely?
- Layers and fossils come from sediments settling and being pressed.
Answer: Sedimentary.
Common mistake
Thinking rocks never change. Rocks change all the time, only slowly: the rock cycle can take millions of years.
Try it at home
- Collect a few rocks from a yard, a park or a path (where collecting is allowed).
- Look at each with a hand lens. Do you see crystals, grains of sand or layers?
- Scratch each with a steel nail. Quartz is harder than steel and will not scratch; chalk and many softer rocks will.
Going further. Agnes Giberne's The World's Foundations is a classic geology book for young readers. Later: The Rock Cycle in Earth Science.