The ground beneath your feet
Earth’s crust is made of rock. Rocks are found in mountains, under soil, at beaches and even inside your house (in countertops, bricks and concrete). Studying rocks helps us learn about Earth’s history.
Minerals: the building blocks
A mineral is a natural, non-living solid with a regular crystal shape. Rocks are made of one or more minerals mixed together, like a cookie is made of flour, sugar and chocolate chips.
Testing minerals
| Property | How to test | Example |
|---|---|---|
| Colour | Look | Gold is yellow |
| Streak | Rub on white tile; check the powder colour | Hematite leaves a red-brown streak |
| Lustre | How it shines | Metallic (pyrite) or glassy (quartz) |
| Hardness | Scratch test (Mohs scale 1–10) | Talc = 1 (softest); diamond = 10 (hardest) |
| Cleavage | Breaks along flat surfaces | Mica peels into thin sheets |
Your fingernail is about 2.5 on the Mohs scale. A steel nail is about 5.5.
Three types of rock
| Type | How it forms | Examples | Clues |
|---|---|---|---|
| Igneous rock | Melted rock cools and hardens | Granite, basalt, obsidian, pumice | Crystals; sometimes glassy or bubbly |
| Sedimentary rock | Layers of sediment pressed and cemented | Sandstone, limestone, shale | Layers; may contain fossils |
| Metamorphic rock | Heat and pressure change existing rock | Marble (from limestone), slate (from shale) | Bands or stripes; very hard |
- Pumice is full of air bubbles, so it can float!
- Obsidian cooled so fast it formed natural glass.
The rock cycle
Rocks change from one type to another over millions of years. This is the rock cycle.
- Igneous rock wears away into sediment → becomes sedimentary rock.
- Sedimentary rock gets buried deep and heated → becomes metamorphic rock.
- Metamorphic rock melts → becomes magma → cools into igneous rock.
Fossils
A fossil is the remains or traces of a plant or animal from long ago. Most fossils are found in sedimentary rock.
How a fossil forms:
- A plant or animal dies and is quickly buried in mud or sand.
- The soft parts rot away.
- Layers of sediment pile up and harden into rock.
- Minerals slowly replace the bones or shell.
- Millions of years later, erosion exposes the fossil.
Types of fossils: bones, shells, footprints, leaf prints, and insects trapped in amber.
Alberta’s Dinosaur Provincial Park is one of the richest dinosaur fossil sites in the world.
Soil
Soil is a mix of tiny bits of rock, decayed plants and animals (called humus), water and air. It takes hundreds of years to form just a few centimetres of soil.
| Soil type | Feel | Holds water? |
|---|---|---|
| Sand | Gritty, loose | Drains quickly |
| Silt | Smooth, floury | Medium |
| Clay | Sticky when wet | Holds a lot |
| Loam (a mix) | Crumbly | Just right for most plants |
Weathering and erosion shape landforms
- Weathering breaks rock into smaller pieces. Water freezes in cracks and expands; plant roots push rocks apart.
- Erosion moves the pieces away by water, wind, ice or gravity.
- Deposition drops them in a new place, building beaches, deltas and sand dunes.
Landforms shaped this way include canyons, valleys, beaches and deltas. Glaciers carved the Great Lakes and many of Canada’s valleys.
Common mistakes
- Thinking rocks never change. They change slowly over very long times.
- Mixing up rocks and minerals. Minerals are the ingredients; rocks are the mixtures.
- Thinking fossils are the actual bones. Usually minerals have replaced them.
Try it yourself
- Collect five rocks. Sort them by colour, texture and whether they have layers or crystals.
- Do a scratch test: can your fingernail, a coin or a steel nail scratch each rock?
- Press a shell into playdough to make a "fossil" print.
Quick recap
- Rocks are made of minerals.
- Igneous rocks form from cooled melted rock; sedimentary from layers; metamorphic from heat and pressure.
- The rock cycle changes rocks from one type to another.
- Fossils form mostly in sedimentary rock.
- Weathering, erosion and deposition shape landforms.