The solid ground
Earth’s crust is made of rock. A rock is a natural solid made of one or more minerals. Granite countertops, brick walls, gravel roads, mountains and pebbles on the beach are all rock.
Geologists sort rocks into three groups based on how they formed.
Igneous rocks: born of fire
Igneous rock forms when melted rock cools and hardens. Igneous comes from the Latin word for fire.
| Intrusive igneous | Extrusive igneous | |
|---|---|---|
| Where it forms | Underground, from magma | At the surface, from lava |
| Cooling | Slow | Fast |
| Crystals | Large, easy to see | Tiny or none |
| Examples | Granite, gabbro | Basalt, obsidian, pumice |
- Granite has visible crystals of pink feldspar, clear quartz and black mica.
- Obsidian cooled so fast it became natural glass. Ancient peoples made sharp tools from it.
- Pumice is full of gas bubbles. It’s so light it can float on water.
- Basalt makes up most of the ocean floor.
Sedimentary rocks: layer upon layer
Sedimentary rock forms from sediment: bits of rock, sand, mud, shells and plant material.
- Weathering breaks rock into small pieces.
- Water, wind or ice carries them away (erosion).
- They settle in layers, often in lakes or oceans (deposition).
- Over time, the weight of layers above presses them together (compaction).
- Minerals in water glue the grains together (cementation).
| Rock | Made from | Notes |
|---|---|---|
| Sandstone | Sand | Gritty texture |
| Shale | Mud and clay | Splits into thin layers |
| Conglomerate | Rounded pebbles in sand | Looks like natural concrete |
| Limestone | Shells and skeletons of sea life | Fizzes in vinegar |
| Coal | Ancient plants | A fossil fuel |
Sedimentary rocks often have visible layers and are the only rocks that usually contain fossils. Alberta’s badlands and Drumheller’s dinosaur fossils are found in sedimentary rock.
Metamorphic rocks: changed rocks
Metamorphic rock forms when existing rock is changed by heat and pressure deep underground, without melting. Metamorphic means "changed form."
| Original rock | Metamorphic rock | Uses |
|---|---|---|
| Shale | Slate | Roof tiles, chalkboards |
| Limestone | Marble | Statues, buildings |
| Sandstone | Quartzite | Very hard; used in construction |
| Granite | Gneiss | Has bands of light and dark minerals |
Metamorphic rocks often have bands or stripes (called foliation) and are usually very hard. The Canadian Shield, which covers about half of Canada, is made mostly of ancient igneous and metamorphic rocks, some more than 4 billion years old.
The rock cycle
Rocks don’t stay the same forever. The rock cycle shows how they change from one type to another over millions of years.
| Process | Change |
|---|---|
| Melting | Any rock → magma |
| Cooling | Magma → igneous rock |
| Weathering and erosion | Any rock → sediment |
| Compaction and cementation | Sediment → sedimentary rock |
| Heat and pressure | Any rock → metamorphic rock |
There’s no single path. An igneous rock can become sedimentary, then metamorphic, then melt and become igneous again.
The rock cycle is driven by Earth’s inner heat (which melts rocks and moves plates) and by the Sun (which powers wind, rain and rivers that erode rock).
Identifying rocks
| Clue | Likely type |
|---|---|
| Interlocking crystals | Igneous (or metamorphic) |
| Glassy or full of bubbles | Extrusive igneous |
| Layers, rounded grains or fossils | Sedimentary |
| Bands or stripes; very hard | Metamorphic |
Common mistakes
- Mixing up rocks and minerals. Minerals are the ingredients; rocks are the mixtures.
- Thinking metamorphic rocks form by melting. They change without melting.
- Thinking rocks never change.
Try it yourself
- Collect 10 rocks. Sort them using the identification table.
- Put a drop of vinegar on a piece of chalk or limestone. What happens?
- Make "sedimentary rock" by layering sand, gravel and soil in a clear jar of water and letting it settle.
Quick recap
- Rocks are made of minerals and come in three types.
- Igneous rocks form from cooling magma or lava.
- Sedimentary rocks form from compacted, cemented layers of sediment.
- Metamorphic rocks form when heat and pressure change existing rocks.
- The rock cycle shows how rocks change from one type to another.