Earth is always changing
Mountains rise and wear down. Rivers carve canyons. Glaciers scrape valleys. Most of these changes are too slow to notice in a lifetime, but over millions of years, they transform the planet.
The three rock types
| Type | Formation | Texture clues | Examples |
|---|---|---|---|
| Igneous | Magma or lava cools and crystallizes | Interlocking crystals; large crystals = slow cooling underground (intrusive); small or glassy = fast cooling at the surface (extrusive) | Granite (intrusive), basalt (extrusive), obsidian |
| Sedimentary | Sediment is compacted and cemented | Layers; rounded grains; fossils | Sandstone, shale, limestone, conglomerate |
| Metamorphic | Heat and pressure change existing rock without melting it | Bands (foliation); recrystallized minerals | Slate, schist, gneiss, marble, quartzite |
The rock cycle
The rock cycle shows how rocks change form. There is no fixed order; any rock can become any other type.
- Melting → magma → cooling → igneous rock
- Weathering and erosion → sediment → compaction and cementation → sedimentary rock
- Heat and pressure → metamorphic rock
Energy for the rock cycle comes from Earth’s internal heat (driving plate tectonics and melting) and the Sun (driving weather, water and wind).
Weathering
Weathering breaks rock down where it sits.
| Type | Process | Example |
|---|---|---|
| Mechanical (physical) | Frost wedging: water freezes in cracks and expands about 9% | Potholes; broken rocks on mountains |
| Mechanical | Root wedging | Tree roots cracking sidewalks |
| Mechanical | Abrasion | Wind-blown sand polishing rocks |
| Mechanical | Exfoliation | Layers peeling off granite domes |
| Chemical | Dissolution: slightly acidic water dissolves limestone | Caves and sinkholes |
| Chemical | Oxidation | Iron-rich rocks rusting red |
| Biological | Lichens release acids; burrowing animals | Lichen on old gravestones |
Frost wedging is especially important in Canada, where temperatures cross 0 °C many times each year.
Erosion and deposition
Erosion carries weathered material away. Deposition drops it somewhere else when the carrying agent slows down.
| Agent | Erosion landforms | Deposition landforms |
|---|---|---|
| Running water | V-shaped valleys, canyons, waterfalls | Deltas, floodplains, alluvial fans |
| Glaciers | U-shaped valleys, fjords, cirques | Moraines, drumlins, eskers, erratics |
| Wind | Desert pavement, carved rock arches | Sand dunes, loess |
| Waves | Sea cliffs, sea arches, sea stacks | Beaches, sandbars, spits |
| Gravity (mass wasting) | Landslides, rockfalls, slumps | Talus slopes |
Canada’s landscape was dramatically shaped by glaciers during the last ice age. The Laurentide Ice Sheet, up to 3 km thick, covered most of the country until about 12 000–10 000 years ago. It carved the Great Lakes basins and left boulders (erratics) scattered across the Prairies.
Hopewell Rocks in New Brunswick are sea stacks carved by the Bay of Fundy’s huge tides.
Mountain building and uplift
Rock cycle processes are balanced by uplift:
- Plate collisions fold and fault rocks into mountains (the Rockies, the Himalayas).
- Volcanoes build new land.
- As heavy ice melts, the land slowly rebounds. Parts of Hudson Bay are still rising about 1 cm per year.
Reading the rocks
Geologists use simple principles to interpret rock layers:
- Uniformitarianism: "the present is the key to the past." Processes we see today, like rivers depositing sand, also happened in the past. James Hutton proposed this in the late 1700s.
- Superposition: in undisturbed layers, the oldest are at the bottom.
- Original horizontality: sediments are deposited in flat layers; tilted layers were disturbed later.
- Cross-cutting relationships: a fault or intrusion is younger than the rocks it cuts.
Common mistakes
- Mixing up weathering and erosion. Weathering breaks; erosion moves.
- Thinking metamorphic rock forms by melting. Melting makes magma, which forms igneous rock.
- Thinking the rock cycle has one fixed path.
Try it yourself
- Fill a plastic container with water, seal it, and freeze it. What happened? Relate it to frost wedging.
- Shake sugar cubes in a jar for 2 minutes. Describe what happens. Which kind of weathering is this?
- Build a sand slope in a tray and pour water down it. Sketch the erosion and deposition features.
Quick recap
- Igneous, sedimentary and metamorphic rocks form in different ways.
- The rock cycle has many paths, driven by Earth’s heat and the Sun.
- Weathering breaks rock; erosion moves it; deposition drops it.
- Water, ice, wind, waves and gravity shape landforms.
- Superposition and uniformitarianism help geologists read Earth’s history.