North of Toronto, a ridge of sand and gravel about 160 km long holds the groundwater that feeds dozens of streams. In Ottawa, the ground sometimes shakes, even though the city is nowhere near the edge of a tectonic plate. Out in the Rockies, fossil seashells sit near mountaintops. Each of these puzzles has the same kind of answer: the land is not fixed. Slow geological processes make and move rock, ice sheets scrape and pile it, and water and weather wear it away. Those processes built Canada's landform regions and they are still at work today.
The rock cycle
Geologists sort rocks into three families by how they formed. Igneous rock forms when molten rock (magma underground, lava at the surface) cools and hardens. Sedimentary rock forms when bits of rock, shells or plant matter (sediment) settle in layers and are squeezed and cemented together. Metamorphic rock forms when existing rock is changed by intense heat and pressure deep underground, without melting. The rock cycle links them: any rock can be weathered into sediment, buried and changed, or melted and cooled into new rock.
| Rock family | How it forms | Examples | Where in Canada |
|---|---|---|---|
| Igneous | Molten rock cools and hardens (slowly underground gives big crystals; quickly at the surface gives tiny crystals) | Granite, basalt | Much of the Canadian Shield; volcanic rock in BC |
| Sedimentary | Sediment is deposited in layers, then compacted and cemented | Limestone, sandstone, shale, coal | Interior Plains, Great Lakes–St. Lawrence Lowlands, Hudson Bay Lowlands; fossils are common |
| Metamorphic | Existing rock is changed by heat and pressure | Gneiss (from granite), marble (from limestone), slate (from shale) | The Shield (gneiss), roots of old mountain belts |
Plate tectonics and mountain building
Earth's outer shell is broken into tectonic plates (seven very large ones and many smaller ones) that move a few centimetres a year, driven by heat from Earth's interior: slow movement in the hot mantle below and the pull of cold, heavy plate edges sinking into it. Almost all of Canada sits on the North American Plate. Where plates meet, they pull apart, push together or slide past each other. Collisions crumple rock into folds and break it along faults, slowly building mountains; this is how the Cordillera, the Appalachians and the Innuitian Mountains formed. The sudden slip of rock along a fault releases energy as an earthquake.
| Boundary | Plate motion | What happens | Canadian example |
|---|---|---|---|
| Convergent | Plates move toward each other | One plate can sink (subduct) under the other; mountains, volcanoes and very large earthquakes | The Juan de Fuca Plate dives under North America along the Cascadia Subduction Zone off Vancouver Island |
| Divergent | Plates move apart | Magma rises to fill the gap, making new crust and a ridge | The Mid-Atlantic Ridge, where North America slowly moves away from Europe |
| Transform | Plates slide past each other | Rock locks, then slips suddenly in earthquakes | The Queen Charlotte Fault off Haida Gwaii, between the Pacific and North American plates |
Glaciation: how ice shaped Ontario
About 20,000 years ago, at the peak of the last ice age, the Laurentide Ice Sheet, more than 2 km thick in places, covered almost all of Canada east of the Rockies, while a separate ice sheet covered much of the Cordillera. A glacier moves under its own weight. As it slides, it erodes: rocks frozen into its base scrape and pluck the land, gouging out basins and stripping soil off the Shield. When it melts, it deposits everything it carried. The Great Lakes formed this way: ice deepened old river valleys and basins in softer rock, and meltwater filled them as the ice melted back. Southern Ontario was largely free of ice by roughly 12,000 to 13,000 years ago.
- Moraine: a ridge of unsorted rock, sand and clay (till) dumped at the edge of a glacier.
- Drumlin: a smooth, egg-shaped hill of till, streamlined in the direction the ice moved; common near Peterborough.
- Esker: a long, winding ridge of sand and gravel left by a river that flowed in a tunnel under the ice; a valuable source of gravel.
- Kettle lake: a round lake formed where a buried block of ice melted and left a hollow.
- Erratic: a boulder carried far by ice and dropped on different bedrock.
- U-shaped valley: a river valley widened and deepened by a valley glacier, common in the Rockies.
- Isostatic rebound: the slow rise of land after the weight of the ice is removed; land near Hudson Bay is still rising by around a centimetre a year.
The land is still changing. Weathering breaks rock down where it sits: water freezes in cracks and pries them apart (frost wedging, very common in Canada's freeze–thaw climate), slightly acidic rain dissolves limestone, and tree roots split rock. Erosion then carries the pieces away by water, wind, ice or gravity. Niagara Falls is a vivid example: by eroding the soft shale under the hard caprock, the falls has worn its way back roughly 11 km from the edge of the escarpment over about 12,000 years. Rivers carve valleys, waves eat at coastlines, and landslides move whole hillsides, as at Frank, Alberta, in 1903.