B. Physical Geography and Physical Processes in Canada · Canada's Physical Environment: Landforms, Climate, Hazards and People

Canada's Rocks and Geologic Time: How the Land Was Built

B1.2 B1.1 A1.5About 70 minutes

Learning goals

I can describe how igneous, sedimentary and metamorphic rocks form, and give a Canadian example of each.
I can place the Precambrian, Paleozoic, Mesozoic and Cenozoic on the geologic time scale.
I can explain how the Canadian Shield, the sedimentary lowlands and Canada's mountain ranges formed at different times.
I can use the law of superposition and cross-cutting to put rock layers in order.

Words to know

geological process
A natural process that forms or changes rock and the Earth's crust, such as volcanic activity, mountain building, deposition of sediment, weathering and erosion.

igneous rock
Rock formed when melted rock (magma or lava) cools and hardens. Examples: granite, basalt.

sedimentary rock
Rock formed when layers of sediment (sand, mud, shells) are pressed and cemented together. Examples: sandstone, shale, limestone.

metamorphic rock
Rock changed by great heat and pressure without melting. Examples: gneiss, marble, slate.

geologic time scale
The calendar of Earth's 4.6-billion-year history, divided into eons, eras and periods.

Precambrian
The time from Earth's formation (about 4.6 billion years ago) to about 541 million years ago; nearly 90% of Earth's history.

bedrock
The solid rock under the soil and loose material at the surface.

law of superposition
In undisturbed layers of sedimentary rock, the lowest layers are the oldest and the top layers are the youngest.

orogeny
A mountain-building event, usually caused by plates colliding.

fossil fuel
Fuel such as coal, oil or natural gas formed from the remains of ancient living things buried in sedimentary rock.

Holding four billion years in your hand

In the Northwest Territories, about 300 km north of Yellowknife, there is a grey, striped rock called the Acasta Gneiss (say it "nice"). It is about 4 billion years old, one of the oldest known rocks on Earth. Canada is a great place to study the Earth's history, because our land holds rocks from almost every chapter of it, from the ancient Shield to young volcanic rock in British Columbia.

The landscape you see today, flat or rugged, rocky or covered in deep soil, depends on what rock is underneath and what happened to it. That is what this lesson is about.

Three families of rock

In Grade 4 science you met the three families of rock. Here is a closer look at each.

Igneous rock forms when molten rock cools. If it cools slowly underground, crystals grow large (like granite, common on the Canadian Shield). If it cools quickly on the surface, crystals are tiny (like basalt from lava flows).
Sedimentary rock forms in layers. Rivers, wind and waves carry bits of rock, mud and shells into lakes and seas. Over millions of years, the layers are buried, squeezed and cemented into sandstone, shale and limestone. Sedimentary rock often contains fossils, and it is where coal, oil and natural gas are found. The Niagara Escarpment is capped by a hard sedimentary rock called dolostone.
Metamorphic rock forms when existing rock is changed by heat and pressure deep in the crust, usually during mountain building. Shale can become slate; limestone can become marble; granite can become gneiss.

Rocks move between these families over very long times in the rock cycle: weathering breaks rock into sediment, sediment becomes sedimentary rock, burial and heat make metamorphic rock, melting makes magma, and magma cools into igneous rock.

Rock family	How it forms	Canadian example
Igneous	Molten rock cools and hardens	Granite of the Canadian Shield; lava rock at the Nisga'a Memorial Lava Bed in northwestern B.C.
Sedimentary	Layers of sediment are pressed and cemented	Limestone of the Great Lakes–St. Lawrence Lowlands; the oil-bearing rock of the Interior Plains
Metamorphic	Heat and pressure change existing rock	Gneiss of the Canadian Shield (including the Acasta Gneiss)

The three rock families, with examples from Canada.

The geologic time scale

Earth is about 4.6 billion years old. Geologists divide that time into a calendar called the geologic time scale. The four big divisions you need for this course are:

Precambrian (4.6 billion to 541 million years ago): almost 90% of all Earth's history. Simple life only, for most of it. The rock of the Canadian Shield formed and was folded into mountains during this time.
Paleozoic Era (541 to 252 million years ago): life explodes in the oceans; fish, then plants and animals on land. Shallow seas cover much of what is now Canada and lay down thick sedimentary layers. The Appalachian Mountains form as plates collide.
Mesozoic Era (252 to 66 million years ago): the age of dinosaurs. Sediments pile up in the Interior Plains (Alberta's dinosaur fossils come from these rocks). Mountain building begins in the west.
Cenozoic Era (66 million years ago to today): mammals spread; the Western Cordillera keeps rising; and in the last 2.6 million years or so, great ice sheets advance and retreat across Canada.

Earth's history drawn to scale on one bar. The Precambrian takes up almost the whole bar. Everything with dinosaurs, mammals and people is squeezed into the thin slices at the right end.

How the big building blocks of Canada formed

1. The Canadian Shield: the ancient core. The Shield is made mostly of Precambrian igneous and metamorphic rock. Long ago it was the root of huge mountain ranges. Over hundreds of millions of years, weathering and erosion wore those mountains down to the rolling, rocky surface you see today. Because it formed through heat, pressure and melting, the Shield is rich in metallic minerals such as nickel, copper, gold and iron. (The nickel and copper of the Sudbury area are linked to a huge meteorite impact about 1.85 billion years ago.)

2. The lowlands and plains: layers on top. Around the edges of the Shield, shallow seas covered the land many times in the Paleozoic and Mesozoic. Sediment settled on the sea floors and became flat-lying sedimentary rock. That is why the Hudson Bay Lowlands, Arctic Lowlands, Great Lakes–St. Lawrence Lowlands and Interior Plains are mostly flat and why the Interior Plains contain coal, oil and natural gas. The Shield rock is still there underneath, like the bottom layer of a cake.

3. The mountains: built when plates collided.

The Appalachians formed in the Paleozoic when ancient continents collided. They have been eroding for hundreds of millions of years, so they are low and rounded today.
The Innuitian Mountains of the High Arctic formed much later than the Appalachians, when the crust there was squeezed and folded.
The Western Cordillera is the youngest. Over roughly the last 200 million years, pieces of crust were pushed against the western edge of North America, folding and lifting rock into high, sharp, rugged ranges such as the Rockies and the Coast Mountains. Plate movement there has not stopped (next lesson). Even fossil-rich sedimentary rock got lifted up: the famous Burgess Shale fossils, about half a billion years old, are high in the mountains of Yoho National Park, B.C.

A schematic west-to-east slice through Canada (not to scale). Shield rock lies under everything. On the plains it is buried under flat sedimentary layers; on the Shield it is bare at the surface; in the west it has been pushed up and folded into mountains.

Pattern to remember

Old mountains are low; young mountains are high. A mountain range starts to wear down as soon as it rises. The older the range, the longer weathering and erosion have worked on it. Compare Mount Logan in the young St. Elias Mountains of Yukon (5959 m, the highest point in Canada) with the old, worn Appalachians, whose highest peak in Canada, Mont Jacques-Cartier on Quebec's Gaspé Peninsula, is only about 1270 m.

🔎 Geographer's toolkit: reading rock layers

Geologists read rock layers like pages in a book. Two simple rules help:

Law of superposition: in undisturbed sedimentary layers, the bottom layer is oldest and the top is youngest (each layer was laid on top of the one before).
Cross-cutting: anything that cuts through a layer (a crack, a fault, or a vein of igneous rock called a dike) must be younger than the layer it cuts.

This is the concept of patterns and trends in action: you are finding a sequence (a trend over time) from evidence you can see in space.

A made-up rock outcrop. Layer A is at the bottom, then B, then C. The solid-coloured igneous dike D cuts through A and B but stops below C.

Worked example

Worked example: Putting rock events in order

Use the outcrop drawing above. List events A, B, C and D from oldest to youngest, and explain each step.

Superposition: the layers were laid down from the bottom up, so A is older than B, and B is older than C.
Cross-cutting: the dike D cuts through A and B, so D is younger than both.
D stops at the bottom of C. That means C was laid down after the dike formed (otherwise the dike would cut C too).
Put it together: A → B → D → C.

Oldest to youngest: A (sandstone), B (shale), D (igneous dike), C (limestone). The trilobite fossils in A also tell us it formed in a sea during the Paleozoic, since trilobites lived in the oceans of that era.

Hands-on activity

Make a sediment jar

You need: a clear plastic jar or bottle with a lid, a mix of garden soil, sand and small pebbles (about one-third of the jar), water.

Put the soil, sand and pebbles in the jar. Fill it almost to the top with water and close the lid tightly.
Shake it hard for 30 seconds, then set it down where it will not be moved.
Observe after 1 minute, after 1 hour and the next day. Sketch what you see.
Answer: Which material settled first? Which is on top? Which layer is "oldest"?
Connect: How is this like the shallow Paleozoic seas that covered the lowlands?

What to look for: pebbles settle first (bottom), then sand, then silt and clay on top, sometimes with bits of plant floating. The bottom layer settled first, so it is the "oldest", just like the law of superposition. In ancient seas, layer after layer like this was buried and slowly cemented into sedimentary rock. Wash your hands after handling soil.

Just for fun

Funny but true: if Earth's whole history were squeezed into one 24-hour day, dinosaurs would roam for about 50 minutes near the end of the day, and modern humans would show up only in the last 6 seconds before midnight. We are the newest kids in the class.

Your turn

Name the three families of rock and one way each forms.

Which family of rock is most likely to contain fossils? Why?

Put these in order from oldest to youngest: Mesozoic, Precambrian, Cenozoic, Paleozoic.

Why is the Canadian Shield rich in metallic minerals?

Why are the Interior Plains and the Great Lakes–St. Lawrence Lowlands mostly flat?

Why are the Appalachians lower and more rounded than the Western Cordillera?

In an undisturbed cliff of sedimentary rock, where is the oldest layer?

More practice: warm-up

About how old is the Earth?

Name the rock family of granite.

What is the youngest mountain region in Canada?

In which era did the dinosaurs live?

Coal, oil and natural gas are found in which rock family?

More practice: on your own

Limestone is changed by heat and pressure during mountain building. What rock does it become, and what family is that?

Explain, as an interrelationship, why Alberta has both dinosaur fossils and oil.

A layer of rock has a fault (crack) running through it. Is the fault older or younger than the layer? Explain.

The Precambrian lasted from about 4600 to 541 million years ago. About what percent of Earth's history is that?

Burgess Shale fossils of sea creatures are found high in the Rocky Mountains. What does that tell you about how the mountains formed?

Challenge

A cliff shows (bottom to top): layer 1 sandstone, layer 2 shale, layer 3 limestone. A fault cuts layers 1, 2 and 3, and a thin layer of river gravel (layer 4) lies flat across the top of the fault without being broken. List the five events from oldest to youngest and explain.

If the 4.6 billion years of Earth's history were a 24-hour day, at about what time would the Mesozoic Era begin (252 million years ago)? Show your work.

Explain how the geologic history of the Canadian Shield helps explain BOTH its value for mining AND its poor value for farming.

Check your understanding

Quiz

Which rock family forms from layers of sand, mud or shells pressed together?

The Canadian Shield is made mostly of what kind of rock?

About how much of Earth's history does the Precambrian take up?

Why are the Appalachians lower than the Rockies?

An igneous dike cuts through layers A and B. What can you conclude?

Where would you most likely find oil and natural gas in Canada?

For parents and teachers

Big idea: Expectation B1.2 asks students to analyse how geological processes formed Canada's landscape. This lesson builds the foundation: rock types, the geologic time scale, and how the Shield, the sedimentary lowlands and the mountain ranges formed at very different times. The next lessons add plate tectonics, glaciers, water and climate.

Common difficulties:

Huge time spans. Millions and billions are hard to picture. The 24-hour-day model helps; so does a long strip of paper or a hallway (4.6 m = 4.6 billion years, so the Cenozoic is the last 6.6 cm).
Thinking the lowlands have no Shield rock. The Shield rock continues underneath the sedimentary layers.
Cross-cutting. Ask: "Could you cut a cake that doesn't exist yet?" The thing that cuts must come after.

How to help: Look at stone around your home or town: granite countertops (igneous), slate tiles (metamorphic), limestone buildings (sedimentary; look for fossils). Road cuts along highways in Ontario show the Shield's pink and grey rock and the flat layers of the lowlands; stop only where it is safe and legal.