Grade by Grade
A. Physical Patterns in a Changing World · Physical Patterns in a Changing World

Changing Landforms: Weathering, Erosion, Glaciers and People

A3.2About 45 minutes
Learning goals
  • I can explain how weathering, erosion and deposition change landforms.
  • I can describe how glaciers shaped much of Canada, including Ontario.
  • I can describe human activities, such as mining and land reclamation, that create and change landforms.

Words to know

weathering
The breaking down of rock into smaller pieces where it is, by water, ice, plants, chemicals or temperature changes.
erosion
The carrying away of rock and soil by water, wind, ice or gravity.
deposition
When water, wind or ice drops the sediment it was carrying, building up new land.
glaciation
The shaping of land by glaciers, huge slow-moving masses of ice.
moraine
A ridge or hill of rock, sand and gravel left behind by a glacier.
delta
Flat land built from sediment where a river slows and enters a lake or ocean.
land reclamation
Making new land, for example by draining wetlands or filling in part of a lake or the sea.

Mountains don't last forever

Plate tectonics builds land up. But at the same time, other forces are always wearing it down. The Canadian Shield has some of the oldest rock on Earth. Long, long ago it had high mountains. Today it is mostly low, rocky and rounded. Where did the mountains go?

They were worn down by weathering and erosion over an enormous amount of time, and scraped by glaciers.

Smooth pink and grey granite with long parallel scratches slopes down to a lake, with pine trees on the rocks.
The Canadian Shield, rounded and worn down. The long parallel scratches were carved by rocks dragged along under a glacier thousands of years ago. AI-generated image

Remember from Grade 4

In Grade 4 science you learned that weathering breaks rock into pieces, erosion carries the pieces away, and freeze-thaw can crack rocks. In Grade 7 you'll use these ideas to explain landforms around the world.

Breaking down, moving, building up

  1. Weathering breaks rock where it is.
  • Mechanical: water seeps into cracks, freezes and expands, and splits the rock (freeze-thaw, very common in Canada in spring and fall). Tree roots can also split rock.
  • Chemical: rainwater that is slightly acidic slowly dissolves some rocks, like limestone, making caves.
  1. Erosion moves the pieces away.
  • Running water is the biggest eroder. Rivers carve V-shaped valleys and canyons, like the Grand Canyon in the United States, carved by the Colorado River.
  • Wind blows sand and shapes sand dunes in deserts.
  • Waves wear away coasts and make cliffs.
  • Gravity pulls rock down slopes in landslides and rockfalls.
  1. Deposition builds new land where sediment is dropped. When a river slows down at its mouth, it drops mud and sand and builds a delta, such as the Mackenzie River Delta in the Northwest Territories.
1. Weatheringbreaks rock where it is2. Erosioncarries the pieces away3. Depositiondrops them in a new placeiceice widens the crackrunning waterriverseadelta
Three steps that reshape land: weathering cracks rock in place, erosion moves the bits, and deposition drops them somewhere new, such as a delta. Not to scale.

Glaciers: giant bulldozers of ice

During the last ice age, huge ice sheets, up to a few kilometres thick, covered almost all of Canada. The ice slowly melted back over thousands of years (southern Ontario was free of ice more than 10 000 years ago), and it left its mark everywhere:

  • Glaciers scraped the Canadian Shield down to bare rock and gouged out thousands of lake basins. That is one reason Ontario has so many lakes.
  • Glaciers deepened the basins of the Great Lakes.
  • In mountains, glaciers turned V-shaped river valleys into wide U-shaped valleys.
  • When the ice melted, it deposited rock, sand and gravel in ridges and hills. The Oak Ridges Moraine, north of Toronto, is a long ridge of sand and gravel left by glaciers.
V-shaped valleycarved by a riverU-shaped valleycarved by a glacier
A V-shaped valley carved by a river (left) and a wide U-shaped valley carved by a glacier (right). Not to scale.

People change landforms too

Humans are now one of the biggest forces changing Earth's surface.

  • Mining and quarrying dig huge pits and pile up rock. Open-pit mines can be hundreds of metres deep.
  • Land reclamation makes new land. In the Netherlands, people built dikes (walls of earth) and pumped out the water to create farmland called polders. About a quarter of the country is below sea level. Much of Toronto's waterfront is built on land filled into Lake Ontario; the old shoreline was near today's Front Street.
  • Building roads, railways and cities cuts through hills and fills in valleys and wetlands.
  • Removing plants (cutting forests, overgrazing) leaves soil bare, so wind and water erode it much faster.
Just for fun

The Oak Ridges Moraine is a ridge of sand and gravel that the glaciers left behind when they melted. It's the world's slowest case of "Sorry, we'll come back for our stuff later": more than 10 000 years and counting.

Worked example

Let's explain a landform's story

Ontario's Canadian Shield has thousands of lakes and lots of bare, rounded rock. Explain how it got that way using the processes from this lesson.

  1. Start with the rock. The Shield is made of very old, hard rock. Over a very long time, weathering and erosion wore down the mountains that were once there.
  2. Add the glaciers. During the last ice age, thick ice sheets moved over the Shield. They scraped off soil and loose rock, leaving bare, rounded rock.
  3. Explain the lakes. The ice gouged out hollows in the rock. When the ice melted, water filled them, making thousands of lakes.
  4. Notice what's still happening. Today, freeze-thaw keeps cracking the rock, and rivers keep eroding it, slowly.

The Shield was worn down by weathering and erosion, then scraped and hollowed out by glaciers; meltwater filled the hollows to make lakes.

Hands-on activity

Erosion in a baking pan

You need: a baking pan or plastic tub, sand or soil, a few small stones, a cup of water, and a book to prop up one end. Do this outside (or on a tray you can empty outside, not down a drain), and wash your hands afterwards.

  1. Pack the sand into a slope at one end of the pan. Prop that end up on the book.
  2. Slowly pour water at the top of the slope. Watch where it flows.
  3. Look for: a channel (a little river valley), erosion (sand being carried away) and deposition (sand piling up at the bottom, like a delta).
  4. Now press a few stones into the slope and add some grass or leaves on one side. Pour again. Which side eroded less?
  5. Bonus (glacier): freeze some water with sand and pebbles mixed in, in a small container. Rub the ice block across a bar of soap or a pan of flattened modelling clay. What marks does it leave?

What to look for: your child uses erosion and deposition correctly and notices that plants and rocks slow down erosion.

Your turn

1

What is the difference between weathering and erosion?

2

Explain freeze-thaw weathering.

3

Name four things that cause erosion.

4

What shape of valley does a glacier carve? What shape does a river carve?

5

What is a delta, and how does it form?

6

What is the Oak Ridges Moraine made of, and what made it?

7

Give one way people in the Netherlands created new land.

More practice: warm-up

1

What is the biggest eroder of land?

2

What landform does wind build in deserts?

3

What covered almost all of Canada during the last ice age?

4

Name a canyon carved by a river.

5

Name one human activity that changes landforms.

More practice: on your own

1

Why does Ontario have so many lakes?

2

Explain why freeze-thaw weathering is very common in Canada.

3

How can cutting down a forest on a hillside speed up erosion?

4

A river flows fast down a mountain, then slowly across a plain into a lake. Where will most erosion happen, and where will most deposition happen? Why?

5

Is Toronto's waterfront an example of a natural or a human-made change to the land? Explain.

Challenge

1

Plate tectonics builds mountains up, and erosion wears them down. Use this idea to explain why the Rocky Mountains are high and jagged, but the Canadian Shield is low and rounded.

2

Land reclamation creates useful land, but it can cause problems. Give one benefit and one possible problem.

3

Look at the two valleys in the diagram. A hiker finds a wide valley with steep sides and a flat bottom in the Rockies. What can she infer about its history?

Check your understanding

Quiz

1.

Which process carries away pieces of rock?

2.

A U-shaped valley was most likely carved by...

3.

A delta forms by...

4.

Water freezing in cracks and splitting rock is called...

5.

Making new farmland by building dikes and pumping out water is an example of...

6.

The Oak Ridges Moraine was left by...

For parents and teachers

Big idea: Expectation A3.2 asks students to describe natural processes (tectonic forces, erosion, glaciation) and human activities (such as mining and land reclamation) that create and change landforms. This lesson builds on Grade 4 science (weathering, erosion, glaciers).

How to help: Go for a walk and look for erosion: soil washed onto a sidewalk, a gully on a slope, cracked rocks, a stream bank. Ask, "What wore this away? Where did it go?" If you visit a park on the Canadian Shield, look for bare, rounded rock and scratches left by glaciers.

Common difficulties:

  • Mixing up weathering and erosion: weathering breaks, erosion moves.
  • Thinking landforms are permanent: they change constantly, mostly too slowly to see.
  • Forgetting that people change landforms: mining, building and land reclamation are major forces today.

Safety: supervise the water-and-sand activity; keep sand away from drains.