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

Plate Tectonics: How Moving Plates Build Landforms

A3.2 A3.1About 45 minutes

Learning goals

I can describe Earth's layers and explain what tectonic plates are.
I can describe divergent, convergent and transform plate boundaries and the landforms each one makes.
I can explain why most earthquakes and volcanoes happen in certain places, such as the Ring of Fire.

Words to know

crust
Earth's thin, rocky outer layer.

mantle
The very thick layer under the crust. Its rock is hot and can flow very slowly.

tectonic plate
A giant slab of Earth's crust and upper mantle that moves slowly. Earth's surface is broken into several large plates and many smaller ones.

plate boundary
The place where two tectonic plates meet.

subduction
When one plate sinks under another plate into the mantle.

fault
A crack in Earth's crust where rocks move past each other.

Ring of Fire
A horseshoe-shaped zone around the Pacific Ocean with many earthquakes and volcanoes.

magma
Melted rock under the ground. Above ground it is called lava.

A puzzle on a world map

Look at a world map. Does the east coast of South America look like it could fit into the west coast of Africa, like puzzle pieces? Scientists noticed this long ago. Today we know why: the continents sit on huge moving plates, and they have moved apart over millions of years.

This big idea is called plate tectonics. It explains where many mountains, volcanoes and earthquakes are.

A long, deep crack runs through dark, moss-covered lava rock across flat land in Iceland. — In Iceland two plates are pulling apart, so the ground cracks open. This is a divergent boundary that you can actually see on land. AI-generated image

Remember from Grade 4

In Grade 4 science you learned about igneous rock made from cooled magma and lava, and that earthquakes are a geological process. Now you'll learn what causes them.

Earth's layers

Crust: the thin, solid outer layer we live on. It is thicker under continents and thinner under oceans.
Mantle: a very thick layer of hot rock. It is solid, but over long periods it can flow very slowly, a bit like very stiff putty.
Outer core: liquid metal (mostly iron and nickel).
Inner core: solid metal at the centre, extremely hot.

The crust and the top of the mantle are broken into tectonic plates. They move only a few centimetres per year, about as fast as your fingernails grow. That is slow, but over millions of years it moves whole continents.

Earth's four layers from the outside in: crust, mantle, outer core, inner core. The plates are made of the crust plus the very top of the mantle.

Three kinds of plate boundaries

Divergent boundary (moving apart): magma rises to fill the gap and makes new crust. Under the ocean this makes long underwater mountain chains called mid-ocean ridges, such as the Mid-Atlantic Ridge. On land, it can make a rift valley, like the Great Rift Valley in eastern Africa. Iceland sits on the Mid-Atlantic Ridge.
Convergent boundary (pushing together):
When an ocean plate meets a continental plate, the heavier ocean plate sinks underneath (subduction). Rock melts, magma rises and makes volcanoes. This made the Andes.
When two continental plates collide, neither sinks easily, so the crust crumples and folds upward into fold mountains. The Himalayas formed when the plate carrying India pushed into Asia, and they are still rising slowly.
Transform boundary (sliding past): plates grind sideways past each other along faults. They can get stuck, then slip suddenly, causing earthquakes. Example: the San Andreas Fault in California.

Three kinds of plate boundaries seen from above: divergent (plates move apart), convergent (plates push together) and transform (plates slide past each other). Not to scale.

Subduction at a convergent boundary: the ocean plate (under the blue ocean) sinks under the continental plate, and magma rises to form a volcano. Not to scale.

Boundary	Plate movement	Landforms and events	Example
Divergent	Apart	Mid-ocean ridges, rift valleys, volcanoes, earthquakes	Mid-Atlantic Ridge, Iceland, Great Rift Valley
Convergent (ocean–continent)	Together; ocean plate sinks	Volcanic mountains, deep ocean trenches, strong earthquakes	Andes; coast of British Columbia
Convergent (continent–continent)	Together; crust crumples	Fold mountains, earthquakes	Himalayas
Transform	Slide sideways past	Faults, earthquakes	San Andreas Fault

The Ring of Fire and Canada

Around the edges of the Pacific Ocean, many plates meet. This horseshoe-shaped zone is called the Ring of Fire. Most of the world's earthquakes and many of its active volcanoes happen there, in places like Japan, Indonesia, Chile, Alaska and the west coast of North America.

Canada is part of it! Off the coast of British Columbia, the small Juan de Fuca Plate is slowly sliding under the North American Plate. This area is called the Cascadia Subduction Zone. It can cause strong earthquakes, so B.C. has strict building rules and earthquake drills in schools.

Ontario is in the middle of the North American Plate, far from a plate boundary. Ontario does have small earthquakes sometimes, but strong ones are much less common than on the B.C. coast.

Worked example

Let's explain a landform using plate tectonics

Why are there volcanoes all along the Andes in South America?

Find the plates. The ocean plate in the Pacific (the Nazca Plate) meets the South American Plate along the west coast.
Name the boundary. They are moving together, so it is a convergent boundary between an ocean plate and a continental plate.
Describe the process. The heavier ocean plate sinks under the continental plate (subduction). As it sinks deep into the hot mantle, rock melts and makes magma.
Connect to the landform. The magma rises through the crust and erupts, building volcanoes. The pushing also squeezes and lifts the land into mountains.

The Andes have volcanoes because they are at a convergent boundary where an ocean plate is subducting under South America; melting rock rises as magma and builds volcanoes.

Just for fun

Funny but true: the Atlantic Ocean gets a few centimetres wider every year, about as fast as your fingernails grow. So Europe and North America are drifting apart at the speed of a manicure.

Hands-on activity

Cracker plate boundaries

You need: 2 graham crackers (or any flat crackers), a plate, a small cup of water, and a spoonful of thick frosting or peanut-free spread. Check for food allergies first.

Divergent: spread frosting on the plate (this is the mantle). Lay two crackers side by side on it, touching. Slowly pull them apart. What fills the gap? (The "mantle" shows through, like magma rising to make new crust.)
Transform: put the crackers side by side again and slide one forward and one back while pressing them together. Do they move smoothly or stick and jump? (Stick-and-jump is like an earthquake.)
Convergent (two continents): dip one edge of each cracker in water for 2 seconds so the edges get soft. Lay them with wet edges touching and push them slowly together. The soft edges crumple up, like fold mountains.
Draw and label a diagram of each boundary.

What to look for: your child names each boundary type and connects it to a landform (rift valley, earthquake, fold mountains). Eat the crackers afterward if you like!

Your turn

Name Earth's four layers from the outside in.

What is a tectonic plate?

About how fast do plates move?

What happens at a divergent boundary? Name one landform it makes.

How did the Himalayas form?

What is the Ring of Fire?

Which Canadian province is near a subduction zone?

More practice: warm-up

What is melted rock under the ground called?

What is the thin outer layer of Earth called?

Name the three types of plate boundaries.

At which type of boundary do plates slide past each other?

What is a fault?

More practice: on your own

Explain what subduction is and what landform it often creates.

Why do transform boundaries cause earthquakes?

Iceland has many volcanoes. Use plate tectonics to explain why.

Why does British Columbia have more strong earthquakes than Ontario?

Look at a world map. Which ocean is surrounded by the Ring of Fire, and name two countries in it.

Challenge

Compare the formation of the Andes and the Himalayas. How are they alike and different?

The Atlantic Ocean is getting a little wider each year. Explain why, using the correct vocabulary.

Using spatial thinking, explain why a map of the world's earthquakes looks a lot like a map of plate boundaries.

Check your understanding

Quiz

Which layer of Earth do we live on?

At a divergent boundary, plates...

The Himalayas formed when...

The San Andreas Fault is an example of a...

The Ring of Fire surrounds which ocean?

Volcanoes along the Andes come mainly from...

For parents and teachers

Big idea: Expectation A3.2 asks students to describe key natural processes, including tectonic forces, that create and change landforms. Plate tectonics explains where many mountains, volcanoes and earthquakes are.

How to help: Do the cracker activity together and ask your child to name each boundary as they model it. Look up a world earthquake map from Natural Resources Canada or the United States Geological Survey and compare it with a map of plate boundaries.

Common difficulties:

Thinking the mantle is a liquid ocean of lava: it is mostly solid rock that flows extremely slowly.
Mixing up divergent and convergent: diverge = go different ways (apart); converge = come together.
Thinking plates move quickly: they move centimetres per year; earthquakes happen when stuck plates slip suddenly.

If your child is worried about earthquakes, reassure them that Ontario is far from plate boundaries and that schools in higher-risk places like B.C. plan and practise for them.