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

Moving Plates: Earthquakes, Volcanoes and Mountains in Canada

B1.2 B1.3 A1.4About 70 minutes

Learning goals

I can identify the three types of plate boundaries off Canada's west coast and the landforms and events each produces.
I can explain why most of Canada's large earthquakes and all its young volcanoes are in the west, and why eastern Canada still has some earthquakes.
I can compare earthquake magnitudes and explain what one step on the magnitude scale means.
I can use evidence (including Indigenous oral histories and records from Japan) to explain how scientists know about the great earthquake of 1700.

Words to know

tectonic plate
A huge slab of the Earth's crust and upper mantle that moves slowly (a few centimetres per year).

divergent boundary
Where two plates move apart and new crust forms, such as the Juan de Fuca Ridge.

convergent boundary
Where two plates move toward each other; one may sink under the other (subduction).

transform boundary
Where two plates slide past each other sideways, such as the Queen Charlotte Fault.

subduction
The sinking of a denser oceanic plate beneath another plate.

megathrust earthquake
A very large earthquake on the boundary of a subduction zone, where the plates are locked and then suddenly slip.

magnitude
A number that describes the size (energy released) of an earthquake. Each whole step is about 32 times more energy.

epicentre
The point on the surface directly above where an earthquake starts (the focus).

intraplate earthquake
An earthquake in the middle of a plate, far from a boundary, often along old faults.

volcanic belt
A line of volcanoes formed by the same plate process, such as the Garibaldi Volcanic Belt in B.C.

An earthquake recorded in Japan... caused in Canada

On the night of January 26, 1700, the ground off the coast of what is now British Columbia, Washington and Oregon lurched violently. Huge waves raced across the Pacific Ocean. About ten hours later, a tsunami struck the coast of Japan, where people wrote down the date and the damage, puzzled because they had felt no earthquake.

Centuries later, scientists put the evidence together: Japanese records, "ghost forests" of dead trees drowned when the coast suddenly dropped, layers of tsunami sand, and the oral histories of First Nations on the west coast, such as the Nuu-chah-nulth nations (including the Huu-ay-aht), which tell of a great shaking and flooding. Together these showed that a magnitude 9 earthquake struck the Cascadia Subduction Zone. It can happen again. This lesson explains why.

Remember from Grade 7

In Grade 7 you learned that the Earth's crust is broken into tectonic plates that move a few centimetres a year, that there are divergent, convergent and transform boundaries, and that Canada's west coast is part of the Pacific Ring of Fire. Now you will zoom in on Canada.

Three kinds of boundaries, all off British Columbia

Almost all of Canada sits on the North American Plate. But just off the B.C. coast, it meets the Pacific Plate and the small Juan de Fuca Plate, and all three types of boundary are found there:

Divergent: the Juan de Fuca Ridge. Far offshore, the Juan de Fuca and Pacific plates move apart. Magma rises and makes new sea floor.
Convergent: the Cascadia Subduction Zone. From northern Vancouver Island south to California, the Juan de Fuca Plate is pushed east and sinks (subducts) under the North American Plate. The plates are locked together where they touch. Strain builds for centuries until they suddenly slip in a megathrust earthquake, which can lift or drop the sea floor and cause a tsunami. Deeper down, the sinking plate heats up, rock melts, and magma rises to form volcanoes such as those in the Garibaldi Volcanic Belt (Mount Garibaldi and Mount Meager, north of Vancouver).
Transform: the Queen Charlotte Fault. Off Haida Gwaii, the Pacific Plate slides northwest past the North American Plate. Canada's largest recorded earthquake, magnitude 8.1, happened on this fault in 1949. In 2012 a magnitude 7.7 earthquake struck off Haida Gwaii.

A schematic map of the plate boundaries off British Columbia (not to scale, shapes simplified). Divergent, convergent and transform boundaries are all found within a few hundred kilometres of the coast.

Inside the Cascadia Subduction Zone (schematic, not to scale). The sinking Juan de Fuca Plate causes three things: megathrust earthquakes where the plates are locked, deeper earthquakes in the sinking slab, and magma that feeds volcanoes inland.

Why the west has mountains, volcanoes and big earthquakes

Mountains: The squeezing and lifting along the western edge of North America over millions of years built the Western Cordillera. That process is still going on, very slowly.
Volcanoes: Canada's young volcanoes are all in B.C. and Yukon. The Garibaldi Volcanic Belt is linked to Cascadia subduction. Other volcanic areas in northern B.C. and Yukon, such as Mount Edziza and the Tseax Cone, come from the crust being stretched. The Tseax Cone erupted roughly 300 years ago, and the Nisga'a people's oral histories describe the lava flow; today it is protected in the Nisga'a Memorial Lava Bed Provincial Park, which the Nisga'a Nation co-manages.
Earthquakes: Most of Canada's large earthquakes happen along the B.C. and Yukon coast because that is where plates meet.

But eastern Canada shakes too

Eastern Canada is in the middle of the North American Plate, far from any boundary. Yet it has intraplate earthquakes along old, weak zones in the crust, where ancient faults are squeezed by the slowly moving plate. Active zones include the Ottawa Valley (Western Quebec Seismic Zone) and Charlevoix, along the St. Lawrence River in Quebec. In 1929, an earthquake on the Grand Banks set off an underwater landslide and a tsunami that struck Newfoundland's Burin Peninsula.

Natural Resources Canada's Earthquakes Canada program records thousands of earthquakes in Canada every year. Most are too small for anyone to feel.

🔎 Geographer's toolkit: the magnitude scale

Earthquake magnitude measures the energy released. The scale is logarithmic:

Each whole step up means the ground shakes about 10 times more (measured on a seismograph).
Each whole step up releases about 32 times more energy.

So a magnitude 7 earthquake releases about 32 × 32 ≈ 1000 times as much energy as a magnitude 5. Magnitude is different from intensity, which describes how strongly shaking is felt and how much damage happens in one place. Intensity depends on distance from the epicentre, depth, type of ground, and how buildings are built.

Magnitude	Typical effect	Canadian example
under 3	Usually not felt; recorded by instruments	Most of the thousands recorded in Canada each year
about 5	Felt widely; light damage possible near the epicentre	Ottawa Valley, 2010 (about magnitude 5)
about 7 to 8	Major; serious damage possible near the epicentre	Haida Gwaii 2012 (7.7); Queen Charlotte 1949 (8.1)
about 9	Great; can cause huge tsunamis	Cascadia, January 26, 1700

Comparing magnitudes. Damage also depends on depth, distance, ground type and building quality.

Worked example

Worked example: How much bigger?

Compare the 1949 Queen Charlotte earthquake (magnitude 8.1) with a magnitude 6.1 earthquake. How many times more ground shaking and how many times more energy?

Difference in magnitude: 8.1 − 6.1 = 2 whole steps.
Ground shaking: ×10 for each step, so 10 × 10 = 100 times greater amplitude.
Energy: ×32 for each step, so 32 × 32 = 1024, about 1000 times more energy.

The 1949 earthquake shook the ground about 100 times more and released about 1000 times more energy than a magnitude 6.1 earthquake.

Worked example

Modelled example: Weighing evidence for the 1700 earthquake

How do we know a great earthquake struck Cascadia in 1700 when no written records exist from British Columbia at that time?

Physical evidence: ghost forests of dead red cedars along the coast; tree rings show they died between the 1699 and 1700 growing seasons.
Sediment evidence: layers of sand washed inland by a tsunami lie on top of buried marsh soil.
Written records: Japanese records describe an "orphan tsunami" (no earthquake felt in Japan) at the end of January 1700. Computer models of wave travel time point back to Cascadia.
Oral histories: First Nations stories along the coast describe a great earthquake and flood on a winter night.
Conclusion: several independent kinds of evidence agree, which makes the conclusion strong.

Different sources (tree rings, sediments, written records and Indigenous oral history) all point to the same event. When independent sources agree, the evidence is much more reliable (A1.3).

Hands-on activity

Model the plates and practise Drop, Cover and Hold On

Part A: plate model. You need two thick bath towels on a smooth table.

Lay the towels flat, edges touching. Push them toward each other: one may ride over the other, or both crumple up into "mountains" (convergent boundary).
Pull them apart (divergent boundary): what would fill the gap on the real sea floor?
Slide one past the other while pressing down. Notice how the edges catch, then jump (transform boundary, the way strain builds and then releases in an earthquake).

Part B: earthquake safety. Canada's official advice in an earthquake is Drop, Cover and Hold On: drop to your hands and knees, cover your head and neck under a sturdy table or desk, and hold on until the shaking stops. Practise it once. Then walk through one room and list two objects that could fall in a strong earthquake and how to secure them (for example, strap a tall bookcase to the wall with a grown-up's help).

What to look for: the student names each boundary type correctly and can explain that strain builds where plates are locked, then releases suddenly.

Just for fun

I'd tell you a joke about plate tectonics, but it's still in the works. It should be ready in a few million years.

Your turn

Name the three types of plate boundaries off British Columbia and give the name of each one.

Why do volcanoes form inland from a subduction zone, not right at the coast?

Why is the Cascadia Subduction Zone dangerous even though it has been quiet in living memory?

Why does eastern Canada have some earthquakes even though it is far from a plate boundary?

How many times more energy does a magnitude 7 earthquake release than a magnitude 6?

What is the difference between magnitude and intensity?

Name two kinds of evidence for the 1700 Cascadia earthquake.

More practice: warm-up

On which plate is almost all of Canada?

What is the highest magnitude earthquake recorded in Canada with instruments?

In which two areas are Canada's young volcanoes found?

What does "subduction" mean?

What should you do during an earthquake?

More practice: on your own

Compare a magnitude 5.0 and a magnitude 8.0 earthquake: how many times more ground shaking and energy?

Explain the interrelationship between plate tectonics and the shape of the Western Cordillera.

Two towns feel the same magnitude 7 earthquake. One has much more damage. Give three possible reasons.

Why is a tsunami more likely from a megathrust earthquake than from an earthquake on a transform fault?

Why do scientists treat First Nations oral histories of the great shaking and flooding as important evidence?

Challenge

Strain builds on the Cascadia Subduction Zone as the plates converge at roughly 4 cm per year. If the plates have been locked since 1700, about how much "slip" could be stored up by 2026? Give your answer in metres.

Assess this claim: "Ontario has no earthquake risk, so its buildings do not need any earthquake design."

Using the concept of spatial significance, explain why the same magnitude 9 Cascadia earthquake would have very different effects on Tofino (on the west coast of Vancouver Island) and on Calgary.

Check your understanding

Quiz

Which boundary type is the Queen Charlotte Fault?

What causes the volcanoes of the Garibaldi Volcanic Belt?

A magnitude 8 releases about how much more energy than a magnitude 6?

Which event in 1700 is known partly from Japanese tsunami records?

Earthquakes in the Ottawa Valley are best described as:

What is the recommended action during an earthquake in Canada?

For parents and teachers

Big idea: This lesson continues B1.2 (geological processes that formed and still shape Canada) and starts B1.3 (patterns in the distribution and severity of physical events). Students should see that Canada's earthquakes and volcanoes follow a clear spatial pattern tied to plate boundaries.

Common difficulties:

Magnitude arithmetic. Each step is ×10 shaking and ×32 energy; students often add instead of multiply. Have your teen write out 32 × 32.
"Eastern Canada can't have earthquakes." Intraplate earthquakes are less frequent but real.
Valuing evidence. Point out that Indigenous oral histories are treated by scientists as serious evidence, especially when they agree with physical data.

How to help: Visit Natural Resources Canada's Earthquakes Canada website together and look at the map of recent earthquakes. Ask: "What pattern do you see? Why there?" Practise Drop, Cover and Hold On as a family, and check that heavy furniture is secured.