Plate Tectonics Shape (and Shake) British Columbia

Plate tectonics have shaped the continents for millions

of years. In British Columbia, they have created

the mountains, and are the source of

frequent earthquakes along the coast.

To understand plate tectonics, we

must rst understand the internal structure of the Earth. Figure 1 (left) shows a

cross-section of the Earth with its layers exposed. The solid inner core is

surrounded by the liquid outer core which is surrounded by the mantle.

Heat from the Earth’s core rises toward the surface, which causes the mantle

to circulate. The outermost layer of the Earth is broken into eight major plates and

dozens of smaller ones, and they move up to fteen centimetres a year.

The movement and interaction of these plates is referred to as plate tectonics.

Plate Tectonics Shape

(and Shake)

British Columbia

Earthquakes, Volcanoes

and Plate Boundaries

Many of the world’s earthquakes and

volcanoes are concentrated in distinct

zones along the edges of tectonic

plates. Most are the result of plates

colliding (at convergent boundaries),

pulling apart (at divergent boundaries)

or sliding past each other (at strike-

slip or transform boundaries). Shallow

earthquakes are in yellow, the deepest

are in red.

A map of many of the world’s earthquakes.

Government of Canada work on plate tectonics is undertaken by Natural Resources Canada’s Earth Sciences Sector

For more information visit the

Geological Survey of Canada, NRCan website:

EarthquakesCanada.nrcan.gc.ca

ISBN: Paper - Eng: M4-59/4-2010E 978-1-100-12100-0

PDF - Eng: M4-59/4-2010E-PDF 978-1-100-12101-7

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PAC IF IC

PLATE

NORTH

AMERICA N

PLATE

M A N T L E

PA R T I A L

ME LT I NG

CON TI NEN TA L

CRU ST

Mt. Baker

Mt. Meager

Mt. Garibaldi

OC EA NIC

CR UST

MELTING

1. Convergent

When two tectonic plates collide, one of two things happens. If they are

both continental plates, the edges of the plates crumple and deform,

creating mountain ranges like the Himalayas. However if one is a thin,

dense oceanic plate, like the Juan de Fuca Plate, and the other is a thick,

less dense plate like the North American Plate, the thin dense oceanic

plate sinks, or is subducted beneath the continental plate. This is known

as subduction. Scientists believe this gravity-controlled sinking is the driving

force behind plate tectonics. The Cascadia Subduction Zone (CSZ) extends

approximately 1000 km from northern Vancouver Island to northern

California. The world’s largest earthquakes occur in subduction zones,

like the March 2011 magnitude 9.0 earthquake in Japan.

2. Divergent

Roughly 280 km offshore of Vancouver Island, the Juan de Fuca Ridge is a “spreading

ridge” or long, linear submarine mountain range where new ocean oor is being

produced. Hot magma from the Earth’s mantle is injected into the fractures that run

the length of the ridge. New ocean oor crust is added to the Pacic Plate to the west

and to the Juan de Fuca Plate to the east. Each plate is forced away from the ridge

like a conveyer belt, as new ocean oor material is added. There are swarms of small

earthquakes along the ridge produced by this volcanic activity.

3. Strike-slip or Transform

At the Queen Charlotte Fault, the North American Plate is moving southeast and

the Pacic Plate is moving northwest relative to one another. They move past each

other, with many earthquakes along the transform boundary marking their passage,

including the largest earthquake ever recorded in Canada (1949, magnitude 8.1).

In California, the San Andreas Fault is a similar transform plate boundary where,

again, the Pacic Plate and the North American Plate are slowly sliding past

each other.

The line of volcanoes along the Pacic coast, including Mount Meager and Mount Garibaldi in Canada and

the Cascade Mountains in the U.S. (e.g. Mount Baker and Mount St. Helens), are a product of the subduction

zone. As the Juan de Fuca Plate subducts under the North American Plate, water from the Juan de Fuca Plate

rises, and causes the hot rocks above to melt into magma. This magma erupts through the North American

Plate surface as volcanoes. The most recent eruption in B.C. was at Mount Meager 2350 years ago.

Plate Tectonics in British Columbia

Off the west coast of British Columbia there are examples of

convergent, divergent and strike-slip or transform plate boundaries.

The Cascadia

Subduction

Zone.

Three types of

plate boundaries:

1. Convergent

2. Divergent

3. Strike-slip or

Transform.

Geological Survey of Canada

9860 West Saanich Road

Sidney, BC V8L 4B2

Tel: 250-363-6500

earthquakeinf[email protected].ca