1. We'll be talking a lot about 'Cascadia' (the Pacific Northwest including southwest BC) through the
earthquakes/volcanoes sections. Here is a sketch of a cross-section of the Cascadia area that we are part
of here in Vancouver (I'd expect you to be able to roughly sketch and label the basic types of plate
boundaries like this).

Note that this could be a cross-section through just about any oceanic-continental subduction zone.
Cascadia is a little unusual (but not too unusual) in that there is a ridgecrest within a few hundred
kilometres of the trench.

The sketch isn't exactly to scale but is reasonable. The bottom of the sketch is at roughly 150-250km. The
different shades/textures represent major differences in either composition or rheology of the rock.

Rather than label it by hand, your job is to match the labels on the drawing with a list I've provided. It's
fine to use labels more than once if you need to.

North American Plate, Pacific Plate, Juan de Fuca Plate, Nazca Plate, Farallon Plate, Atlantic Plate
Plate A =___________________, Plate B = __________________, Plate C _____________________

2. Same sketch as in the previous question, a cross-section through the ridge offshore and our
subduction zone here in Cascadia. Choose the best answer to the following layers.
[Note: be careful - There isn't anything tricky, but you have to look carefully to see some of the arrows that define
where the layer in question extends to/from. It's easy to go too quickly and make a mistake...]

Name of Layers 1, 2, 4, 5, 6, 7, 8
1.
2.
4.
5.
6.
7.
8.

3. This is a cross-sectional sketch of the Cascadia area. Compare this with the many different similar
figures in the pdf notes to make sure you are oriented as to what the layers are. The dashed arrows
indicate the motion of the plates and of the asthenosphere.
The sketch isn't exactly to scale but is reasonable. Match them up:

Name of Location X
Name of feature shown by Y
Thickness of Layer 1 (black layer)
Thickness of Layer 6
Fault type most likely to occur at f
Fault type most likely to occur near W
Fault type most likely to occur at e (see below Y, on the interface btw the black/grey)

4. Which of the following events has a similar physical mechanism to the rupture that causes an
'earthquake'?
A. A glass breaking when you drop it
B. Tearing a sheet of paper.
C. A broken elastic snapping back.

5. The most complete and meaningful way of characterising the size of an earthquake is the moment
magnitude. It requires more time to calculate than the simpler methods (e.g. Local (or Richter)
magntiudes) as several parameters must be determined.
Which of the following are necessary to calculate the moment magnitude?
[There are several correct answers]
a.

the area of the rupture zone along the fault

b.

the distance the fault plane slipped along the rupture

c.

maximum ground motion measured at the surface

d.

the physical properties of the fault zone rocks (e.g. rigidity, compressibility)

e.

the gradient of damage moving away from the epicenter

f.

the focal mechanism of the earthquake

6. On January 21st 2012, there was a M4.4 quake beneath the Aleutian Islands.
For more information, see: Fox Islands, Aleutian Islands, Alaska
There are maps and other information (like depth) on those pages, including historical seismicity maps.
From the location of the earthquake and its depth (see the map, you could plug the coordinates into
Google Earth if you like), it was most likely related to:
a.

Subduction-related faulting within the downgoing plate

b.

Subduction thrusting in the overriding plate

c.

Subduction thrusting on the plate interface

d.

Ridgecrest spreading (normal faulting)

e.

Transform motion (strike-slip faulting)

7. On Jan. 2nd 2012, there was a M4.8 quake in the Northern Atlantic Ocean basin.
For more information, see: Northern Mid-Atlantic Ridge There are maps and other information (like
depth) on those pages, including historical seismicity maps. From the location of the earthquake and its
depth (see the map, you could plug the coordinates into Google Earth if you like), it was most likely
related to: a. Subduction thrusting on the plate interface
b. Ridgecrest spreading (normal faulting)
c. Subduction thrusting in the overriding plate.
d. Subduction-related faulting within the downgoing plate
e. Transform motion (strike-slip faulting)

8. On November 17th 2009, there was a M6.6 quake near the southern Haida Gwaii (Queen Charlotte)
Islands. For more information, see: Queen Charlotte Islands Region
There are maps and other information (like depth) on those pages, including historical seismicity maps.
I recommend also referring to the pdf notes for this one to double check the nature of the plate
boundaries.
From the location of the earthquake and its depth (see the map, you could plug the coordinates into
Google Earth if you like), it was most likely related to:
a.

Subduction-related faulting within the downgoing plate

b.

Ridgecrest spreading (normal faulting)

c.

Subduction thrusting on the plate interface

d.

Transform motion (strike-slip faulting

e.

Subduction thrusting in the overriding plate.

9. Which of the following points are TRUE?
[There may be more than one correct answer, penalty for wrong answers]
When oceanic lithosphere is subducted into the asthenosphere and lower mantle, _________.
a. the entire lithospheric plate eventually melts and is reincorporated back into the convecting mantle
b. the seawater-saturated oceanic crust melts a little and releases water into the mantle above, which can
in turn melt, generating a volcanic arc on the surface above.
c. the subducting lithosphere gradually heats up to the temperature of the surrounding mantle and is
reincorporated back into the lower mantle.
d. earthquakes can occur within the subducting lithospheric plate down to the base of the transition
zone (~660-670 km depth)
e. Earthquakes commonly occur on the interface between the upper surface of the downgoing plate and
the asthenosphere and lower mantle. These earthquakes define the Wadati-Benioff zone.

10. Continental and island arc volcanism are quite similar, but have a few important differences. Which
of the following points are true (more than one may be correct, points deducted for wrong answers).
1. Many islands in the Caribbean are island arc volcanoes (look at a map of plate boundaries to decide)
2. Island arc and continental arc volcanoes are both related to subduction.
3. One main difference is that island arc volcanoes tend to be more mafic in composition than
continental arc volcanoes. Island arcs are more mafic>>intermediate while continental volcanoes would
be more intermediate>>felsic.
4. Hawaii is an island arc volcano.
5. Continental arc volcanoes are subduction related, island arc volcanoes are caused by plumes (or
hotspots).

11. Subducting plates produce a chain of volcanoes called a volcanic arc. Approximately how deep is
the upper surface of a subducting plate beneath the volcanoes?
a. 10 km
b. 50 km
c. 100 km
d. 200 km
e. 660 km
f. Anywhere from 0 to ~660 km

12. The image below is a cartoon of a young island arc. I say 'young' in that there are just individual
volcanoes built on top of the overriding oceanic plate - it hasn't developed into larger islands with
thickened crust yet (like say the Philippines or some of the Indonesian islands like Java and Sumatra well developed, older island arcs). So perhaps only 5-10 million years along in development.

Which of the points below are correct for island arc volcanoes?

[More than one answer may be correct. Marks deducted for wrong answers.]

a.

lavas are intermediate to mafic in composition

b.

lavas mostly felsic

c.

lavas very high in silica (SiO2) content: around 75%

d.

lavas mid-low Silica (SiO2) content: around 60%

e.

lavas very low silica content: < 50%

f.

Moderately viscous magma and gas in the magma, possibility for water to get into the magma,

potential to be quite explosive.
g.

Very fluid magma, little gas, not explosive

h.

. Volcano age (the age of the youngest lava erupted) gets progressively older from left to right
down the chain.

i.

Very gently sloped, shield volcanoes

j.

Composite volcanoes - steeper and built from layers of lava and ash on the exposed parts of the
volcanoes (above water)

k.

Composite volcanoes - steeper and built from layers of lava and ash on the exposed parts of the
volcanoes (above water)

l.

Melt feeding the volcanoes formed by melting in the mantle just above the subducting plate.

13. Which of the following statements about magma are FALSE?
a. The viscosity of a magma increases as the silica content decreases.
Note: high viscosity = thick, low viscosity = fluid
b. High temperature magmas are more fluid (less viscous) than low temperature magmas.
c. The more gas a magma contains, the less violent the eruption
d. Mafic magmas are very explosive
e. A magma can contain solids (unmelted minerals) and gas

14. Mount Baker (just southeast of Vancouver) and Mount Garibaldi (just south of Whistler and close to
Squamish) are examples of: (more than one choice is possible)
a. a cinder cone.
b. a stratovolcano (composite volcano)
c. a shield volcano
d. plume (or hotspot) volcanism
e. continental arc
f. island arc

15. You live 15 km away from the base of a glacier-covered stratovolcano (perhaps Mt. Baker, or Mt.
Rainier, or Mt. Fuji in Japan). Your house is in a lovely, scenic river valley (the river formed by drainage
from the runoff and snow/ice melt from the volcano). And, just because of the prevailing wind patterns,
your house is generally downwind from the volcano.
Which order of hazards (from extremely hazardous to not very) correctly describes the situation?
Most hazardous:
A likely problem, but not extremely hazardous
Hazardous but not that likely:
Not a problem:

16. The Cascades volcanoes present many hazards. Mount Rainier is the largest of the Cascades
volcanoes and it is located just east of Seattle/Tacoma. There is a very large population living to the west
of the mountain.

Like all the Cascades volcanoes, it is potentially explosive (and has explosively erupted in the past). So
that hazard is always there. However, lahars present a very different hazard - and one that also has a
very long, well-documented geological record.

Which of the following statements are true?
(Mark all correct answers, marks deducted for wrong answers).
a.

As long as Mt. Rainier remains dormant (not erupting), then the people in the region don't need to
be worried about lahars.

b.

One of the big problems with a lahar is that they flow too quickly to warn people in its path.

c.

Lahars are potentially dangerous for the entire area around the mountain within about 40 km.

d.

Mt. Rainier has the ingredients required for lahars to form: glaciers (providing water), major
rainfall/snow zone, highly unstable edifice.

e.

Lahars are mudflows that have quite a low percentage of water (so very high density).

17. Partial melting is a very important process for modifying the continental crust.
Think about what happens in a subduction zone environment, what causes the melting and how that
process works in relation to the subducting lithosphere.
A range of magmas erupt from continental volcanoes (like Mt.Baker, Mt. Garibaldi, Mt. St.Helens), from
quite mafic to quite felsic. It is the felsic lavas that are the most explosive.
a.

It represents melted oceanic crust (melted during subduction).

b.

It represents the melted mantle rock above the subducting oceanic plate, which was triggered to
melt by water released by the subducting oceanic crust.

c.

It is a melt or partial melt of the continental crust (overriding plate).

d.

It is a partial melt of the subducting continental crust.

18. Hot spot volcanism is caused by mantle plumes - relatively narrow columns of buoyant, hot
ultramafic mantle rock, rising up to the crust. When the pressure decreases enough, there is partial
melting and basaltic magma rises up through the crust.
There are hot spot volcanoes on oceanic and continental crust.
Which of the following statements are true?
(more than one may be correct)
a. Mantle plumes or hotspots can generate felsic volcanism at both oceanic or continental volcanoes.
b. The type of volcano (stratovolcano) produced by a mantle plume is the same on both oceanic and
continental crust
c. Mantle plumes or hotspots can generate mafic volcanism at both oceanic or continental volcanoes.

d. Hot spot volcanoes built on continental crust tend to be much more explosive than those on oceanic
crust.

19. A chain of volcanoes that exhibits a clear progression in eruption age from recent at one end to older
and older along the chain is most likely a/an:
a. island arc
b. continental arc
c. hot spot track
d. sequence of impacts
e. supervolcano
f. mid-ocean spreading ridge

20. Which of the following statements about isostasy are true?
[Note: More than one answer may be possible]
a.

Isostasy represents how an object will reach a state of buoyancy balance that depends on
its density and that of the fluid (liquid or ductile solid) that it 'floats' in.

b.

An object that is less dense than the material it is floating in will float completely on top of the fluid
(no part will be submerged because it is less dense).

c.

Normal continental crust, even when it is cold, is less dense than the ultramafic mantle rock.
Therefore, it 'floats' in isostatic balance on/in the mantle and cannot subduct.

d.

The lithosphere is rigid and therefore supports all loads without bending into the more ductile
asthenosphere.

e.

The lithosphere is rigid and therefore supports all loads without bending into the more ductile
asthenosphere.

21. My friend Laurent is skiing below the beautiful granitic peak called Mt. Thiassi (about 30 km north
of Pemberton). Several of the following statements are true, which ones?
A. Granite is an igneous rock and because it is at the surface it is an
extrusive rock.
B. The mountain is likely part of an old batholith or pluton that is
now exposed at the surface
C. Erosion from the top of the thickened crust results in the crust
buoyantly rising in response (due to isostacy). This exposes rocks at
the surface that were originally deeper in the crust.
D. All the good powder skiing is in the Rockies, not the Coast Range
(very important you get this one correct...)