GEOL 106 – Winter 2025

ASSIGNMENT 2– Natural Hazards in Western North and Central America

Due Date: February 14* (11:59 PM, Kingston time)

Mark: /30

A few things before starting.

• I provide an answer sheet (in Word format) where you can write your answers. Please use this sheet, it makes it easier for the TAs to be consistent in their marking. You need to save as a PDF to submit.

• note that you will be only able to upload the answer sheet as a PDF in OnQ

• information I give you to read through prior to the questions can be useful.

• don’t forget to include units in your answer, TA’s will take marks off for no units.

• I am looking for answers that fall within a range; don’t stress if you can’t get an ‘exact’ value

• information I give you to read through prior to the questions can be useful

*this is the due date without the 3-day grace period

For this assignment you will be visiting various locations along the west coast of North America (and into Central America), including the Pacific Northwest (PNW). There is a long history in the PNW (both in the geological record, historical record, and First Nation’s oral traditions) of catastrophic earthquakes, volcanic eruptions, and tsunamis. Even in the present day, this is a seismically and volcanically active region, which is also prone to mass wasting.

To help you in completing this assignment, you need to read/view these resources (PDF and links available in OnQ).

• Mount Rainier – Living Safely With A Volcano in Your Backyard USGS Fact Sheet

• https://www.canadiangeographic.ca/article/what-canada-can-learn-eruption-mount-st-helens

Part 1 - Earthquakes and Plate Tectonics

For the first part of the assignment, you will be using the IRIS Earthquake Browser. A link can be found in OnQ.

When you access the browser, you should see a screen that looks like the image below. You may need to zoom out.

A few things to note:

• on right hand side, you can set how many earthquakes you are viewing

• you can also select earthquake by largest or newest – in this view, I have it set at ‘Largest’

• further down on the right hand side there is an option to turn plate boundaries on/off

• there is a ‘Select New Region’ on the left-hand side, you will need to click it when ‘zooming in’ on one region in the world.

In the bottom left-hand corner you should see a column indicating earthquake depths in km (by colour, key below in questions as well) and circle sizes for different magnitudes. Earthquakes have a focus, the point inside the Earth’s crust where the earthquake originates. Magnitude refers to the overall size or ‘strength’ of an earthquake.

First ‘zoom’ into a region that includes the west coast of the US and Central America. Draw a box around the region shown to the left. You can click on the ‘Zoom to Region’ – on the left to zoom in.

There will be a lot more earthquakes showing – I have maximum earthquakes set to 10 for this example. I suggest setting maximum earthquakes to 500. Remember to select earthquakes by largest.

1. Select a region that includes the entire world (all continents except Antarctica) Select for a maximum of 10 earthquakes and select earthquakes by largest. Make sure you have checked the box for all magnitudes. You have created a map of the 10 largest earthquakes since 1970.

a. Turn the ‘show plate boundaries’ on. What type of plate boundary is associated with the 10 largest earthquakes? (1 mark)

Earthquake depths are often classified as shallow (0 to 70 km in depth), intermediate (70 to 300 km in depth) and deep (>300 km in depth).

b. Are the majority of the 10 largest earthquakes shallow, intermediate or deep? (1 mark) depth key: purple: 0 to 32.9 km, blue: 33 to 69.9 km, green: 70 to 150 km, yellow: 150 to 299.9 km, orange: 300 to 499.9 km, red: 500 to 800 km.

2. Look at the earthquake pattern along the west coast of the US, Mexico, and Central America.

a. As you move from the west coasts of Mexico, Guatemala, and El Salvador towards the interiors of these countries, do earthquake foci get deeper or shallower? (1 mark).

b. Is there a correlation between deeper earthquakes and the type of plate boundary? (1 mark). Describe this correlation? (1 mark)

Part 2 - Mount St. Helens, Washington and the Cascadia Volcanic Arc

On May 18, 1980, Mount St. Helens exploded in a cloud of ash, lava, pyroclastic flows and mud flows (lahars). What had been an iconic symmetrical snow-covered mountain with heavily forested slopes became an apocalyptic landscape of ash, mud and downed trees surrounding a fractured, asymmetrical peak. Geologists had been actively monitoring the mountain, and access to the area was restricted; still, 57 people were killed by the eruption and subsequent lahars, including a geologist who had been observing from a nearby ridge. The ash cloud reached 19 km (12 miles) into the atmosphere, where strong east winds scattered it across numerous states and cities, as well as into western Canada.

left image - ash column from the Mount St. Helens eruption pre-and-post eruption images of Mount St. Helens

The map below shows the direction in which the ash cloud travelled following the eruption. It also shows the time at which the ash reached a certain area. For example, if you lived in Butte, Montana, the ash cloud reached you after 3:45 PM (15:45). It is a good example of how a volcanic eruption can affect people living far away from the volcanic eruption. In the case of Mount St. Helens, 2 to 3 days later, fine ash was detected by systems used to monitor air pollution in several cities of the north-eastern United States. Some of the ash drifted around the globe within about 2 weeks. Airborne volcanic ash is also a threat to the global aviation industry.

Above image. Ash collected 39 km downwind in Randle, WA. Volcanic ash looks soft and powdery, but it actually consists of tiny sharp shards of volcanic glass and is harmful to people, machines, and structures. Photo by the USGS.

Part 2 - Questions

1. Examine the image of Mount St. Helens pre-eruption. What type of volcano is it? (1 mark)

Use the ashfall map above to answer the following questions.

2. Which series of cities were directly affected by the ashfall? (1 mark)

a. Yakima, Moscow, Boise and Jackson

b. Yakima, Moscow, Missoula and Jackson

c. Yakima, Ritzville, Missoula and Pocatello

3. In which city affected by the ashfall would you expect the accumulated ash layer to be thickest? (1 mark)

a. Jackson

b. Seattle

c. Pasco

d. Yakima

4. In which city affected by the ashfall would you expect the accumulated ash layer to be thinnest? (1 mark)

a. Pasco

b. Jackson

c. Yakima

d. Missoula

5. How fast the ash cloud was travelling. To get a sense, find out the rate as the ash cloud travelled from Mount St. Helens to Pasco, Washington? (1 mark)

Rate = distance/time

Distance – if you were an ash particle – 590 km

Time – eruption began at 8:32 AM, reach Pasco at 10:45 AM – what was the length of time in hours? I’ll supply the value – 2.2 hours

Using Google Earth or a similar program, search for Mount St. Helens (46.192342°, -122.196122°).

6. During the eruption event at Mount St. Helens there was a lateral blast. Examine the image of the mountain and determine if the blast occurred on the north, south, east, or west side of the mountain. (1 mark)

7. Go to the north end of Spirit Lake (46.284517°, -122.130506°; don’t forget the negative sign) and zoom into the grey area. What objects are present in the lake? These objects were knocked over as a result of the lateral blast. If you aren’t sure of what you are looking at, spend a couple minutes searching up Spirit Lake, Mount St. Helens on the web. (1 mark)

8. Zoom out and examine the region (from the Canadian border to California). Are there any other volcanos nearby? Describe the tectonic setting of the volcanoes in this region. Use the IRIS Earthquake Browser to help you answer this question. (2 marks)

9. Next, it is time to take a look at volcanoes in Canada. Go to 49.974982, -123.042626°. What you are looking at here is the remains of an extinct stratovolcano that last erupted 170,000 years ago. What is the name given to this feature? (not looking for a scientific name, also zoom in and take a look at this feature, it’s really cool). (1 mark)

10. Finally, travel north of Pemberton, B.C. to 50.633089°, -123.505161°. You are now looking at the Mount Meager Volcanic Complex (MMVC). Although there hasn’t been any recent volcanic eruptions, this complex is the location of many mass wasting events (we will learn more later on in the semester). The MMVC is referred to as ‘Canada’s Mount St. Helens’. Read the Canadian Geographic article available in OnQ and explain why the MMVC is compared to Mount St. Helens. (2 marks)

11. Natural hazards are linked. After reading the Canadian Geographic article, what natural hazard immediately preceded the Mount St. Helen’s eruption? (1 mark) There are concerns that a similar scenario may play out at Mount Meager.

Part 3 - Mount Rainier, Washington

If you travel ~ 144 km to the north-east of Mount St. Helens, you will reach Mount Rainier, another volcanic located in the Cascade Volcanic Arc. It is located ~ 95 km south/southeast of Seattle and Tacoma. It is the tallest mountain in the volcanic arc and is considered by geologists and emergency planners as one of the most dangerous volcanoes in the world. The most recent recorded volcanic eruption of the mountain was between 1820 and 1854.

Part 3 - Questions

1. Examine Mount Rainier (46.849560°, -121.759733°). What type of volcano is it? (1 mark)

2. Seattle is too far from Mount Rainier to receive direct damage from a volcanic eruption. Other nearby towns are not so lucky. Using the volcanic hazards map, what sort of volcanic hazard would you need to be concerned about if you lived in Ashford, Washington. (1 mark)

In 2 sentences, describe the volcanic hazard that you chose. (2 marks)

Volcanic Hazards Map

3. Sumner is located along the Puyallup River and is a community that will be alerted by the Mount Rainier lahar-warning system. If a lahar is detected, people have up to 3 hours to move to safety.

The image above is an elevation profile of the valley close to Sumner. What location do you think the people should evacuate to? (1 mark) Explain why. (1 mark)

4. List 2 reasons why Mount Rainier is considered one of the most dangerous volcanoes in North America. (2 marks)

Part 4 - Seattle, Washington

You have decided that Seattle is a relatively safe from volcanic hazards, so you decide to live there. But…you then discover that you need to worry about earthquakes and tsunamis. After all, you are living above the Cascadia Subduction Zone, which is capable of producing an earthquake with a magnitude of >Mw9.

Use the provided seismic hazards and tsunami inundation maps (next page) to answer the following questions.

Part 4 – Questions

1. Although not in Seattle, go to 46.692864°, -123.734812° and use Street View. Find and describe the blue and white sign that you find at these coordinates. Note – you might need to move around a bit to find the sign (1 mark)

2. Using the tsunami inundation map, which location will experience the greatest inundation by water during a tsunami? (1 mark)

a. Location A

b. Location B

c. Location C

3. Although it is inland from the coast, why do you think Location B will still experience flooding associated with a tsunami? (1 mark)

4. Which of these Seattle neighbourhoods has the lowest shaking risk? (1 mark)

a. Magnolia

b. Rainier Valley

c. Georgetown

d. Capitol Hill