Lab 6 - Volcanism online
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Western University *
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1083F
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Geology
Date
Jan 9, 2024
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5
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5 5. Exercise 1.
The diagram below shows an actively erupting volcano. Years after eruption has ceased, magma, lava, ash, and pyroclastic deposits will cool and solidify into various rock types, in, under, and around the volcano. Imagine you visited this volcano 100 years after eruption, and collected 5 groups of rock samples. a.
Study the 5 groups of samples (A, B, C, D, E on next pages), and make note of textures and composition. Then, based on your observations, decide whether the rock is intrusive or extrusive, and assign a rock name to each group of samples. (10 marks) Sample Textures Composition Intrusive/Extrusive Rock Name A B C D E
11 b.
On the diagram below, please indicate the areas where each of the 5 sample rocks (A,B,C,D,E) were likely collected. Remember, samples were collected AFTER eruption ceased and cooling took place (3 marks). Figure 1. Cross-section of an erupting volcano. 2.
The government of Columbia has hired you to assess the danger posed by a recent eruption. Although volcanic activity has subsided, several feet of ash have fallen near the volcano and the rainy season is beginning. The town of Armero is located 20 km from the base of the volcano in a narrow river valley and surrounded by small hills.
a.
What is the greatest volcanic hazard under these conditions? (1 mark) b.
Why? (1 mark) c.
Suggest a course of action to help the 23,000 inhabitants of Armero. (1 mark)
12 3.
The May 18, 1980 eruption of Mount St. Helens generated a horizontally directed series of explosions that formed a lateral blast. This blast destroyed an area of 370 square kilometers. Trees 2 m in diameter were mowed down like blades of grass as far as 24 km from the volcano. The blast exhibited characteristics of both pyroclastic flows and surges.
On the day of the main Mt. St. Helens eruption, seismometers 6 km from the blast origin stopped transmitting 77 seconds after the eruption, when they were overridden by a dense pyroclastic blast cloud. Based on this information, how fast (in km per hour) was the blast cloud moving? Show your math. (1 mark) 4.
Figure 2 shows the paths of two lahars that descended from Mt. Rainier in the past. What makes this a little scary is that neither of these events seems to be related to a major eruption of the volcano – a lahar can happen simply because of increased heat flow from some deep magmatic source melting the glaciers.
Figure 2. (left) Osceola and Electron Mudflows, Mt. Rainier, Washington (right)
Geological map of the area https://www.dnr.wa.gov/programs-and-
services/geology/geologichazards/volcanoes-and-lahars#washington-volcanoes.3
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13 The Osceola Mudflow downstream from Mt. Rainier, Washington, (~5000 years ago) is the maximum lahar at Mt. Rainier (see Figure 2) and is believed to have a recurrence interval ~10,000 years. It would have had a mean flow velocity of ~40 meters per second (m/s) at the base of the volcano, ~20 m/s at the Lowland boundary, and 10 m/s on the Lowland. a.
Assuming an average recurrence interval of 10,000 years for an Osceola-sized lahar event, how would you advise city planners in nearby Enumclaw, WA, regarding site location within city limits for an elementary school? a nuclear power plant? (2 marks) (There isn’t a right or wrong answer, but please explain your recommendations)
b.
From the map and using a piece of string (or equivalent), measure the distance the Osceola Mudflow traveled. Assume the lahar began at the summit of Mt. Rainier. (1 mark) c.
The speed of the front of a lahar has been measured for several recent events. The average speed seems to be about 40 m/s. Using this speed, how long after the initial event that triggered the mud to start moving at the summit did it take for the lahar to get to Greenwater? to Auburn? (Good thing that no one was living around there at the time!) (1 mark) 5.
Case I lahars (e.g., Electron Mudflow) have a recurrence interval of 500-1000 years. Even one event equal to or greater than a flow with a 1000-year recurrence interval has a 9.5% probability of occurring at least once in the next century.
What would you recommend to Orting city planners regarding the site location for an elementary school within city limits? Would it differ from your advice to Enumclaw regarding their elementary school? Explain. (2 marks) TOTAL: 23 marks