Please type your answers directly into this document and submit the assignment to your Open Learning Faculty Member for grading when you are finished. Do not remove the questions, or the number of points for each question, from the document.
Please remember to reference concepts that you get from outside sources. Concepts from the textbook and course units do not need to be referenced, but images, tables, or copied fragments of text longer than a few words from any source need to have in-text citations and references. Refer to the “TRU Citation Guides” for citing references in your assignments. If you’re not sure about what you need to reference, please see Student Academic Integrity and refer specifically to the plagiarism section (VI, 4).
Part A: Short-Answer Questions (20 points in total)
Answer the following questions as succinctly as you can. None of the answers should be more than a couple of sentences (100 words or less).
- Describe the components of the lithosphere. (2 points)
- Explain how the behaviour of P waves differs from that of S waves. (2 points)
- According to Figure 9.4.5 in the textbook, the sea floor around Canada is generally subsiding. Explain why this is the case. (3 points)
- Explain what the differences between the left and right parts of Figure 10.3.1 tell us about plate tectonics. (2 points)
- The land area of the continent of North America is underlain by three different plates. Name those plates, and describe where you’d have to go to stand on the one that makes up the least part of the continent. (3 points)
- Discuss the evidence that suggests that “slab pull” plays an important role in the movement of tectonic plates. (2 points)
- What is a rupture surface in the context of an earthquake, and how is it related to the magnitude of the earthquake? (2 points)
- In the context of earthquakes, explain the difference between magnitude and intensity. (2 points)
- Explain why it matters what type of geological material you are situated on (or that a building is constructed on) when an earthquake strikes. (2 points)
Part B: Exercises (45 points in total)
B1: Interpreting seismic records (15 points)
Figure A3-1 is a record—from a seismic station just north of Nanaimo—of a small earthquake that occurred near to Vancouver Island on June 24, 1997. Features that are important from the perspective of using seismic data to locate earthquakes and determine their magnitude are labelled in red, including the times of the arrival of the first P wave and the first S wave, and the minimum and maximum amplitudes of the S wave.
Figure A3-1. Seismogram for the June 24, 1997 earthquake from the Nanaimo station. Used with permission of Government of Canada.
The important values that can be interpreted from this seismogram are listed in the following table.
Table A3-1. Seismogram parameters for the June 24, 1997 earthquake.
| Station | P arrival (sec) | S arrival (sec) | S-P interval (sec) | Maximum amplitude | Minimum amplitude | Average amplitude |
| Nanaimo | 64.1 | 67.7 | 3.6 | 1,000,000 | 850,000 | 925,000 |
| Bowen Island | ||||||
| Port Renfrew |
- Seismograms for the same event from stations at Bowen Island and Port Renfrew are shown in Figure A3-2. Using the Nanaimo seismogram as a model, complete the two remaining rows of Table A3-1. (6 points).
Figure A3-2. Seismograms for the June 24, 1997 earthquake from the Bowen Island and Port Renfrew stations. Used with permission of Government of Canada.
- For crustal rock in the BC southwest, the relationship between the S-P interval and distance is considered to be D = (T × 9.3) – 8.5, where D is the distance from the earthquake hypocentre to the seismic station, and T is the S-P time. For the Nanaimo station, that works out to 25.0 km. Determine the equivalent distances for Bowen Island and Port Renfrew. (2 points)
| Nanaimo | 25.0 km | Bowen Island | Port Renfrew |
- The locations of the three seismic stations are shown on Figure A3-3. Based on the distances estimated for the previous question, show the approximate epicentre of the earthquake on the map, and explain your rationale for choosing that location. (4 points)
Figure A3-3. Map showing the Port Renfrew, Nanaimo, and Bowen Island seismic stations. © Steven Earle. Used with permission.
- Comment on what you think accounts for the significant difference in amplitude of the S waves at Port Renfrew compared with the S waves at Nanaimo and Bowen Island. (3 points)
B2: Understanding patterns of sea-floor magnetism (30 points)
A magnetometer is an instrument used to measure very small variations in the magnetic intensity of the upper part of the Earth’s crust. Magnetometers can be moved around on land (usually by a person on foot), used in the air (towed beneath an aircraft), or used at sea (towed behind a ship). Regional studies of magnetic variations are useful for geological mapping because they provide general information about variations in rock types (e.g., granite versus basalt) and the presence of rocks that have significantly more magnetic minerals than other rocks (e.g., iron ores with magnetite). Magnetic surveys were first carried out at sea in the 1940s, but the results showed confusing variations between high and low intensities that appeared to bear no relationship to the geology of the ocean floor.
In the mid-1950s, the US Office of Naval Research undertook a systematic oceanographic survey of an area off the west coast of the US and Canada. After much persuasion, they agreed to a request from the Scripps Institute of Oceanography to tow a magnetometer behind the vessel.
The results of this survey, which, for the first time, included many precisely located parallel survey lines, are shown in Figure 10.3.7 in the textbook—a pattern of contrasting strips of positive magnetism (black areas) and negative magnetism (white areas).
In the following years, similar surveys were done in other areas—with similar results—but the origin of the patterns remained a mystery until 1963 when a solution was proposed by a Cambridge University graduate student (Fred Vine) and his thesis advisor (Drummond Matthews), and (independently) by a Geological Survey of Canada geologist (Lawrence Morley).
Vine, Matthews, and Morley (VMM) suggested that the patterns could be related to the creation of new oceanic crust at a spreading centre, and to the periodic reversals of the Earth’s magnetic field. Their theory suggested that as new basaltic crust is created, its minerals (particularly magnetite) become magnetized in alignment with the existing magnetic field of the Earth. Rock formed during a period of normal magnetism will have a positive magnetic anomaly because the rock has the same polarity as the Earth’s existing magnetic field, whereas rock formed during a period of reverse magnetism will have a negative magnetic anomaly. The stripes on the ocean floor, it was suggested, represent different ages of oceanic basaltic rocks that have been pushed away to either side of a spreading centre and replaced by younger basaltic rock, as illustrated in Figure A3-4.
Figure A3-4. Typical magnetic profile across a spreading ridge (adapted from Shea, 1988).
In the beginning, the VMM hypothesis was largely ignored, first because in the early 1960s, the idea of sea-floor spreading itself was not well accepted; second, because the chronology of magnetic-field reversals was not well known; and third, because not enough sea-floor magnetic data was available to test the hypothesis. However, within a few years, a lot more data became available, and after other researchers had the opportunity to verify the phenomenon in different locations, the VMM theory became widely accepted, and in fact became a crucial piece of evidence for continental drift and plate tectonics a few years later.
For this exercise, we need to start by making some predictions based on the VMM hypothesis, and then use the available magnetic data to test them. Some useful predictions are as follows (although you might be able to think of others as well):
- Since the spreading at a ridge is expected to be symmetrical on either side of the ridge axis, the pattern of positive and negative magnetism also should be symmetrical.
- Since magnetic field polarity reversals have a global effect, magnetic profiles at various points along a ridge, and at different ridges around the world, should be generally comparable.
- The positive and negative magnetic features should correlate with the known chronology of magnetic field reversals.
- The corresponding rates of spreading (as determined from the magnetic chronology) should be consistent with typical oceanic-ridge spreading rates (i.e., a few cm/year).
Symmetry across the ridge
Profiles of the magnetic patterns on either side of the East Pacific Rise at 51.6°S are shown in Figures A3-5 and A3-6. Compare the profiles peak for peak and valley for valley. (In each case, the 0 km point is where the spreading ridge is located.)
Figure A3-5. Magnetic profile across the East Pacific Rise at 51.6˚ S (east side; adapted from Shea, 1988).
Figure A3-6. Magnetic profile across the East Pacific Rise at 51.6˚ S (west side; adapted from Shea, 1988).
- Is there a reasonable degree of mirror-image symmetry in these patterns? In other words, do you think that the patterns show the same general shape on opposite sides of the ridge? Describe the evidence that supports your answer. (3 points)
Correlation along the ridge
A magnetic profile on the same ridge at 47.7° S (approximately 450 km from the other profile) is shown in Figure A3-7. Compare this profile with those of the preceding figures. Identify the same five peaks (A through E) that are labelled on the other profiles.
Figure A3-7. Magnetic profile across the East Pacific Rise at 47.7˚ S (east side; adapted from Shea, 1988).
- For each of the labelled peaks on the two east-side profiles, measure the distance from the ridge, and record the information in the first two rows of following table. Then calculate the ratio of the distance on the 51.6° profile over that of the 47.7° profile (east side), and record that in the ratio row.The first one (A) is done for you. (8 points)
| A | B | C | D | E | |
| Distance at 51.6˚ S (east side) (km) | 52 | ||||
| Distance at 47.7˚ S (east side) (km) | 50 | ||||
| Ratio (distance at 51.6˚/distance at 47.7˚) | 1.04 | ||||
| Date of magnetic event (Ma) | 0.945 | ||||
| Plate motion rate at 51.6˚ S (cm/y) | 5.50 | ||||
| Plate motion rate at 47.7˚ S (cm/y) | 5.29 |
*You will complete the last three rows of the table in questions 4 and 5.
- What does the ratio information tell you about the relative rates of spreading at these two points 450 m apart on the same ridge? (2 points)
Correlation with the magnetic time scale
The magnetic reversal time scale for the past 4.5 Ma, which is primarily derived from careful work carried out on rocks of the continental crust, is shown in Figure A3-8.
- Correlate the peaks that you selected on the magnetic profiles with the various events described on the magnetic chronology scale, and record the dates of the features in the fourth row of the table above. Note that points labelled A to E are the tips of the magnetic peaks. They correspond, therefore, with the centre points of the various positive (normal) magnetic events.
For example, peak A represents the midpoint of the Jaramillo event (as shown in blue), and the time should be halfway between 0.92 and 0.97 Ma, which is 0.945 Ma, or 945,000 years. Identify the normal events that correspond with peaks B through E. If you do not understand how to answer this question, please ask your Open Learning Faculty Member for assistance. (5 points)
Figure A3-8. Magnetic chronology scale for the past 4.5 Ma (adapted from Shea, 1988).
- Estimate the plate motion rates of the east side of the East Pacific Rise at 47.7° and 51.6°. Divide the distances (km) by the number of years, and then convert those numbers to cm/year (multiply by 100,000), and put those numbers into rows five and six of the table above. Calculate the average rates at the two locations, and report them here. (5 points)
- Are the calculated plate motion rates reasonable? Explain your answer. (2 points)
- Are you satisfied that Vine, Matthews, and Morley were correct in their interpretation of the sea-floor magnetic patterns? Explain why or why not.
(5 points)
- If you are satisfied, then you have confirmed the VMM hypothesis. Although this does not mean that it has been proven to be correct, if it can pass several such tests, it can be upgraded from a hypothesis to a theory. In fact, the VMM hypothesis regarding the interpretation of sea-floor magnetic anomalies has been part of the comprehensive theory of plate tectonics since the late 1960s.
Reference
This exercise, including the various figures, has been adapted with permission from:
Shea, J. H. (1988). Understanding magnetic anomalies and their significance. Journal of Geoscience Education, 36, 298–305.
Part C: Longer Questions (35 points in total)
Please answer the following questions. Write as much as you think is necessary to answer each question, but don’t forget that someone has to read what you write, so be as concise and clear as possible. You do not need to reference the textbook or the material in the course units (except images and quotations), but if you use any outside sources, provide in-text citations. Use any referencing style that you are comfortable with.
- Summarize the composition and physical properties of the major components of the Earth, including the continental crust, oceanic crust, lithosphere, asthenosphere, rest of the mantle, outer core, and inner core. You can use the following table if that works for you. (15 points)
- Describe the plate-tectonic setting of the southwestern part of British Columbia (from southern Vancouver Island to Haida Gwaii), including the nearby offshore regions, and explain how plate tectonics is responsible for earthquakes and volcanoes in this region. Include a map if you think it would be helpful. (20 points)
