GEOL1011_Major_AS3_13.11.2020

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GEOL 1011: Introduction to Geoscience 1 Jordy Major T00532765 GEOL 1011 Assignment 1 November 13 th , 2020 Assignment 3 This assignment is worth 12%of your total course mark. Please type your answers directly into this document and submit the assignment to your Open Learning Faculty Member. 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). 1. Describe the components of the lithosphere. ( 1 point ) The crust and outermost rigid mantle make up the lithosphere. The crust is composed of granite (continents) or basalt (beneath the oceans). The upper mantle is usually composed of peridotite (olivine and pyroxene). The property of the lithosphere is solid. 2. Explain how the behaviour of P waves differs from that of S waves. ( 2 points ) P waves are compression waves or a “push” wave while S waves have back and forth vibrations known as a shear wave. P waves travel faster and will pass through liquid while S waves will not and are slower. 3. Determine the latitude at your current location, and describe, in general terms, the orientation of the Earth’s magnetic field in this location. ( 2 points ) The latitude of my current location is 50.65. From this location, the Earth’s magnetic field is angled into the Earth. 4. According to Figure 9.19 in the textbook, the sea floor around Canada is generally subsiding. Explain why this is the case. ( 3 points ) The sea floor around Canada is generally subsiding because the mantle is slowly flowing back towards areas that are experiencing post glacial rebound. This is because during glaciation, mantle rock flowed away from areas below main ice sheets. 5. Explain what the differences between the left and right parts of Figure 10.6 tell us about plate tectonics. ( 2 points ) The difference between the left and right depicts that tectonic plates under North America and Europe have moved relative to each other, displaying continental drift in the right part. 6. The land area 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. ( 2 points ) TRU Open Learning

2 GEOL 1011, Major T00532765, AS3, 13/11/2020 Assignment 3 The North American continent is divided into the Juan De Fuca plate, the North American plate, and the Cocos plate. The Cocos plate is the smallest and you’d have to travel to the West coast of Mexico. 7. Describe the evidence that suggests that “slab-pull” plays an important role in the movement of tectonic plates. ( 2 points ) The evidence that suggests that “slab-pull” plays an important role in the movement of tectonic plates is that plates attached to subducting slabs move the fastest, and plates that are not subducting move slower. 8. 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 an earthquake, rapture surface is the displacement of rock. The magnitude is in proportion to the area of the rupture surface and the amount of rock that was displaced. 9. In the context of earthquakes, explain the difference between magnitude and intensity. ( 2 points ) In the context of earthquakes, the magnitude is the estimate of the energy released and the intensity is the assessment of what people felt and how much damage was done (specific to location). 10. Explain why it matters what type of geological material you are situated on when an earthquake strikes. ( 2 points ) It matters what type of geological material you are on when an earthquake strikes because loose sediment is less stable. The loose sediment causes the ground to shake more, creating a vibration that is amplified in weak material, leading to the collapse of structures. 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 th , 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. TRU Open Learning

GEOL 1011: Introduction to Geoscience 3 Used with permission of Government of Canada. Figure A3-1. Seismogram for the June 24, 1997 earthquake from the Nanaimo station. TRU Open Learning

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4 GEOL 1011, Major T00532765, AS3, 13/11/2020 Assignment 3 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) Maximu m amplitude Minimum amplitud e Average amplitude Nanaimo 64.1 67.7 3.6 1,000,000 850,000 925,000 Bowen Island 64.4 68.3 3.9 1,000,000 850,000 925,000 Port Renfrew 74.3 85.3 11 300,000 500,00 400,000 1. 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 ). Used with permission of Government of Canada. Figure A3-2. Seismograms for the June 24, 1997 earthquake from the Bowen Island and Port Renfrew stations. 2. For crustal rock in the BC southwest, the relationship between the S-P interval and distance is considered to be D = (T x 9.3) – 8.5 , where D is the distance from the earthquake hypocentre to the seismic station, and T is the S-P time. TRU Open Learning

GEOL 1011: Introduction to Geoscience 5 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 D = (3.9x9.3) – 8.5 D =27.77 km Port Renfrew D = (11x9.3)-8.5 D = 93.8 km 3. The locations of the three seismic stations are shown on Figure A3-3. Based on the distances estimated for the previous question, show on the map, or describe in words, the approximate epicentre of the earthquake. ( 4 points ) Based on the estimated distances, the approximate epicentre of the earthquake was beneath the Strait of Georgia, somewhere between Nanaimo and Bowen Island. Figure A3-3. Map showing the Port Renfrew, Nanaimo, and Bowen Island seismic stations. © Steven Earle. Used with permission. 4. 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 ) The significant difference in the amplitude of the S waves at Port Renfrew compared to Nanaimo and Bowen Island is that Port Renfrew is about 67km farther from the epicentre, causing the amplitude to be less because it is inversely proportional to distance. TRU Open Learning

6 GEOL 1011, Major T00532765, AS3, 13/11/2020 Assignment 3 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 U.S. Office of Naval Research undertook a systematic oceanographic survey of an area off the west coast of the U.S. 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 10.11 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 Morely). Vine, Matthews, and Morely (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. TRU Open Learning

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GEOL 1011: Introduction to Geoscience 7 Adapted from Shea, J.H. (Nov 1988). Understanding Magnetic Anomalies and Their Significance. Journal of Geological Education, 36(5), p298-305. Figure A3-4. Typical magnetic profile across a spreading ridge. 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). TRU Open Learning

8 GEOL 1011, Major T00532765, AS3, 13/11/2020 Assignment 3 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.) Adapted from Shea, J.H. (Nov 1988). Understanding Magnetic Anomalies and Their Significance. Journal of Geological Education, 36(5), p298-305. Figure A3-5. Magnetic profile across the East Pacific Rise at 51.6˚ S (east side). Adapted from Shea, J.H. (Nov 1988). Understanding Magnetic Anomalies and Their Significance. Journal of Geological Education, 36(5), p298-305. Figure A3-6. Magnetic profile across the East Pacific Rise at 51.6˚ S (west side). 1. 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? Explain. ( 3 points ) Yes, there is a reasonable degree of mirror image symmetry from the same general shape is being formed on opposite sides of the ridge. This happens from polarity reversal which is when the magnetic North flips to the South Pole, creating symmetry from there being opposite polarity on each side of the ridge. TRU Open Learning

GEOL 1011: Introduction to Geoscience 9 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. Adapted from Shea, J.H. (Nov 1988). Understanding Magnetic Anomalies and Their Significance. Journal of Geological Education, 36(5), p298-305. Figure A3-7. Magnetic profile across the East Pacific Rise at 47.7˚ S (east side). 2. 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 47.7  profile over that of the 51.6  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 90 65 210 230 Distance at 47.7˚ S (east side) (km) 50 85 130 160 210 Ratio (distance at 51.6˚/distance at 47.7˚) 1.04 1.06 .5 1.31 1.10 Date of magnetic event (Ma) 0.945 1.77 2.02 2.13 3.93 Spreading rate at 51.6˚ S (cm/y) 5.50 5.08 3.22 9.86 5.85 Spreading rate at 47.7˚ S (cm/y) 5.29 4.8 6.44 7.51 5.85 3. 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) The ratio information tells me that the 47.7 profile is spreading, on average, more than double than the 51.6 profile. TRU Open Learning

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10 GEOL 1011, Major T00532765, AS3, 13/11/2020 Assignment 3 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. 4. 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 half way between 0.92 and 0.97 Ma, which is 0.945 Ma, or 945,000 years. ( 5 points ) Adapted from Shea, J.H. (Nov 1988). Understanding Magnetic Anomalies and Their Significance. Journal of Geological Education, 36(5), p298-305. Figure A3-8. Magnetic chronology scale for the past 4.5 Ma. 5. Estimate the spreading rates of the Antarctic ridge 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 5 and 6 of the table above. Calculate the average rates at the two locations and report them here. ( 5 points ) Average of 51.6 = 5.9 cm/year Average of 47.7 = 6 cm/year 6. Are the calculated spreading rates reasonable? Explain your answer. ( 2 points ) The calculated spreading rates are reasonable because plates spread at rates of 1cm- 20cm per year. 7. Are you satisfied that Vine, Matthews, and Morely were correct in their interpretation of the sea-floor magnetic patterns? Explain. ( 5 points ) I am satisfied with their interpretation of the sea-floor magnetic patterns because evidence that the same general magnetic patterns are present straddling each ridge and TRU Open Learning

GEOL 1011: Introduction to Geoscience 11 its correspondence to the chronology of Earth’s magnetic field reversals is strong support for the VMM hypothesis. 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 hypotheses 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. TRU Open Learning

12 GEOL 1011, Major T00532765, AS3, 13/11/2020 Assignment 3 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, but if you use any outside sources, provide in-text citations. Use any referencing style that you are comfortable with. 1. Summarize the composition and physical properties of the major components of the Earth, including the continental crust, oceanic crust, asthenosphere, rest of the mantle, outer core, and the inner core. You can use a table if that works for you. ( 10 points ) Continental crust Granitic and low density. Composed of igneous, metamorphic, and sedimentary rocks (mostly granite). Oceanic crust Basaltic. Composed of magma. asthenosphere Partially liquid and low velocity. Made of the partially molten upper mantle that is plastically. mantle Upper and lower mantle are solid but plastic. The properties are of the rock peridotite (iron, magnesium-rich silicate minerals). The mantle surrounding the outer core is solid rock but plastic enough to slowly flow. Outer core Liquid because convection currents keep it from solidifying. Made of 85% iron metal and 15% nickel metal. Inner core Solid because S-waves stop at the inner core. Made of 85% iron metal and 15% nickel metal. 2. 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 help. ( 15 points ) The plate-tectonic setting of southwestern British Columbia is characterised by a pattern of sea-floor magnetism (the zebra striped area in the map below). The magnetized area causes seafloor spreading, which is when tectonic plates split apart from each other. Earthquakes and volcanos are created along transform faults which are connecting to spreading ridges. The sea floor magnetism is the responsible for the creating an environment for earthquakes and volcanos. TRU Open Learning

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GEOL 1011: Introduction to Geoscience 13 3. The 2010 Haiti earthquake caused around 100,000 deaths and billions of dollars in damage from which the country still has not fully recovered. Write a brief summary of the tectonic setting of this earthquake (names of plates and faults, type of fault motion) and include an explanation about why some parts of Port au Prince were severely devastated. Include a map if you think it would help. ( 10 points ) The tectonic setting of the 2010 Haiti earthquake was between the Caribbean and North American plates along the Enriquillo-Plantain Garden fault zone, which are left lateral-moving stroke slip faults (plates moves past each other horizontally). Some parts of Port-au-Prince were severely damaged because the epicenter was close to the city and the earthquake did not happen far beneath the surface (a close hypocentre), meaning it had more energy and caused more damage to buildings that were not built to withstand earthquakes. TRU Open Learning