INSTRUCTOR GUIDE Chapter 4 Paleomagnetism and Magnetostratigraphy

Transcription

1 INSTRUCTOR GUIDE Chapter 4 Paleomagnetism and Magnetostratigraphy SUMMARY Reversals of our planet s magnetic field throughout geologic time have provided Earth scientists with a distinct barcode record of normal and reversed polarity that is preserved in deep-sea sediments and in oceanic crust. In Part 4.1, you will consider the nature of Earth s magnetic field today and explore the nature of a paleomagnetic record preserved in deep-sea sediments. Part 4.2 explores the nature of the paleomagnetic record preserved in ocean crust as positive and negative magnetic anomalies and relates the anomaly pattern ( barcode ) observed in the ocean basins to the paleomagnetic record preserved in deep-sea sediments. In Part 4.3, you will see how paleomagnetism was used to test the Seafloor Spreading Hypothesis. In Part 4.4, you will learn how paleomagnetism has been used to create the geomagnetic polarity timescale of the past 160 million years and how paleomagnetic, radiometric, and biostratigraphic data are integrated to provide methods of age determination. FIGURE 4.1. The Earth s magnetic field protects our planet from much of the deadly radiation and charged particles (solar wind) streaming away from the Sun, as depicted in this NASA diagram. This magnetosphere is compressed on the day (Sun) side of the Earth and smeared out on the night side. Times of strong solar wind interact with the magnetosphere near the north and south magnetic poles and are observable on Earth as aurora ( Northern and Southern Lights ). Figure courtesy of NASA. Page 1 of 40

2 Goal: to explore the nature of Earth s magnetic field, the paleomagnetic record of deep-sea cores, and its applications for age determination. Objectives: After completing this exercise your students should be able to: 1. Describe the nature of the Earth s magnetic field, polarity, and the relationship between magnetic inclination and latitude. 2. Compare and contrast magnetostratigraphic records from the Northern and Southern Hemispheres. 3. Describe the nature of magnetic reversals using evidence from marine sediments and ocean crust. 4. Explain how paleomagnetic evidence was used to support the sea floor spreading hypothesis. 5. Explain how the Geomagnetic Polarity Timescale (GPTS) was developed, its value, and its organization. 6. Determine the age of a sedimentary sequence by interpreting a paleomagnetic record and correlating it to the GPTS. I. How Can I Use All or Parts of this Exercise in my Class? (based on Project 2061 instructional materials design.) Title (of each part) How much class time will I need? (per part) Can this be done independently (i.e., as homework)? Part 4.1 Part 4.2 Part 4.3 Part 4.4 Earth s Magnetic Field & Paleomag Record of Deep Sea Seds Paleomagnetism in Oceanic Crust Testing Seafloor Spreading Hypothesis Geomagnetic Polarity Timescale minutes 20 minutes 30 minutes 25 minutes Yes, but best to do as an in-class scaffolding intro to paloemag Yes, with followup discussion Yes, with followup discussion Yes, with follow-up discussion What content will students be introduced to in this exercise? Science as human endeavor X X X X Science as evolving process/nature of science X X X X Earth history archives (nature of the sedimentary record) X X X X Stratigraphic principles X X X X Magnetism as an age X indicator X X X Geologic timescale and/or Geomagnetic polarity timescale X Sed accum rates, age-depth plots X Calibration and correlation of different types of data X X X What types of transportable skills will students practice in this exercise? Make observations (describe what you see) X X X X Page 2 of 40

3 Plot data, determine lines of best fit, interpret graphs, diagrams, photos, tables Pose hypotheses or predictions Perform calculations & develop quantitative skills Decision-making, problem solving & pattern recognition What general prerequisite knowledge & skills are required? What Anchor Exercises (or Parts of Exercises) should be done prior to this to guide student interpretation & reasoning? What other resources or materials do I need? (e.g., internet access to show online video; access to maps, colored pencils) What student misconception does this exercise address? What forms of data are used in this? (e.g., graphs, tables, photos, maps) What geographic locations are these datasets from? How can I use this exercise to identify my students prior knowledge (i.e., student misconceptions, commonly held beliefs)? How can I encourage students to reflect on what they have learned in this exercise? [Formative Assessment] How can I assess student learning after they complete all or part of the exercise? [Summative Assessment] X X X X X X X X X X X Basic geography Basic geography Basic math and geography Chapter 1 Intro to Paleoclimate records; Chapter 2 Seafloor Sediments X None Part 4.1 Part 4.2 Parts 4.2 and 4.3; Chapter 3 Microfossils and Biostrat. None None None None Earth s magnetic field has not been stable through geologic time Plots of magnetostrat data vs. depth in deep sea drill sites NW and SW The seafloor does not have black and white stripes Bathymetric profiles with data; magnetostrat in deep sea drill sites East Pacific and Bathymetric map; x-y data plots South Atlantic X Paleomagnetic barcode can be used to build a geologic time scale Geomagnetic Polarity Time Scale; magnetostrat in drill sites NW Pacific Pacific Ocean NW Pacific Part 1 of this exercise module is designed as an initial inquiry aimed at drawing out student beliefs and prior knowledge. This can be very effective as an in-class (lecture) exercise. Parts 2-4 can be done in class but are probably best assigned as homework, or used in a laboratory section. The various parts often lead with tasks or questions that can further identify student prior knowledge. Exercise Parts can be concluded by asking: On note card (with or without name) to turn in, answer: What did you find most interesting/helpful in the exercise we did above? Does what we did model scientific practice? If so, how and if not, why not? See suggestions in Summative Assessment section below. Page 3 of 40

4 Where can I go to for more information on the science in this exercise? See the Supplemental Materials and Reference sections below. II. Annotated Student Worksheets (i.e., the ANSWER KEY) This section includes the annotated copy of the student worksheets with answers for each Part of this chapter. This instructor guide contains the same sections as the student book chapter, but also includes additional information such as: useful tips, discussion points, notes on places where students might get stuck, what specific points students should come away with from an exercise so as to be prepared for further work, as well as ideas and/or material for mini-lectures. Page 4 of 40

5 Part 4.1. Earth s Magnetic Field Today and the Paleomagnetic Record of Deep-Sea Sediments 1 Do the following to describe the Earth s magnetic field as you currently understand it: (a) The circle below represents the Earth. Draw a sketch of the Earth and its magnetic field by putting in the magnetic lines of force; label the equator and the North and South poles. You can ask for a volunteer to go to the board or overhead projector and draw a sketch of the Earth with its magnetic field. Then ask if anyone wants to add to, or modify the first volunteer s interpretation/representation. You can expect a range of possibilities, but below is a sketch that can help you focus a classroom discussion. The class is likely to grasp the concept of dipolar field with north and south magnetic poles. But two other important concepts: 1) the Earth s magnetic field lies close to the spin/rotational axis of Earth, and 2) magnetic lines of force strike the Earth at different angles (= magnetic inclination or magnetic dip); this latter concept will be explored more below in Question 2. (b) Do the magnetic lines of force intersect the Earth s surface everywhere at the same angle? No. The magnetic lines of force intersect the Earth s surface at various angles. The inclination at the equator is 0º and at the poles it is 90º. Page 5 of 40

6 EARTH S MAGNETIC FIELD Figure 4.1 depicts the Earth s magnetosphere and Figure 4.2 depicts the behavior of iron filings in the presence of a magnetic field (you may remember doing this little experiment in elementary or middle school). Note the general similarity in the shape of the magnetic field in these two diagrams. The Earth s magnetic field approximates a dipole bar magnet and is generated within the liquid outer core owing to our planet s rotation about an axis defined by the North and South geographic (true) poles. The magnetic poles are close to Earth s rotational axis (i.e., the magnetic field approximates the spin axis of the Earth), but today the magnetic north pole and magnetic south pole are offset from true North and South by approximately 11. The Earth s magnetic field is dynamic and complex and the magnetic poles are not stationary. In 2005, the north magnetic pole was at 82.7 N, W, moving northwest at approximately 40 km/yr, while the south magnetic pole (2001) was at 64.7 S, E (NOAA National Geophysical Center, see also Frequently Asked Questions on the NGDC site ). A compass points to the magnetic north pole in response to the magnetic lines of force generated by our magnetic field, similar to the bar magnet and iron filings (Figure 4.2). Likewise, magnetic minerals in sediments and rocks will behave like tiny compass needles that align with the Earth s magnetic field at the time they are deposited (in the case of sediments), or crystallized as molten magma cools (in the case of igneous rocks). In other words, magnetic minerals in sediments and rocks become locked in and therefore capture and preserve the nature of the magnetic field at that particular location for that particular time in Earth history. The study of the geologic record of Earth s magnetic field and its changes is called Paleomagnetism. FIGURE 4.2. Bar magnet with iron filings showing magnetic lines of force. From = Here is another link Inclination (or magnetic dip) is the angle between the orientation of a magnetic mineral grain in a rock or in sediments and the (horizontal) surface of the Earth at a particular location. Based on the similarity in behavior of the Earth s magnetic field and the magnetic lines of force generated by a bar magnet, predict the following: Page 6 of 40

7 (a) Where on Earth s surface would you expect magnetic minerals found in sediments and igneous rocks to have very little or no inclination? Why? Little or no inclination: near the equator, where the magnetic lines of force are roughly parallel to the surface of the Earth. (b) Where on Earth s surface would you expect the magnetic minerals to have the steepest inclination? Why? Steepest inclination: near the (magnetic) poles, as observed in the bar magnet analogy. See diagram from 1(a) above. 3 If the inclination is into the Earth at or near the magnetic north pole, what is the nature of the inclination at or near the south magnetic pole? Explain. Today, the magnetic lines of force are going into the Earth near the magnetic north pole (= positive values), therefore they must be coming out of the Earth near the magnetic south pole (= negative values). This idea of inclination into and out of Earth will become relevant in the questions and discussion that follow regarding the preserved record of magnetic field reversals. 4 Drawing from your answers in Questions 2 and 3 above, and your general knowledge of plate tectonics, predict how the inclination (magnetic dip) preserved in older rocks might be used to determine past lithospheric plate positions? Inclination is a function of latitude (tan I = 2 tanλ, where I = inclination, and λ = latitude at the time of rock formation; solve for λ), therefore past plate positions in a north-south reference frame can be inferred, assuming a dipolar field similar to today s field. Natural Remanent Magnetization in Sediments and Igneous Rocks The magnetic signal that geoscientists are interested in measuring is called natural remanent magnetization (NRM). This signal is carried by magnetic minerals such as magnetite. The NRM is preserved in ocean crust (basalt) as it cools and crystallizes past the Curie temperature (approximately 580 C for the mineral magnetite), or in sediments as detrital magnetic mineral grains (eroded from another source) accumulate on the seafloor. In both situations, the magnetic minerals become aligned with the Earth s magnetic field at the time of crystallization and deposition, respectively. Not all rocks are good carriers of a magnetic signal. The basalt that makes up oceanic crust is a magnetite-bearing rock, while granite, a typical igneous rock of continental crust, has a generally poor content of magnetic minerals. Likewise, terrigenous sediments (e.g., sand, mud, and clay) derived from the erosion of continental rocks have a much higher concentration of detrital magnetic mineral grains than does a pure biogenic sediment, such as calcareous or siliceous ooze. Figures 4.3(a) and (b) depict the magnetic character of two deep-sea sediment cores. Site 1149 (Figure 4.3a) is located in the northwest Pacific (approximately 31 N latitude) near the Marianas Trench. Site 1172 (Figure 4.3b) is located in the southwest Pacific (approximately 44 S latitude) near Tasmania (Figure 4.4). Both sedimentary sequences represent continuous sediment deposition over the past 6.5 million years or so. The x-axis shows inclination (magnetic dip); positive inclination values are into the Earth, negative values are out of the Earth. The y-axis is depth in the core in meters below seafloor (mbsf). Page 7 of 40

8 Remind students that in this exercise they are examining the paleomagnetic signal preserved in sediments cored in the deep-sea; the youngest sediments are at the top (0 = present day seafloor), with sediment age increasing with increasing depth in the core. Each of these records represents just part of the sedimentary sequence cored at these two sites. Also please note: these plots come straight from their sources; they have not been redrafted here. This emulates how a scientist goes to the primary literature and finds that different authors present their data in different ways (e.g., style of graph, or scale used, etc.). 5 Make a list of observations about the nature of the paleomagnetic record in these two deep-sea sediment sequences. How are they similar and how are they different? Observations: Earth s magnetic field has reversed polarity many times in the past 6.5 Ma These changes in polarity, called reversals, are very rapid (i.e., change in inclination from positive values to negative values, or vise versa). The two sites are mirror images of each other. In other words, both sites reveal a similar pattern of reversals, but in an opposite sense in the Northern and Southern Hemisphere. The inclination angle is steeper at Site 1172 (the site at higher latitude, therefore inclination is expected to be greater). If the pattern at the two sites records the same reversals, then the sedimentation accumulation rates of the two sites are different. FIGURE 4.3. Two paleomagnetic records from deep-sea sediments cored in the Pacific Ocean. Magnetic inclination (degrees; ) is plotted against depth in hole (meters below seafloor; mbsf). (a) ODP Hole 1149A is located southeast of Japan near the north end of the Marianas Trench ( N, E). From Shipboard Scientific Party, (b) ODP Hole 1172B is located east of Tasmania ( S, E). From Shipboard Scientific Party, 2001a. Page 8 of 40

9 Similarities There have been many reversals of polarity at both sites during the past 6.5 Ma Reversals of polarity appear very rapid geologicially speaking Differences Inclination values are greater at Site 1172 The topmost polarity at both sites is opposite; at Hole 1149A (left) the inclination values are positive, at Hole 1172B (right) the values are negative Same pattern of polarity reversals, except that they are a mirror image Page 9 of 40

10 FIGURE 4.4. World map showing all Ocean Drilling Program sites (1985 to 2003; Legs ). Black boxes show locations of Site 1149 (North Pacific) and Site 1172 (South Pacific). Blue box shows location of Site 1208 (Figure 4.11). Courtesy of IPDP. 6 How do your observations of the modern sediments (those closest to the seafloor and at the shallowest depths) at these two sites compare with the predictions that you made for Questions 2 and 3? Inclinations are positive (into the Earth) at the Northern Hemisphere Site 1149 in the most recent sediments, and inclinations are negative (out of the Earth) at the Southern Hemisphere Site Also important to notice the steeper inclination values at the higher latitude site 1172 off Tasmania. 7 Assuming that magnetic reversals are geologically instantaneous, the sediments at approximately mbsf at Site 1149 were deposited at the same time as what interval at Site 1172? (By doing this you are essentially correlating an interval at Site 1149 with an age equivalent interval at Site 1172, just as paleomagnetists do!) ~12-17 mbsf; thus, sediments at mbsf at Site 1149 are same age as those at mbsf at Sitel (a) What is the average inclination of this interval (25-33 mbsf) at Site 1149? -50º Page 10 of 40

11 (b) What is the average inclination of this interval (12-17 mbsf) at Site 1172? +65º (c) How does the average inclination of this interval at each site compare with the average inclination of the modern sediments (those at the top of these cores) at each site? 1149 modern inclination = +50º 1172 modern inclination = -65º It has the same inclination values, but with the opposite polarity. The magnetic field reversed. (d) Do these observations support or refute the notion that the Earth s magnetic north and south poles generally approximate the geographic (true) north and south poles over geologic time? Support. Explain. The Earth s magnetic field at the two intervals is reversed (opposite sign) from the nature of the field today indicating that magnetic north during this time was located near geographic south. NOTE: This series of questions also illustrates how we can use paleomagnetism (magnetostratigraphy) to correlate from one site to another. In addition, the differences in depth in the core and differences in thickness in the interval between the two sites illustrates that the rate of sediment accumulation is not the same at the two sites. PALEOMAGNETIC REVERSALS (REVERSALS OF MAGNETIC POLARITY The Earth s magnetic field episodically reverses polarity. Prior to a reversal of the magnetic field, the field fluctuates and becomes weaker and there may be multiple poles for a short time. The process is geologically rapid, taking several thousand years for the field to completely reverse polarity and stabilize again (Figure 4.5). Today s field is referred to as normal polarity. The last reversal of the magnetic field occurred approximately 780,000 years ago. Refer to for additional information about Earth s magnetic field (see also Frequently asked questions ). between reversals during a reversal Page 11 of 40

12 FIGURE 4.5. Earth s magnetic field simulated with a supercomputer model of the geodynamo by G.A. Glatzmaier and P.H. Roberts, University of California (es.ucsc.edu/~glatz/geodynamo). Yellow magnetic field lines are directed outward and blue magnetic field lines are directed inward. The stable normal dipolar magnetic field is shown in the picture on the left between reversals. The picture on the right during a reversal provides an idea of the complicated, but rapid nature of a reversal of magnetic polarity. Note: the field does not go away during a reversal. PALEOMAGNETISM IN SEDIMENT CORES Paleomagnetic records (i.e., the ancient or fossil magnetic signal preserved in sedimentary layers; Figure 4.3) are measured in deep-sea sediments cored by the Ocean Drilling Program. These data were collected using a cryogenic magnetometer (Figure 4.6). Sediments cored from the seafloor are passed through the magnetometer and the inclination, or magnetic dip, of the magnetic minerals is measured, as well as the magnetic intensity. In Figure 4.3, the data are plotted with the x-axis showing magnetic inclination (in degrees of inclination, or dip, from the horizontal) and the y-axis as depth in the drill hole (in meters seafloor, mbsf). See (or for a detailed discussion of how shipboard paleomagnetic data are collected. FIGURE 4.6. Cryogenic magnetometer aboard the JOIDES Resolution drillship. The 2G 760-R superconducting rock (cryogenic) magnetometer is used primarily for continuous measurements of magnetic properties on 1.5-m long core halves. The wide silver cylinder shields the magnetometer from the present day magnetic field. The photo shows the tray where the core half is placed and the opening into the magnetometer. Photo courtesy of IODP. Part 4.2. Paleomagnetism in Ocean Crust Figure 4.7 shows two transects of ship-towed magnetometer data across the East Pacific Rise, a volcanically active ridge in the eastern Pacific (Pitman and Heirtzler, 1966). Each transect displays two types of data: magnetic intensity (in gammas) and bathymetry (water depth, in kilometers) plotted on the y-axes, and distance (in km) on either side of the ocean ridge plotted on the x-axes. The ridge crest at 0 km is the axis of the ocean ridge. 1 and 1, 2 and 2, and so on correspond to distinctive measurements of the magnetic character of seafloor rocks (called positive magnetic anomalies ) on either side of the ridge. Page 12 of 40

13 See next page. FIGURE 4.7. Two transects (numbers 21 and 19) of magnetometer data collected by the research vessel Eltanin. The East Pacific Rise is the crest (bathymetric high) in both transects. Stronger magnetic intensity values are called positive magnetic anomalies; several examples on either side of the ridge are number 1 4 and 1 4, respectively. From Pitman and Heirtzler, (a) What common features do you see in the magnetic intensity data from these two transects across the East Pacific Rise (Figure 4.7)? The same distinctive pattern of positive magnetic anomalies is seen in both transects. In addition, there is a mirror-image symmetry in the magnetic intensity pattern on each side of the ridge with increasing distance from the ridge axis (0 km). (b) Read the box below on seafloor magnetic anomalies. What do paleomagnetic records from such places as the East Pacific Rise (Figure 4.7) tell us about the nature of Earth s magnetic field preserved in oceanic crust? Page 13 of 40

14 Suggests that reversals of the magnetic field are preserved in the crust, as well as in sediments. SEAFLOOR MAGNETIC ANOMALIES Figure 4.8 depicts a paleomagnetic record of ocean crust across a volcanically active ocean ridge. A ship towing a magnetometer records signals of stronger (positive magnetic anomalies) and weaker (negative magnetic anomalies) magnetic intensities. What the magnetometer measures is the magnetic character of the oceanic crust, consisting of the igneous rock called basalt (which is relatively enriched in magnetic minerals such as magnetite), as well as the much weaker overlying sedimentary cover and its detrital magnetic minerals. Positive magnetic anomalies correspond to ocean crust that has normal polarity, like today (today s field plus the ancient normal field result in a stronger signal); negative magnetic anomalies correspond to ocean crust with reversed polarity (today s field plus the ancient reversed field result in a weaker signal). The magnetic anomalies are mapped as magnetic stripes, which depict an alternating pattern of positive and negative magnetic anomalies preserved in the ocean crust. FIGURE 4.8. An observed magnetic profile (blue) for the ocean floor across the East Pacific Rise is matched quite well by a calculated profile (red) based on the Earth s magnetic reversals for the past 4 million years and an assumed constant rate of movement of ocean floor away from a hypothetical spreading center (bottom). The remarkable similarity of these two profiles provided one of the decisive arguments in support of the seafloor spreading hypothesis. From Reproduced from USGS. Figure 4.9 shows an interpretation of the pattern of positive and negative magnetic anomalies measured by the ship-towed magnetometer on either side of East Pacific Rise and a corresponding geomagnetic polarity time scale (Figure 4.10). Polarity epochs represent times of dominantly normal or dominantly reversed polarity. This barcode pattern of polarity changes led to the development of the geomagnetic polarity time scale. See next page. Page 14 of 40

15 FIGURE 4.9. Magnetic profiles depicting positive and negative magnetic anomalies across the East Pacific Rise at latitude 52 S, longitude 118 W (notice the similarity to magnetic profiles in Figure 4.7). The heavy arrow shows the location of the crest of the East Pacific Rise (i.e., spreading center = zero-age crust). 1 and 1 and 2 and 2 correspond to distinctive positive anomalies. M-B, G-M, G-G = boundaries between the Bruhnes, Matuyama, Gauss, and Gilbert polarity epochs. From Cox, Describe the nature of the magnetic record preserved in the ocean crust of the East Pacific Rise shown in Figure 4.9 by answering the questions below: (a) What pattern do you see in magnetic anomaly data preserved at the crest of the ocean ridge? The pattern of magnetic anomalies is symmetrical about the spreading center. In other words, we hope that the students will observe that each side of the ocean ridge has a distinct pattern of anomalies (positive and negative excursions of magnetic field intensity), and that one side of the ridge is a mirror image of the other side. If the students don t pick up on this observation themselves, point out that the ridge axis is the site of present-day spreading and the point of symmetry for the magnetic anomalies on the flanks of the spreading center. (b) How does this pattern of magnetic anomaly data relate to distance from the crest of the ocean ridge? The pattern refers to the general peak or trough shape of the positive and negative anomalies. The pattern is not cyclical or predictable, but is distinctly random or episodic with increasing distance from the ridge crest. In addition, this same pattern is observed on both sides of the ridge. Polarity (normal or reversed) represented by each anomaly is the same on either side of ridge with increasing distance from ridge; i.e., the pattern is symmetrical on each side of the ridge axis. (c) Provide an explanation for the observed pattern of magnetic anomalies on either side of the ocean ridge. The students should notice that the anomaly closest to the ridge axis is positive, and corresponds with our current normal polarity. Normal polarity (like today) is shown in black, and reversed polarity is shown in white. The important teachable point here is that the spreading centers are the sites of ocean crust production. As new crust is added at the ridge, the previously formed crust moves away from the spreading axis like a conveyor belt. Ocean crust cools and subsides as it moves away from the hot active spreading center. This is why the spreading centers/oceanic ridges stand high relative to the abyssal plains on Page 15 of 40

16 their flanks. Seafloor spreading results in the symmetrical (mirror image) pattern of anomalies on either side of the ridge. Lastly, again emphasize that the Earth s magnetic field has reversed polarity episodically many, many times throughout our planet s history. THE GEOMAGNETIC POLARITY TIME SCALE Figure 4.10 shows the first geomagnetic polarity time scale (GPTS), which was derived from continental lava flows with paleomagnetic records from numerous locations (Cox, 1969). These lava flows and the polarity reversals that they contained were radiometrically dated. This compilation demonstrated the potential of using magnetic reversals as a means of building a geologic time scale. More details on the GPTS are explored in Part 4.4. to FIGURE Time scale for geomagnetic reversals. Normal polarity intervals are shown by the black portions and correspond positive magnetic anomalies on the seafloor; reversed polarity intervals are shown by the white portions and correspond to negative polarity intervals. Note that the Bruhnes Epoch is normal polarity, the Matuyama Epoch is dominantly reversed polarity, the Gauss Epoch is dominantly normal polarity and the Gilbert Epoch is dominantly reversed polarity. From Cox, Page 16 of 40

17 Bruhnes ~42.5 mbsf Matuyama Matuyama ~120 mbsf Gauss FIGURE Paleomagnetic record preserved in the upper 155 m of Hole 1208A, located east of Japan on Shatsky Rise ( N, E). From Shipboard Scientific Party, Page 17 of 40

18 3 Figure 4.11 depicts the paleomagnetic record from ODP Site 1208 in the northwest Pacific Ocean (site location shown in Figure 4.4). (a) How do the paleomagnetic data preserved in the vertical sequence of deep-sea sediments cored at ODP Site 1208 (Figure 4.11) compare with the horizontal pattern of seafloor magnetic anomaly data from the eastern Pacific shown in Figures 4.7 & 4.9? List the similarities and differences. Similarities Changes in magnetic field are rapid in both sediments and ocean crust. On close inspection, the pattern of reversals is very similar (more resolution in the sediment record). Differences Sediment paleomagnetic data (fig. 4.11) are shown as inclination (in degrees positive or negative), whereas seafloor data are shown as intensity (in gammas) The sediment record is displayed vertically with increasing depth into the seafloor (in m), whereas the seafloor data are shown horizontally with increasing distance from the ridge axis (in km). There is greater detail (resolution) in the sediment record compared with the seafloor record (b) The Bruhnes-Matuyama and Matuyama-Gauss boundaries are defined in Figures 4.9 & At what depths would you place these boundaries in Site 1208? Bruhnes-Matuyama ~ 42.5 mbsf Matuyama-Gauss ~ 120 mbsf The pattern of positive values ( normal polarity) and negative values ( reversed polarity) in the Site 1208 sedimentary sequence bears striking resemblance to marine magnetic anomaly patterns shown and discussed above. For example, the Bruhnes-Matuyama boundary occurs at ~43 mbsf at Site 1208, and the Matuyama-Gauss boundary occurs at ~120 mbsf. The pattern is similar, but the ocean crust anomalies are shown horizontally because they are produced as new ocean crust is formed at the spreading center then moves like a conveyor belt away from the ridge axis, while the sediment reversals are shown vertically because they are deposited in this manner by gravity. Page 18 of 40

19 (c) What assumptions did you have to make to assign these two paleomagnetic boundaries to specific depths at Site 1208? Since figures 4.9 and 4.10 depict ocean spreading centers, we know that the crest is where new ocean crust is formed. As we have learned, modern deposits in the Northern Hemisphere yield a positive magnetic anomaly. Site 1208 (Figure 4.11) is not a spreading center, but rather a place where sediments accumulate due to the rain down of material. The shallower deposits will therefore be the youngest, and as a result will reflect the present day positive magnetic anomaly. Since the Bruhnes-Matuyama boundary occurs from the transition of a negative anomaly to the present day positive anomaly (as seen in figure 4.9 and 4.10), we know that the B-H boundary has to be at about 43 mbsf in figure 4.11 where we see such a transition. From that point, we can match the pattern of magnetic reversals in figures 4.9, 4.10 with that in figure 4.11 to determine where to place the G-M boundary. (d) Using the geomagnetic polarity time scale shown in Figure 4.10, what is the age of the sedimentary sequence at 150 mbsf? ~ 3.1 Ma; The two short-lived reversals below the Matuyama/Gauss boundary in the Site 1208 data (Figure 4.11) correlate with the Kaena Event and Mammouth Event within the Gauss Normal Epoch (Figure 4.10). The base of the Mammouth Event at Site 1208 is at 150 mbsf, which correlates to ~3.1 Ma on the Geomagnetic Polarity Time Scale shown in Figure Reflecting on the paleomagnetic data you have worked with in this exercise, explain how paleomagnetic records preserved in sedimentary sequences and/or ocean crust can be used to construct a geologic time scale (e.g., Figure 4.10). In your response be sure to include a description of the assumptions you would need to make, as well as reference to other data that would be needed to support the construction of this time scale. The sequence of magnetic reversals preserved in ocean crust as it forms, and polarity changes preserved in sediments as they accumulate on the seafloor are natural tape recorders of unique events in Earth history. This tape or barcode record could form the basis for a geologic time scale. What do you need to know? We need to be able to date the magnetic anomalies. Ocean crust can be dated radiometrically by measuring the ratio of radiogenic isotopes to their stable isotopic decay products in minerals that constitute the basalt. Some of the marine magnetic anomalies have been calibrated by datable volcanic ash-fall deposits in marine sedimentary sequences with good microfossil biostratigraphy and magnetostratigraphy. Both of these methods provide an absolute age for the anomalies. Correlation between radiometricallydated ocean crust and its contained magnetic anomalies, and fossiliferous marine sediments and its preserved magnetostratigraphy provides a means of dating fossil first and last occurrences. The integration of magnetostratigraphy and biostratigraphy provides the basis of the Geomagnetic Polarity Time Scale (discussed in more detail in Part 4 of this module). Assumptions? The main assumptions are that there are no breaks (unconformities) in sedimentary sequences, and that we can accurately identify all the marine magnetic anomalies. Identification of polarity events and chrons in marine sediments is accomplished in close cooperation with microfossil biostratigraphy. Page 19 of 40

20 Part 4.3. Using Paleomagnetism to Test the Seafloor Spreading Hypothesis In 1969, Deep Sea Drilling Project (DSDP) Leg 3 set out to test the Seafloor Spreading Hypothesis. The drillship Glomar Challenger cored nine sites in the South Atlantic, seven to the west of the Mid-Atlantic Ridge and two to the east of the ridge (Figure 4.12). FIGURE Map of the South Atlantic showing the cruise path and sites drilled during DSDP Leg 3. Notice that Sites and were drilled to the west of the Mid-Atlantic Ridge and Sites 17 and 18 were drilled to the east of the ridge. From Maxwell et al., Table 4.1 summarizes the age of ocean crust ( basement ) at each of the drill sites based on the paleomagnetic age, plus the age of the sediment overlying ocean crust at the bottom of each drill site based on fossil evidence (from Maxwell et al., 1970). The distance from the ridge axis is given as a straight-line (linear) value, as well as a distance along a curve, which is based on an axis of rotation on a spherical surface. Note: DSDP Sites 17 and 18 are on the east flank of the Mid-Atlantic Ridge, while the other sites are on the west flank and adjoining seafloor. 1 What two types of data are used to determine age of each location on the seafloor? See Table 4.1. The marine anomaly pattern on the seafloor at the location of each drill site is compared (correlated) with the Geomagnetic Polarity Time Scale that was available at the time (1970). In addition, a paleontological age assignment is based on the microfossils that directly overlie the basalt at each of the sites. Paleontological ages are also correlated with the Geomagnetic Polarity Time Scale. Page 20 of 40

21 TABLE 4.1. Estimates for the age of ocean crust (basement) at the DSDP Leg 3 drill sites and distance of each site from the Mid-Atlantic Ridge axis. From Maxwell et al., Site No. Magnetic age of basement (million years) Paleontological age sediment above basement (million years) Distance from ridge axis (km) Linear Rotation at 62 N, 36 W ±1 191±5 211± ±1 380±10 422± ±1 506±0 506± ±2 643±20 718± ± ±10 745± ±1 990± ± ±1 1270± ± ± ±10 2 On the graph paper provided, plot paleomagnetic age of basement (oceanic crust) vs. distance from the ridge axis using solid circles. On the same graph, plot the paleontological age of sediment overlying basement vs. distance from the ridge axis using open circles. Draw a visual best-fit line through the data points. See plots on next page. First is hand drawn as students might do and second is the age-distance plot as shown in DSDP Volume 3. NOTE: If you don t specify the x and y axes, you will find that some students will put km on the x-axis and others age. This can result in a classroom discussion about the pros and cons of both ways of presenting the data. Placing distance from the ridge on the x-axis depicts the ocean floor as a horizontal surface, which may seem more intuitive (as seen above from the original DSDP volume), but it yields a graphical slope (y = mx + b) that is inverse of the spreading rate. Placing distance on the y-axis and age on the x-axis, on the other hand, results in a graphical slope value that represents the spreading rate. Page 21 of 40

22 Ch 4 Paleomagneticm and Magnetostratigraphy Page 22 of 40

23 3 What is the relationship between the age of basement and distance from ridge axis? Basement rocks increase in age with increasing distance from the ridge. In other words, there is a direct relationship between distance from the ridge axis and age of the oceanic crust. 4 What is the relationship between the age of sediment overlying basement and distance from ridge axis? The sediment (and its contained microfossils) sitting directly on the basalt at each site increases in age with increasing distance from the ridge. So, like basement age, there is a direct relationship between distance from the ridge axis and age of the sediments in contact with the underlying oceanic crust. The basalt that makes up oceanic crust basement crystallized at the ridge axis as volcanic lava cooled. This is also the time when the ocean crust acquired its magnetic character. Over time, the ocean crust at each of the drill sites has moved away from the ridge axis with slow conveyor-like motion to its present geographic location on the adjacent seafloor. 5 Hypothesize why the ages of sediment immediately overlying the basement (i.e., the basalt) also show increasing age from the ridge axis: Pelagic sediment (microfossils, silt, and clays that have settled out of suspension) can only accumulate on the oceanic crust (basalt) once the basalt actually exists. In other words, the basalt has to first form at the ridge before the sediment can accumulate on it. As the sediment-draped basalt moves away from the ridge due to seafloor spreading, sediment continues to accumulate on the moving seafloor so that the sediment section typically gets thicker and thicker with increasing ocean crust age and distance from the ridge. In addition, the oldest (basal) sediment overlying the basalt is older the further the drill site is now from Page 23 of 40

24 the ridge crest because the basalt is also older with increasing distance from the spreading center. 6 The average rate of seafloor spreading in the South Atlantic can be calculated from the data provided in Table 4.1 and in the graphs you made. In preparation for this calculation, do the following: In the space below sketch a profile across the Mid-Atlantic Ridge in the South Atlantic depicting the relationships of distance and age that you plotted. Label the spreading axis with its age. Provide units on your profile (distance in km, age in myr) Based on this sketch, write a formula to calculate the average spreading rate. Students can think of this rate term as being analogous to the velocity of their car in miles/hour or km/hour. Simply stated: distance (d) divided by time (t): d/t. Therefore, spreading rate = kilometres per million years (km/myr) (or centimetres per year; cm/yr). 7 Use your graph with best-fit line through the data points (Question 2 above) to calculate seafloor spreading rates: (a) What was the average rate of seafloor spreading on the western side of the Mid-Atlantic Ridge in km/myr (kilometers per million years)? Show your work. In the plot above, an approximate best-fit line through the data (simple rise/run) reveals a slope of 1200 km/65 myr, or km/myr. (b) What was the average rate of seafloor spreading in cm/yr (centimeters per year)? Show your work. Spreading rates are more typically described in units of cm/yr, so, an exercise of unit conversion is helpful here: km/myr x 1 myr/10 6 yr x 10 3 m/1 km x 10 2 cm/1 m = cm/yr This number reflects the spreading rate on one side of the ridge, the so-called half Page 24 of 40

25 spreading rate. 8 Calculate how fast the South Atlantic Ocean is widening (i.e., opening). Show your work. Half-spreading rate x 2 = cm/yr x 2 = cm/yr, which is the full spreading rate. THE SEAFLOOR SPREADING HYPOTHESIS In 1960, Professor Harry Hess of Princeton University advanced the hypothesis that new ocean crust is produced at the mid-ocean ridges such as the Mid-Atlantic Ridge and the East Pacific Rise. The youngest ocean crust should lie in the axis of each volcanically active ocean ridge and the oldest ocean crust should lie farthest from the ridge axis. According to his hypothesis, the seafloor moves laterally away from these spreading centers and explains why North and South America have moved away from Europe and Africa over time. This became known as the seafloor spreading hypothesis. By the early 1960s, abundant paleomagnetic data from ship-towed magnetometers had demonstrated the existence of alternating positive (normal polarity) and negative (reversed polarity) magnetic anomalies on the seafloor (e.g., Vine and Matthews, 1963). When mapped, they reveal a pattern of magnetic stripes that form a symmetrical pattern on either side of a spreading center (i.e., the pattern on one side is a mirror image of the pattern on the other side). For example, Figure 4.13 is a seafloor map showing magnetic anomalies on the Mid-Atlantic Ridge south of Iceland. FIGURE Map of magnetic stripes on the seafloor south of Iceland. The straight lines mark the ridge axis and the central positive magnetic anomaly. From Vine, How could these magnetic stripes be used to test the hypothesis of seafloor spreading? Make a list of observations based on the patterns of magnetic stripes that would be useful for evaluating this hypothesis. Recognizing the mirror-image symmetry of the anomalies on either side of the mid-ocean ridges was a critical observation. The students should note the linearity of the anomalies, Page 25 of 40

26 which parallel the ridge axis. The pattern of anomalies can be compared to the anomalies preserved in other ocean basins or spreading centers. 10 What additional data would you like to collect in order to test the seafloor spreading hypothesis? Additional data needed would include age of the individual magnetic anomalies. This could be accomplished by radiometric dating of oceanic crust. Marine magnetic anomalies can also be calibrated by datable volcanic ash-fall deposits in marine sedimentary sequences with good microfossil biostratigraphy and magnetostratigraphy. For example, the Joides Resolution could sail to the location of one of these magnetic anomalies, core through the sediments to the underlying ocean crust, and then sample the basalt for radiometric analysis. The microfossils in the sediments overlying the basalt will also provide some relative age information. Both of these methods provide an absolute age for the anomalies. Correlation between radiometrically-dated ocean crust and its contained magnetic anomalies, and fossiliferous marine sediments and its preserved magnetostratigraphy provides a means of dating fossil first and last occurrences. The integration of magnetostratigraphy and biostratigraphy provides the basis of the Geomagnetic Polarity Time Scale (discussed in more detail in Part 4 of this module). Part 4.4. The Geomagnetic Polarity Time Scale The Geomagnetic Polarity Time Scale (GPTS) is a composite geomagnetic polarity sequence that has been calibrated with radiometric age dates. The Late Cretaceous and Cenozoic (83 to 0 Ma) GPTS is based on an analysis of marine magnetic profiles from the Atlantic, Pacific, and Indian ocean basins (Cande and Kent, 1992, 1995). The marine magnetic anomaly sequence recorded in the South Atlantic Ocean serves as the principal polarity sequence for this interval (Heirtzler et al., 1968; Cande and Kent, 1992). Radiometric dating of volcanic ash fall deposits (bentonites) in land-based and marine sections with reliable biostratigraphic and magnetostratigraphic control provides the numerical calibration for most of the anomaly pattern (paleomagnetic barcode ). An orbitally-tuned age provides the calibration age of the Gauss/Matuyama boundary (Figure 4.14). Nine age calibration points, plus the zero-age ridge axis, provide the age control on the Late Cretaceous-Cenozoic GPTS (Cande and Kent, 1992). Page 26 of 40

27 FIGURE GPTS for the past 5 myr. Ages (in Ma) of the chron and subchron boundaries are shown as well as the names of the chrons and subchron events. From Mankinen and Wentworth, Figure 4.15 shows a portion of the geomagnetic polarity time scale (GPTS). The GPTS is divided into magnetic epochs, which are themselves divided into chrons. Distinctive normal polarity events occur within the chrons. The suffix n after the anomaly number refers to normal polarity and the r refers to reversed polarity. Page 27 of 40

28 FIGURE Portion of the geomagnetic polarity time scale used during ODP Leg 191. From Shipboard Scientific Party, 2001b. In the investigation that follows, you will use the GPTS to make an interpretation of the paleomagnetic data recovered from ODP Site 1208 in the northwest Pacific (Figure 4.4). In other words, you will correlate the Site 1208 data to the GPTS (Figure 4.15). Page 28 of 40

29 1 Interpret the paleomagnetic record of the upper 300 m of sediments cored at ODP Site 1208 (Figure 4.16) by correlating with the GPTS shown in Figure Use the blank columns on the right of each panel of paleomagnetic inclination data to interpret normal polarity (color in) or reversed polarity (leave blank/white). As much as possible, determine the magnetic chrons and the polarity events and label these on your polarity interpretation. Assume that the top of the sequence at 0 mbsf is the top of the Brunhes Magnetic Epoch (= Chron C1n). Note that there are missing data at , , and mbsf. NOTE: Interpreted magnetostratigraphy of ODP Site 1208, from the Leg 198 Initial Reports volume ( Note that the image from the URL has a typo in the depth scale of the right panel; instead of 50, 100, and 150 those values should be 200, 250, and 300 mbsf. It has been corrected here. The gray areas are intervals of uncertain polarity due to unrecovered section or poor data quality. See next page. Page 29 of 40

30 Figure Paleomagnetic data from ODP Hole 1208A. Left graph: mbsf; right graph: mbsf. From Shipboard Scientific Party, What assumption(s) must you make in order to correlate the Site 1208 paleomagnetic data with the GPTS? That the sediment section is continuous and complete (i.e., no breaks in the section; no unconformities). Physical sedimentology, biostratigraphy, physical properties, and sediment geochemistry can all provide clues about the presence of an unconformity in a sedimentary section. Page 30 of 40

31 3 What is the approximate age of the sedimentary sequence at 150 and 300 mbsf (meters below seafloor)? What paleomagnetic chrons do these levels correlate with? Referring to the Magnetic Epoch column (figure 4.15): Age Chrons 150 mbsf ~3.33 Ma C2An 300 mbsf ~ Ma C5Ar.2n Notice that the age at 150 mbsf is older than the ~3.1 Ma age determined in Part 4.2, question 3d. This is because the geomagnetic polarity time scale used for that question is based on the time scale from 1970 (Figure 4.10). There have been improvements in the geomagnetic polarity time scale since then. 4 As a summary of what you have learned in this chapter, list several ways that magnetic reversal data can be useful to Earth scientists. Magnetostratigraphy is very valuable for establishing the age of sedimentary sequences provided there is additional first-order age information such as biostratigraphy to initially calibrate the normal-reversed polarity pattern. Highly resolved magnetostratigraphic records like Site 1208 (or Sites 1149 and 1172 examined in Part 4.1) provide reliable correlation to other sites around the world. High-resolution magnetostratigraphic records also provide valuable age control for calculation of rates, such as sediment accumulation rates (see below; from Page 31 of 40

32 Integrating Paleomagnetic and Biostratigraphic Age Control Data THE GEOMAGNETIC POLARITY TIME SCALE (GPTS) Figure 4.17 shows the record of Earth s magnetic polarity changes back approximately 160 Ma compiled from seafloor magnetic anomalies and uplifted marine sequences. Black=normal polarity, white=reversed polarity. The left plot is from Lowrie (1982). The right plot is a composite record of seafloor magnetic anomalies that extends back to the top of the Cretaceous Long Normal, approximately 83.0 Ma, compiled by Cande and Kent (1995). This is the currently accepted geomagnetic polarity time scale or GPTS for the later part of the Cretaceous Period and Cenozoic Era (Gradstein et al., 2004). Nine radiometric age tie points, plus the zero-age ridge axis, are used to calibrate this part of the time scale (ages for the marine magnetic anomalies between the calibration points are interpolated by a cubic spline function; Cande and Kent, 1992; Berggren et al., 1995). For the Late Cretaceous to present, the prominent positive magnetic anomalies (i.e., those corresponding to time intervals, or chrons, of normal geomagnetic polarity) have been numbered from Chron 1n in the central axis of the spreading centers to Chron 34n at the younger end of the Cretaceous Long Normal (also called the Cretaceous Quiet Zone; 83.0 Ma). Many of the younger chrons are divided into shorter polarity intervals, or subchrons. Page 32 of 40

33 The GPTS based on marine magnetic anomalies extends back through the late Middle Jurassic (Callovian stage, approximately 162 Ma; e.g., Kent and Gradstein, 1986), while land-based polarity records go back through the Triassic (e.g., Gradstein et al., 1995). A nomenclature called the M-sequence is used for marine magnetic polarity chrons of the Early Cretaceous to late Middle Jurassic extending from Chron M0r at the base the Cretaceous Long Normal (Chron 34n; base of the Aptian stage in the Early Cretaceous, approximately 121 Ma) to Chron M39 in the Jurassic (Callovian stage). 5 Make a list of observations about the characteristics of the paleomagnetic record for the past 160 million years based on Figure 4.17 (next page). Students will notice that the polarity changes are random. They will also notice that the frequency of reversals has been increasing from the Late Cretaceous through the Paleogene and through the Neogene. The randomness of the barcode is a major reason why the paleomagnetic record is such a powerful tool for correlation and chronostratigraphy. 6 Have magnetic reversals occurred in a regular or cyclic fashion (i.e., periodic), or have the reversals occurred in an irregular or random fashion (i.e., episodic)? Students should recognize the random or episodic nature of polarity changes. Explain. This randomness yields a record of reversals that resembles a familiar barcode. The distinct pattern is useful for correlation, but it may also provide important insights into the long-term nature of Earth s magnetic field. For example, the Cretaceous Long Normal was initiated ~120 Ma and is contemporaneous with the emplacement of the Large Igneous Province preserved in the western equatorial Pacific as Ontong Java Plateau. This feature formed from a mantle plume. Such deep-seated mantle plumes may be related to thermal discontinuities at or near the Core-Mantle boundary, which are suspected to have somehow disrupted Earth s magnetic field. Hopefully the students will also recognize the increasing frequency of reversals since ~83.5 Ma in the Late Cretaceous. Page 33 of 40

34 FIGURE Geomagnetic polarity time scale. Plot on left shows magnetic polarity changes back to the Middle Jurassic (nearly 160 Ma; from Lowrie, 1982). Note the long interval in the Cretaceous Period with no magnetic reversals; this is called the Cretaceous Long Normal chron. The plot on the right shows the details of the marine anomaly record of the South Atlantic (from Cande and Kent, 1995). Red lines show how the South Atlantic anomaly record correlates with the longer Lowrie (1982) record. Since the late 1960s, the geomagnetic polarity time scale has been extended further back into the Cenozoic and Mesozoic Eras, refined with greater resolution (i.e., recognition of subchrons and polarity events; Figure 4.15) and calibrated with radiometric age dates. In addition, biostratigraphic datum levels (first occurrences or bases and last occurrences or tops) have been integrated (correlated) with the geomagnetic polarity time scale (Figure 4.18). In doing so, microfossil datum levels have become primary indicators of geologic age in deep-sea sediments. Recall from Chapter 3 that biostratigraphic datum levels define the boundaries of biostratigraphic zones, or biozones. Page 34 of 40

35 FIGURE Portion of geomagnetic polarity time scale from 0 to 25 Ma, including geologic epochs and ages (see time scale inside front cover of book), and showing integration with microfossil biozones for planktic forams (N zones) and calcareous nannofossils (CN zones). Microfossil datum levels are shown with ages in parentheses. B = base or first occurrence; T = top or last occurrence. Shipboard Scientific Party, 2002, Explanatory notes. Page 35 of 40

36 7 Drawing from your work in Chapters 3 and 4, explain how biostratigraphic datum levels can be dated using the geomagnetic polarity time scale. In other words, how can ages be assigned to microfossil first occurrences (bases) and last occurrences (tops) as illustrated in Figure 4.18? It s based on relative age dating. The age of oceanic crust can be determined by radiometric dating. Any sedimentary material deposited on top of this will have a relative age to the oceanic crust. Spreading centers capture magnetic field normal and reverse polarities that can all be dated using radiometric dating. Because microfossils are deposited shortly after the crust is created, wherever scientists find first or last occurrences of the microfossil, they can determine a relative age of their occurrence in relation to the radiometric age of the crust. Also, depending on which polarity interval the microfossil is found, one can correlate its age to whichever chron it lies in. 8 Reconsider Question 2 (Part 4.4) which asked about the assumption(s) you needed to make in order to correlate the Site 1208 paleomagnetic data with the GPTS. How could you test your assumption(s) using microfossils? The relative age of the microfossils at Site 1208 can be used to correlate Site 1208 with the GPTS. III. Summative Assessment The following questions could be used to assess student learning after completing this exercise: 1. The Earth has a magnetic field that protects our planet from dangerous cosmic energy. What is the nature of the magnetic lines of force relative to the surface of the Earth? (A sketch may be helpful in answering this question.) a. Magnetic lines of force are parallel to the Earth s surface at all latitudes b. Magnetic lines of force are perpendicular to the Earth s surface at all latitudes c. Magnetic lines of force are parallel at the equator d. Magnetic lines of force are perpendicular near the poles (Arctic and Antarctic) e. C and D only 2. Earth s magnetic field has episodically (randomly) and rapidly switched polarity. Such magnetic reversals have occurred throughout Earth history. These ancient magnetic lines of force were "frozen" (recorded) into igneous and sedimentary rocks. How might the orientation of these "paleomagnetic" lines of force be useful to geoscientists? a. They can help determine age b. They can help determine longitude c. The can help determine latitude d. All of the above e. A and C only Page 36 of 40

37 3. Examine the magnetic inclination record (figure on right) measured from a sedimentary sequence in the North Pacific Ocean. At what depth in the core was a polarity change (magnetic field reversal) recorded? a. at 7 mbsf b. at 20 mbsf c. at 27 mbsf d. at 39 mbsf 4. The Earth s magnetic field a. has maintained the same polarity throughout Earth s history b. has reversed polarity over regularly spaced intervals of time, about every 50 million years. c. has reversed polarity many times, but these reversal do not occur in any regularly spaced intervals of time. 5. Which of the following statements about interpreting paleomagnetic data is true? a. Rocks of the same core depth and latitude in the North Atlantic should show the same change in paleomagnetic inclination. b. Rocks of the same age and latitude in the North Atlantic should show the same change in paleomagnetic inclination. c. Rocks of the same age and longitude in the South Pacific should show the same change in paleomagnetic inclination d. All of the above Short Answer: 6. How is the Earth s magnetic field like a bar magnet? 7. What is Declination? Inclination? 8. How would a freely moving compass needle (or a magnetic mineral) align with the magnetic field if it was at the equator? At the north pole? The south pole? 40 S? 40 N? 9. How does inclination relate to latitude at the time of rock formation? Does it have any relationship to longitude at the time of rock formation? 10.What types of materials record the Earth s magnetic signature? 11.What is Natural Remanent Magnetism (NRM)? 12.What is magnetic intensity? How does magnetic intensity change perpendicular to a midocean ridge? How do these changes relate to NRM? Page 37 of 40

38 13.What is a magnetic reversal? Are these fast or slow? Global events or regional? Periodic (repeated at a regular interval) or random in time? 14.How are paleomagnetic records for the same time period from the northern and southern hemisphere similar and how would they be different? 15.What is the process for determining age using paleomagnetic data? What assumptions must be made? 16.Why is it important to integrate paleomagnetic data with both radiometric age data and biostratigraphic data? 17.Examine the figure (to the right) showing a segment the magnetic polarity timescale. Note that normal polarity (i.e., like today s magnetic field) is depicted black, and reversed polarity is depicted as white. What was the dominant polarity in the middle Pliocene? of as IV. Supplemental Materials To learn more about the Earth s magnetic field go to the frequently asked questions page on the National Geophysical Data Center site To view a model of the convection and magnetic field generation in the fluid outer core surrounding the solid inner core go to: In 2003, NOVA produced a 1-hour public television show, Magnetic Storms, which explores the generation of Earth s magnetic field, ancient and recent changes in Earth s magnetic field, and its role in protecting Earth from cosmic radiation: To take a panoramic virtual tour of the paleomagnetic lab on board the JOIDES Resolution go to: For a nice review of our current understanding about the unstable nature of Earth s magnetic field: Gubbins, D., Geomagnetic reversals. Nature, 452 (13 March 2008): To learn more about magnetic anomalies in the crust: For a video tour of the paleomagnetic laboratory aboard the JOIDES Resolution go to: and select tour the Cryogenic Magnetometer Lab w/ Klayton Curtis. Page 38 of 40