With a group, get a bar magnet, some plastic wrap, iron filings and a compass.

Transcription

1 Name: EPS 50 Lab 8: The Earth's Magnetic Field Chapter 2, p : The Seafloor as a Magnetic Tape Recorder Chapter 7, p. 213: Paleomagnetic Stratigraphy Chapter 14, p : Earth s Magnetic Field and the Geodynamo Introduction The Earth's magnetic field is one of the most important properties of this planet. Studies in the 1960's of the earth's magnetic field and magnetic anomalies led to the discovery of plate tectonics, which is one of the most important theories about Earth's dynamics. Earth's magnetic field also shields us from most space-derived cosmic radiation. In some ways, Earth s magnetic field behaves in the same way that magnetic fields on ordinary magnets behave. In this lab we will be exploring some of the properties of Earth's magnetic field. Objective In the first part of this laboratory we will visualize a magnetic field by observing the alignment of iron filings around a magnet. We will then extrapolate this to Earth, which to a first approximation has a dipole magnetic field. You will explore what a dipole field is, and how Earth's magnetic field is similar and different from that of a dipole. We will then look at how we use Earth's magnetic properties as a record of tectonics and geologic time. Answers All answers should be your own, but we encourage you to discuss and check your answers with 2-3 other students. This labs is graded out of 100 points Part 1: Magnetic Field Lines Magnetic fields are produced by moving charges. Magnetic field lines describe the structure of magnetic fields in three dimensions. They are defined as follows: if at any point on such a line we place an ideal compass needle, free to turn in any direction (unlike the usual compass needle, which stays horizontal), then the needle will always point along the field line. Field lines converge where the magnetic force is strong, and spread out where it is weak. For instance, in a compact bar magnet, which approximates a dipole, field lines spread out from one pole and converge towards the other. The magnetic force is strongest near the poles where they come together. The behavior of field lines in the Earth's magnetic field is very similar. With a group, get a bar magnet, some plastic wrap, iron filings and a compass. Place one of your bar magnets on a piece of loose-leaf paper. Place the plastic wrap over the magnet (this is so that the iron fillings do not stick directly to the magnet!). Sprinkle the iron filings from a height of about 10 cm. Continue sprinkling until a distinct pattern emerges. The iron filings fall on the plastic and align themselves with the magnetic field. 1) Draw a simplified version of the field pattern that emerges when the iron filings are 1

2 placed on the bar magnet. Be sure to show the overall shape of the field and the density of the field lines. (10 pts). Place your compass in your magnetic field in the spots illustrated by circles below. 2) Does the compass needle line up with the field lines? (2 pts). 2

3 3) Draw an arrow into each circle above to show the direction of the north pole of your compass in the field. (5 pts). 4) Where along the bar magnet is the magnetic field strongest, and how can you tell? (3 pts). Place the north end of a bar magnet about 4 cm from the south end of another bar magnet. Place a piece of plastic over the two magnets and sprinkle iron filings in the region between the magnets. 5) Draw a simplified version of the field pattern that emerged when the iron filings were placed on the bar magnets in these configurations. Only show the field lines as they appear between the bar magnets. (6 pts). 6) Use the north pole of your compass to determine the direction of the magnetic field 3

4 lines. Add arrows to your picture above, showing the direction of the magnetic field as determined by your compass. (4 pts). 7) How does the magnetic field line pattern differ between your single bar magnet and the double bar magnet configuration? (3 pts). 4

5 Part 2: Earth's Magnetic Field Left: the magnetic field of a bar magnet revealed by iron filings on paper. Right: Earth s magnetic field viewed as if a giant bar magnet were placed at the Earth s center and slightly inclined (11 ) from the axis for rotation. A magnetic dipole field is the field produced by a north magnetic pole and a south magnetic pole in close proximity. The above figure shows an idealized dipole field for Earth. The magnetic field lines show the direction of the magnetic field and presently point from the southern geomagnetic pole to the northern geomagnetic pole. There is something surprising about this fictitious bar magnet in Earth: Its south (negative) pole lies beneath Earth s north magnetic pole, or magnetic north! You can see why this must be so by considering that, in the absence of any significant magnetic fields other than the Earth s, the north end of a compass needle points toward Earth s magnetic north, which is in the general direction of the geographic North Pole. Since the magnetic field along the fictitious bar magnet s field lines points from the north pole to the south pole, it has to be that the south pole of the Earth s magnetic field lies beneath Earth s north magnetic pole. The magnetic pole is not located at the same position as the North Pole. Declination is the angle between magnetic north (the direction in which a compass needle points) and true north (the direction of the north geographic pole). It is often noted on maps, and changes from year to year. 8) Draw a sketch to illustrate declination on a hypothetical map, below. (2 pts). 5

6 Magnetic Inclination In general, the magnetic field lines of Earth are not parallel with the surface of the planet. The angle between the magnetic field and the horizontal is called inclination, which is positive if the field lines point into the ground, and negative if the field lines point out of the ground. In the above figure, inclination is shown as i, magnetic latitude is shown as ϴ. Magnetic inclination varies from 90 (perpendicular to the surface) at the magnetic poles to 0 (parallel to the surface) at the magnetic equator. How can we characterize this relationship between the change in inclination with a change in latitude for a magnetic dipole? It can be proven theoretically (but not in this lab) that i and ϴ for an idealized dipole field are related by the following expression: tan i = 2 tan ϴ This means that for a location on an Earth with an ideal dipole field, if you know the latitude (ϴ) you can find the inclination (I), and vice versa. Refer back to your first bar magnet diagram and sketch from Part 1. Imagine that your drawing now represents the Earth. Where is the magnetic North pole? 9) From your drawing in Part 1, measure (not calculate) the inclination angle I at the 'latitudes' of 0, 30, 45, 60 and 90 around the 'globe'. Record your measurements in the following table under "experimental inclination". (5 pts). 6

7 Dipole Field Latitude (ϴ) Experimental Inclination (Ix) Theoretical Inclination (I) 10) Now use a calculator to find the theoretical inclinations for a dipole field at latitudes of 0, 30, 45, 60 and 90. Record your results in 'theoretical inclination row' of the table above. (5 pts). 11) Explain any significant difference between the "theoretical" and the "experimental" values. (4 pts). 12) If the Earth's magnetic field is that of an ideal dipole and if the magnetic poles are aligned with the geographic poles, what would be the expected pattern of lines of equal inclination on a world map? (5 pts). 7

8 Below is a world map of the magnetic inclination of the real Earth as determined by measurementbased models. This is a snapshot of the field a few years ago ( ). The contour lines are 20 degrees apart, positive in the northern hemisphere, and negative in the southern hemisphere. 13) What is the current magnetic inclination of Berkeley? (2 pts). 14) Describe the similarities and differences between this map and the expected pattern for a hypothetical earth with an ideal dipole field. What do you think causes the differences between your calculations, observations, and this map? (5 pts). 8

9 Earth's magnetic field is slowly changing over time, on time scales that range from years to millennia. Such changes are referred to as secular variation. Secular variation was first recognized in 1634 when Gellibrand compared magnetic declination observations he had made in London with earlier observations. The observations of declination made in London over the years constitute one of the best records of secular variation. The figure below shows that declination has changed from approximately 10 E in the late 16th century to 25 W in the early 19th century before returning to a current value of about 3 W. All elements of the magnetic field change with time not just the declination. For example, the total field intensity (strength) in Toronto has decreased 14% during the last 160 years. Magnetic poles are just the time-averaged direction of the magnetic field, where the effects of secular variation are (ideally) averaged out. Generally, many magnetic measurements spanning long periods of time (thousands to tens of thousands of years) are needed to derive a statistically robust magnetic pole. 15) What is the average declination for London over the last 400 years? (2 pts). 16) Is secular variation fast or slow compared with magnetic reversals? (3 pts). 17) Does secular variation lead to magnetic reversals? Why or why not? (3 pts). 9

10 Part 3: Paleomagnetism A key factor in establishing the theory of plate tectonics was the recognition of past reversals in the Earth s magnetic field and the mapping of normal and reverse magnetic striping on the ocean floor. The motion of the Earth s molten outer core produces a magnetic field that is characterized by force lines emanating from near the geographic poles. At present, the magnetic field causes compasses to point toward magnetic north. This polarity is called normal polarity. Paleomagnetism is the study of Earth's past magnetic field as it is recorded in the rocks. The basic premise of paleomagnetism is that a rock sample can acquire and hold the direction of the magnetic field that existed at the time and place the rock was formed; this is termed the magnetic remanence of the rock. The earth contains numerous elements that are generally classified as ferro-magnetic (their electrons will align in the presence of magnetic field). These elements can form minerals and thus rocks with strong magnetic properties. A small proportion, 5% or less, of a typical crustal rock will be made up of iron-bearing magnetic minerals such as magnetite and hematite. There are two principal ways by which the ancient magnetic field direction can become "frozen" into rocks during their formation. As magma cools to solidify and form igneous rocks, it cools through the Curie points of its magnetic minerals (about 580 C for the crystal magnetite). Above the Curie point these crystals are magnetized in the direction of the ambient magnetic field, but they do not stay aligned. As the crystals cool below the Curie point, their magnetizations become locked in, parallel to the local magnetic field. Therefore, as a rock cools, the ancient field direction becomes "frozen" in. This 10

11 magnetization can persist for millions and even billions of years. This processes is called 'thermal' remanent magnetization (TRM). Sedimentary rocks can also preserve a record of the earth's past magnetic field, but since they have not been cooled from a high temperature they contain no TRM. They do, however, contain grains of magnetite and hematite that have been eroded from igneous rocks. These fine magnetized grains behave like small magnets or compasses. As sediments slowly settle through a water column in lakes or in the oceans, particles of magnetite are free to align with the ambient magnetic field. As the sediments accumulate on the lake or sea floor and gradually become compacted and cemented into sedimentary rocks, this magnetic field gets frozen in. This is called 'depositional' remanent magnetization (DRM). 18) Which type of rock and characteristic remanent magnetization is better for studying a detailed record of secular variation: TRM or DRM? Why? (Note it is best to discuss this answer with a group first!) (5 pts). New oceanic crust inherits the Earth s magnetic field at the time it solidifies. Reversals of the Earth s magnetic field therefore cause reversals of the oceanic crust's magnetization. Thus the seafloor has regions where its magnetic minerals point north (normal) and areas where they point south (reverse). These magnetic rocks produce highs and lows in the local magnetic field. Because they were originally produced at the mid-ocean ridge, the anomalies are oriented parallel the midocean ridge. We can map these anomalies by towing an instrument called a magnetometer behind a ship and measuring the strength of the local magnetic field. The resulting map of the sea floor will have a distinctive magnetic signature. The width of a normally or reversely polarized strip depends on two factors: 1. The length of time the field is in a given polarity, and 11

12 2. The rate of spreading of the ocean floor along a mid ocean ridge axis. Imagine that the field is reversed for 1 million years. During that time all the basalt that forms at the ridge will be reversely polarized. If the spreading rate is 50 mm/yr you will wind up with a band 50 km wide of reversely polarized rock. Alternatively, if the spreading rate were 100 mm/yr you would have a 100 km wide zone. Likewise, if the reversed epoch lasted 500,000 years, at 50 mm/yr, you would have 25 km of reversely magnetized rock, etc. Marine magnetic anomalies (A) The black line shows positive anomalies as recorded by a magnetometer towed behind a ship. In the cross section of the oceanic crust, positive anomalies are drawn as black bars (normal polarity) and negative anomalies are drawn as white bars (reverse polarity). (B) Perspective view of magnetic anomalies shows that they are parallel to the rift valley and symmetric about the ridge crest. Magnetic Time Scale - Magnetic anomaly identification numbers are given on the top and absolute time scale in millions of years (Ma) is given on the bottom. Normal magnetic anomalies are shown 12

13 in black, and reversed polarity is shown in white. The absolute age of the anomalies and their polarities can be read directly from the chart. For example, anomaly 4 is 7 million years old, and it is a positive anomaly. Restoration of the South Atlantic Coastline 50 Million Years before Present From the magnetic striping on either side of the Mid-Atlantic Ridge, we can deduce that the African and South American plates are moving away from each other, carrying the continents of Africa and South America with them. By using the pattern of magnetic lineations shown on the map on the next page, we can reverse spreading process and restore the positions of the African and South American coastlines to a time when a particular set of magnetic lineations was being formed on the Mid-Atlantic Ridge. For the purposes of this exercise we will use anomaly number 21, which, according to the magnetic lineation time scale, was formed roughly 50 million years ago. 19) On the map on the South Atlantic Ocean, draw a red line over each of the magnetic lineations of anomaly number 21 on the South American side of the Mid-Atlantic Ridge. Connect the segments of the number 21 anomaly with a red line drawn along the fracture zones against which they terminate. Start with the point where anomaly 21 touches the Ascension F. Z. Follow Anomaly 21 with your red pencil southward until it reaches the Bode Verde F. Z., then along the Bode Verde F. Z. westward to the northern end of the next fracture zone. Continue until you have reached the southernmost fracture zone on the map. (2 pts). Attach a piece of tracing paper over the map with tape or paper clips, and repeat the process described above for Anomaly 21 on the African side of the Mid-Atlantic Ridge. Draw this line in red pencil on the tracing paper. (2 pts). With the tracing paper still in place, trace the coastlines of Africa and South America on the tracing paper with black pencil. Also, trace on the tracing paper the boundaries of map and the 20 South latitude line in black pencil. (2 pts). Detach the tracing paper and slide it toward South America until the red line on the tracing paper matches the red line on the map. When the two lines are matched as closely as possible, hold the tracing paper in place and trace the coastline of South America in red pencil on the tracing paper. Trace also the 20 South line on the tracing paper in red pencil. (2 pts). The map you have constructed on the tracing paper shows the Mid-Atlantic Ridge as it existed when magnetic anomaly 21 was being formed. Your tracing paper also shows the relative positions of segments of the coastlines of Africa in black pencil and South America in red pencil as they were approximately 50 million years ago. This reconstruction is based on the assumption that the continents of Africa and South America were fixed to their respective plates during the spreading process over the past 50 million years. The continents moved with respect to each other because the tectonic plates to which they were attached moved as spreading continued along the Mid-Atlantic Ridge. 13

14 20) What is the evidence that the movement of the two plates was not strictly in an eastwest direction? (2 pts). 21) Was the earth's magnetic field normal or reversed at the time represented by your map on the tracing paper? (2 pts). Map of the South Atlantic Ocean showing part of the Mid-Atlantic Ridge in black bars, with eastwest fracture zones, and selected magnetic anomalies. The ages of the numbered anomalies or magnetic lineations can be determined from the magnetic time scale on the next page. (From Magnetic Lineations of the World s Ocean Basins, 1985 the American Association of Petroleum Geologists. 14

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16 Since the polarity of the magnetic field at a given time is recorded (or "frozen") in rocks that formed at that same time, polarity can act as a chronometer ( time-measurer ), helping to tell us when the rock was formed. In addition, the inclination angle i of the "frozen" ancient magnetic field will tell us the ancient magnetic latitude or paleolatitude ϴ of the location where the rock was formed. To get ϴ from i, use the above use the relationship you learned in Part 2 to calculate the paleolatitude, ϴ, using the inclination angle, i. tan i = 2 tan ϴ Solving for ϴ: ϴ = tan -1 (1/2 tan i) The measured declination of a sampled rock (corrected for present day declination) tells us the angle between the present north and paleomagnetic north, indicating the rotation that the rock has experienced. Note that we have no method to determine paleolongitude. Paleomagnetic information is important in plate tectonics because if the magnetic latitude determined from the rock is not the same as it is today for the site where the rock was collected, then either the magnetic pole has moved or the site has moved, or both. These types of data form the main observations that are used in plate reconstructions. 22) Describe some of the main sources of error in plate reconstruction models. (3 pts). 23) Give two reasons why, if you find some nice 2.5 Ga (billion-year-old) rocks, good paleomagnetic data might be difficult to obtain from them. (3 pts). 16

17 Laboratory analysis of the paleomagnetic direction in rock samples of Pennsylvanian age from South America indicates that their magnetic inclination is 70 and their declination is 83. The samples were taken at 20 S, 55 W. 24) At what paleolatitude were these samples formed? (2 pts). 25) Draw a sketch showing the present day sample location in South America with a vector indicating magnetic declination. (2 pts). 26) What can you say about the plate tectonic drift of the South American continent since Pennsylvanian time given this paleopole? (Discuss magnitude of movement plus rates of any migrations and rotations.) (4 pts). 17