The class was again alerted to the midterm exam which will be given from 9:25 to 10:40 AM on Tuesday, March 15 in room 3127N. The exam format will be essay/short answer. There will be a choice of four questions out of six. Topics covered will be 1 through 6 on the revised syllabus. That is, everything through today's lecture (Lecture 6) will be included. (See the revised syllabus on the class website.)

1. Earth Magnetism: As anyone who has ever used a compass knows, the Earth acts as if it was a giant magnet. As a compass needle is moved about the earth's surface, its orientation varies in systematic fashion similar to that of a compass needle moved about a bar magnet. This behavior has prompted geologists to describe the earth as behaving as if it had a bar magnet implanted in its core. The length of the (imaginary) bar magnet is pictured as being almost but not exactly parallel to the earth's rotation axis.

2. The orientation of the compass needle in any locality is described as being parallel to the local 'lines of magnetic force'. These imaginary lines may be envisioned as curving 'railway tracks' in space along which the compass needle, like a train, must lie. The orientation of the lines of force varies in a gradual, systematic fashion, from horizontal (near the equator) to vertical (near the rotational poles of the earth).

3. Earth's Magnetic Poles: The point on the earth's surface where the N end of the compass needle points straight down is called the magnetic north pole; the point on the earth's surface where the N end of the compass needle points straight up (and the S end of the needle points straight down) is the magnetic south pole. The magnetic poles lie close to the rotational poles of the earth. The positions of the magnetic poles are not fixed; they vary through time but always stay fairly close to the rotational poles.

4. Over a small region of the earth's surface, the lines of force are approximately parallel, and therefore the orientation of a compass needle remains constant (assuming the absence of natural or artificial local concentrations of iron and of electric currents).

5. Paleomagnetism: Paleomagnetism is the study of earth magnetism as it has changed through time.

6. Remanent Magnetism: Some rocks are capable of recording the local orientation of the Earth's lines of magnetic lines of force as they existed at the time the rocks were formed. This record of the Earth's former magnetic field preserved in rocks is called 'remanent magnetism'.
   1. Igneous Rocks. Amongst the minerals that form as molten magma or lava cools and crystallizes, is the mineral magnetite. The magnetite grains may be envisioned as tiny compass needles floating freely in the molten rock. As such, they can rotate until they line up parallel to the local lines of force. As the magma or lava turns to rock, the magnetite 'needles' are frozen in position, remaining parallel to one another and to the lines of force. The aligned magnetite needles make the rock behave like a very weak bar magnet whose length is aligned parallel to the lines of force. If the orientation of the lines of force subsequently changes, the orientation of the 'rock magnet' remains unchanged, since the magnetite 'needles' are locked in position. When a sample of the rock is examined today, its magnetic orientation (and thus the orientation of the lines of force that existed when the molten material cooled and turned into rock) may be determined. The ancient magnetism 'preserved' within the rock is called 'remanent magnetism.'

   2. Sedimentary Rocks. In a similar fashion, some sedimentary rocks that contain magnetite grains may show remanent magnetism. In this case, the magnetite grains become oriented parallel to local lines of force as they settle out through water to accumulate on a lake or sea floor. If the sediment is turned into rock, the parallelism of the magnate grains is 'locked in', and the rock may display very weak but determinable magnetic properties.

7. Magnetic Polarity Reversals: A common approach to studying the Earth's magnetic field through time is to examine a series of volcanic lava flows. The age of each lava flow (as will be discussed in an upcoming lecture) and the character its remanent magnetism may be determined. In the course of studying such series of lava flows, it was discovered that in some of the lava flows, the 'polarity' of the remanent magnetism was opposite to that of the rest. That is, for those lava flows, it was as if the 'magnets' they contained were reversed, with the N ends becoming the S ends, and vice versa. Since the remanent magnetism of the rocks records the character of the Earth's magnetic field at the time the rocks were formed, it was concluded that every now and then the Earth's magnetic field reverses itself, changing from 'normal polarity' (such as we have today) to 'reverse polarity' (opposite to what we have today). A reversal takes place quite rapidly (perhaps over a few thousand years). Then, the field remains stable for tens or hundreds of thousands of years until it reverses again.

8. Origin of the Earth's Magnetic Field: The Earth's magnetic field may be generated by electric currents formed by in the Earth's outer core. As the molten iron-nickel of the outer core swirls around, it generates electric currents. Electric currents create associated magnetic fields. Polarity reversals may result from changes in the pattern of motion of the molten metal.

9. Magnetic Anomalies: Where the strength of the Earth's magnetic field is greater or lesser than the average strength, a 'magnetic anomaly' is said to exist. 'Positive anomalies' are where the field is stronger than average; 'negative anomalies' are where the field is weaker than average.
   1. The Pattern of Magnetic Anomalies over Continents: Using an instrument called a 'magnetometer', the strength of the Earth's magnetic field in any location may be determined. Upon investigation, it was discovered that on the continents, the strength varies from place to place in a seemingly random fashion, controlled by the concentration of iron in the rocks. In fact, commercially valuable deposits of iron were sometimes located using a magnetometer.

   2. The Pattern of Magnetic Anomalies over Ocean Basins: After World War 2 and the development of more advanced magnetometers that were able to measure the strength of the magnetic field in oceanic areas (used by the military to detect enemy submarines), it was discovered that a dramatically different pattern of anomalies existed. Instead of forming a random pattern, positive and negative anomalies alternated to form a pattern of stripes that paralleled the mid ocean ridges.

10. Origin of the Ocean Floor Magnetic Anomaly Stripes:
    1. Sea Floor Spreading and the Formation of Remanent Magnetism. As sea floor spreading occurs, new oceanic crust is formed at mid ocean ridges and then moves away to either side as part of the diverging plates. The newly formed crust is igneous, the result of the cooling and solidification of magma injected into and lava erupted onto the mid oceanic ridges. As these new igneous rocks form, they become weakly magnetized parallel to the lines of force that exist at the time and place of their formation. If the Earth's magnetic field has normal polarity, then the remanent magnetism imprinted in the rock is 'normal'. If the Earth's magnetic field has reverse polarity, then the remanent magnetism imprinted in the rock is 'reverse'.

    2. Factors that Contribute to the Total Strength of the Magnetic Field in Oceanic Areas: The total strength of the magnetic field in an oceanic area today depends upon two factors:
       1. the principal factor is the strength at that location of the Earth's general magnetic field (that is, the field generated in the Earth's core)
       2. a minor but significant additional factor is the contribution from remanant magnetism of the rocks underlying that location.

       Combining the Factors The polarity of the Earth's general magnetic field today is by definition 'normal'. If the polarity of the remanent magnetism of the underlying rock is also 'normal', then the effect is additive: the total strength of the magnetic field at a particular location equals the sum of the strength of the general magnetic field plus the strength of the remanent magnetism associated with the local underlying rocks.

       However, if the polarity of the remanent magnetism of the underlying rock is 'reverse', then the effect is subtractive: the total strength of the magnetic field equals the strength of the general magnetic field minus the strength of the magnetic field associated with the underlying rock. The result in the first (additive) case is a total strength that is greater than 'average' (a positive magnetic anomaly); the result in the second (subtractive) case is a total strength that is less than the 'average' (a negative anomaly).

    3. Oceanic Crust as Recorder of Reversing Polarity: As new crust is steadily generated along a mid ocean ridge, it 'records' the polarity of the Earth's magnetic field, either normal or reverse, by developing normal or reverse remanent magnetism. During the whole length of time that the Earth's magnetic field polarity remains the same, a widening strip of rock on each side of the ridge displays that particular polarity. If the Earth's magnetic polarity switches, then the new polarity is recorded in the rocks as new crust is generated along the ridge. The older crust, displaying the former, opposite polarity, is found further away on each side.

    4. Thus, as time goes by and the Earth's magnetic field keeps switching polarity, the oceanic crust develops alternate stripes on each side of the ridge (and parallel to it) that display normal and reverse polarity. Each polarity stripe, once it forms, moves away from the ridge.

    5. Width of the Magnetic Anomaly Stripes: If the Earth's magnetic polarity is stable for a long time, the stripe that records that polarity is correspondingly wide. If it stable for a short time, the stripe is correspondingly narrow.

    6. Relationship Between Polarity and Magnetic Anomalies: Since the present polarity of the Earth's magnetic field is by definition 'normal', the total magnetic strength above any 'stripe' that has imprinted 'normal' polarity will be relatively high, and a positive magnetic anomaly results. Similarly, the total magnetic strength above any 'stripe' that has imprinted 'reverse' polarity will be relatively low, and a negative magnetic anomaly results.

    7. Thus, the alternating pattern of normal and reverse polarity stripes results in an alternating pattern of positive and negative magnetic anomaly stripes.

11. Support for Plate Tectonics: The professor noted that the pattern in oceanic areas of magnetic anomaly stripes parallel to mid oceanic ridges is a strong confirmation of sea floor spreading, plate tectonics, and continental drift.

12. Why the Pattern of Magnetic Anomalies is Different in Continental Regions: Because oceanic crust is essentially all igneous (that is, it all formed from solidifying magma or lava), all of the crust is charcaterized by the presence of remanent magnetism. In contrast, continental crust is highly diverse: it contains sedimentary and metamorphic rocks as well as igneous. The metamorphic rocks and many of the sedimentary rock cannot develop remanent magnetism. In the partial or total absence of remanent magnetism, magnetic anomalies in continental areas are formed primarily due to the amount of iron in underlying rocks. Since the concentration of iron in the rocks is random, the pattern of magnetic anomalies is also random.

13. In an upcoming lecture, we will learn how the paleomagnetic properties of rocks are useful in determining their ages.