Magnetic reversals

Investigation of the earth's magnetic field history, as recorded by diverse rock types, has disclosed that the magnetic field changes position relative to the surface of the earth. The information accumulated from the study of these reversals is used to explain many of the events that have occurred over the course of the earth's history, such as continental collisions.

Normal and Reverse Polarities

Research into the history of the earth's magnetic field has revealed that the field has flipped polarity many times in the past. Presently, the field is oriented so that a compass needle points toward the Northern Hemisphere of the earth. This orientation is known as normal polarity. If a compass needle were to point toward the south, that would indicate a reverse polarity. The flipping, or reversal, of the field involves the exchange of pole positions from Northern Hemisphere to Southern Hemisphere, either normal to reverse or reverse to normal.

To determine whether the polarity change is a real field change or simply a modification in a rock's magnetic-recording mechanism, numerous rocks were analyzed to ascertain their magnetic characteristics and to determine whether these change over time. Only a very small percentage of the rocks studied, including an igneous rock from Japan, displayed a self-reversing tendency. This finding persuaded geophysicists that self-reversing tendencies in rocks do not need to be considered in the study of the field's history. Therefore, geophysicists do not have to test every rock to determine whether it self-reverses.

Thermal Remanent Magnetization

Geologists must still verify that the polarity changes are real phenomena that are consistent from one region of the earth to another. They make use of the fact that when magnetic grains form in magma, they magnetically align themselves with the magnetic field present at that time. This type of rock recording of magnetic direction is known as thermal remanent magnetization (TRM). The best recorder of TRM is rock of basaltic composition.

The Hawaiian Islands are an example of basaltic rock formed from magma that has sporadically erupted from the Hawaiian volcanoes over a period of millions of years. The island of Hawaii is a large volcano that sits several kilometers below sea level and rises several kilometers above sea level. Measured from base to summit, Hawaii is the highest mountain in the world.

A detailed polarity history of the island is difficult to develop because volcanic eruptions are intermittent, with several thousands of years between eruptions. However, an overall appreciation of the field changes can be acquired by sampling the distinct layers located in the eroded sides of the volcanoes. In the laboratory, an “absolute” date for the rocks can be obtained using radioactive-dating procedures. Relative dates—the sequence of occurrence for the samples—can also be established, as sample A is from a layer that lies below sample B, and so on. Relative dating helps assure that the absolute dates are correct. If sample A has an absolute date of 120,000 years and sample B a date of 140,000 years, but sample B is located physically above sample A in the volcano, then something is wrong. Accurate dating is an important aspect in the establishment of the polarity time scale.

By using a magnetometer, the polarity and the field direction of the sample can be determined. If the field points down, the polarity is normal. A field that points up indicates a reversed polarity. Once enough data have been collected (several hundred samples), the sample polarities can be plotted against the sample date. In this manner, the polarity history can be determined for the past 4 million years.

The polarity scale shows that from 4 million to 3.3 million years ago, the field was reversed. This period is called the Gilbert reverse epoch (the major periods are named for scientists who have advanced the discipline of magnetism). The field was normal until 2.5 million years ago during the Gauss normal epoch, except for a brief period of reversed polarity around 3 million years ago, known as the Mammoth reverse event. The Matuyama reverse epoch continued until 700,000 years ago. This epoch contained two normal events: the Olduvai, around 2 million years ago; and the Jaramillo, about 1 million years ago. The Brunhes is the present-day normal epoch. Other normal or reverse events may have been present in these epochs.

Detrital Remanent Magnetization

Geophysicists are compelled to find other methods that verify the validity of the polarity scale and to extend and add more detail to the existing scale. One technique utilizes the sediment layer covering most of the ocean basin. This sediment can record magnetic field direction by the mechanism of detrital remanent magnetization (DRM). Long sediment cores are obtained from various areas in the ocean, and the magnetic polarity of areas along the length of the individual cores is measured. Again, a pattern of polarity changes is evident.

Radioactivity cannot efficiently date the layers of the core. Fortunately, the sediment is laid down very slowly, and this rate is measurable. The rate is on the order of millimeters per 1,000 years; thus, a layer 10 millimeters from the surface was deposited approximately 15,000 years ago. Polarity is plotted against calculated age, and analysis shows that the sediment-based data correspond well with the land-based scale.

Magnetic Seafloor Stripes

In the 1950s, magnetometers were towed behind ships that sailed over the oceanic ridge to the south of Iceland. The data were plotted on a map of the research area, and something strange became evident: The recorded magnetic field varied over the area. The map revealed a striped pattern of weaker and stronger field intensities that was aligned parallel to the ridge, now known as magnetic seafloor stripes. Fred Vine and Drummond Matthews, working together, and Lawrence Morley, working alone, realized that polarity changes caused the stripes.

In the mid-1960s, a revolution in the earth sciences was occurring with the development of the theory of plate tectonics. Scientists theorized that the earth's surface rock was split into plates of thin but considerable area. These plates had boundaries that interacted in several possible ways: They could move together, or converge; they could move apart, or diverge; or they could slide past each other in an area known as a transform fault. At the diverging boundary, the motion should produce a breach between the plates, but none was found. Investigation disclosed that the volcanically active oceanic ridge was the diverging boundary and that basaltic magma quickly filled any gap. New plate material is formed at this diverging boundary, and the cooling magma records the magnetic field present at the time of cooling by thermal remanent magnetization. The cooled magma moves away parallel to the ridge as the plates diverge. The magnetic field of basaltic rock that recorded the earth's magnetic field during a period of the reversed polarity cancels some of the earth's present-day field. This cancellation produces an area of lower-intensity field parallel to the ridge. The rock recording normal polarity adds to the earth's field, resulting in a strong intensity stripe.

The last polarity change 700,000 years ago was represented by rock that was located many kilometers from the center of the ridge. By dividing that distance by 700,000 years, the rate at which the plate is forming can be calculated. That value is approximately 2-5 centimeters per year depending on which portion of the ridge is being measured. This rate is comparable to how fast human fingernails grow. As a result of this movement, North America, which is west of the ridge, is now about 25 meters farther west of Europe, which is east of the ridge, than when Christopher Columbus sailed in 1492.

Identifying Trends

Other research has extended the polarity scale back hundreds of millions of years. This extension permits the identification of long-term trends depicting the manner in which the field has changed polarity. The field has remained very stable, with few polarity changes, several times in the past. In the Pennsylvanian period of the late Paleozoic era, for example, the field was predominantly in the reversed position; the field was normal for a long time in the Cretaceous period of the Late Mesozoic era. At other times, including the present Cenozoic era, the field has flipped many times.

Analyzing Rock Samples

The procedure used to study magnetic reversals depends on the area of investigation. Land-based investigations are straightforward. The researcher chooses a likely site and conducts preliminary research to ascertain whether others have studied the area and whether the site will yield samples appropriate for study. If an area displays promise, the required rock samples are obtained. The rock should not be severely weathered, as that may alter the magnetic or radioactive components of the rock, which could lead to incorrect results. The sample is collected using a gasoline-powered drill with a tube-shaped, diamond-tipped drill bit. The resulting sample is a cylinder of rock still attached at its base to the original rock. A brass tube, the size of the drill bit, is slipped over the cylinder. Brass is used because it is nonmagnetic and will not alter the sample's magnetic characteristics. Attached to the tube is a small platform on which is placed a Brunton compass, used to measure the orientation of the sample. The compass is very important, as sample orientation, the sampling site latitude and longitude, and the magnetic field direction of the sample are needed to calculate the sample's pole position. An orientation mark is made on the sample with a brass rod, and the sample is broken from the original rock. An identification number is assigned, which is carefully recorded along with all other pertinent information.

In the laboratory, the samples are prepared for the measurement of the rock's magnetic field direction by cutting them into lengths of 2.5 centimeters; thus, several small samples are obtained from each core, which can be used for dating purposes and for verifying the sample's polarity. Scientists do not rely on one measurement but make multiple assessments of a characteristic to ensure sample integrity. Because they are basalt samples, the rocks' magnetic directions are measured by a spinner magnetometer. A spinner works similarly to an electrical generator in that the sample is spun at high speed near coils of wire. The magnetic sample induces an electric current in the coils that is proportional to the strength of the sample's magnetic field. The rock's magnetic field direction is determined from these signals.

Computers perform the calculation of the final pole position using sample orientation, site latitude and longitude, and rock field direction. This pole position is plotted on a graph known as a stereonet, which is a two-dimensional representation of the earth's surface. Normal polarity poles plot in the Northern Hemisphere of the stereonet and those of reversed polarity in the Southern Hemisphere.

The basalt is also analyzed to determine its age. The amount of a suitable radioactive element (the parent isotope) and the amount of the element into which the radioactive element decays (the daughter isotope) are measured. The sample's age is calculated from these measurements.

Obtaining the Sediment Polarity Scale

The sediment polarity scale is more difficult to obtain, because the cores come from the ocean bottom—3 kilometers below sea level, in some places—using a coring device that is dropped from a ship. Back in the laboratory, samples are taken along the length of the core. Their position on the core is measured, as their distance from the top of the core determines the age of the sample. The original core is not oriented as it is taken from the sea floor, because the scientist is not interested in the sample's pole position. The scientist is interested in the sample's polarity, and an unoriented core will yield that information.

The sediment has a weak magnetic field, so a superconducting magnetometer is used for the measurements. When some materials are cooled close to absolute zero, near the boiling point of liquid helium, they have no resistance to electrical current. Superconducting magnetometers that can detect small magnetic fields, such as those of sediment, employ these materials. Again, the polarity of the sediment sample is determined from the magnetometer readings.

Detecting Seafloor Stripes

The detection of the seafloor stripes is a simple but tedious endeavor that requires the towing of a magnetometer “fish” several hundred meters behind a ship along parallel tracks across the area of interest. The magnetometer is towed to prevent the detection of ship-related magnetic fields. The readings—signal strength and ship's position—are plotted on a map and the stripe patterns are observed.

Geologic Applications

The fact that field reversals are a worldwide occurrence and instantaneous from a geological perspective led to the establishment of the field of magnetostratigraphy for correlating rock layers from various continents. In piecing together the earth's history as revealed in the rocks, it is necessary to know what is occurring around the world at approximately the same time. To accomplish this feat, ways have been developed to correlate rocks in one area with rocks in another area. One technique involves using index fossils, which are fossils that are widespread and common but for which the organisms lived for only a short time. The problem is that index fossils are limited in distribution and cannot be used for correlating between continents. Magnetostratigraphy bypasses this difficulty.

Magnetic reversals require that the strength of the magnetic field decrease to near zero and then build back with reversed polarity. This change in polarity can take anywhere from 2,000 to 20,000 years. The magnetic field strength of the past has been both greater and weaker than it is at present. The magnetic field of the earth shields living organisms from damaging cosmic radiation. Periods of reduced magnetic field strength allow increased exposure of life-forms to radiation. Periods of mass extinctions or accelerated genetic changes may accompany these periods of weakened magnetic field strength.

The fact that the earth's magnetic field has reversed many times in the past is important because it verifies the theory of plate tectonics. Before the plates rupture, the rock layers along the potential rift area dome upward; to relieve the pressure, the dome splits in a three-armed rift. Two of the arms expand in length and join arms from other domed areas. These joined arms form the boundary between the two plates; the third arm fails to enlarge but forms a wedge-shaped basin that fills with sediment. Over time, organic material in the sediment is converted to petroleum. These sources of petroleum were unknown until the development of the theory of plate tectonics. Understanding plate tectonics also increases the understanding of earthquakes and their origin, which could lead to their prediction and even to their control.

Principal Terms

basalt: dark-colored, fine-grained igneous rock frequently found beneath the sediment covering the ocean floor

detrital remanent magnetization (DRM): sedimentary rock magnetization acquired by magnetic sediment grains aligning with the magnetic field

normal polarity: orientation of the earth's magnetic field so that a compass needle points toward the Northern Hemisphere

polarity: orientation of the earth's magnetic field relative to the earth

reverse polarity: orientation of the earth's magnetic field so that a compass needle points toward the Southern Hemisphere

thermal remanent magnetization (TRM): magnetization acquired as a magma's magnetic material becomes permanently magnetized

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