Rock magnetism
Rock magnetism is a subdiscipline of geophysics that explores how rocks record the Earth's magnetic field, the reliability of this recording process, and the conditions that can alter these records, potentially leading to misinterpretations. The study of the Earth's magnetic field dates back to the 1600s, but this relatively brief period provides limited insight into the planet's magnetic history spanning 4.6 billion years. Magnetic minerals within rocks, such as magnetite and hematite, serve as recorders of ancient magnetic fields, offering vital information about the Earth’s surface and interior conditions.
Two primary types of magnetism are of interest: ferromagnetism, where materials can retain magnetization after exposure to a magnetic field, and paramagnetism, where materials lose their magnetic orientation in the absence of an external field. The magnetic recording process occurs through mechanisms such as thermal remanent magnetization in igneous rocks and detrital remanent magnetization in sedimentary rocks, both of which rely on the alignment of magnetic minerals during formation.
Understanding rock magnetism is crucial for reconstructing geological history, including the movement of continents and the implications for natural resources and geological events. Advanced instruments, such as superconducting rock magnetometers, are employed to analyze the magnetic properties of rock samples, contributing to our comprehension of the Earth’s dynamic processes over geological time.
Rock magnetism
Rock magnetism is the subdiscipline of geophysics that has to do with how rocks record the magnetic field, how reliable the recording process is, and which conditions can alter the recording and therefore raise the possibility of a false interpretation being rendered by geophysicists.
Magnetic Field Production
The direct study of the earth's magnetic field began in the 1600's. This study involves the measurement of the field with scientific instruments and subsequent analysis of the resulting data. Four centuries is a very small fraction of the 4.6 billion years that the earth has existed; thus, direct study affords scientists very little understanding of the nature of the field over long periods of time. It is useful to know what happened to the earth's magnetic field in those billions of years before the present, because the field can be a source of information about conditions on the earth's surface and its interior. Magnetic minerals in rocks serve as recording devices, giving scientists clues regarding the nature of the ancient magnetic field.
A moving electric charge, such as an electron, produces a magnetic field that is the ultimate source of any larger magnetic field. An atom is composed of a nucleus, with its protons and neutrons, and the electrons that surround the nucleus. The protons do not orbit within the nucleus, but their spinning does produce a small magnetic field, which is cancelled out if there is an even number of protons. The electrons, however, orbit the nucleus, and this movement produces a weak magnetic field. In addition, the electrons spin on their axes, and this activity also gives rise to a small magnetic field.
Types of Magnetism
Because all atoms have electrons orbiting and spinning, one might think that all materials should have a permanent magnetic field, but the situation is more complicated. Strictly speaking, every material is magnetic, but there are different types of magnetism. Some materials are paramagnetic: When they are placed in an external magnetic field, the atoms align with the field. The atoms act as small compasses, orienting with the field, and the material is magnetized; the magnetic fields produced by the atom's electrons add to the intensity of the external field. When the external field is removed, however, the atom's orientation becomes randomized because of vibrations caused by heat, and the material is consequently demagnetized. Many materials, such as quartz, are paramagnetic and are not able to record the earth's magnetic field.
A much smaller number of minerals are ferromagnetic. There are various types of ferromagnetism, but the underlying principle is the same. In ferromagnetic materials, an external magnetic field again aligns the atoms parallel to the field, and the material is magnetized. When the field is removed, however, the atoms remain aligned, and the substance retains its magnetization; it is “permanently” magnetized. Actually, the substance can be demagnetized by heating or stress. Dropping a bar magnet on the floor or striking it with a hammer will demagnetize it slightly. The shock randomizes some of the atoms so that they cease to contribute to the overall magnetic field. The heating of a magnet above its Curie temperature also destroys its magnetization by randomizing the atoms and making the material paramagnetic. As the temperature drops below the Curie point, the material becomes slightly re-magnetized, because the weak field of the earth aligns some of the atoms.
In ferromagnetic materials, atoms are not all aligned in one direction; rather, they are found in aligned groups, called domains. Under a microscope, the domains are barely visible. Within a particular domain, the atoms are aligned, but all the domains are not aligned in the same direction. A “permanent” magnetic material that is unmagnetized has all the domains randomly aligned, and the overall field cancels to zero. When placed in a magnetic field, some of the domains realign parallel to the direction of the field and stay aligned after the field is removed. It is these domains that give the material its overall magnetization. If a high enough magnetic field is applied, all the domains align with the field, and the magnetization has reached its saturation point; the strength of the material's magnetic field is at a maximum. One of the areas of research for physicists is the quest for materials that have high magnetic field strengths but with less material. Such materials are useful in making small, but powerful, electric motors.
Magnetic Minerals
Rocks are classified into three main groups: igneous, formed from crystallized molten rock; sedimentary, formed from weathered rock material; and metamorphic, produced when other rock is modified with heat, pressure, and fluids. Most magnetic minerals occur in igneous and sedimentary rocks.
Materials such as iron, cobalt, and nickel are ferromagnetic. For this reason, they are used in making various permanent magnets. These metals are not found naturally on the earth's surface in the uncombined state, so they do not contribute to rocks' recording ability. Most of the minerals that make up rocks, such as quartz and clay, are not ferromagnetic. These minerals are useless as recorders, but many rocks contain magnetite or hematite, which are good recorders. These common magnetic minerals are oxides of iron.
Hematite is Fe2O3, which means that there are two iron atoms for every three oxygen atoms. Hematite is red in color, similar to rust on a piece of iron. Most reddish-brown hues in sedimentary rock are caused by hematite. This magnetic mineral is not a very strongly magnetized compound, but it is a very stable recorder in sedimentary rocks. Unfortunately, in many cases, its formation postdates that of the rock in which it occurs, so it does not necessarily record the magnetic field at the time of the rock's formation. Magnetite (Fe3O4) has been known as lodestone for several millennia. It is a strongly magnetized iron compound that makes some igneous rocks very magnetic and supplies some of the recording ability of sedimentary rocks. The magnetite in rocks can record the field direction by one of several methods.
Thermal Remanent Magnetization
In igneous rocks, magnetite crystals form as the magma cools. As the crystals grow, they align themselves with any magnetic field present. This process is called thermal remanent magnetization (TRM). If the crystals are quite small or quite large, they cannot permanently record the field direction; after a short time the recording fades and becomes unreadable. The magnetism of such small grains is called superparamagnetism: They do align with a magnetic field, but they easily lose their orientation. The larger grains contain many magnetic domains that become misaligned over time so that the recording fades.
Grains the size of fine dust are good recorders. Unfortunately, not all igneous rocks have grains of the proper size. The size of the mineral crystal depends on the rate of cooling: When magma is cooled very slowly, large crystals are produced, while a rapid cooling results in smaller crystals. Granite is coarse-grained and thus is not the best recorder. The best igneous recorder is basalt, a black, fine-grained rock. Basalt is fairly common on the surface of the earth, particularly in the ocean basins, where nothing but basalt underlies the sediment on the basin floor.
A useful magnetic recorder must provide information about how old it is. Basalt again fills this requirement, as its crystallization can be dated by measuring the amount of radioactive elements and their daughter products it contains. Clearly, basalt is an ideal source of information on the magnetic field. Unfortunately, it does not occur everywhere on the earth; moreover, as a recorder, it covers only times of eruptions of magma. Some other recorder must be used to fill in the blanks.
Detrital Remanent Magnetization
Sedimentary rock is formed from the products of the rock weathering that accumulate mostly in watery environments, such as rivers, lakes, and oceans. Clastic sedimentary rocks are formed from fragments of rock and mineral grains, such as grains of quartz in sandstone. Chemical sedimentary rock is derived from chemical weathering products, such as calcium carbonate or calcite, which is the major constituent of limestone. Most of the material in sedimentary rocks is not ferromagnetic, but there are a few grains of magnetite and other ferromagnetic compounds. As the grains fell through the water, they aligned with the magnetic field present at that time. When they hit the bottom, they retained the orientation, for the most part, and were subsequently covered by more sediment. This process is termed detrital remanent magnetization (DRM).
An interesting aspect of DRM is the role that organisms play in its formation. The grains of magnetic minerals that fall through the water are oval-shaped, and when they strike the surface of the sediment they become misaligned with the field. Organisms such as worms disturb the sediment in a process known as bioturbation, which moves the sediment around and realigns the magnetic grains with the field. In the mid-1980's, it was discovered that certain varieties of bacteria have small grains of magnetite in their bodies. The bacteria use the grains like compasses to find their way down into the sediment on which they feed. The bacteria eventually die, and the magnetite grains become part of the sediment, aligned with the magnetic field; this phenomenon is known as biomagnetism.
The grain-size problem also occurs in DRM, given that a sediment particle can be the size of a particle of clay, a boulder, or anything in between. Conglomerate, a rock composed of rounded pebbles and other large particles, is not a good recorder, nor is coarse sandstone. Finer sandstones, shales, siltstones, and mudstones are much better. Most chemical rocks, such as halite (common table salt), are poor recorders; limestone may or may not be good, depending on the conditions of formation.
The magnetization in sedimentary rocks is generally between one thousand and ten thousand times weaker than is the magnetization in a basalt. Very sensitive magnetometers are needed to measure the magnetic field in these specimens. To be useful in geomagnetic studies, sedimentary rocks must be dated, but this is a difficult task, as they cannot be dated using radioactive methods. By a complex method of determination, fossils can act as indicators of the age of the rock in which they are found. If igneous rock layers are located above and below the rock layer of interest, and if these igneous rock layers can be dated, an intermediate age can be assigned to the sedimentary layer.
Study of Rock Magnetism
A magnetometer useful in the study of rock magnetism is the superconducting rock magnetometer (SCM). Superconductivity is the phenomenon of a material's losing its resistance to electric current at low temperatures. Liquid helium is used to cool a portion of the magnetometer, composed of a cylinder of lead closed at one end. As the lead cools, it becomes superconducting, and if done in a region of low magnetic-field intensity, this low field is “trapped” inside the cylinder. Magnetic field sensors known as SQUIDS (or superconducting quantum interference devices) are very sensitive to low-intensity magnetic fields. The sample is lowered into the device, and its electronic display shows the intensity of the sample's magnetization. Such devices are useful in studying the rock magnetism of low-intensity sedimentary rocks.
The Curie temperature is important for establishing the thermal remanent magnetization for igneous rocks. A sample of a particular ferromagnetic material in a magnetic field is heated and the temperature is measured; the sample's Curie temperature is determined when the pull of the magnetic field on the sample weakens. The Curie point for various ferromagnetic materials is established by this method. Once that is done, the procedure is reversed. A sample of an unknown ferromagnetic material can be heated in a magnetic field to determine its Curie point, which can then be compared with the established table of values to identify the magnetic mineral. This method does not establish the exact composition of the material, but it does narrow down the possibilities, which is of value because other methods for determining composition are more expensive. In addition, it has been discovered that Curie temperature is not the only factor critical to the recording process. At the Curie point, the material is ferromagnetic but the recording ability is weak. The material has to cool through the blocking temperature for recording stability. Thereafter, magnetic minerals are magnetically stable for periods of billions of years.
Another area of study is the determination of the best grain size and shape for magnetic recording. Researchers experiment with different sizes and shapes of magnetic grains in magnetic fields of various strengths and directions and measure their responses to changes. It was found that crystals of magnetic materials such as magnetite develop features known as domains. These are areas where the atoms are aligned in one direction and produce the unified magnetic field for the domain. A small crystal has only one domain that can easily shift to another direction; therefore small crystals are poor recorders. If the crystal is quite large, it has many domains in which it is again easy to shift direction. Crystals with one large domain or several small domains are magnetically “hard” in that it is more difficult to shift the magnetic alignment. For magnetite, these are dust-sized particles, around 0.03 micron in diameter.
Methods of “Magnetic Cleaning”
Other research reveals that a rock's recording of the magnetic field is not as “neat and clean” a process as portrayed in the previous paragraphs. Many events can lead to the alteration of the magnetic alignment. If the rock is heated above the Curie point and then cooled, the magnetic alignment is that of the field present at that time, and the old alignment is erased. The rock may be changed chemically, and old magnetic minerals may be destroyed and new ones produced. This process is referred to as chemical remanent magnetization (CRM).
These secondary magnetizations can be removed in some cases, and they can even provide more information on the rock's history. One method of magnetic cleaning or demagnetization involves subjecting the rock sample to an alternating magnetic field while other magnetic fields are reduced to zero. This “cleaning” will remove that portion of the mineral's magnetization that is magnetically “softer” than the maximum alternating field. The magnetization above this level is unaffected and should represent the original magnetization. Heating a sample to a certain temperature is another method of demagnetization.
Role in Study of Earth's History
The study of the earth's magnetic field history, and all the inferences about the earth drawn from that study, depends on the ability of rocks to record information about the magnetic field at the time of the rocks' formation. That ability, in turn, is dependent upon the magnetic characteristics of a few permanently magnetized minerals, such as magnetite.
The study of rock magnetism is rather esoteric; only a few individuals worldwide are involved in this subdiscipline of geomagnetism. Yet, such study has shown that rocks can faithfully record the history of the earth's magnetic field. This record is used to infer conditions on the earth hundreds of million years ago. Such studies have lent support to the idea that the continents have actually moved over the surface of the globe—and thus the theory of plate tectonics was born, with all its implications for the formation and location of petroleum and ore deposits, the origin of earthquakes and volcanoes, and the formation of mountain ranges such as the Himalaya. Such is an example of the odd twists and turns that science can take. Seemingly inconsequential findings can lead to a theory with great potential for making the earth and its workings much more understandable.
Principal Terms
basalt: a very common, dark-colored, fine-grained igneous rock
blocking temperature: the temperature at which a magnetic mineral becomes a permanent recorder of a magnetic field
Curie temperature: the temperature above which a permanently magnetized material loses its magnetization
daughter product: an isotope that results from the decay of a radioactive parent isotope
detrital remanent magnetization: the magnetization that results when magnetic sediment grains in a sedimentary rock align with the magnetic field
ferromagnetic material: the type of magnetic material, such as iron or magnetite, that retains a magnetic field; also called a permanent magnet
granite: a low-density, light-colored, coarse-grained igneous rock
magnetite: a magnetic iron oxide composed of three iron atoms and four oxygen atoms
radioactivity: the spontaneous disintegration of a nucleus into a more stable isotope
thermal remanent magnetization: the magnetization in igneous rock that results as magnetic minerals in a magma cool below their Curie temperature
Bibliography
Butler, Robert F. Paleomagnetism: Magnetic Domains to Geologic Terranes. Boston: Blackwell Scientific Publications, 1992. Butler's exploration of the earth's magnetic fields begins with basic descriptions of what paleomagnetism is and how it occurs. The book is filled with illustrations to back up difficult concepts covered in the text.
Cox, Allan, ed. Plate Tectonics and Geomagnetic Reversals. San Francisco: W. H. Freeman, 1973. Cox provides fascinating introductions to chapters that are composed of seminal papers concerning magnetic reversals and their contribution to the development of the theory of plate tectonics. Information on rock magnetism is scattered throughout the book in discussions on baked sediments, magnetization of basalt, magnetic intensity, and self-reversals in rocks. The papers are advanced for the average reader, but there are many graphs, diagrams, and figures that merit attention.
Dunlop, D. J., and O. Ozdemir. Rock Magnetism. New York: Cambridge University Press, 2001. A valuable resource for graduate students and researchers. This text covers topics of general earth magnetism as well as putting rock magnetism into the context of sedimentary, igneous and metamorphic rock. The focus of these topics is on the mathematics and physics aspects; a strong understanding of mathematics is recommended.
Glen, William. The Road to Jaramillo: Critical Years of the Revolution in Earth Science. Stanford, Calif.: Stanford University Press, 1982. This book gives a history of the plate tectonics revolution of the mid-1950's to the mid-1960's. Rock magnetism is specifically covered on pages 103-109. Other aspects of rock magnetism are discussed in various portions of the book—for example, those dealing with the magnetic minerals associated with rock magnetism and deep-sea core work. Of particular interest are the sections devoted to instruments used to measure rock magnetism. Best suited for students of geology, the writing is very technical.
Hamblin, William K., and Eric H. Christiansen. Earth's Dynamic Systems. 10th ed. Upper Saddle River, N.J.: Prentice Hall, 2003. This geology textbook offers an integrated view of the earth's interior not common in books of this type. The text is well organized into four easily accessible parts. The illustrations, diagrams, and charts are superb. Includes a glossary and laboratory guide. Suitable for high school readers.
Hargraves, R. B., and S. K. Banerjee. “Theory and Nature of Magnetism in Rocks.” In Annual Review of Earth and Planetary Sciences, Vol. 1, edited by F. Donath. Palo Alto, Calif.: Annual Reviews, 1973. The article covers the theories of the natural remanent magnetization of rocks (NRM). NRM is the combined magnetization of all magnetic minerals in a rock, such as those resulting from TRM. The various carriers of remanence are also covered, with a table that lists each mineral and its composition, crystal structure, magnetic structure, and other pertinent information. The paleomagnetic potential of rocks is also discussed. A few mathematical equations are included, but nothing too formidable. There are numerous figures and a long list of references.
Lapedes, D. N., ed. McGraw-Hill Encyclopedia of Geological Sciences. New York: McGraw-Hill, 1978. Pages 704-708, under the heading “Rock Magnetism,” provide concise descriptions of how rock magnetization occurs, the present field, magnetic reversals, secular variation, and apparent polar wandering. The text is very readable, with no mathematics and a fair number of graphs, tables, and figures.
Merrill, R. T., and M. W. McElhinney. The Magnetic Field of the Earth: Paleomagnatism, the Core, and the Deep Mantle. San Diego: Academic Press, 1998. The authors cover the basic material associated with the earth's magnetic field. Chapters deal with the origin of the magnetic field, as well as the origin of secular variation and field reversals. A strong background in mathematics is recommended. Bibliography and index; numerous tables, figures, and mathematical equations.
O'Reilly, W. Rock and Mineral Magnetism. New York: Chapman and Hall, 1984. O'Reilly covers the atomic basis for magnetism, the magnetization process, the various remanent magnetizations such as TRM, the magnetic properties of minerals, and the applications of rock and mineral magnetism. Many tables, figures, and photographs of minerals are included. Some mathematics and chemistry are also included but should not be too difficult. References are included at the end of each chapter.
Plummer, Charles C., and Diane Carlson. Physical Geology. 12th ed. Boston: McGraw-Hill, 2007. A college-level introductory geology textbook that is clearly written and wonderfully illustrated. An excellent sourcebook of basic information on geologic terminology and fundamentals of geologic processes. An excellent glossary.
Stacey, F. D., and Paul M. Davis. Physics of the Earth. 4th ed. New York: Cambridge University Press, 2008. Under Section 25.2, “Magnetic Properties of Minerals and Rocks,” the author provides a short, technical description of rock magnetism. Several figures show the various types of magnetic alignments. The domain structure of ferromagnetic materials is also discussed. Many other aspects of the earth's magnetic field are covered at a technical level as well. Reformatted into units to make topics more accessible.
Tarling, D. H. Paleomagnetism: Principles and Applications in Geology, Geophysics, and Archeology. New York: Chapman and Hall, 1983. A very good resource on the subject of paleomagnetism, or the ancient magnetic field. Chapter 2 is devoted to the “physical basis” for the magnetization of material, with a discussion of the atomic level and the resulting magnetic domains. The various remanent magnetizations are covered in detail. Chapter 3 deals with the various magnetic minerals and their identification. The magnetization of the various rock types is covered in Chapter 4. Chapter 5 discusses instruments used in paleomagnetic work. The remainder of the text deals with mathematical analysis used in paleomagnetic work and paleomagnetic applications.
Yamaguchi, Masuhiro, and Yoshifumi Tanimoto, eds. Magneto-Science: Magnetic Field Effects on Materials: Fundamentals and Applications. Berlin: Springer-Verlag, 2010. Provides an overview of magnetism and its applications in earth science, chemistry, and physics. An excellent source of basic knowledge of magnetic forces and magnetic fields.