What is the geomagnetism of earth? Information about the properties, origin of the field, polarity reversals of earth.
Geomagnetism Of Earth
The early English physicist William Gilbert demonstrated in 1600 that the geomagnetic field of the earth is similar to the field of a magnetized sphere. Subsequent observations have borne out his view that the earth behaves as a big magnet, but they have indicated that its magnetic field is not quite regular and that it fluctuates with time.
Properties. In the first half of the 19th century the German mathematician Karl Friedrich Gauss demonstrated that the earth generates its own main field internally, and that only some minor and rapid fluctuations in the field are due to solar activity. This main field is not placed symmetrically within the earth. A magnetic needle points in a direction that usually is a few degrees east or west of true, or geographic, north, depending on the observer’s longitude; this angle is known as the angle of declination. If the needle is free to move vertically, it also dips downward at an angle known as the angle. of inclination. The dip is not vertical at the geographic poles but at the magnetic dip poles—one of which was over Bathurst Island in the Canadian Arctic Ocean and the other near the coast of Antarctica south of New Zealand in the late 1960’s. The poles are not exactly apposite to each other geographically, nor are they stationary.
The earth’s field can be compared to that of a dipole magnet in the earth that neither passes through the earth’s center nor points along thc| axis of rotation. The north dip pole is moving northwest at an average rate of 5 miles (8 km) per year. It seems probable that the magnetic field nevertheless is loosely coupled to the axis of rotation, so that over many centuries it would be seen to have a mean position oriented along the earth’s axis.
The strength and direction of the earth’s magnetic field have been mapped in detail by nonmagnetic ships and their towed instruments, by aircraft, and by observations on land. The patterns of the maps show irregularities that either are very large or are very small; there are no anomalies of intermediate size. The small anomalies have been found to be caused by permanent magnetization of geological features within the crust, the temperature of which is below the Curie point—the temperature at which ferromagnetic materials lose their permanent magnetization. The large anomalies, which apparently are formed in the earth’s core, move slowly westward at a rate of about 0.3° per year.
Origin of the Field. These properties of the geomagnetic field are puzzling. The earth cannot be a huge permanent magnet; it is much too hot. All magnetic substances lose their magnetism above the Curie point, which never is more than a few hundred degrees centigrade. Nor could a permanent magnet shift about as the earth’s field does. The generally accepted answer is that the earth is a dynamo; it has been demonstrated that the main magnetic field could be caused by electric currents flowing in the fluid, metallic core. The escape of heat generated by the decay of small amounts of radioactive elements could maintain convection currents in the core, rather like currents in a pot boiling on a stove. The convection currents would carry the electric currents that generate the earth’s field and produce the large anomalies and motions of the field. (The lack of irregularities of intermediate size is ascribed to the absence of any magnetic influence in the mantle, which is both solid and above the Curie point.)
The small anomalies related to geological features are due to a phenomenon called rock magnetism. When a melt of iron-rich lava such as basalt solidifies and cools below its Curie point in a magnetic field, grains of iron oxides in the solidified melt retain a slight but easily measured magnetization along the direction of the field at the time of solidification. (The same property can be observed in some sedimentary rocks.) This means that measurements made today can determine the direction in which the earth’s field lay in past times.
One important piece of information provided by such study of many lavas formed in the recent past is that their directions of magnetization are scattered but that their average value coincides with the present field. This evidence supports the view that the dynamo in the core has a mean position symmetrically arranged around the rotational axis; it also suggests that by the averaging of several determinations, the orientation of old rocks and their geographical latitude at time of formation can be found. This is the basis of the study of paleomagnetism.
Polarity Reversals. Work on rock magnetism also has shown, surprisingly, that the earth’s magnetic field has reversed its polarity many times. This happens irregularly and for reasons that are not fully understood. All that can be said is that the phenomenon is conceivable in terms of the dynamo theory. The reversals in polarity were discovered as early as 1906, but they have attracted much attention only since it has become possible to date young lavas; by measuring radioactive decay. Using the potassium-argon method, geophysicists have established the time scale for the past 5 million years; the periods are irregular, but are the same for every site sampled.
The ratios of this time scale have been found in two other connections since 1965. Thus, it has been shown that if the magnetization is measured at close intervals along undisturbed cores of sediments taken in the ocean floors, the cores show reversals at depths that are in the same ratios to each other as are the ratios of the time scale. This can be readily understood if it is assumed that ocean floors were deposited uniformly and each layer has preserved the polarity of its time of deposition. Even more remarkable are the magnetic patterns found over ocean floors. The patterns consist of very regular strips that run parallel to mid-ocean ridges and are arranged symmetrically on either side of them. It has been established that the widths of the successive strips, when measured outward from the ridges, are in the same ratios to one another as are the timescale ratios found in rock magnetism.
As a result of these discoveries it has been proposed as an explanation by various scientists that the ocean floor is being generated by up-welling at mid-ocean ridges, that the basalt in layer 2 of the crust is being imprinted magnetically with the polarity of the geomagnetic field at the time of upwelling, and that the whole ocean floor is being carried away in either direction from the ridges at uniform velocities. Therefore the widths of the strips of anomalies correspond to the periods between successive reversals of the field. This is strong evidence for continental drift, and it also provides a precise way to determine the recent motions of continental blocks.