IV. Internal Heat Flow


Intense heat from the inner core is continually radiated outward, through the several concentric shells that form the solid portion of the planet. The source of this heat is thought to be energy released by the radioactive decay of uranium and other radioactive elements. Convection currents within the mantle transfer most of this heat energy from deep within the earth to the surface and are the driving force behind continental drift. Convective flow supplies hot, molten rock to the worldwide system of midocean ridges  and feeds the lava that erupts from volcanoes on land.


V. Age and Origin of the Earth


Radiometric dating has enabled scientists to arrive at an estimate of 4.65 billion years for the age of the earth . Although the oldest earth rocks dated this way are not quite 4 billion years old, meteorites, which correlate geologically with the earth's core, give dates of about 4.5 billion years, and crystallization of the core and meteorites is considered to have occurred at the same time, some 150 million years after the earth and solar system first formed .

After originally condensing, by gravitational attraction of cosmic dust and gas, the earth would have been almost homogeneous and relatively cool. But continued contraction of these materials caused them to heat, as did the radioactivity of some of the heavier elements. In the next stage of its formation, as the earth became hotter, it began melting under the influence of gravity. This caused the differentiation into crust, mantle, and core, with the lighter silicates moving up and outward to form the mantle and crust and the heavier elements, mainly iron and nickel, sinking downward toward the center of the earth to form the core. Meanwhile, by volcanic eruption, light, volatile gases and vapors continually escaped from the mantle and crust. Some of these, mainly carbon dioxide and nitrogen, were held by the earth's gravity and formed the primitive atmosphere, while water vapor condensed to form the world's first oceans.


VI. Terrestrial Magnetism


The phenomenon of terrestrial magnetism results from the fact that the entire earth behaves as an enormous magnet. The English physician and natural philosopher William Gilbert was the first to demonstrate this similarity in about 1600, although the effects of terrestrial magnetism had been utilized much earlier in primitive compasses.

A. Magnetic Poles


The magnetic poles of the earth do not correspond with the geographic poles of its axis. The north magnetic pole is presently located off the western coast of Bathurst Island, in the Canadian Northwest Territories, almost 1290 km (almost 800 mi) northwest of Hudson Bay. The south magnetic pole is presently situated at the edge of the Antarctic continent in Adélie Coast about 1930 km (about 1200 mi) northeast of Little America.

The position of the magnetic poles is not constant and shows an appreciable change from year to year. Variations in the magnetic field of the earth include secular variation, the change in the direction of the field caused by the shifting of the poles. This is a periodic variation that repeats itself after 960 years. A smaller annual variation also exists, as does a diurnal, or daily, variation that can be detected only by sensitive instruments.


B. Dynamo Theory


Measurements of the secular variation show that the entire magnetic field has a tendency to drift westward at the rate of 19 to 24 km (12 to 15 mi) per year. Clearly the magnetism of the earth is the result of a dynamic rather than a passive condition, which would be the case if the iron core of the earth were solid and passively magnetized. Iron does not retain a permanent magnetism at temperatures above 540° C (1000° F), however, and the temperature at the center of the earth may be as high as 6650° C (12,000° F). The dynamo theory suggests that the iron core is liquid (except at the very center of the earth where the pressure solidifies the core), and that convection currents within the liquid core behave like the individual wires in a dynamo, thus setting up a gigantic magnetic field. The solid inner core rotates more slowly than the outer core, thus accounting for the secular westward drift. The irregular surface of the outer core may help to account for some of the more irregular changes in the field.


C. Inner Core Structure


Another theory that may explain some variations in the earth's magnetic field concerns the structure of the very inner core of the earth. In 1995 scientists at the Carnegie Institute of Washington announced that computer models of the earth's inner core appear to show one huge, remarkably aligned iron crystal. Scientists think that the atoms in the core are arranged so that each atom is packed with 12 neighboring atoms in a tightly packed hexagonal structure . The molten outer core still provides the earth's magnetic field in this theory, but the inner core would have some effect, probably causing the magnetic field to warp slightly and causing especially large variations in the position of the magnetic poles during times when the outer core's effect is weaker, such as during a magnetic reversal. A crystalline inner core would also explain why shock waves caused by earthquakes take about four seconds longer to go from east to west through the earth than from north to south, because the waves would travel more quickly with the "grain" than across the grain of the crystal.