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Principles of Physical Science

Terrestrial magnetism

Fields of Study

Geophysics; Electromagnetism

Key Terms and Concepts

  • center of mass: the mass weighted center of an object or system of objects.

  • current: the rate at which an electric charge, usually in the form of electrons, moves through a wire or other conductive material.

  • electromagnetic field: a physical field consisting of a combined electric field (generated by stationary electric charges) and magnetic field (generated by moving electric charges) that affects the behavior of charged objects in its vicinity.

  • friction: the force created by resistance to relative motion between solid surfaces.

  • paleomagnetism: the magnetic signature remaining in rocks that indicates the strength and direction of Earth's magnetic field in the past.

  • sea-floor spreading: the gradual movement of sea-floor bedrock away from an undersea rift where magma continuously wells up from the mantle and solidifies due to tectonic activity in Earth's crust.

Summary

Terrestrial magnetism has been known and used for navigation purposes for thousands of years, though it was not understood in any scientific sense until 1600, when William Gilbert determined that Earth is in fact a huge magnet. Sea-floor spreading and continental drift demonstrated the dynamic nature of Earth's interior, and subsequent research indicates that 90 percent of Earth's magnetism is generated by the outer core due to the induced electrical current caused by the rotation of the solid inner core of the planet. Terrestrial magnetism reverses its polarity about every 500,000 years, possibly as the result of the Earth colliding with large meteors.

How It Works

Earth's magnetic field. People have understood for hundreds of years that this planet possesses a magnetic field, although it was not appreciated as such for much of that time. A naturally occurring form of iron ore called 'lodestone' was used to make a guidance device, or compass, during the Han Dynasty in China as much as 2,300 years ago. Similar devices may have been used by the Olmecs, in Central America, as much as a thousand years earlier, although this is not yet conclusively proven. European navigators were using lodestone, and later, dry magnetic compasses more than fifteen hundred years later. Yet, in all that time, the principle that made a compass function remained unrealized. It wasn't until 1600 that William Gilbert (1540 – 1603) systematically investigated the phenomenon of magnetism, and determined that Earth is itself a source of magnetism. Still, a working model of the cause of magnetism was not determined until the development of the model of atomic structure based on quantum mechanics early in the 20th century. Although Gauss and Maxwell had been able to describe the behavior of an electromagnetic field more than a century earlier, that work did not posit a viable cause for magnetism. In the intervening years, it came to be understood that magnetic fields are caused by the movement of electrons, as an electric current, through a conductive material. Earth's magnetic field must therefore be generated by the same cause somehow, within Earth's interior, and although the intensity of the magnetic field is weak, it is nevertheless global in scale. It was also believed to be constant over time, with only minor variations due to the local composition of Earth's crustal matter. In the mid-1960s, this concept was proven false.

Continental drift and magnetic pole reversals. In 1915, Alfred Wegener (18890 – 1930) published a theory to explain the observation that the coastlines of the continents can be fitted together like the pieces of a planet-sized jigsaw puzzle. According to that theory, the continents, particularly Africa and South America, were at one time joined together in a single 'supercontinent' and through the phenomena of sea-floor spreading and continental drift have moved away from each other to their present locations. The theory was not accepted at the time, partly because Wegener was a meteorologist and not a geologist, but primarily because the scientific establishment maintained a strong adherence to the 'steady state' principle. Earth was deemed to be in an unchanging condition, rather than as a dynamic structure. Wegener's theory did not gain universal acceptance until the mid-1960s, as a direct result of mapping the magnetic signature, or paleomagnetism, of sea-floor bedrock along the Pacific coast of California. The mapping revealed a pattern of bands of reversed magnetic polarities that could be matched perfectly to the pattern obtained on the opposite side of a seismic faultline. Similar research in the Atlantic Ocean revealed the existence of the Mid-Atlantic Ridge, about midway between Africa and the American continents. The mechanism of sea-floor spreading was observed directly at that location. Taken together, continental drift and the polarity reversals in paleomagnetism demonstrate that the interior of Earth is a dynamic system, rather than a steady-state system. This dynamism is therefore responsible for the generation of Earth's magnetic field. Given that magnetism is generated by the movement of electrons through a conductive medium, the internal structure of Earth must therefore function as a dynamo.

Earth's interior as a dynamo. The interior structure of Earth cannot be examined directly for several reasons. Firstly, the crustal layer is tens of kilometers thick (1 kilometer = 0.625 mile). Secondly, the material lying below the crust is too hot for any material to withstand. But most importantly, the interior is dynamic and in constant motion, however slow. Essentially all information about the interior of Earth is obtained from analysis of seismic waves. This has indicated that the interior of Earth has a layered structure below the crust. There are basically three layers below the crust: the mantle, the outer core, and the inner core. Sea-floor bedrock is essentially the solidified surface material of the mantle. The friction caused by rotation of the core about the center of mass of the planet is sufficient to keep the mantle layer in a fluid state. The core itself consists mostly of iron, with some nickel and a few minor elements. The inner and outer cores are separated by a 'transition layer' that is several hundred kilometers in thickness. The outer core is in a fluid state, and is believed to produce more than 90% of the terrestrial magnetism. The inner core is believed to be a single enormous crystal of solid iron, at a temperature well above its melting point but kept in a solid state by pressure. Seismic analysis indicates that the inner core rotates at a slightly faster rate than the outer core, and so generates the electrical current in the outer core that produces the terrestrial magnetism. The cause and mechanism of magnetic polarity reversals are still unknown, though it is known that they occur approximately every half-million years.

Why Terrestrial Magnetism is Important

Earth is under a constant state of bombardment from charged particles emitted by the Sun, and essentially the entire universe. These 'cosmic rays' are capable of doing great harm to living systems as they are able to pass through matter and damage the DNA molecules that define living organisms on Earth. The atmosphere is incapable of absorbing them and preventing them from reaching the surface. But because they are electrically charged, they interact with magnetic fields such as the terrestrial magnetism. As they encounter Earth's magnetic field, the charged particles become trapped by the interaction and do not reach the surface of the planet. Individual charged particles enter into a spiral motion about the particular particular magnetic 'lines of force' that they have matched with, and move along those lines until their kinetic energy has dissipated. They then are able to enter the atmosphere as 'cosmic dust'. It is estimated that hundreds of tonnes of cosmic dust add to the mass of Earth each year. While losing their energy, however, the charged particles emit energy as light, the Northern and Southern Lights (the Aurora Borealis and the Aurora Australis).

Fascinating Facts about Terrestrial Magnetism

  • The study of past magnetic field of the Earth is known as paleomagnetism.

  • Humans have used compasses for direction finding since the 11th century ad and for navigation since the 12th century.

  • The geomagnetic field changes on time scales from milliseconds to millions of years. Shorter time scales mostly arise from currents in the ionosphere (ionospheric dynamo region) and magnetosphere, and some changes can be traced to geomagnetic storms or daily variations in currents. Changes over time scales of a year or more mostly reflect changes in the Earth's interior, particularly the iron-rich core.

  • Studies of lava flows on Steens Mountain, Oregon, indicate that the magnetic field could have shifted at a rate of up to 6 degrees per day at some time in Earth's history, which significantly challenges the popular understanding of how the Earth's magnetic field works.

  • The Earth's magnetic field is believed to be generated by electric currents in the conductive material of its core, created by convection currents due to heat escaping from the core. However the process is complex, and computer models that reproduce some of its features have only been developed in the last few decades.

—Richard M. Renneboog MSc

Further Reading

1 

Merrill, Ronald T., McElhinny, Michael W. and McFadeen, Phillip L., The Magnetic Field of the Earth. Paleomagnetism, the Core, and the Deep Mantle San Diego, CA: Academic Press, 1998

2 

Tauxe, Lisa, Essentials of Paleomagnetism Berkeley, CA: University of California Press, 2010.

3 

Cox, A. and Hrt, Robert Brian, Plate Tectonics. How It Works Palo Alto, CA: Blackwell Scientific Publications, 2008.

4 

Merrill, Ronald T., Our Magnetic Earth. The Science of Geomagnetism Chicago, IL: University of Chicago Press, 2010.

5 

Lowrie, William A Student's Guide to Geophysical Equations New York, NY: Cambridge University Press, 2011.

6 

Rikitake, Tsuneji Electromagnetism and the Earth's Interior New York, NY: Elsevier Publishing, 1966.

Citation Types

Type
Format
MLA 9th
"Terrestrial Magnetism." Principles of Physical Science, edited by Donald R. Franceschetti & nullnull, Salem Press, 2017. Salem Online, online.salempress.com/articleDetails.do?articleName=POPS_0112.
APA 7th
Terrestrial magnetism. Principles of Physical Science, In D. R. Franceschetti & null (Eds.), Salem Press, 2017. Salem Online, online.salempress.com/articleDetails.do?articleName=POPS_0112.
CMOS 17th
"Terrestrial Magnetism." Principles of Physical Science, Edited by Donald R. Franceschetti & nullnull. Salem Press, 2017. Salem Online, online.salempress.com/articleDetails.do?articleName=POPS_0112.