Further Reading

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Abbott, P. L. Natural Disasters. 3rd ed. Boston: McGraw Hill, 2002.

Bryant, E. A. Natural Hazards. Cambridge: Cambridge

University Press, 1993. Erickson, Jon. Quakes, Eruptions, and Other Geologic Cataclysms: Revealing the Earth's Hazards. New York: Facts On File, 2001. Griggs, Gary B., and J. A. Gilchrist. Geologic Hazards, Resources, and Environmental Planning. Belmont, Calif.: Wadsworth, 1983. Kusky, Timothy M. Geologic Hazards: A Sourcebook. Greenwood Press, 2003.

-. Earthquakes: Plate Tectonics and Earthquake

Hazards. New York: Facts on File, 2008.

-Volcanoes: Eruptions and Other Volcanic Hazards.

New York: Facts On File, 2008.

-. Tsunami: Giant Waves from the Sea. New York:

Facts On File, 2008. -. Landslides: Mass Wasting, Soil, and Mineral Hazards. New York: Facts On File, 2008.

-. Climate Change: Shifting Deserts, Glaciers, and

Climate Belts. New York: Facts On File, 2008.

-. The Coast: Hazardous Interactions within the

Coastal Zone. New York: Facts On File, 2008. -. Floods: Hazards of Surface and Groundwater Systems. New York: Facts On File, 2008.

-. Asteroids and Meteorites: Catastrophic Collisions with Earth. New York: Facts On File, 2008. Murck, Barbara W., Brian J. Skinner, and Stephen C. Porter. Dangerous Earth: An Introduction to Geologic Hazards. New York: John Wiley & Sons, 1997.

geomagnetism, geomagnetic reversal

The Earth has a magnetic field generated within the core of the planet. The field is generally approximated as a dipole, with north and south poles and magnetic field lines emerging from the Earth at the south pole and extending toward the north pole. The field is characterized at each place on the planet by an inclination and a declination. The inclination is a measure of how steeply inclined the field lines are with respect to the surface, with low inclinations near the surface, and steep inclinations near the poles. The declination measures the apparent angle between the rotational north pole and the magnetic north pole.

The magnetic field originates in the liquid outer core of the Earth and is thought to result from electrical currents generated by convective motions of the iron-nickel alloy that the outer core is made from.

Magnetic field lines of Earth approximate the shape of the field produced by a bar magnet. Magnetic field lines point upward out of the south magnetic pole and form imaginary elliptical belts of equal intensity around Earth and plunge back into Earth at the magnetic north pole. Note how the magnetic poles are not coincident with the rotational poles. The orientation of the magnetic field at any point on Earth can be expressed as an inclination (plunge into Earth) and a declination (angular distance between the magnetic and rotational north poles).

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The formation of the magnetic field by motion of the outer core is known as the geodynamo theory, pioneered by the German-American geophysicist and biologist Walter M. Elsasser (1904-91) of Johns Hopkins University in the 1940s. A dynamo generates electrical energy from mechanical energy. The basic principle for the generation of the magnetic field is that mechanical energy from the motion of the liquid outer core, which is an electrical conductor, is transformed into electromagnetic energy of the magnetic field. The convective motion of the outer core, maintained by thermal and gravitational forces, is necessary to maintain the field. If the convection stopped or the outer core solidified, the magnetic field would disappear. Secular variations in the magnetic field have been well documented by examination of the paleomagnetic record in the seafloor, lava flows, and sediments. Every few thousand years the magnetic field changes intensity and reverses, with the north and south poles abruptly flipping.

See also Earth; geophysics; paleomagnetism.

geomorphology Geomorphology is the description, classification, and study of the physical properties of and the origin of the landforms of the Earth's surface. Most studies in geomorphology include an analysis of the development of landforms and their relationships to underlying structures, and how the surface has interacted with other Earth systems such as the hydrosphere, cryosphere, and atmosphere. Geomorphologists have become increasingly concerned with the study of global climate change and the development of specific landforms associated with active deformation and tectonics. These relatively new fields of global change and active tectonic

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End of Days Apocalypse

End of Days Apocalypse

This work on 2012 will attempt to note them allfrom the concepts andinvolvement by the authors of the Bible and its interpreters and theprophecies depicted in both the Hopi petroglyphs and the Mayan calendarto the prophetic uttering of such psychics, mediums, and prophets asNostradamus, Madame Blavatsky, Edgar Cayce, and Jean Dixon.

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