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| The Earth has always been changing... |
At
left are the original drawings of geographer Antonio Snider-Pellegrini
made in 1858. These two maps showing his version of how the American
and African continents may once have fit together, then later separated.
This idea was ridiculed at the time, however as knowledge grew,
the motion of the continents was verified by several independent
facts. |
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changing magnetic field of the Earth has been recorded. In the 1950s, zebra stripe-like magnetic patterns were found "frozen" in the rocks of the ocean floor. Obviously, the ocean floor had a story to tell, but what? A theoretical
model of the formation of magnetic striping. New oceanic crust forming
continuously at the crest of the mid-ocean ridge cools and becomes
increasingly older as it moves away from the ridge crest with seafloor
spreading (see text): a. the spreading ridge about 5 million years
ago; b. about 2 to 3 million years ago; and c. present-day. |
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In
1962, scientists of the U.S. Naval Oceanographic Office prepared a
report summarizing available information on the magnetic stripes mapped
for the volcanic rocks making up the ocean floor. Two young British
geologists, Frederick Vine and Drummond Matthews, and also Lawrence
Morley of the Canadian Geological Survey, suspected that the magnetic
pattern was no accident. In 1963, they hypothesized that the magnetic
striping was produced by repeated reversals of the Earth's magnetic
field. About the same time as these exciting discoveries were being
made on the ocean floor, new techniques for determining the geologic
ages of rocks ("dating") were also developing rapidly. The direction of the magnetism in the rocks on the ocean floor reversed over millions of years. the measurements matched the theory. The remarkable similarity of these two profiles provided one of the clinching arguments in support of the seafloor spreading hypothesis. |
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| Actual magnetic striping in the Pacific Northwest. The center part of the figure -- representing the deep ocean floor with the sea magically removed -- shows the magnetic striping mapped by oceanographic surveys offshore of the Pacific Northwest. Thin black lines show transform faults that offset the striping. |  |
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 As noted by Snider-Pellegrini and Wegener, the locations of certain fossil plants and animals on present-day, widely separated continents would form definite patterns (shown by the bands of colors), if the continents are rejoined. |
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 USGS Information Services Box 25286, Building 810 Denver Federal Center Denver, CO 80225 303-202-4700; Fax 303-202-4693 ISBN 0-16-048220-8 |
The above information
is from This Dynamic Earth: The Story of plate Tectonics
by W. Jacquelyne Kious and Robert I. Tilling. Available online.
This book was originally published in paper form in February 1996
(design and coordination by Martha Kiger; illustrations and production
by Jane Russell). It is for sale for $7 from: |
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Measurements made with the Global Positioning System are sensitive enough to detect motion of the Earth's tectonic plates. 
The Global Positioning System (GPS) is a constellation of 24 satellites which is used for navigation and precise geodetic position measurements. Daily position estimates are determined from satellite signals which are recorded by GPS receivers on the ground. Data from a global receiver network were collected by the International GPS Service for Geodynamics (IGS) and analyzed at the Jet Propulsion Laboratory, California Institute of Technology under contract with the National Aeronautics and Space Administration. Horizontal velocities, mostly due to motion of the Earth's tectonic plates, are represented on the maps by arrows extending from each site. This technology is now being applied to study earthquakes in the Los Angeles basin. Additional information can be obtained from mbh@cobra.jpl.nasa.gov. |
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