"Piecing Continents Together"

Magazine article

By: Anonymous

Date: August 18, 1967

Source: "Piecing Continents Together." Time 90 (August 18, 1967).

About the Author: This article was published without a byline, and was written by staff writers for Time, a widely circulated U.S. weekly news magazine.

INTRODUCTION

The theory of continental drift explains that the continents are constantly changing position relative to one another. Continental drift is part of a larger theory known as plate tectonics, which describes the processes that result in many of the major geological features on Earth.

Geologists Alfred Wegener and Frank Taylor first proposed the theory of continental drift in 1912. They believed that all of the continents had been joined together in a massive super continent called Pangaea approximately 200 million years ago. They suggested that pieces of Pangaea slowly broke apart and moved into the positions that the continents occupy today.

Wegener and Taylor used several different lines of evidence to support the idea of continental drift. The first involves the simple observation that the modern continents can be assembled so that they fit together like pieces of a puzzle. This idea was actually first mentioned by Sir Francis Bacon more than 300 years earlier. He noticed that the bulge of Africa can fit into a notch in the North American continent and that the bulge in South America where Brazil is located fits in a notch in the continent of Africa.

Wegener also noticed that identical fossil species are found on South America and on Africa. He reasoned that it would be very difficult for many of these plant and animal species to have traveled the great distance across the ocean and that the species must have existed on the same continent at one point in time. In addition, Wegener reasoned that the coal deposits on Antarctica represented fossil plants that must have grown when the continent was located in a more tropical latitude. Finally, Wegener pointed out that the locations of large grooves, called striations, in rocks made by glaciers in South America and Africa are best explained if one assumes that the Atlantic Ocean did not separate the continents.

PRIMARY SOURCE

[This text has been suppressed due to author restrictions]

[This text has been suppressed due to author restrictions]

SIGNIFICANCE

Lines of evidence supporting the theory of continental drift continued to accumulate throughout the 1960s and 1970s. The magnetic field striping found around mid-ocean ridges suggested that the ridges are actually weak parts of the ocean floor. Eventually these places were recognized as an edge of a tectonic plate. New crust is continually added to the plate along the mid-ocean ridges as magma rises from deep in the Earth to the ocean floor. This explains the observation that rocks are always youngest closest to mid-ocean ridges.

Princeton University geologist Harry H. Hess suggested that if new crust forms along mid-ocean ridges, it must be shrinking in other places. In the 1960s, the field of seismology was developed. Seismologists measure the location and energy of earthquakes. Extensive seismographic surveys indicated that earthquakes are found in zones that are generally adjacent to deep undersea canyons called trenches. These trenches are found along the edges of the Pacific Ocean.

In particular, the Atlantic Ocean is continually growing, moving the Americas farther from Europe and Africa; while the Pacific is shrinking, moving North and South America towards Australia and Asia. These ideas helped crystallize the theory of plate tectonics, which states that massive irregular slabs of the Earth's crust float on magma beneath the Earth's surface and slowly move over time. Collisions of plates result in mountain ranges. Regions where plates slip past each other, like trenches, produce dynamic geologic activity, such as earthquakes.

In the 1970s, NASA developed satellites that were able to measure the precise location of points on Earth. In particular, the Global Positioning System or GPS has been very instrumental in determining movement along the faults between different tectonic plates. These measurements, which are made over the course of several years, compare well with estimated movement rates of tectonic plates made over millions of years.

The significance of the theories of continental drift and plate tectonics are vast. Understanding the relative motion of the Earth's crust provides an explanation for the presence of many geologic features such as mountain ranges, volcanoes, ocean trenches, and mid-ocean ridges. It also explains why certain regions are much more prone to earthquakes and geothermal activity than other regions. Plate tectonics also leads to an understanding of where natural resources such as fossil fuels and ore deposits are likely to be found. Finally, the theory unites a variety of scientific disciplines in the understanding that the Earth functions as a dynamic planet.

FURTHER RESOURCES

Web sites

National Aeronautics and Space Administration. "On the Move: Continental Drift and Plate Tectonics." 〈http://kids.earth.nasa.gov/archive/pangaea〉 (accessed January 6, 2006).

Pangaea.org. "The Meteorologist Who Started a Revolution." 〈http://pangaea.org/wegener.htm〉 (accessed January 6, 2006).

United States Geological Survey. "This Dynamic Earth: The Story of Plate Tectonics." 〈http://pubs.usgs.gov/publications/text/dynamic.html〉 (accessed January 6, 2006).