In the early 1900s, most geologists thought that Earth's appearance, including the arrangement of the continents, had changed little since its formation. This ...
Discovering Women: "Earth Explorer"
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©2006,© 1994 WGBH Educational Foundation. All rights reserved.
Adapted from Discovering Women: "Earth Explorer". A Production of Middlemarch Films, Inc. for WGBH. Animation courtesy of Michael Saz. Music courtesy of Richard Einhorn.
The similarity between jigsaw puzzles and maps is undeniable. Straight lines, wavy lines, jagged lines, and often arbitrary lines delineate political borders just as they define the edges of puzzle pieces. Continents appear to be cut from the same piece of Earth's crust. At least that's the picture the earliest mapmakers saw as they created the first images of our world.
As early as 1596, the Dutch mapmaker Abraham Ortelius wrote in his work Thesaurus Geographicus that the Americas must have been "torn away from Europe and Africa." He suggested that catastrophic earthquakes and floods must have been responsible for rearranging the continents. By the early 20th century, geologists still believed that such forces were responsible for many of the major changes to Earth's surface since its formation 4.6 billion years ago. However, the suggestion that these forces actually split and moved continents had been widely dismissed.
A few scientists, however, were convinced that the continents had once been connected and had then broken apart and moved to their present positions. One such scientist, Alfred Wegener, an astronomer and meteorologist by training, set out to find evidence to support this notion. Wegener applied his wide-ranging interests and scientific knowledge to support what he would ultimately call his theory of "continental drift". In addition to detailing the similarity between the coastlines of continents on opposite sides of the Atlantic, Wegener compared assemblages of fossilized plants and animals, as well as similar rock types found oceans apart. The patterns he saw helped support his theory that all continents were once connected and formed a single supercontinent, which has since split up and drifted apart.
How the supercontinent Pangaea broke apart and how the individual continents moved to their present locations were questions Wegener couldn't answer before he died. Despite an impressive collection of data on similarities among Earth's continents, Wegener had no knowledge of mid-ocean ridges, patterns of magnetic polarity, and many other pieces of evidence that led geologists to the current theory of plate tectonics.
- Is Earth's surface stable and stationary now? Was it ever in the past? Do you think it will be stable in the future?
- Do you think that the way continents fit together is convincing evidence for the theory of plate tectonics? Why or why not?
- What was the supercontinent called that once contained nearly all of the continental crust? Research what the name means.
- Which ocean is growing in size? Which is shrinking? Explain why this is occurring.
- How does the theory of plate tectonics help us explain natural phenomena such as earthquakes and mountains, which geologists had difficulty accounting for prior to the development of the theory?