July 2002 AGGMAN Geology

Continental Drift

The spark that changed our view of the Earth

Author’s Note: For the past few years this column has taken on issues of concern to the aggregate industry and has described how geology relates to those issues. While most folks who read this column probably have some knowledge of geology, it is a science with a history worth sharing. These next few articles will provide brief descriptions of some of the theories that have evolved into the science of geology.

Map prepared by geographer Antonio Snider-Pellegrini (1858) showing his version of how the American and African continents may have fit together.

What causes great mountain ranges to rise to incredible heights? Last month, this column described some of the quaint theories that scientists, philosophers, and theologians devised over the centuries to answer this question. As recently as the early 1960’s, the origin of mountains was still a mystery. But studies that took place during the first half of the 20th century, combined with ideas spawned during the 19th and 20th centuries, led to a theory called “Continental Drift” which, in turn, has led to what we have come to accept as the current theory of mountain building — a process called “Plate Tectonics”.
One of the first clues that the continents may have once been connected has existed for as nearly as long as there have been relatively accurate maps of the world. Dutch mapmaker Abraham Ortelius noticed that the east coasts of North and South America fit nicely with the west coasts of Europe and Africa and suggested in his work Thesaurus Geographicus (1596) that the Americas were “torn away from Europe and Africa …by earthquakes and floods.” The Scottish natural philosopher Thomas Dick made similar observations, which he published in his 1838 Celestial Scenery. Dick wrote that it was “not altogether improbable that these continents were originally conjoined, and that, at some former physical revolution or catastrophe, they may have been rent asunder by some tremendous power.”
Another clue about how the continents fit together surfaced in 1858, when French scientist Antonio Snider-Pelligrini, in his Creation and its Mysteries Revealed, noted the similarity of fossil plants in coal beds of Europe and North America. Of particular interest was a fossil fern called Glossopteris (because of its tongue-like leaves) that occurs in coal seams on both sides of the Atlantic Ocean. Similarly, in 1885, Australian geologist Edward Seuss documented similarities between plant fossils from South America, India, Australia, Africa, and Antarctica. Seuss gave the name “Gondwanaland” (named for the Gonds, an aboriginal tribe in India that inhabit an area where Glossopteris fossils are abundant) to a proposed ancient super-continent that was comprised of these land masses.
A number of people speculated on how Gondwanaland might have been broken apart. Seuss proposed that parts of the continent sank into the Earth. Frank B. Taylor (1908), an American scientist, proposed that the moon was captured by the Earth’s gravity and came so close to the Earth that its tidal pull dragged the continents toward the equator. About the same time, Howard Baker, another American scientist, proposed that a close approach of Venus plucked the moon from the Earth’s surface and the continents shifted to fill the void left by the moon.
But the person who put all of these ideas together into one theory was German meteorologist Alfred Wegener. During 1912, he presented a proposal to the Frankfurt Geological Association that the continents on Earth drift with respect to one another. His research was interrupted by the war until, in 1914, he was wounded in battle. During his convalescence, he was able to continue his research and during 1915 published The Origin of Continents and Oceans. Wegener contended that at one time there was just one supercontinent that he named Pangea (meaning “all lands” in Greek). Then, about 200 million years ago, Pangea split and the continents began to drift apart.
According to Wegener, continental drift not only explained the matching fossil occurrences, but also explained evidence of dramatic climate changes on some continents. For example, the discovery of fossils of tropical plants in Antarctica led to the conclusion that this frozen land must have at one time been situated in a more temperate climate closer to the equator, and the occurrence of glacial deposits in present-day arid areas such as the Vaal River valley of South Africa, indicate those areas must have been closer to the poles.
Wegener’s proposal was not well received and his theory of continental drift was largely dismissed as being eccentric, preposterous, and improbable. The scientific community firmly believed the continents and oceans to be permanent features on the Earth’s surface. Former president of the American Philosophical Society, W.B. Scott, even called the theory of continental drift “utter damned rot.”
Wegener devoted the rest of his life pursuing additional evidence to defend his theory. He froze to death in 1930 during an expedition crossing the Greenland ice cap. Decades later, in the 1950’s and 1960’s, new evidence from ocean floor exploration and other studies revived the debate about Wegener’s theory, ultimately leading to the development of the theory of plate tectonics. Long after his death, Wegener’s theory of continental drift ultimately would become the spark that ignited a new way of viewing the Earth.

William H. Langer is a geologist with the Mineral Resources Team of the U.S. Geological Survey.

July 2002 Carved in Stone Continental Drift The spark that changed our view of the Earth				Continental Drift The spark that changed our view of the Earth By Bill Langer Author’s Note: For the past few years this column has taken on issues of concern to the aggregate industry and has described how geology relates to those issues. While most folks who read this column probably have some knowledge of geology, it is a science with a history worth sharing. These next few articles will provide brief descriptions of some of the theories that have evolved into the science of geology. Map prepared by geographer Antonio Snider-Pellegrini (1858) showing his version of how the American and African continents may have fit together. What causes great mountain ranges to rise to incredible heights? Last month, this column described some of the quaint theories that scientists, philosophers, and theologians devised over the centuries to answer this question. As recently as the early 1960’s, the origin of mountains was still a mystery. But studies that took place during the first half of the 20th century, combined with ideas spawned during the 19th and 20th centuries, led to a theory called “Continental Drift” which, in turn, has led to what we have come to accept as the current theory of mountain building — a process called “Plate Tectonics”. One of the first clues that the continents may have once been connected has existed for as nearly as long as there have been relatively accurate maps of the world. Dutch mapmaker Abraham Ortelius noticed that the east coasts of North and South America fit nicely with the west coasts of Europe and Africa and suggested in his work Thesaurus Geographicus (1596) that the Americas were “torn away from Europe and Africa …by earthquakes and floods.” The Scottish natural philosopher Thomas Dick made similar observations, which he published in his 1838 Celestial Scenery. Dick wrote that it was “not altogether improbable that these continents were originally conjoined, and that, at some former physical revolution or catastrophe, they may have been rent asunder by some tremendous power.” Another clue about how the continents fit together surfaced in 1858, when French scientist Antonio Snider-Pelligrini, in his Creation and its Mysteries Revealed, noted the similarity of fossil plants in coal beds of Europe and North America. Of particular interest was a fossil fern called Glossopteris (because of its tongue-like leaves) that occurs in coal seams on both sides of the Atlantic Ocean. Similarly, in 1885, Australian geologist Edward Seuss documented similarities between plant fossils from South America, India, Australia, Africa, and Antarctica. Seuss gave the name “Gondwanaland” (named for the Gonds, an aboriginal tribe in India that inhabit an area where Glossopteris fossils are abundant) to a proposed ancient super-continent that was comprised of these land masses. A number of people speculated on how Gondwanaland might have been broken apart. Seuss proposed that parts of the continent sank into the Earth. Frank B. Taylor (1908), an American scientist, proposed that the moon was captured by the Earth’s gravity and came so close to the Earth that its tidal pull dragged the continents toward the equator. About the same time, Howard Baker, another American scientist, proposed that a close approach of Venus plucked the moon from the Earth’s surface and the continents shifted to fill the void left by the moon. But the person who put all of these ideas together into one theory was German meteorologist Alfred Wegener. During 1912, he presented a proposal to the Frankfurt Geological Association that the continents on Earth drift with respect to one another. His research was interrupted by the war until, in 1914, he was wounded in battle. During his convalescence, he was able to continue his research and during 1915 published The Origin of Continents and Oceans. Wegener contended that at one time there was just one supercontinent that he named Pangea (meaning “all lands” in Greek). Then, about 200 million years ago, Pangea split and the continents began to drift apart. According to Wegener, continental drift not only explained the matching fossil occurrences, but also explained evidence of dramatic climate changes on some continents. For example, the discovery of fossils of tropical plants in Antarctica led to the conclusion that this frozen land must have at one time been situated in a more temperate climate closer to the equator, and the occurrence of glacial deposits in present-day arid areas such as the Vaal River valley of South Africa, indicate those areas must have been closer to the poles. Wegener’s proposal was not well received and his theory of continental drift was largely dismissed as being eccentric, preposterous, and improbable. The scientific community firmly believed the continents and oceans to be permanent features on the Earth’s surface. Former president of the American Philosophical Society, W.B. Scott, even called the theory of continental drift “utter damned rot.” Wegener devoted the rest of his life pursuing additional evidence to defend his theory. He froze to death in 1930 during an expedition crossing the Greenland ice cap. Decades later, in the 1950’s and 1960’s, new evidence from ocean floor exploration and other studies revived the debate about Wegener’s theory, ultimately leading to the development of the theory of plate tectonics. Long after his death, Wegener’s theory of continental drift ultimately would become the spark that ignited a new way of viewing the Earth. William H. Langer is a geologist with the Mineral Resources Team of the U.S. Geological Survey.