Continental Drift

Continental Drift

History of Wegener’s theory

Evidence of the theory

Formation of Pangaea

Pangaea splits

Resources

The relative movement of the continents is explained by modern theories of plate tectonics. These theories describe the processes by which lithospheric plates—of which the visible continents are a part—move over the asthenosphere (the molten, ductile, upper portion of Earth’s mantle). In a historical sense, the now discarded explanations of continental drift were rooted in antiquated concepts regarding Earth’s structure.

Explanations of continental drift that persisted well into the twentieth century made the improbable geophysical assertion that the continents moved through and across an underlying oceanic crust much as ice floats and drifts through water. Eventually multiple lines of evidence allowed modern tectonic theory to replace continental drift theory.

In the 1920s, German geophysicist Alfred Wegener’s writings advanced the hypothesis of continental drift, which depicted the movement of continents through an underlying oceanic crust. Wegner’s hypothesis met with wide skepticism, but found support and development in the work and writings of South African geologist Alexander Du Toit, who discovered a similarity in the fossils found on the coasts of Africa and South America, indicating that the fossils were seemingly derived from a common source. Other scientists also attempted to explain orogeny (mountain building) as a result of Wegner’s continental drift.

Technological advances necessitated by the World War II made possible the accumulation of significant evidence regarding Wegener’s hypothesis, eventually refining and supplanting Wegner’s theory of continental drift with modern plate tectonic theory. Although Wegener’s theory accounted for much of the then existing geological evidence, Wegener’s hypothesis was specifically unable to provide a verifiable or satisfying mechanism by which continents—with all of their bulk and drag—could move over an underlying mantle that was solid enough in composition to be able to reflect seismic S waves.

History of Wegener’s theory

At one time—estimated to be 200 to 300 million years ago—all of the continents were united in one supercontinent or protocontinent named Pangaea (or Pangea, from the Greek pan, meaning all, and gaea, meaning world) that first split into two halves. The two halves of the protocontinent were the northern

continent Laurasia and the southern continent named Gondwanaland or Gondwana. These two pieces were separated by the Tethys Sea. Laurasia later subdivided into North America, Eurasia (excluding India), and Greenland. Gondwana is believed to have included Antarctica, Australia, Africa, South America, and India. Two scientists, Edward Suess and Alexander Du Toit, named Pangaea, Gondwanaland, and Laurasia.

In Wegener’s 1915 book, The Origin of Continents and Oceans, he cited evidence that Pangaea had existed; most of the evidence came from Gondawana, the southern half of the supercontinent, and included the following: glacially gouged rocks in southern Africa, South America, and India; the fit of the coastlines and undersea shelves of the continents, especially eastern South America into western Africa and eastern North America into northwestern Africa; fossils in South America that match fossils in Africa and Australia; mountain ranges that start in Argentina and continue into South Africa and Australia; and other mountain ranges like the Appalachians that begin in North America and trend into Europe. He even measured Greenland’s distance from Europe over many years to show that the two are drifting slowly apart.

Although Wegener’s ideas are compelling today, scientists for decades dismissed the continental drift

theory because Wegener could not satisfactorily explain how the continents moved. His assertion that continents plowed through oceanic rock, riding tides in the Earth like an icebreaker through sea ice, brought derision from the world’s geophysicists (scientists who study the physical properties of Earth including movements of its crust). Harold Jeffreys, a leading British geophysicist of the 1920s, calculated that if continents did ride these Earth tides, mountain ranges would collapse and Earth would stop spinning within a year.

Wegener’s fossil arguments were countered by a widely-held belief that defunct land bridges (now sunken below sea level) once connected current continents. These bridges had allowed the small fossil reptiles Lystrosaurus and Mesosaurus (discovered on opposite sides of the Atlantic) to roam freely across what is now an ocean too wide to swim. The cooling and shrinking of Earth since its formation supposedly caused the flooding of the bridges. Furthermore, critics explained that Wegener’s fossil plant evidence from both sides of the Atlantic resulted from windblown seeds and plant-dispersing ocean currents.

Measurements of Greenland’s movements proved too imprecise for the equipment available to Wegener at that time. The fit of continents could be dismissed as coincidence or by a counter theory claiming that Earth is expanding. Like shapes drawn on an expanding balloon, the continents move farther from each other as Earth grows.

Evidence of the theory

Technological improvements after World War II supported many of Wegener’s ideas about continental drift. New methods of dating and drilling for rock samples, especially from deep-sea drilling ships like the Glomar Challenger, have allowed more precise matching of Pangaea’s rocks and fossils. Data from magnetometers (instruments that measure the magnetism of the iron in sea floor rocks) proved that the sea floors have spread since Pangaea’s breakup. Even satellites have clocked continental movement.

Geologists assume that, for the 100 million years that Pangaea existed, the climatic zones were the same as those operating today: cold at the poles, temperate to desert-like at the mid-latitudes, and tropical at the equator. The rocks and fossils deposited in the early days of Pangaea show that the equator crossed a clockwise-tilted North America from roughly southern California through the mid-Atlantic United States and into Northwestern Africa. Geological and archaeological evidence from the Sahara desert indicate the remains of a tropical world beneath the sands. Rock layers in southern Utah point to a warm sea that gradually altered into a large sandy desert as the west coast of the North American section of Pangaea slid north into a more arid latitude. Global climates changed as Pangaea rotated and slowly broke up over those 100 million years.

Meanwhile, dinosaurs, mammals, and other organisms evolved as they mingled across the connected Earth for millions of years, responding to the changing climates created by the shifting landmass. Fossil dinosaurs unearthed in Antarctica proved that it was connected to the rest of Pangaea, and dinosaur discoveries in the Sahara desert indicated that the last connection between Laurasia (the northern half of Pangaea) and Gondwana was severed as recently as 130 million years ago.

All these discoveries also helped to develop the companion theory of plate tectonics, in which moving plates (sections of Earth’s outer shell or crust) constantly smash into, split from, and grind past each other. Wegener’s theory of continental drift predated the theory of plate tectonics. He dealt only with drifting continents, not knowing that the ocean floor drifts as well. Parts of his continental drift theory proved wrong—such as his argument that continental movement would cause the average height of land to rise— and parts proved correct. The continental drift theory and plate tectonics, although demonstrating many interrelated ideas, are not synonymous.

Formation of Pangaea

With improved technology, geologists have taken the continental drift theory back in time to 1, 100 million years ago (Precambrian geologic time) when another supercontinent had existed long before Pangaea. This supercontinent, named Rodinia, split into the two half-continents that moved far apart to the north and south extremes of the planet. About 514 million years ago (in the late Cambrian), Laurasia drifted from the north until 425 million years ago when it crashed into Gondwana. By 356 million years ago (the Carboniferous period), Pangaea had formed. The C-shaped Pangaea, united along Mexico to Spain/Algeria, was separated in the middle by the Tethys Sea, an ancient sea to the east, whose remnants include the Mediterranean, Black, Caspian, and Aral Seas. Panthalassa, a superocean (“All Ocean”), covered the side of the globe opposite the one protocontinent.

Pangaea splits

During the formation of Pangaea, the collision of North America and northwestern Africa uplifted a mountain range 621 mi (1,000 km) long and as tall as the Himalayas, the much-eroded roots of which can still be traced from Louisiana to Scandinavia. The Appalachians are remnants of these mountains, the tallest of which centered over today’s Atlantic Coastal Plain and over the North American continental shelf. Pangaea’s crushing closure shortened the eastern margin of North America by at least 161 mi (260 km).

The internal crunching continued after the formation of Pangaea, but most of the colliding shifted from the east coast of North America to the western edge of the continent. Pangaea drifted northward before it began to break up, and it plowed into the Panthalassa ocean floor and created some of today’s western mountains. The rifting of Pangaea that began 200 million years ago (the end of the Triassic period) forced up most of the mountain ranges from Alaska to southern Chile, as North and South America ground west into and over more ocean floor.

The tearing apart of Pangaea produced long valleys that ran roughly parallel to the east coasts of the Americas and the west coasts of Europe and Africa. The floors of these valleys dropped down tremendously in elevation. One of the valleys filled with seawater and became the Atlantic Ocean, which still grows larger every year. Other valleys gradually choked with sediment eroded off the ridges on either side. Today, many of these former low spots lie buried beneath thousands of feet of debris. The formation of the valleys did not occur along the same line as the collision 100 million years before; a chunk of Gondwana now sits below the eastern United States.

Pangaea began to break up around 200 million years ago, with the separation of Laurasia from Gondwana, and the continents we know began to take shape in the late Jurassic—about 152 million years ago. New oceans began to open up 94 million years ago (the Cretaceous period). India also began to separate from Antarctica, and Australia moved away from the still united South America and Africa. The Atlantic zippered open northward for the next few tens of millions of years, until Greenland eventually

KEY TERMS

Continental drift theory— Alfred Wegener’s theory that all the continents once formed a giant continent called Pangaea and later shifted to their present positions.

Gondwana (Gondwanaland)— The southern half of Pangaea, which included today’s South America, Africa, India, Australia, and Antarctica.

Laurasia— The northern half of Pangaea, which included today’s North America, Greenland, Europe, and Asia.

Pangaea (Pangea)— The supercontinent from approximately 200–300 million years ago, which was composed of all today’s landmasses.

Panthalassa— The ocean covering the opposite site of the globe from Pangaea. Panthalassa means “All Ocean.”

tore from northern Europe. By about 65 million years ago, all the present continents and oceans had formed and were sliding toward their current locations, while India drifted north to the south side of Asia.

Current Pangaea research no longer focuses on whether or not it existed, but refines the matching of the continental margins. Studies also center on parts of Earth’s crust that are most active, in order to understand both past and future movements along plate boundaries (including earthquakes and volcanic activity) and continuing continental drift. For example, the East African Rift Valley is a plate boundary that is opening like a pair of scissors, and, between Africa and Antarctica, new ocean floor is being created, so the two African plates are shifting further from Antarctica. Eventually, possibly 250 to 300 million years in the future, experts theorize that Pangaea will unite all the continents again. In another aspect of the study of continental drift, scientists are also trying to better understand the relatively sudden continental shift that occurred 500 million years ago that led to the formation of Pangaea. The distribution of the land mass over the spinning globe may have caused continents to relocate comparatively rapidly and may also have stimulated extraordinary evolutionary changes that produced new and diverse forms of life in the Cambrian period, also about 500 million years ago.

See also Earth’s interior; Planetary geology.

Resources

BOOKS

Hancock P. L. and Skinner B. J., editors. The Oxford Companion to the Earth. New York: Oxford University Press, 2000.

Oreskes, Naomi. Plate Tectonics: An Insider’s History of the Modern Theory of the Earth. New York: Westview Press, 2003.

Tarbuck, Edward D., Frederick K. Lutgens, and Tasa Dennis. Earth: An Introduction to Physical Geology. 7th ed. Upper Saddle River, NJ: Prentice Hall, 2002.

Winchester, Simon. The Map That Changed the World: William Smith and the Birth of Modern Geology. New York: Harper Collins, 2001.

PERIODICALS

Buffett, Bruce A., “Earth’s Core and the Geodynamo.” Science. (June 16, 2000): 2007–2012.

Hellfrich, George, and Bernard Wood. “The Earth’s Mantle.” Nature. (August 2, 2001): 501–507.

OTHER

United States Department of the Interior, U.S. Geological Survey. “This Dynamic Earth: The Story of Plate Tectonics.” February 21, 2002. <http://pubs.usgs.gov/publications/text/dynamic.html> (accessed January 3, 2007).

Ed Fox

K. Lee Lerner