Continental Drift
Continental Drift
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
Continental Drift
Continental drift
The relative movement of the continents is explained by modern theories of plate tectonics . The relative movement of continents is explained by the movement of lithospheric plates—of which the visible continents are a part—over the athenosphere (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 depicting the movement of continents through an under-lying 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 that were seemingly derived from a common source. Other scientists also attempted to explain orogeny (mountain building) as resulting from Wegner's continental drift.
Technological advances necessitated by the Second World War 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—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 the 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 wind-blown 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 indicate 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 (what is now Eurasia, Greenland, and North America) drifted from the north until 425 million years ago when it crashed into Gondwana (also called Gonwanaland and composed of South America, Africa, Australia, Antarctica, India, and New Zealand). 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 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 smack 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 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, 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 B.J. Skinner, eds. The Oxford Companion to the Earth. New York: Oxford University Press, 2000.
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: WilliamSmith 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 Wood, Bernard. "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 (cited February 5, 2003). <http://pubs. usgs.gov/publications/text/dynamic.html>.
Ed Fox
K. Lee Lerner
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 that included today's South America, Africa, India, Australia, and Antarctica.
- Laurasia
—The northern half of Pangaea that 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."
- Wegener, Alfred
—German meteorologist (1880–1930), developer of the Continental Drift theory.
Continental Drift
Continental Drift
Introduction
Continental drift is the slow movement of continents over the surface of Earth. The geological theory explaining continental drift is plate tectonics. The large, more or less rigid rafts of rock on which the continents float are called plates. Tectonics are any large-scale processes that shape Earth's crust, from the Greek tekton for “builder.” Earth-shaping processes that involve plates are therefore termed “plate tectonics.”
Over geological time—millions or billions of years— continental drift has a strong effect on climate, both local and global. Rearrangement of the layout of oceans and continents changes the ocean circulation pattern, leading to warming or cooling. In addition, the regional climate of a land mass changes gradually as the land moves toward or away from the equator. When continents temporarily stick together in larger masses, climate is changed over large areas by shifted rainfall patterns (the interiors of large continents tend to be dry). Volcanoes, earthquakes, and the creation of mountain ranges are all caused by plate tectonics, and these can also affect regional or global climate. Volcanoes, for example, can affect climate by adding greenhouse gases to the atmosphere.
Historical Background and Scientific Foundations
During the nineteenth century, it seemed obvious to geologists that continents are too big and stable to move. German scientist Alfred Wegener (1880–1930) proposed the concept of continental drift in his 1915 book The Origins of Continents and Oceans. He suggested that the puzzle-piece match between the east and west sides of the Atlantic Ocean, as well as other coastal coincidences, was too close to be a mere accident, and explained this correspondence by proposing that the continents had once been joined together in a single huge landmass. He proposed that continental drift could explain the mystery of paleoclimate—the fact that some parts of Earth, as shown by their fossils, were once much warmer or colder than they are today. If they were once located near the equator or poles and had since drifted to other latitudes, paleoclimate would be explained.
Wegener's theory was rejected by American scientists, who condemned it for trying to explain too much in a single framework—fossils, paleoclimate, mountain-building, and more. Wegener had no convincing explanation for why the continents might move about, which counted against his view. However, European scientists tended to be more receptive. Scientific research in the 1920s through the 1940s helped make Wegener's theory more plausible by showing that low-level radioactivity is present throughout Earth's rocks, which could supply heat to make the interior of Earth soft and mobile. British geologist Arthur Holmes (1890–1965) proposed that slow, gigantic convection currents in the earth might drive continental drift.
Convection currents occur in any liquid that is heated from below. The heated liquid expands and rises to the surface, where it gives up some of its heat. It becomes denser as it cools, then sinks. A heated fluid is thus often self-organized into cells or rotating masses of fluid—convection currents. The Earth's interior, heated from within by radioactive elements and cooled on the surface by radiation of heat into space, acts like such a fluid, although a slow-moving one.
In Holmes's view, which is now widely accepted, the continents float on convection currents in Earth's mantle like patches of scum in a boiling pot—drifting about, bumping against each other, separating again in continuous movement. In the 1950s, studies of rock magnetism showed that either the continents or the poles had moved in the deep past. (In fact, both have.) It was theorized that fresh crust wells up along the midocean mountain ridges, flowing away in twin sheets in opposite directions from a central line. So, for example, the Old World on one side of the Atlantic Ocean is being pushed eastward by a sheet of ocean floor generated at the mid-Atlantic ridge, while the New World is being pushed the other way by a twin sheet. If this is so, then as Earth's magnetic field reverses every few hundred thousand years, the magnetism in the rocks of the ocean floor should record these reversals in alternating stripes parallel to the central ridge.
In 1966, data showed that the predicted magnetic striping of the Atlantic Ocean floor does exist. From that time on, the theory of plate tectonics became a fundamental principle of modern geology.
As Wegener had believed, continental drift explains some aspects of ancient climate change (paleoclimate). About 225 million years ago, all the continental plates were collected in a single giant mass, a supercontinent that geologists have named Pangea (Greek for “all lands”). Because atmospheric moisture comes primarily from ocean evaporation, the central parts of large continents tend to be dry, and this was the case with Pangea. It was surrounded by a single giant world-ocean, Panthalassa (Greek for “all-sea”).
The formation of this supercontinent—the latest in a long series of supercontinents that have formed and broken up over the 4.5-billion year history of Earth— caused a global ice age. Mountains were pushed up as continental plates crammed together. This caused an increase in erosion from streams running downhill, which changed ocean biogeochemistry so that carbon dioxide was removed from the atmosphere. Removing carbon dioxide reduced the natural greenhouse effect, chilling the planet. Glaciers spread from mountainous areas and covered large parts of Pangea.
WORDS TO KNOW
BIOGEOCHEMISTRY: The study of how substances and energy are exchanged between living things and the nonliving environment.
CONVECTION CURRENT: Circular movement of a fluid in response to alternating heating and cooling.
CRUST: The hard, outer shell of Earth that floats upon the softer, denser mantle.
JURASSIC PERIOD: Unit of geological time from 200 million years ago to 145 million years ago, famous in popular culture for its large dinosaurs. Global average temperature and atmospheric carbon dioxide concentrations were both much higher during the Jurassic than today.
MANTLE: Thick, dense layer of rock that underlies Earth's crust and overlies the core.
MILANKOVITCH CYCLES: Regularly repeating variations in Earth's climate caused by shifts in its orbit around the sun and its orientation (i.e., tilt) with respect to the sun. Named after Serbian scientist Milutin Milankovitch (1879–1958), though he was not the first to propose such cycles.
PALEOCLIMATE: The climate of a given period of time in the geologic past.
PANGEA: A supercontinent that was assembled from Gondwana, Euramerica, Siberia, and the Cathaysian and Cimmerian terranes. The assembly of Pangea lasted from the Late Carboniferous to the Middle Triassic, while the break up of this supercontinent began in the Jurassic and has continued to the present. The term was first used by Alfred Wegener to refer to the supercontinent of the Mesozoic. It can also be spelled “Pangaea” and comes from the Greek, meaning “all lands.”
PERMIAN PERIOD: Geological period from 299 to 250 million years ago. During the Permian, all of Earth's continental plates were united in the supercontinent Pangea.
PLATE TECTONICS: Geological theory holding that Earth's surface is composed of rigid plates or sections that move about the surface in response to internal pressure, creating the major geographical features such as mountains.
THERMOHALINE CIRCULATION: Large-scale circulation of the world ocean that exchanges warm, low-density surface waters with cooler, higher-density deep waters. Driven by differences in temperature and saltiness (halinity) as well as, to a lesser degree, winds and tides. Also termed meridional overturning circulation.
TRIASSIC PERIOD: Geological period from 251 million years ago to 199 million years ago. Global climate was particularly warm during the Triassic; the poles were ice-free and all Earth's continental plates were clumped into the supercontinent Pangea.
The location of land and sea areas is a major influence on global climate, because water (being darker) is a more effective absorber of solar energy than is land. Therefore, the more of the global ocean area that happens to be in the tropics, the warmer the world will tend to be. Another effect of continental layout is that the continents determine where ocean currents can flow. The ocean's largest currents are thermohaline, that is, governed by temperature (thermo-) and saltiness (-haline). Tropical waters gain heat, then eventually flow northward as surface currents to the poles, where they lose heat and gain salt that has been pushed out of freezing sea-ice. Denser because it is cooler and saltier, this northern water sinks and flows back to the tropics through the deep ocean, eventually to rise again in the tropics and complete the loop. If continents shift so that north-south thermohaline circulations are blocked, heat transfer from the tropics to the poles will be slowed, making the polar regions colder.
During the Triassic period, about 200 million years ago, Pangea began to break up. Ocean circulations changed so that tropical waters could now circle the globe around the equator without running into a supercontinent. This allowedthe oceanwater to spendmore timeinthe tropics and grow warmer. It is not understood how these changes affected the poles, but it is known that from 100 to 70 million years ago, during the Jurassic period, the polar regions were warm enough to support forests. As the continents continued to drift apart, the world's present system of ocean currents arose, establishing today's climate system.
Impacts and Issues
Whether land or sea dominates in the tropical areas is what geophysicists call a first-order control; that is, like the geometry of the solar system and the energy output of the sun, it has a strong effect on Earth's climate. Changes in land-sea layout due to continental drift can cause regional average temperature changes greater than 45°F (25°C) even over geologically brief time-spans, that is, over a few million years. Changes in ocean circulation caused by rearrangement of the continents are second-order controls, that is, they produce changes up to 27°F (15°C). Milankovitch cycles—which are determined by regular, recurrent shifts in Earth's orbit happening over many thousands of years—are third-order controls, causing temperature changes no greater than 18°F (10°C). Volcanic eruptions, El Niño, La Niña, and atmospheric carbon-dioxide concentrations are fourth-order controls, producing temperature fluctuations usually less than 9°F (5°C).
Continental drift is too slow to be an influence on the global climate changes that are now being seen. The distance between North America and Europe, for example, is increasing at only about 1 in (2.54 cm) per year.
Primary Source Connection
Incoming solar radiation can either be reflected, absorbed, or reradiated. The amount of solar radiation absorbed compared to that reflected or reradiated is referred to as the global heat budget. The following excerpt from an article in the journal Geografiska Annalerdiscusses the potential effects of plate tectonics on land-mass area, ocean currents, and Earth's heat budget. Dramatic changes in each could affect climate change.
GLOBAL HEAT BUDGET, PLATE TECTONICS, AND CLIMATE CHANGE
Introduction
Knowledge of the controlling factors of the heat budget of the Earth is critical to the understanding of the past, present and future climate of our environment. This includes the nature and origin of ice ages, causes of climatic changes and the potential effects of the works of man, e.g., the increasing CO2 content of the atmosphere…. Many theories have been suggested, ranging from the effects of massive clouds of volcanic dust, …the presence of an open Arctic Ocean, …the rise of the Tibetan Plateau, …to the Milankovitch theory of orbital cycles, but none satisfactorily explains how the heat budget works.
Fortunately, recent advances in several fields have yielded additional information that suggests a solution to the problem. Since the Earth has suffered periodic, widespread glaciations with intervening warmer periods for at least 2000 Ma,… it must be in some form of thermal equilibrium, with the climate fluctuating in response to changes in certain critical controls. This paper will show that the new evidence concerning the difference in heat absorption by land and water, the transport of excess heat polewards from the tropics, and the changes in distribution of land and sea resulting from plate tec-tonics, appear to explain the major fluctuations in the temperatures recorded in the geological record during the last 350 Ma. However, no sequence of climatic change in one place can be used to determine the variations in mean global heat budget during this time period.
Plate tectonics and the heat budget of the Earth
The original theory of continental drift of Wegener (1924) has evolved into the concept of plate tectonics, which regards the surface of the Earth as consisting of plates, most of which are in motion. This implies that the distribution of land and water is always changing. The area between the two tropics is the primary heat-absorbing zone: the larger the percentage of ocean in the primary heat-absorbing zone, the higher the absorbed heat and the sea-surface temperatures will be there, and the greater the potential for heat transfer to the poles. At present, about 30% of the heat absorbed in the primary heat-absorbing zone is transferred polewards.
The actual distribution of land and sea will also be critical to the efficient transfer of heat. If land areas block the movement of the warm and cold ocean currents, heat transfer polewards will be greatly reduced. Conversely, if the ocean currents can travel freely into and out of the polar regions, the heat transfer will be very efficient, producing warm temperature conditions in the adjacent polar lands.
Stuart A. Harris
harris, stuart a. “global heat budget, plate tectonics, and climate change.” geografiska annaler 84a(2002): 1–9.
See Also Great Conveyor Belt; Milankovitch Cycles; Snowball Earth.
BIBLIOGRAPHY
Periodicals
Harris, Stuart A. “Global Heat Budget, Plate Tectonics and Climatic Change.” Geografiska Annaler 84A (2002): 1–9.
Web Sites
Sleep, Norman H. “Plate Tectonics and the Evolution of Climate.” Review of Geophysics, Vol. 33 Suppl. (1995). < http://www.agu.org/revgeophys/sleep00/sleep00.html> (accessed August 6, 2007).
Larry Gilman
Continental Drift
Continental Drift
If you have ever looked at a map of the Atlantic Ocean, you have probably noticed that the coastlines of Africa and South America seem to fit together like pieces of a jigsaw puzzle. The fit between the two coastlines is even better when the edges of the continental shelf are compared. For many years, scientists thought this was just a coincidence, because no one could think of a way that the continents could slide around.
Evidence for Continental Drift
Evidence that South America and Africa might once have been joined to each other came from the research of the German geographer, Alexander von Humboldt. Von Humboldt traveled throughout South America, Africa, and other parts of the world, collecting plant and animal specimens and studying geography and geology. He observed many similarities between South America and Africa in addition to the apparent fit of continental coastlines. For example, von Humboldt noticed that the mountain ranges near Buenos Aires, Argentina, match mountain ranges in South Africa.
Other mountain ranges in Brazil extend to near the seashore and stop. Similar mountain ranges begin at the corresponding seashore in Ghana in Africa. All of these mountain ranges appear to have the same age and to be formed of the same kinds of rock. The rock strata in these and other mountain ranges would match perfectly if the coastlines of the two continents were lined up. Von Humboldt also observed similar patterns among mountain ranges in Europe and North America.
Von Humboldt and other naturalists also noticed many similarities among fossils of plants and animals on either side of the Atlantic. Although fossil species in eastern South America are somewhat different from fossil species in western Africa, their similarities are often striking. Before long, similarities across other oceanic gaps were observed. Plant and animal fossils found in India, for example, are often remarkably similar to those found in Australia.
Another important piece of evidence was discovered in the early twentieth century. When molten lava freezes, it preserves traces of Earth's magnetic field. Basalt, which freezes deep underground, also records Earth's magnetic field at the time the basalt cooled. Measurements of the direction of Earth's magnetic field from many different rocks of different ages on different continents indicate either that Earth's magnetic poles have moved all over the planet or that the continents themselves have moved.
Continental drift was first proposed in 1908 by American geologist Frank B. Taylor. However, Taylor's paper was mostly ignored and soon forgotten. Then a German meteorologist, Alfred Wegener, began working on a theory of continental drift. By 1912 Wegener had developed a theory suggesting that continental rocks were stronger and lighter than seafloor rocks. He also suggested that the seafloor rocks were like very thick tar. He concluded that the stronger continents were able to drift around on the weaker seafloor rocks.
Furthermore, Wegener thought the continents had once been part of a single large land mass, which he called Pangaea. Initially, he asserted, the original land mass had broken into two parts, two supercontinents, which he called Gondwanaland and Laurasia. Over millions of years, he suggested, Gondwanaland had broken apart into South America, Africa, India, Australia, and Antarctica, while Laurasia separated into North America and Eurasia.
Unfortunately, Wegener could not suggest any mechanism that would have caused the continents to break apart and move around in this way. In contrast to Taylor's experience, Wegener's theory was met with rejection and open hostility by other scientists, probably because Wegener was a meteorologist, not a geologist.
In the mid 1930s, however, Wegener's ideas were resurrected and rehabilitated. Scientists had discovered a ridge down the middle of the Atlantic seafloor through which hot lava was flowing upward and spreading outward. Stripes of lava on either side of this ridge were progressively older the farther away they were from the ridge. This pattern of stripes of lava strongly suggested to the scientists that the floor of the Atlantic Ocean was getting steadily wider. The discovery of this spreading of the seafloor, along with other discoveries, eventually led to the modern theory of plate tec-tonics .
Plate Tectonics
Wegener's idea of continental drift had the continents floating around on semisolid oceanic rock. In contrast, plate tectonics suggests that Earth's entire crust is composed of a number of large plates that are in constant motion relative to each other. Some plates are sliding under other plates, some are sliding past each other, others are pulling apart, and still others are colliding. Each of these types of interactions produces unique geological consequences. The Himalayas are formed as two continental plates collide. Along the northwest coast of North America, an oceanic plate is sliding under the North American plate. The resulting geological characteristic is a chain of volcanoes. As one plate is forced under the other, friction causes enormous amounts of heat that builds up until a volcano forms and erupts. Earthquakes are often the result of sudden movement of two adjacent plates. The plates "lock up" until enough force is generated to break them apart, causing the quake. One of the world's most famous earthquake zones, the San Andreas Fault, lies at the boundary of the Pacific and the North American plates.
After being initially rejected and ridiculed, the concept of continental drift (and plate tectonics) is now widely accepted as one of the fundamental unifying ideas of geology. This shift in thinking among geologists depended not only on the discovery of an adequate explanation for continental movement (seafloor spreading, rifts, and trenches) but also on the discovery of more and more similarities between continents.
Evolution and Biological Diversity
Early explorers, mapmakers, and traders were often accompanied on their travels by naturalists (people who studied all the natural sciences). These naturalists made two striking observations. They found that fossils of exactly the same plants and animals were located on continents that are separated by thousands of miles of oceans. For example, the tropical fossil fern, Glossopteris, was found in South America, Africa, India, and Australia. Similarly, fossils of the land vertebrate, Kannemeyrid, were found in Africa, North and South America, and Asia.
While the ancient fossils on different continents were often similar or identical, the exploring naturalists were finding out that living plants and animals on the different continents were often very different. The naturalists were discovering whole new groups of animals and plants on nearly every island and continent they visited. Most biological species seemed to be unique to the region or continent in which they were found. How could these seemingly contradictory observations be reconciled? Plate tectonics provided the answer. When the different land masses were connected, the same or closely related plants and animals inhabited each. After the land masses were separated, the different populations were geographically isolated from each other by great distances of ocean. Life on the different continents had apparently evolved into different species, because the populations were isolated from each other by such great distances.
It is possible to correlate, or link, the breakup of the continents with the types of animals found on each. The longer the period of separation, the more differences between species were found. For example, all of the indigenous (native) mammals found in Australia are marsupials. There are no naturally occurring placental mammals. This suggests that Australia broke away before placental mammals had evolved. In geographic isolation from the rest of the world, Australia's mammals were able to evolve into many highly sophisticated forms found nowhere else.
Has the diversity of life on Earth increased as a result of the breakup of the supercontinents? This idea was first proposed in 1970 by the American geologists James W. Valentine and Eldridge M. Moores. They suggested that the diversity of life increased as continents broke up and moved apart and decreased as land masses moved together and joined.
Since 1970 the study of plate activity as a force in the evolution of life has substantially added to our understanding of evolution. For example, during the Permian period (around 286 million years ago), there was a decrease in the variety of species of animals living in the shallow seas around Pangaea. In contrast, when the Atlantic Ocean began to open during the middle Mesozoic era (144 million years ago), the differences between the species living on opposite shores gradually increased. The greater the distance, the smaller the number of families in common. Differences accumulated more rapidly in the South Atlantic than in the North Atlantic, because a land connection between Europe and North America remained until the Cenozoic era (66 million years ago). The opposite happened when North and South America became connected at the Isthmus of Panama. In South America, there were many different marsupials and few large predators. After the isthmus emerged, many large herbivores migrated south. They adapted well to the new environment and were more successful than the local fauna in competing for food. Large predators also moved south and contributed to the extinction of at least four orders of South American land mammals. Only a few species, such as the armadillo and the opossum, migrated in the opposite direction. Many of the invading northerners, such as the llama and tapir, died out in North America and are now found only in the south.
see also Biogeography.
Elliot Richmond
Bibliography
Cox, Allan, ed. Plate Tectonics and Geomagnetic Reversals. San Francisco: W. H. Freeman, 1973.
Dott, Robert H., Jr., and Roger L. Batten. Evolution of the Earth, 3rd ed. New York: McGraw-Hill, 1994.
Faul, Henry, and Carol Faul. It Began with a Stone: A History of Geology from the Stone Age to the Age of Plate Tectonics. New York: Wiley, 1983.
Foster, Robert. Geology, 3rd ed. Columbus, OH: Charles E. Merrill, 1976.
Hallam, Anthony. A Revolution in the Earth Sciences: From Continental Drift to Plate Tectonics. Oxford, U.K.: Clarendon Press, 1973.
Matthews, William H., et al. Investigating the Earth, 3rd ed. Boston: Houghton Mifflin, 1981.
Stanley, Stephen. Earth and Life through Time. New York: W. H. Freeman, 1989.
Toulmin, Stephen, and June Goodfield. The Discovery of Time. New York: Harper and Row, 1965.
Uyeda, Seiya. The New View of the Earth: Moving Continents and Moving Oceans. San Francisco: W. H. Freeman, 1978.
Wilson, J. Tuzo, ed. Continents Adrift and Continents Aground. San Francisco: W. H. Freeman, 1976.
Wood, Robert Muir. The Dark Side of the Earth: The Battle for the Earth Sciences, 1800-1980. London: Allen and Unwin, 1985.
Wyllie, Peter J. The Way the Earth Works: An Introduction to the New Global Geology and Its Revolutionary Development. New York: Wiley, 1976.
A marsupial is a member of the mammalian subclass Metatheria, which includes a wide variety of mammals that give birth to undeveloped young. The young complete their development outside the mother's body, attached to a nipple. Most marsupials have a pouch that covers the nipples and protects the young while they are developing.
continental drift
continental drift
continental drift
continental drift
continental drift
con·ti·nen·tal drift • n. the gradual movement of the continents across the earth's surface through geological time.See plate tectonics.