15

• Prior to the 1960s most geologists held the view that the ocean basins and continents had fixed geographic positions and were of great antiquity
• Less than a decade later researchers came to realize that Earth's continents are not static, instead they gradually migrate across the globe
• Because of these movements, blocks of continental material collide, deforming the intervening crust, thereby creating Earth's great mountain chains
• Landmasses occasionally split apart. As the continental blocks separate, a new ocean basin emerges between them
• Other portions of the seafloor plunge into the mantle
• Tectonic processes


  Processes that deform Earth's crust to create major structural features such as mountains, continents, and ocean basins
• This profound reversal in scientific thought has been appropriately described as a scientific revolution
• The revolution began early in the 20th century as a relatively straightforward proposal called continental drift
• For more than 50 years the idea that continents were capable of movement was categorically rejected by the scientific establishment
• Continental drift was particularly distasteful to North American geologists, perhaps because much of the supporting evidence had been gathered from the continents of Africa, South America, and Australia, with which most North American geologists were unfamiliar
• Following World War II, modern instruments replaced rock hammers as the tools of choice for many researchers
• Geologists and a new breed of researchers, including geophysicists and geochemists, made several surprising discoveries that began to rekindle interest in the drift hypothesis
• By 1968 these developments led to the unfolding of a far more encompassing explanation known as the theory of plate tectonics
• The idea that continents, particularly South America and Africa, fit together like pieces of a jigsaw puzzle came about during the 1600s as better world maps became available
• Little significance was given to this notion until 1915, when Alfred Wegener (1880-1930), a German meteorologist and geophysicist, wrote The Origin of Continents and Oceans
• Pangaea


  A single supercontinent consisting of all Earth's landmasses that once existed
• Wegener further hypothesized that about 200 million years ago, during the early part of the Mesozoic era, this supercontinent began to fragment into smaller landmasses
• These continental blocks then "drifted" to their present positions over a span of millions of years
• The inspiration for continental drift is believed to have come to Wegener when he observed the break-up of sea ice during a Danish-led expedition to Greenland
• Wegener and others who advocated the continental drift hypothesis collected substantial evidence to support their point of view
• The fit of South America and Africa and the geographic distribution of fossils and ancient climates all seemed to buttress the idea that these now separate landmasses were once joined
• Wegener's use of present-day shorelines to fit these continents together was challenged immediately by other Earth scientists
• Scientists later determined that a much better approximation of the outer boundary of a continent is the seaward edge of its continental shelf
• In the early 1960s, Sir Edward Bullard and two associates constructed a map that pieced together the edges of the continental shelves of South America and Africa at a depth of about 900 meters
• The remarkable fit that was obtained was more precise than even these researchers had expected
• Wegener learned that most paleontologists (scientists who study the fossilized remains of ancient organisms) were in agreement that some type of land connection was needed to explain the existence of similar Mesozoic age life forms on widely separated landmasses
• Fossil organisms found on different landmasses

  • Mesosaurus
  • Glossopteris
  • Lystrosaurus
• Rafting, transoceanic land bridges (isthmian links), and island stepping stones were the most widely invoked explanations for the existence of identical fossil organisms in places separated by thousands of kilometers of open ocean
• Wegener found evidence of 2.2-billion-year-old igneous rocks in Brazil that closely resembled similarly aged rocks in Africa
• The mountain belt that includes the Appalachians trends northeastward through the eastern United States and disappears off the coast of Newfoundland
• Mountains of comparable age and structure are found in the British Isles, and Scandinavia
• When these landmasses are reassembled, the mountain chains form a nearly continuous belt
• Wegener described how the similarities in geologic features on both sides of the Atlantic fit current continental positions
• Mountains


  • Comparable age and structure found in the British Isles and Scandinavia
  • When reassembled, form a nearly continuous belt
• Alfred Wegener: '"It is just as if we were to refit the torn pieces of a newspaper by matching their edges and then check whether the lines of print run smoothly across. If they do, there is nothing left but to conclude that the pieces were in fact joined in this way."'
• Evidence for a glacial period that dated to the late Paleozoic had been discovered in southern Africa, South America, Australia, and India
• About 300 million years ago, vast ice sheets covered extensive portions of the Southern Hemisphere as well as India
• During the same span of geologic time, large tropical swamps existed in several locations in the Northern Hemisphere
• Fossils found in these coal-bearing rocks were produced by tree ferns that possessed large fronds, consistent with a warm, moist climate
• These fern trees lacked growth rings, a characteristic of tropical plants that grow in regions having minimal yearly fluctuations in temperature