Continental Drift

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.