6.2+Continental+Drift

Lesson Objectives

 * Explain the continental drift hypothesis.
 * Describe the evidence Wegener used to support his continental drift idea.
 * Describe later evidence for continental drift.

Introduction
The continental drift hypothesis was developed in the early part of the 20th century, mostly by Alfred Wegener. Wegener said that continents move around on Earth’s surface and that they were once joined together as a single supercontinent (**Figure** [|below]). While Wegener was alive, scientists did not believe that the continents could move.

The Continental Drift Idea
Find a map of the continents and cut each one out. Better yet, use a map where the edges of the continents show the continental shelf. That’s the true size and shape of a continent. Can you fit the pieces together? The easiest link is between the eastern Americas and western Africa and Europe, but the rest can fit together too (**Figure** [|below]). The continents fit together like pieces of a puzzle. This is how they looked 250 million years ago. Alfred Wegener proposed that the continents were once united into a single supercontinent named Pangaea, meaning //all earth// in ancient Greek. He suggested that Pangaea broke up long ago and that the continents then moved to their current positions. He called his hypothesis **continental drift.**

Evidence for Continental Drift
Besides the fit of the continents, Wegener and his supporters collected a great deal of evidence for the continental drift hypothesis. Wegener proposed that the organisms had lived side by side, but that the lands had moved apart after they were dead and fossilized. Wegener used fossil evidence to support his continental drift hypothesis. The fossils of these organisms are found on lands that are now far apart. Although Wegener’s evidence was sound, most geologists at the time rejected his hypothesis of continental drift. Why do you think they did not accept continental drift? Scientists argued that there was no way to explain how solid continents could plow through solid oceanic crust. Wegener’s idea was nearly forgotten until technological advances presented even more evidence that the continents had moved and gave scientists the tools to develop a mechanism for Wegener’s drifting continents.
 * Identical rocks, of the same type and age, are found on both sides of the Atlantic Ocean. Wegener said the rocks had formed side-by-side and that the land had since moved apart.
 * Mountain ranges with same rock types, structures, and ages are now on opposite sides of the Atlantic Ocean. The Appalachians of the eastern United States and Canada, for example, are just like mountain ranges in eastern Greenland, Ireland, Great Britain, and Norway. Wegener concluded that they formed as a single mountain range that was separated as the continents drifted (**Figure** [|below]).
 * Ancient fossils (**Figure** [|below]) of the same species of extinct plants and animals are found in rocks of the same age, but on continents that are now widely separated. Wegener suggested that the organisms would not have been able to travel across the oceans.
 * Fossils of the seed fern //Glossopteris// were too heavy to be carried so far by wind.
 * //Mesosaurus// was a swimming reptile, but only could only swim in fresh water.
 * //Cynognathus// and //Lystrosaurus// were land reptiles and were unable to swim.
 * Grooves and rock deposits left by ancient glaciers are found today on different continents very close to the equator. This would indicate that the glaciers either formed in the middle of the ocean and/or covered most of the Earth. Today glaciers only form on land and nearer the poles. Wegener thought that the glaciers were centered over the southern land mass close to the South Pole and the continents moved to their present positions later on.
 * Coral reefs and coal-forming swamps are found in tropical and subtropical environments, but ancient coal seams and coral reefs are found in locations where it is much too cold today. Wegener suggested that these creatures were alive in warm climate zones and the fossils and coal later had drifted to new locations on the continents.
 * An animation showing that Earth’s climate belts remain in roughly the same position while the continents move is seen here: @http://www.scotese.com/paleocli.htm.
 * An animation showing how the continents split up can be found here: @http://www.exploratorium.edu/origins/antarctica/ideas/gondwana2.html.

Magnetic Polarity Evidence
Puzzling new evidence came in the 1950s from studying Earth's magnetic history (**Figure** [|below]). Scientists used **magnetometers** to look at the magnetic properties of rocks in many locations. Earth Magnetite crystals are like tiny magnets that point to the north magnetic pole as they crystallize from magma. The crystals record both the direction and strength of the magnetic field at the time. The direction is known as the field’s **magnetic polarity.**

Magnetic Polarity on the Same Continent with Rocks of Different Ages
Geologists noted important things about magnetic polarity of different aged rocks on the same continent: Earth In other words, although the magnetite crystals were pointing to the magnetic north pole, the location of the pole seemed to wander. Scientists were amazed to find that the north magnetic pole changed location through time (**Figure** [|below]). The location of the north magnetic north pole 80 million years before present (mybp), then 60, 40, 20 and now. There are three possible explanations for this.
 * Magnetite crystals in fresh volcanic rocks point to the current magnetic north pole no matter what continent or where on a continent the rocks are located (**Figure** [|below]).
 * Older rocks that are the same age and are located on the same continent point to the same location, but that location is not the current north magnetic pole.
 * Older rock that are of different ages do not point to the same locations or to the current magnetic north pole.
 * 1) The continents remained fixed and the north magnetic pole moved.
 * 2) The north magnetic pole stood still and the continents moved.
 * 3) Both the continents and the north pole moved.

Magnetic Polarity on Different Continents with Rocks of the Same Age
Geologists noted that in rocks of the same age on different continents, the little magnets pointed to different magnetic north poles. The scientists looked again at the three possible explanations. Only one can be correct. If the continents had remained fixed while the north magnetic pole moved, there must have been two separate north poles. Since there is only one north pole today, the only reasonable explanation is that the north magnetic pole has remained fixed but that the continents have moved. To test this, geologists fitted the continents together as Wegener had done. It worked! There has only been one magnetic north pole and the continents have drifted (**Figure** [|below]). They named the phenomenon of the magnetic pole that seemed to move but actually did not **apparent polar wander.** On the left: The apparent north pole for Europe and North America if the continents were always in their current locations. The two paths merge into one if the continents are allowed to drift. This evidence for continental drift gave geologists renewed interest in understanding how continents could move about on the planet’s surface.
 * 400-million-year-old magnetite in Europe pointed to a different north magnetic pole than the same age magnetite in North America.
 * 250 million years ago, the north poles were also different for the two continents.

Lesson Summary

 * In the early part of the 20th century, scientists began to put together evidence that the continents could move around on Earth’s surface.
 * The evidence for continental drift included the fit of the continents; the distribution of ancient fossils, rocks, and mountain ranges; and the locations of ancient climatic zones.
 * Although the evidence was extremely strong, no mechanism for how solid continents could move around on the solid earth and scientists rejected the idea.
 * The discovery of apparent polar wander renewed scientists interest in continental drift.

Review Questions

 * 1) Why can paper cutouts of the continents including the continental margins be pieced together to form a single whole?
 * 2) Explain how each of these phenomena can be used as evidence for continental drift:
 * 3) The fit of the continents
 * 4) The distribution of ancient fossils
 * 5) The distribution of similar rock types
 * 6) Rocks from ancient climatic zones
 * 7) Alignment of mountain ranges on different continents
 * 8) Apparent polar wander
 * 9) To show that mountain ranges on opposite sides of the Atlantic formed as two parts of the same range and were once joined, what would you look for?
 * 10) What are the three possible explanations for apparent polar wander? Considering all the evidence, which explanation is the only one likely to be true and why?
 * 11) With so much evidence to support continental drift, how could scientists reject the idea?
 * 12) Look at a world map. Besides the coast of west Africa and eastern South America, what are some other regions of the world that look as they could be closely fit together?

Further Reading / Supplemental Links

 * Animation of Pangaea splitting apart: @http://www.youtube.com/watch?v=gO2qYMsNHGk

Vocabulary
magnetometer An instrument that measures the magnetic field intensity. magnetite A magnetic mineral that takes on Earth’s magnetic polarity as it crystallizes. magnetic polarity The direction of the Earth’s magnetic field. A compass today will point north, which is normal polarity; south is reversed. magnetic field The region around a magnet that is susceptible to the magnetic force. Earth’s magnetic field is like a magnet. continental drift The early 20th century hypothesis that the continents move about on Earth’s surface. apparent polar wander The path on the globe showing where the magnetic pole appeared to move over time.

Points to Consider

 * Why is continental drift referred to as a hypothesis (or idea) and not a theory?
 * What did Wegener’s idea need for it to be accepted?
 * What other explanations did scientists come up with to explain the evidence Wegener had for continental drift?