An important piece of plate tectonic theory is the continental drift idea. This was developed in the early part of the 20th century, mostly by a single scientist, Alfred Wegener. His hypothesis states that continents move around on Earth's surface and that they were once joined together as a single supercontinent (Figure 6.5). Wegener's idea eventually helped to form the theory of plate tectonics, but while Wegener was alive, scientists did not believe that the continents could move.
- Be able to explain the continental drift hypothesis.
- Describe the evidence Wegener used to support his continental drift idea.
- Describe how the north magnetic pole appeared to move, and how that is evidence for continental drift.
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. In this case, your continent puzzle piece includes all of the continental crust for that continent and reflects the true size and shape of the 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!
Alfred Wegener, an early 20th century German meteorologist believed that the continents could fit together. He proposed that the continents were not stationary but that they had moved during the planet's history. He suggested that at one time, all of the continents had been united into a single supercontinent. He named the supercontinent Pangaea, meaning entire earth in ancient Greek. Wegener further 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 found that this evidence was best explained if the continents had at one time been joined together.
Wegener discovered that identical rocks could be found on both sides of the Atlantic Ocean. These rocks were the same type and the same age. Wegener understood that the rocks had formed side-by-side and that the land has since moved apart. Wegener also matched up mountain ranges that had the same rock types, structures, and ages, but that 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.
Wegener also found evidence from ancient fossils (Figure 6.6). He found fossils of the same species of extinct plants and animals in rocks of the same age, but on continents that are now widely separated. Wegener suggested that the continents could not have been in their current positions because the organisms would not have been able to travel across the oceans. For example, fossils of the seed fern Glossopteris are found across all of the southern continents. But the plants' seeds were too heavy to be carried across the ocean by wind. Mesosaurus fossils are found in South America and South Africa, but the reptile only could only swim in fresh water. Cynognathus and Lystrosaurus were reptiles that lived on land. Both of these animals were unable to swim, let alone swim across wide seas! Their fossils have been found across South America, Africa, India and Antarctica. Wegener proposed that the organisms had lived side by side, but that the lands had moved apart after they were dead and fossilized.
Wegener also looked at evidence from ancient glaciers. Large glaciers are most commonly found in frigid climates, usually in the far northern and southern latitudes. Using the distribution of grooves and rock deposits left by ancient glaciers on many different continents, Wegener traced the glaciers back to where they must have started. He discovered that if the continents were in their current positions, the glaciers would have formed in the middle of the ocean very close to the equator. Wegener knew that this was impossible! However, if the continents had moved, the glaciers would have been centered over the southern land mass much closer to the South Pole.
Wegener also found evidence for his hypothesis from warm climate zones. Coral reefs and the swamps that lead to the formation of coal are now found only in tropical and subtropical environments. But Wegener discovered ancient coal seams and coral reefs in parts of the continents that were much too cold today. The coral reef fossils and coal had drifted to new locations since the coal and coral formed.
Although Wegener's evidence was sound, most geologists at the time rejected his hypothesis of continental drift. These scientists argued that there was no way to explain how solid continents could plow through solid oceanic crust. At the time, scientists did not understand how solid material could move. Wegener's idea was nearly forgotten until technological advances presented puzzling new information and gave scientists the tools to develop a mechanism for Wegener’s drifting continents.
Magnetic Polarity Evidence
The puzzling new evidence came from studying Earth's magnetic field and how it has changed. If you have ever been hiking or camping, you may have used a compass to help you find your way. A compass uses the Earth’s magnetic field to locate the magnetic North Pole. Earth's magnetic field is like a bar magnet with the ends of the bar sticking out at each pole (Figure 6.7). Currently, the field's north and south magnetic poles are very near to the Earth's north and south geographic poles.
Some iron-bearing minerals, like tiny magnetite crystals in igneous rocks, point to the north magnetic pole as they crystallize from magma. These little magnets record both the strength and direction of the Earth's magnetic field. The direction is known as the field's magnetic polarity. In the 1950s, scientists began using magnetometers to look at the magnetic properties of rocks in many locations.
Geologists noted that magnetite crystals in fresh volcanic rocks pointed to the current magnetic north pole. This happened no matter where the rocks were located, whether they were on different continents or in different locations on the same continent. But for older volcanic rocks, this was not true. Rocks that were the same age and were located on the same continent pointed to the same point, but that point was not the current north magnetic pole. Moving back in time, rocks on the same continent that were the same age pointed at the same point. But these rocks did not point to the same point as the rocks of different ages or the current magnetic pole. In other words, although the magnetite crystals were pointing to the magnetic north pole, the location of the pole seemed to wander. For example, 400 million year old lava flows in North America indicated that the north magnetic pole was located in the western Pacific Ocean, but 250 million year old lava flows indicated a pole in Asia, and 100 million year old lava flows had a pole in northern Asia. Scientists were amazed to find that the north magnetic pole changed location through time!
There were three possible explanations for this puzzling phenomenon: (1) the continent remained fixed and the north magnetic pole moved (2) the north magnetic pole stood still and the continent moved (3) both the continent and the north pole moved.
The situation got stranger when scientists looked at where magnetite crystals pointed for rocks of the same age but on different continents. They found these rocks pointed to different magnetic north poles! For example, 400 million years ago the European north pole was different from the North American north pole at that same time. At 250 million years, the north poles were also different for the two continents. The scientists again looked at the three possible explanations. If the correct explanation was that the continents had remained fixed while the north magnetic pole moved, then there had to be two separate north poles. Since there is only one north pole today, they decided that the best explanation had to involve only one north magnetic pole. This meant that the second explanation must be correct, that the north magnetic pole had remained fixed but that the continents had moved.
To test this, geologists fitted the continents together as Wegener had done. They discovered that there had indeed been only one magnetic north pole but that the continents had drifted. They renamed the phenomenon of the magnetic pole that seemed to move but actually did not apparent polar wander. This evidence for continental drift gave geologists renewed interest in understanding how continents could move about on the planet's surface. And we know that the magnetic pole wanders, too, so the correct explanation was that both the continents and the magnetic poles move.
- 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, scientists could not think of a mechanism that could drive solid continents to move around on the solid earth and most rejected the idea.
- Continental drift would resurface after World War II when a mechanism was discovered.
- Why can paper cutouts of the continents including the continental margins be pieced together to form a single whole?
- How can the locations where ancient fossils are found be used as evidence for continental drift?
- 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?
- What are the three possible explanations for apparent polar wander when the rocks are all on one continent? If the rocks are on more than one continent, which explanation is the only one likely to be true and why?
- In the face of so much evidence in support of continental drift, how could scientists reject the idea?
- 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?
- apparent polar wander
- The path on the globe showing where the magnetic pole appeared to move over time.
- continental drift
- The hypothesis developed in the early 20th century that states that the continents move about on the surface.
- A magnetic mineral that takes on the polarity of the Earth’s magnetic field at the time it forms.
- magnetic field
- The region around a magnet that is susceptible to the magnetic force. Earth's magnetic field is like a magnet.
- magnetic polarity
- The direction of the Earth's magnetic field, north is normal or south is reversed.
- An instrument that measures the magnetic field intensity.
Points to Consider
- Why is continental drift referred to as a hypothesis (or idea) and not a theory?
- Why was Wegener's continental drift idea rejected by the scientific community and why is it accepted today?