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Exploring Geology4th EditionChuck Carter, Julia Johnson, Paul Morin, Stephen Reynolds 832 solutions The Continental Drift IdeaFigure 1. The continents fit together like pieces of a puzzle. This is how they looked 250 million years ago. 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 1). 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 DriftBesides the way the continents fit together, Wegener and his supporters collected a great deal of evidence for the continental drift hypothesis.
Take a look at this animation showing that Earth’s climate belts remain in roughly the same position while the continents move and this animation showing how the continents split up. 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 moved and gave scientists the tools to develop a mechanism for Wegener’s drifting continents. Magnetic Polarity EvidenceFigure 4. Earth’s magnetic field is like a magnet with its north pole near the geographic North Pole and the south pole near the geographic South Pole. Puzzling new evidence came in the 1950s from studies on the Earth’s magnetic history (figure 4). Scientists used magnetometers, devices capable of measuring the magnetic field intensity, to look at the magnetic properties of rocks in many locations. 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 AgesGeologists noted important things about the magnetic polarity of different aged rocks on the same continent:
Figure 6. The location of the north magnetic north pole 80 million years before present (mybp), then 60, 40, 20, and now. 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 6). There are three possible explanations for this:
Magnetic Polarity on Different Continents with Rocks of the Same AgeGeologists noted that for rocks of the same age but 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 7). They named the phenomenon of the magnetic pole that seemed to move but actually did not apparent polar wander. Figure 7. 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. Lesson Summary
Contribute!Did you have an idea for improving this content? We’d love your input. Improve this pageLearn More What does this discovery indicate about the land near the South Pole?What does this discovery indicate about the land near the South Pole? The land was much closer to the equator in Earth's past. Unlike today, climate all over the Earth was similar in the past. All plant and animal species originated in the polar regions of the Earth.
What can be inferred about the Archaeopteryx?What can be inferred about the Archaeopteryx? The Archaeopteryx may have been an ancestor of modern-day birds. The Archaeopteryx could not fly very high. The Archaeopteryx was an omnivorous animal.
Could archaeopteryx fly?Now, analysis of the creature's forelimb bones finds that their structure closely resembles that of wing bones in today's quails and pheasants, species that can fly for short bursts. The discovery, published in Nature Communications on Tuesday, strengthens the case that Archaeopteryx could indeed take to the air.
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