The author begins by drawing a picture of Alfred Wegener who, in the early 1900s, proposed the idea of continental drift, the idea that the continents have moved around in relation to one another. His arguments in support of continental drift include:

• the fit of the continents
• paleoclimatic studies that showed evidence of past glaciation, coal deposits, ancient reef deposits, ancient sand dunes, and salt beds that make no sense in today’s world climate belts
• the occurrence of the same fossils on lands now separated by oceans
• the matching of geologic units (distinct assemblages of rocks) on lands now separated by oceans

For several reasons, Wegener’s ideas were not accepted for decades. First, although he was a scientist, he was not a geologist. Second, accepting his ideas would have meant huge changes in geologic thought. And third, he couldn’t supply an explanation of how and why continents moved. New discoveries after his death eventually proved that continents do move. The new areas of study involved:

• changes in the earth’s magnetic field over time (paleomagnetism)
• changes in the sea floor, the shape of its surface, types and ages of its rock, heat flow within it, and sea-floor earthquakes, all of which support the idea of sea-floor spreading

The author spends considerable time developing a historical context for the theory of plate tectonics. Why? Because:

• Working through the reasons for accepting new theories, such as the meaning and significance of paleomagnetism, provides practice in good scientific thinking.
• The gradual acceptance of plate tectonics theory provides an excellent example of the process by which scientific knowledge advances as new evidence and better instruments and techniques are introduced.
• Plate tectonics was a revolutionary idea that caused profound changes in the study of geology and thus merits your thoughtful study and understanding.

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