Plate Tectonics and the Earth’s Geography .....................................

LECTURE

You look at a map of the world, and what you see is what looks like a

jigsaw puzzle whose pieces have been slowly moved apart.

I

n the previous lecture, we saw that the early Earth was very different
from today’s Earth, and distinctly less friendly to life. How did the Earth
acquire today’s geography, a geography that has profoundly shaped the
course of human history? Two ¿ gures, one German and one American, will
play vital roles in our understanding of how the geography of today’s Earth
was constructed. What they showed was that the Earth’s surface also has a
history and has changed profoundly over time.

To understand modern ideas about the history of the Earth’s surface, it will
help to contrast them with more traditional ideas. Traditionally, geologists
assumed that, though mountains might rise (through processes such as
earthquakes or volcanic activity) and fall (by erosion), the basic geography
of the Earth’s surface was ¿ xed.

The idea that the Earth’s surface had changed was ¿ rst proposed seriously
by a German meteorologist, Alfred Wegener (1880–1930). In 1912, Wegener
published a book called The Origins of Continents and Oceans, in which
he proposed a theory that would come to be known as “continental drift.”
Wegener argued that the Earth’s continents had once been joined in a
single supercontinent called Pangaea. What evidence did he offer for this
revolutionary idea, which contradicted most of the basic assumptions of
contemporary geologists?

The ¿ rst modern world maps, created early in the 16th century, showed that the
continents seemed to ¿ t together like pieces of a jigsaw puzzle, particularly
across the Atlantic Ocean. Wegener identi¿ ed geological formations of
a similar date and composition in West Norway, East Greenland, much of
Britain, Northwest Africa, and the eastern seaboard of the U.S. This made
sense only if all these regions had once been joined together. An Austrian
geologist, Eduard Suess (1831–1914), had already proposed that the southern