Page 89 - Earth's Climate Past and Future

CHAPTER 4 • Plate Tectonics and Long-Term Climate 65

predictable changes in glaciations over intervals of tens moving southward across Gondwana, in fact the Gond-
to hundreds of millions of years. wana continent was moving northward across the pole.
Major continent-sized ice sheets existed on Earth How well does the pattern shown in Figure 4-8
during three icehouse eras: a relatively brief interval explain the intervals of glaciation and nonglaciation
near 440 Myr ago, a long interval from 325 to 240 Myr listed in Table 4-1? The position of the south magnetic
ago, and the current icehouse era of the last 35 Myr. pole 440 Myr ago agrees with the evidence of glacia-
During most of the long intervening intervals (425-325 tion in the area of the modern Sahara Desert. The
Myr and 240–35 Myr ago), large ice sheets do not weight of the ice pressing down on the loose rubble
appear to have existed. carried in its base left striations (grooves) cut into
Near 420 Myr ago, small landmasses that were later bedrock (Figure 4-9).
to form modern North America and Eurasia lay scat- At first this match seems to give us a positive confir-
tered across a wide range of latitudes (Figure 4-7A). mation of the polar position hypothesis, but on closer
The other land areas were combined in a southern inspection problems emerge. One problem is that this
supercontinent called Gondwana, equivalent to mod- glacial era was quite brief in terms of geologic time.
ern Africa, Arabia, Antarctica, Australia, South Amer- Although its duration was once thought to be about 10
ica, and India. This continent was located initially in the Myr, new evidence suggests that ice may only have been
southern hemisphere on the opposite side of the globe, present for 1 Myr or less. This brief a glaciation is not
where the Pacific Ocean is now, but Gondwana had easily explained by the slow motion of Gondwana
begun a long trip across the South Pole and then north- across the South Pole (Box 4-1).
ward that would lead to a collision with the northern A more perplexing problem is the lack of glaciations
landmasses and formation of the giant supercontinent between 425 and 325 Myr ago, even though the Gond-
of Pangaea, meaning “All Earth” (Figure 4-7B-D). wana continent was still continuing its slow transit across
A convenient way to represent this motion is to plot the pole (see Figure 4-8). Somehow land existed at the
the changing position of the magnetic south pole in South Pole for almost 100 Myr without ice forming. This
relation to the land (Figure 4-8). Although this presen- observation argues against the hypothesis that a polar
tation makes it look as if the south magnetic pole were position is the only requirement for large-scale glaciation.

A 420 Myr ago B 390 Myr ago

C 350 Myr ago D 260 Myr ago

FIGURE 4-7 Moving continents (A–C) After 450 Myr ago, plate tectonic activity carried the
southern continent of Gondwana across the South Pole on a path headed toward continents
scattered across the northern hemisphere. (D) Subsequent collisions formed the giant continent
Pangaea. (Adapted from S. Stanley, Earth System History, © 1999 by W. H. Freeman and Company.)