An Introduction to Plate Tectonics
Continental Drift - Glaciation
Glaciation in South America, Africa, India, and Australia is best explained if these
continents were once connected. Glaciers covered all or part of each of these
continents during the same time period in the geologic past.
If the continents were in their present position, a major glaciation event that
covered nearly all of the continents and extended north of the equator would be
required. Geologists have found no evidence of glacial action in the northern
hemisphere during this time period. In fact, during this time period, the climate
in North America was warm.
Wegener proposed that the continents were adjacent to each other during the glacial
event. Therefore, glaciers spread over a much smaller area in the southern hemisphere
and probably did not influence the climate of the northern hemisphere.
Wegener used the distribution of specific rock types to determine the distribution of
climate zones in the geologic past. For example, glacial till and striations
(scratches on the rock), sand dunes, and coral reefs, indicate polar, desert, and
tropical climates, respectively. The present climate zones are shown in the above
figure. Note how the distribution of reefs, deserts, and glacial ice constrain the
position of the rotational pole of the Earth.
Using the distribution of rock types, Wegener reconstructed the distribution of
climates zones at specific times in the geologic past. He found that, unlike the
present distribution, in which zones parallel the equator, the past zones occupied
very different positions. This implies that the rotational pole was in very different
locations relative to today. Wegener proposed an alternative interpretation. He
believed that the climate zones remained stationary and the continents drifted to
different locations. The drift of the continents caused the apparent movement of the
climate zones.
Wegener used the distribution of climate zones to determine the location of the poles
at different times in the geologic past. He found that the rotational pole appears to
gradually change location, arriving at its present position only in the very recent
geologic past. The apparent movement in the pole position over time is called polar
wandering. Wegener offered an alternative explanation. He suggested that the poles
remained stationary and that the continents changed their positions relative to the
poles.
Back to main document