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.
