Glaciers
Glaciers are huge sheets of ice that can be hundreds of feet thick. Glaciers have covered much of the surface
of the earth during different periods of history called ice ages. Today glaciers cover much of the earth at the
north and south poles.
As glaciers grow and move across the land, they push huge boulders and rocks along the ground that cause
scratches in the bedrock beneath them. Scientists in Wegener’s time discovered evidence that glaciers once
covered the land in parts of Africa, Australia, India and South America.
If the continents were in their present position (see Figure 1), a major glaciation event that covered nearly all
of the continents and extended north of the equator would be required. This glacier would have had to extend
over vast areas of oceans if the continents were in their present position. 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.
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. 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. (See Figure 2).
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 Figure 3.
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 (See Figure 4). 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 (See Figure 5).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.
1. What evidence is there that glaciers once moved over the continents of South America, Africa, Australia,
and India?
2. Argue the statement that the continents stayed in their present position and one large glacier covered
everything.
3. Ancient climate zones indicate that at one time most of North America had a tropical climate. If the
continents remained stationary, the earth’s rotational axis would have had to change to account for the
climate change. What did Wegener propose?
4. The geographic south pole (and north pole) seems to have changed it’s position over time. According to
Wegener has it?
http://pubs.usgs.gov/publications/text/developing.html
Figure 1
Figure 3
Figure 5
http://pubs.usgs.gov/publications/text/developing.html
Figure 2
Figure 4