Historical Geology Understanding Our Planet\'s Past

the Nazca plate beneath the South American plate. By the time all the con-

tinents had wandered to their present positions and all the mountain ranges

had risen to their current heights, the world was ripe for the coming of the

Ice Age.

TERTIARY TECTONICS

Changing climate patterns resulted from the movement of continents toward

their present positions. Intense tectonic activity built landforms and raised

most mountain ranges of the world. About 57 million years ago, Greenland

began to separate from North America and Eurasia. Prior to about 8 million

years ago, Greenland was largely ice free. However, today the world’s largest

island is buried under a sheet of ice up to 2 miles thick. Alaska connected

with east Siberia and closed off the Arctic Basin from warm-water currents

originating from the tropics,resulting in the formation of pack ice in the

Arctic Ocean.

Except for a few land bridges exposed from time to time, plants and ani-

mals were prevented from migrating from one continent to another. A narrow,

curved land bridge temporarily connected South America with Antarctica and

assisted in the migration of marsupials to Australia. Sediments containing fossils

of a large crocodile, a 6-foot flightless bird, and a 30-foot whale suggest that land

bridges existed as late as 40 million years ago.Antarctica and Australia then broke

away from South America and moved eastward.When the two continents rifted

apart in the Eocene about 40 million years ago, Antarctica moved southward,

while Australia continued moving in a northeastward direction. When Antarc-

Figure 187Active fold
belts in Eurasia, resulting
from the collision of
lithospheric plates.

Historical Geology

Active foldbelts

Arabian Sea

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ASIA

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Black Sea

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Mediterran

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