Travel Reference
In-Depth Information
Figure 39 Ecuador's Cotopaxi volcano, which last erupted in 1975, is typical of the numer-
ous volcanoes that mark the collision of the South American and Nazca tectonic plates. In
1534 a violent eruption of Cotapaxi brought a sudden end to a battle between Incas and
Spaniards.
We now know, as Darwin did not, that the Andes are the result of a collision
between two tectonic plates, regions of the Earth's crust that move slowly rela-
tive to each other. During the past 60 million years the Nazca plate that makes
up part of the Pacifi c's fl oor has moved eastward, sliding gradually beneath the
western edge of the South American plate and crumpling it upwards. Strata
have piled up on top of each other, pushing what had once been seafl oor sedi-
ment higher and higher into the air. The eventual result was the Andes.
Darwin's trip across the Andes shaped his thinking about the natural world in
other important ways. He was amazed by the dif erences that he found between
the animals and plants on the western and the eastern slopes of the mountain
range. It was as if they had come from two separate worlds, with quite dif erent
animals and plants fi lling the same ecological niches on the two sides:
 
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