evolved by a process of upthrusting similar to the plate collision and subduc-
tion that raised the Andes of Central and South America.The Andes continue
to rise due to an increase in crustal buoyancy caused by the subduction of the
Nazca plate beneath the South American plate.
CRETACEOUS WARMING
During the Cretaceous, plants and animals were especially prolific and ranged
practically from pole to pole.The deep ocean waters, which are now near freez-
ing, were about 15 degrees Celsius during the Cretaceous. The average global
surface temperature was 10 to 15 degrees warmer than at present. Conditions
were also much warmer in the polar regions, with a temperature difference
between the poles and the equator of only 20 degrees, or about half that of today.
The drifting of continents into warmer equatorial waters might have
accounted for much of the mild climate during the Cretaceous.By the time
of the initial breakup of the continents about 180 million years ago, the cli-
mate began to warm dramatically.The continents were flatter, the mountains
were lower, and the sea levels were higher.Although the geography during this
time was important, it did not account for all of the warming.
The movement of the continents was more rapid than today, with
perhaps the most vigorous plate tectonics the world has ever known. About
Figure 167The Wasatch
Range of north-central
Utah.
(Photo by R. R.Woolley,
courtesy USGS)CRETACEOUS CORALS