larger continents. Millions of years later, the continents rifted apart, and the
chasms filled with seawater to form new oceans. The regions presently bor-
dering the Pacific Basin apparently have not collided with each other. The
Pacific Ocean is a remnant of an ancient sea called the Panthalassa, which nar-
rowed and widened in response to continental breakup, dispersal, and recon-
vergence in the area occupied by the present-day Atlantic Ocean.
Several oceans have repeatedly opened and closed in the vicinity of the
Atlantic Basin, while a single ocean has existed continuously in the area of
the Pacific Basin.The Pacific plate was hardly larger than the United States after
the breakup of Pangaea in the early Jurassic about 180 million years ago.The rest
of the ocean floor consisted of other unknown plates that disappeared as the
Pacific plate grew. Consequently, no oceanic crust is older than Jurassic in age.
The Jurassic was a tumultuous time. Pangaea split apart into the present-
day continents, leaving a gaping rift that filled with seawater to become the
Atlantic Ocean. Sea level and climate changed dramatically as the continents
tore away from each other.Yet life endured this tremendous geologic upheaval
for millions of years with little effect.
When Pangaea began to separate into today’s continents (Fig. 155), a great
rift developed in the present Caribbean. It sliced northward through the conti-
nental crust connecting North America, northwest Africa, and Eurasia and began
to open the Atlantic Ocean.The process took several million years along a zone
hundreds of miles wide.The breakup of North America and Eurasia might have
resulted from upwelling basaltic magma that weakened the continental crust.
Many flood basalts exist near continental margins, where rifts separated the pre-
sent continents.The episodes of flood basalt volcanism were relatively short-lived
events, with major phases generally lasting less than 3 million years.
India, nestled between Africa and Antarctica, drifted away from Gond-
wana.Antarctica, still attached to Australia, swung away from Africa toward the
southeast, forming the proto–Indian Ocean.The rift separating the continents
breached and flooded with seawater, forming the infant North Atlantic
Ocean. Many ridges of the Atlantic’s spreading seafloor remained above sea
level, creating a series of stepping-stones for the migration of animals between
the Eastern and Western Hemispheres.
About 125 million years ago, the infant North Atlantic obtained a depth
of about 2.5 miles and was bisected by an active midocean ridge system pro-
ducing new oceanic crust. At about the same time, the South Atlantic began
to form,opening like a zipper from south to north.The rift propagated north-
ward several inches per year, comparable to the rate of separation between the
plates.The entire process of opening the South Atlantic was completed in only
about 5 million years.By 80 million years ago, the North Atlantic had become
a fully developed ocean. Some 20 million years later, the Mid-Atlantic rift
progressed into the Arctic Basin, detaching Greenland from Europe.Historical Geology