5 NORTH AMERICA: A HALF BILLION YEARS AGO
䊳 20.5 NORTH AMERICA: A HALF BILLION YEARS AGO
THE APPALACHIAN MOUNTAINS Figure 20–3c shows that eastern North America contin-
Figure 20–5 Eastern North America collided with South ued to separate from South America as the intervening
America in late Devonian time, about 374 million years ago, ocean widened through Middle Cambrian time. Then the
forming great thrust faults and folds such as these in Nova two continents began converging again and collided in
Scotia. (Geological Survey of Canada)
360 CHAPTER 20 T H E G E O L O G I C A L E VO L U T I O N O F N O RT H A M E R I C A
All of these events, beginning with subduction in and flooding low-lying portions of continents. When mid-Ordovician time followed by two collisions with
Pangea II rifted apart in late Precambrian time, a very South America and finally one with Africa, are collec-
long and wide mid-oceanic ridge system formed among tively called the Appalachian orogeny. They built the
the separating fragments of the supercontinent. The new Appalachian mountain chain as well as the Ouachita and
ridge raised sea level for much of the time from the Marathon mountains in Arkansas, Oklahoma, and east-
Cambrian through the Pennsylvanian periods, flooding ern Texas. Related events built the Innuitian mountain
the low, central craton of North America. The sea did not chain along the northern continental margin (Fig. 20–1).
cover the craton continuously, however. Seas advanced Note that the Appalachian orogeny was similar to
and withdrew several times as sea level rose and fell. the events that built the Himalayas—subduction of
Each time sea level rose, the shoreline migrated inland, oceanic crust beneath the continent, followed by a con-
spreading a blanket of marine sand, mud, and carbonate tinental collision. At one time, the Appalachians must
sediment across the craton. As a result, a veneer of sand- have been immense mountains similar to the Himalayas.
stone, shale, and limestone ranging in age from Cambrian Today, however, erosion has worn the Appalachians down
through Pennsylvanian now covers much of the central to maximum elevations of less than 2000 meters.
part of North America. These rocks are less than a few As the Appalachians rose over a period of more than
hundred meters thick in most places and are called plat- 200 million years, all other continents joined the grow-
form sedimentary rocks (Fig. 20–7). They blanket large ing landmass. Thus, Pangea III had assembled by about
areas of the Precambrian igneous and metamorphic rocks 265 million years ago (Fig. 20–3g). North America
of the craton.
formed the northwestern portion of Pangea III. This latest supercontinent was Alfred Wegener’s Pangea, described in Chapter 2.
䊳 20.6 BREAKUP OF PANGEA III
FLOODING OF NORTH AMERICA: PLATFORM OPENING OF THE MODERN SEDIMENTARY ROCKS
ATLANTIC OCEAN
Recall that a long, rapidly spreading mid-oceanic ridge Pangea III remained intact from about 300 to 180 mil- displaces a large volume of seawater, raising sea level
lion years ago and then began to rift apart. As North and
265 MILLION YEARS AGO
A composite map showing the movements of North America from 750 million years ago
550
to 265 million years ago. The other continents are shown in their posi-
530 tions of 260 million years ago. (Redrawn from Ian W. D. Dalziel, “Earth
750
Before Pangea,” Scientific American, January 1995, p. 58)
South America separated from Eurasia and Africa, the modern Atlantic Ocean began to open. The Atlantic Ocean was born, and continues to grow today, as a result of sea-floor spreading along the mid-Atlantic ridge. Passive continental margins developed on both sides of the newly opening ocean basin. Tectonic activity on the eastern margin of North America ceased, and the lofty Appalachians began to wear away.
The new eastern margin of North America devel- oped in about the same place it had been before Pangea
III assembled. Thus, rifting followed the sutures where continents had welded together 120 million years previ- ously (Fig. 20–8). Suture zones may be lines of weak- ness within a supercontinent, like the perforations in tear-out advertisements bound into magazines. The rift- ing did not perfectly follow the old sutures, however. As the supercontinent broke up, small pieces of Europe and Africa remained stuck to the east coast of North America, and parts of North America rode off with Africa and Europe.