4 NORTH AMERICA: 1 BILLION YEARS AGO

䊳 20.4 NORTH AMERICA: 1 BILLION YEARS AGO

After Pangea I split up, the fragments of continen- tal crust reassembled about 1.0 billion years ago, form-

PANGEA III

ing a second supercontinent called Pangea II (some geologists call this supercontinent “Rodinia”).

Geologists have drawn a map of Pangea II by com-

LATE DEVONIAN

EARLY PERMIAN

paring rocks now found on different continents. For ex-

(374 MILLION YEARS AGO)

(260 MILLION YEARS AGO)

ample, 1-billion-year-old sedimentary and metamorphic rocks in western North America are similar to rocks of the same age in both Australia and East Antarctica. Ian

W. D. Dalziel of The University of Texas at Austin re- cently suggested that western North America was joined

NORTH AMERICA

SOUTH AMERICA AND AFRICA

to both Australia and East Antarctica from about 1 bil- lion years ago to 750 million years ago (Fig. 20–3a).

AUSTRALIA, ANTARCTICA

Portions of South America and northern Europe lay ad-

AND INDIA

jacent to eastern North America at the same time.

NORTHERN EUROPE

Dalziel’s maps show that Pangea II then broke apart in late Precambrian time, about 750 million years ago.

SIBERIA

Australia and Antarctica rifted away from western North America, leaving a shoreline at the western margin of North America. This region, however, did not appear as it does today. Parts of Alaska and western Canada and much of Washington, Oregon, western Idaho, and west-

The discontinuities between the provinces of the ern California had not yet become part of western North North American craton are ancient boundaries between

America, as shown in Figure 20–4. the microcontinents. The rocks of each province give dif-

Northern Europe and South America also had rifted ferent radiometric dates because each microcontinent

away from North America by 550 million years ago, as formed at a different time. The sheared and faulted

an ocean basin opened along North America’s eastern

North America: A Half Billion Years Ago 359

mid-Ordovician time (Fig. 20–3d). The convergence of the continents caused subduction of oceanic crust near the east coast of North America. Volcanoes erupted, gran- ite plutons intruded the crust, mountains rose, and great belts of metamorphic rocks formed along the east coast. This first phase of building of the Appalachian mountain chain is called the Taconic orogeny after the Taconic Range on the border of New York State with Connecticut and Massachusetts, where rocks deformed by that orogeny are exposed (the term orogeny refers to the

processes by which mountain ranges are built).

te Following the mid-Ordovician collision, North

rn

America separated from South America a second time

d and then collided with it again in late Devonian time

(Figs. 20–3e and 20–3f). The collision shoved sedimen-

C f tary rocks westward from the continental shelf onto the

tin craton, forming tremendous thrust faults and folds along

the east coast (Fig. 20–5). This second phase of moun-

tain building in the Appalachians is called the Acadian

B orogeny. It affected the northeastern corner of North

n io ill

America from Newfoundland to Pennsylvania, and it is

named for Acadia, the name early settlers gave to that

Ocean

a part of North America.

rs

North g A American

Figure 20–6 is a composite map showing that North

Continent

America moved along the coast of South America as the two continents separated and collided twice. Then about 265 million years ago, North America slid around the upper end of South America and collided with western Africa. This collision built the central and southern Appalachians in the Allegheny orogeny, named for the Allegheny Plateau of the central Appalachian region.

Figure 20–4 Western North America as it appeared fol- lowing rifting in latest Precambrian time.

margin. Thus, by the end of Precambrian time, North America had become isolated from other continents and was surrounded by oceans (Fig. 20–3b).