T he oldest known rocks on Earth are those of the

3.96-billion-year-old Arcasta gneiss in Canada’s North- west Territories. They are of granitic composition and are similar to modern continental crust. Thus, a portion of the North American continent formed early in Earth history.

Sometime in the Archean Era, the Earth’s outer layers had cooled sufficiently that tectonic plates formed and be- gan gliding over the asthenosphere. Those plates were not like modern plates, however. They may have been thinner and may have moved at different rates than modern plates. However, their movements must have built mountains and caused earthquakes and magmatic activity. Over the past

3.96 billion years, tectonic processes have battered, broken, but ultimately created the North American continent.

As you read this chapter, bear in mind that we are dealing with models and hypotheses. They are based on data—facts that can be observed in the rocks of North America. But the models are interpretations of how the rocks formed. The models may change in the future because geologists will discover new facts, or because they will reinterpret old data. It is the nature of geology that our understanding of the Earth changes as we learn more.

Tectonic processes have built the North American continent over the past 3.96 billion years. (Tom Van Sant/Geosphere)

356 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

䊳 20.1 THE NORTH AMERICAN

The third region consists of the continental shelves

CONTINENT

and the coastal plain. They compose the region where young sediments eroded from the continent have accu-

Three major geologic regions make up North America mulated on the continental margin. Geologically, the today (Fig. 20–1). The craton is the continental interior.

shelves and coastal plain are continuous; they differ only It is a tectonically stable region that has seen little or no

in that the shelf lies below sea level whereas the coastal tectonic activity—no deformation, metamorphism, or

plain lies above sea level, between the shore and the magmatic activity—for more than a billion years. The

mountains.

craton consists of two subdivisions: the shield, where very old igneous and metamorphic basement rocks are exposed at the surface, and the platform, where the same types of basement rocks are covered with a veneer of

䊳 20.2 THE CRATON

much younger sedimentary rocks. The second region consists of the mountain chains

The old igneous and metamorphic basement rocks of the bordering the craton to the east and west. All of the

craton consist of several distinct geologic provinces (Fig. mountains are young relative to the craton, although

20–2). Each province contrasts with adjacent provinces some are hundreds of millions of years old.

in the following ways:

Arctic Continental

Innuitian

Shelf

Sheild Shield

Cordilleran n e n n

C Ouachita

Coastal Plain

Marathon

Mtns.

Figure 20–1 Three major geologic regions make up North America. The craton is the tectonically stable continental interior. The mountain ranges surround the craton and are

North America: 2 Billion Years Ago 357

Central Plains

Figure 20–2 The craton consists of several distinct geologic provinces that differ from each other in rock type and age.

1. Geologic relationships seem normal and continuous What caused these abrupt geologic changes and discon- within a single province but abrupt and discontinu-

tinuities at the province boundaries? ous at a boundary between two provinces. For ex- ample, within a single province, low-grade metamorphic rocks may pass gradually to medium and

䊳 20.3 NORTH AMERICA: 2 BILLION

then to high metamorphic grade over a distance of

T he oldest known rocks on Earth are those of the