HISTORY OF THE CONTINENTS

• The Structure of North America 254
• Tectonic Provinces Around the World 259
• How Continents Grow 262
• How Continents Are Modified 266
• The Origins of Cratons 273
• The Deep Structure of ontinents 276

NEARLY TWO-THIRDS OF EARTH’S SURFACE—all of its oceanic crust—was created by seafloor spreading over the past 200 million years, an interval that spans a mere 4 percent of Earth’s history. The stories of earlier events are entirely contained in the continental crust. Thus, to understand how Earth has evolved since its fiery beginnings, we must look to the continents, which contain rocks over 4 billion years old.

The geologic record of the continental crust is very complex, but our ability to read that record has improved immensely in just the last few years. Earth scientists use the theory of plate tectonics to interpret eroded mountain belts and ancient rock assemblages in terms of closing ocean basins and colliding continents. New geochemical tools, such as isotopic dating, help us decipher the history of continental rocks. We can now image the structure of the continents far below Earth’s surface using networks of seismographs and other sensors.

In this chapter, we will describe the structure of Earth’s continents, and we will look back through their 4-billion-year history to see what it tells us about the processes that formed them—and are still modifying them today.

We will see how plate tectonic processes have added new material to the continental crust, how plate convergence has thickened that crust into mountain belts, and how those mountains have been eroded to expose the metamorphic basement rocks found in many older regions of the continents. Then we will reach back into the earliest period of continental evolution, the Archean eon (3.9 billion to 2.5 billion years ago), to ponder two of the great puzzles of Earth’s history: how did continents form, and how have they survived through billions of years of plate tectonics and continental drift?

Continents, like people, show a great variety of surface features that reflect their origins and experience over time. Yet, also like people, continents share many similarities in their basic structure and growth patterns. Before considering continents in general, let’s begin by outlining the major features of one particular continent: North America.