Unraveling Geologic History

The geologic history of a region is a succession of episodes of deformation and other geologic processes. Let’s see how some of the concepts and methods introduced in this chapter can be used to reconstruct that history.

The cross sections in Figure 7.23 represent a few tens of kilometers of a geologic region that underwent a succession of tectonic events. First, horizontal layers of sediment were deposited on the seafloor. Those layers were tilted and folded, and eventually uplifted above sea level, by horizontal compressive forces. There, erosion gave them a new horizontal surface. That surface was covered by lava when forces deep in Earth’s interior caused a volcanic eruption. In the most recent stage, tensional forces resulted in normal faulting, which broke the crust into blocks.

Geologists see only the last stage, but visualize the entire sequence. They begin by identifying and determining the ages of the rock layers and recording the orientation of layers, folds, and faults on geologic maps. Then they use those maps to construct cross sections of the subsurface features. Once the geologists have identified sedimentary beds, they can start with the knowledge that the beds must originally have been horizontal and undeformed at the bottom of an ancient ocean. The succeeding events can then be reconstructed.

Present-day surface topography in young mountain ranges—such as the Alps, the Rocky Mountains, the Pacific Coast Ranges, and the Himalaya—can be traced in large part to deformation that has occurred over the past few tens of millions of years. These young systems still contain much of the information that geologists need to piece together the history of that deformation. Deformation that occurred hundreds of millions of years ago is no longer evident in the form of rugged mountains, however. Erosion has left behind only the remnants of folds and faults, expressed as low ridges and shallow valleys. As we will see in Chapter 10, even older episodes of mountain building are evident only in the twisted, highly metamorphosed formations that constitute the basement rocks of the interiors of continents.