Understanding Mountains Out of Molehills: Why Looking at Old Dirt Can Tell Us How Mountains Form

Cynthia Martinez
School of Earth Sciences
Stanford University
December 2001

You might think that the only way to find out how mountains form is to look at the rocks in the mountains themselves. I take a different approach by looking at the dirt that eroded as the mountains were raising up. By looking at the eroded material that collects at the base of a mountain, I can better understand the processes that created it.

One way that mountains are created is by stretching of the earth. The land extends along faults, or surfaces in rocks that are created by movement of one side against another. Scientists argue about some faults that stretch the earth. One thing they disagree about is the angle at which these faults are dip with respect to horizontal when they move, because some faults are now dipping at very shallow angles. In theory, movement should not occur along a shallowly dipping fault because the forces of friction are too high. That is, the weight from the overlying rocks is too heavy to allow movement on a shallow plane, and extending faults preferentially form at high angles. Yet we see shallow extending faults in mountain ranges all over the world! How did these faults form?

Some people think that these faults may have been rotated to shallow dips after they formed at higher angles. Others think that there might have been fluid in the fault area that lowered the friction and let the faults move. Often, these faults are found in areas that have been severely stretched, so that the surface of the earth appears to be more than twice as long as it once was. We need to know whether the fault started out at a shallow angle in order to figure out why some areas are so extended. If the faults didn't form at a low angle, then we need to know how they evolved to their present dip.

There are many ways to solve this problem. People have mapped out the rocks that are below the fault, and the matching ones above it. They have employed fancy technology to understand the age at which the rocks formed and when the faulting happened, but we still do not know decisively how low-angle extensional faults got to be so shallowly dipping. I take a fresh look at this old question with my research by studying the eroded materials from the mountain. As mountains form, they rise and create topography. The emerging bump is continuously eroded by streams, landslides, wind and ice. These processes dump sand, gravel, and even huge boulders into a basin at the bottom of the hillside. I look at the arrangement of these different types of sediment (the eroded pieces) to try to understand what the fault angle was that controlled the uplift of the mountain. If the fault was at a steep angle when the sediments were deposited, I would expect to find lake sediments close to the mountain range because movement on the fault would create a deep basin. In contrast, movement on a shallowly dipping fault would create a broad basin and I would find lake sediments farther away.

The basins that I study are located in a part of the western United States called the Basin and Range Province, or the Great Basin. It covers most of Nevada, the western edge of California, the eastern edge of Utah, and northern Arizona. I spend my summers making geologic maps of sedimentary basins that formed next to low-angle extensional faults. I map out where the landslides are, and where gravel from streams is located relative to sediments deposited in lakes. Pieces that eroded from the top of the mountain are located in the bottom of the basin, since they eroded first, and vice versa. I study the order in which rocks were eroded so that we can understand how and when the mountain grew higher.

Studying the dirt at the bottom of mountains may not sound glamorous, but it gives us important information that will help us understand how mountains form. It helps us figure out how low-angle extending faults work so that we can some day predict how they will move and when they will cause earthquakes. I love my job, because I am helping to understand how faults and mountains form, and how they impact our landscape and the people in it. (It doesn't hurt that I get to hike and camp in beautiful places while I work, too!)