11.2: Mountains and Valleys

Just to get you started thinking about the Earth’s prominent landforms (there will be more detail later in the chapter), here are some comments about the nature of mountains and valleys.

As you will see soon, mountains are diverse not only in their scale and geometry but also in their processes of origin. I suppose that if you were to ask the proverbial “person in the street” about how mountains originate, he or she would tell you that they are somehow “built” to stand above their surroundings. There are indeed such “constructional” mountains (an only semi-official term), as the next few paragraphs describe.

Volcanic mountains, especially, come readily to mind as constructional mountains. Volcanoes emit lava (liquid, melted rock) and solid particles of a great variety of compositions and sizes. Much of this material is deposited in the vicinity of the fissure or vent, to build a hill or a mountain.

Volcanoes that emit basaltic lava, of relatively low viscosity, build mountains that are very broad, with gentle slopes. Such volcanoes are called shield volcanoes. The great volcanoes of Hawaii and the Galápagos Islands are shield volcanoes. In a certain sense, Kilauea, on the big island of Hawaii, is the world’s tallest mountain—if you measure the vertical distance from its base, in the deep ocean, to its summit!

Many volcanoes emit solid particles rather than liquid lava. Eruptions from such volcanoes tend to be explosive—often catastrophically so. The volcanic ash erupted from the volcano is thrown high into the atmosphere, and much of it settles to the ground over large areas, but some is deposited in the immediate vicinity of the volcano, to build a classic cone-shaped mountain. Such volcanoes are called cone volcanoes. Mount Fuji, in Japan, is a classic cone volcano. Many such volcanoes consist of alternations of layers of lava and ash. The slopes of cone volcanoes are generally much steeper than those of shield volcanoes.

Mountains are also constructed by movements on faults. (A fault is a widespread fracture in bedrock along which the material on the two sides of the fracture have moved relative to one another.) Fault movements that have a vertical component to the movement create topography. Movements of major faults or fault zones can create great mountain ranges. Figure 11-1 shows a common example. The fault surface, usually approximately a plane, is inclined at a steep angle. The movement is such that the mass of rock overlying the fault surface moves downward relative to the mass of rock underlying the fault surface. The result is a mountain range with an adjacent valley. There are many such mountain ranges in the large area in the southwestern U.S. called the basin-and- range province. Death Valley is a classic example: the main fault (called the “range-bounding fault”) is on the east side of the valley; it is inclined to the west (or, in the parlance of geology, it dips to the west) at a moderate angle. The range of mountains east of Death Valley is going up, and the valley is going down, as a result of episodic movement on the fault. Of course, as the mountain goes up it is at the same time being worn down by erosion. The sedimentary products of the erosion are being deposited in the valley. The fill of the valley has attained a thickness of thousands of meters.

Figure 11-1. A mountain range and a valley formed by movement on a fault.

It may seem counterintuitive to you at this point, but you will learn in this chapter that most of the Earth’s mountains are “erosional” mountains rather than constructional mountains. Such mountains come about by broad uplift, over a large area of the Earth’s crust, and then erosion by streams and rivers, to leave elevated remnants of the originally uplifted area as mountains, surrounded by lowlands and valleys.

The central Appalachians of the eastern U.S., in Pennsylvania for example, are an excellent place to study erosional mountains. In much earlier geologic times the entire area was broadly uplifted by several hundred meters. Since then, erosion has excavated valleys, leaving higher ground, underlain by the rocks most resistant to erosion, as mountain ridges. The process has been most advanced in the eastern part of the area; westward, into northwestern Pennsylvania, the progress of erosion has been less advanced, and there is an extensive plateau, called the Allegheny Plateau, that is cut here and there by deep stream valleys.

You can see now that some valleys are produced by fault movements, and are usually occupied by streams and rivers, except in arid regions, whereas other valleys are the result of downcutting into broadly uplifted regions by streams and rivers.