Geology

Mountain belts typically are thousands of kilometers long and hundreds of kilometers across and parallel continental coastlines. The American Cordillera is a series of steep mountain ranges that rim the western edge of North and South America; it is one of the longest mountain belts in the world. In general, the taller mountains are geologically younger than lower mountains (for example, the steeper Rocky Mountains are younger than the lower and more rounded Appalachian Mountains) because older ranges have undergone more weathering and erosion. Most mountain ranges are uplifted, erode to low elevations, and are uplifted again before they become stable.

Major mountain ranges in the United States include the Appalachian Mountains, the Rocky Mountains, the Ozark Mountains, and the many ranges along the West Coast. Fossil evidence and age dating indicate the rounded hills of the Appalachian and Ozark Mountains are some of the oldest mountains in the United States.

Cratons. Billions of years ago the now‐stable interior of North America was a mountainous, tectonically active region that eventually stabilized and weathered to a peneplain (an area reduced by erosion nearly to a plain). A continental interior that has been structurally inactive for hundreds of millions of years is called a craton. It is composed of mostly plutonic and metamorphic rocks. The craton is a “basement” upon which sequences of sedimentary rocks were deposited under marine or nonmarine conditions. The central United States is covered by about 2,000 meters of sedimentary rocks that were deposited in shallow Paleozoic oceans. Continents have grown larger through accretionary episodes in which mostly sedimentary material and volcanic arcs were welded to the craton through plate collisions, usually resulting in mountain‐building.

Rock types. Mountains are typically composed of folded sedimentary strata that may be up to five times as thick as the original sedimentary sequence that covered the cratonic interior. The folded and broken layers indicate the rock has undergone deformation during mountain‐building. Since mountain belts typically form along tectonically active coastlines and above subduction zones, much of the sedimentary rock is marine in origin. The sediments are often parts of the accretionary wedge that have been compressed, folded, and driven onto the continent by plate tectonic processes.

How intensely a mountain belt is folded depends on how great the tectonic forces were. Mountain‐building forces are intensely compressional, and the sedimentary sequence in a basin is often squeezed into a mountain range that is less than half the width of the original basin. Rock layers are typically contorted into tight fold patterns, including overturned or recumbent folds. Fold and thrust belts in many mountain ranges are the result of multiple thrust layers (sheets) of rock that have been thrust forward and stacked vertically along the low‐angle detachment faults that separate the thrust sheets. After uplift has been completed, a later stage of tensional stress develops that forms a series of fault‐block (horst and graben) mountains. The faulting is an adjustment to the extensional stress created by the vertical uplift.

The core of a mountain range tends to be its most intensely metamorphosed part. The metamorphic rocks were originally sedimentary rocks or volcanic rocks that were intensely metamorphosed through deep burial, folding, and tectonic uplift. It is often difficult to recognize the original rock types, and metamorphic rocks are typically mapped as “schist” or “gneiss.” Migmatites are some of the most intensely metamorphosed rocks that are found in the cores of mountain ranges. The large batholithic intrusions that underlie mountain ranges were formed by partial melting during the mountain‐building process. The continental crust under mountain ranges is thicker than that under the cratonic interior; similarly, the crust under younger mountain ranges is thicker than the crust under older ranges. The uplift of these blocks of crust eventually stabilizes through isostatic adjustments. Geologically young, tectonically active mountains have more earthquakes and volcanic activity than the older, more stabilized mountain chains.