United States Geography

There is a distinct association between the location of minerals that meet the needs of heavy manufacturing and the land's subsurface rock structure. Each of the three major forms of rock--sedimentary, metamorphic, and igneous--is capable of containing a type of mineral economically useful to humans. Sedimentary and metamorphic rocks are the most prevalent rock substructure and are more likely to contain minerals of broad utility than are igneous rocks.

Sedimentary rock is the result of the gradual settling of small solid particles in stationary water. For example, if a shallow sea were located adjacent to an arid landscape subject to occasional rainstorms, sand particles would be washed into the sea and spread across its bottom by water currents and the force of gravity. As this process continued, each layer of sand would press down on the layers beneath it, squeezing and solidifying the sandy mass that had been deposited only a few thousand years before. When this seabed was raised and folded into mountains by shifts in the earth's crust, the method by which at least some of the rocks were formed was betrayed by the presence of layers of sandstone.

About 300 million years ago during what earth historians call the Carboniferous period of the Paleozoic Era, conditions present in most existing land areas were such that unusual sedimentary sequences were created. Heavily vegetated and thick, swampy regions were drowned and covered with another layer of sediment. In some cases, the organic matter came to be represented in liquid form, trapped between folds of impermeable rock and eventually drawn off as petroleum. Most of these petroleum deposits are found in conjunction with another by-product of the period--natural gas. In other cases, the organic matter became solid layers of coal that were sometimes only centimeters thick but occasionally found dozens of meters thick.

In North America, vast regions are underlaid by sediments formed during the Carboniferous period. These areas where coal, oil, or natural gas might be found are located in the interior and Great Plains, sections of the Gulf coastal plain, portions of the Pacific mountains and valleys, the Arctic rimland, and in folded and broken form along the western margins of the Appalachian Highlands and into the eastern Rockies.

Large deposits of mineral fuels have been identified across extensive portions of these sedimentary lowlands. The most important coal deposits in America have been mined in the more rugged Appalachian field. Mines throughout this nearly continuous field in eastern Kentucky, West Virginia, and western Pennsylvania were the earliest to be brought into production, and they continue to supply over half of America's coal needs.

Until recently, much of the remaining coal mined in the United States has been obtained from the Eastern Interior Field, which underlies most of Illinois and extends into western Indiana and western Kentucky. Although some of the Eastern Interior Field's coal has been used in iron and steel production, its higher sulfur content has restricted most use to heating and electric-power generation.

The Western Interior Field is also large, located under Iowa and Missouri with a narrowing extension southward into eastern Oklahoma. The coal found in this field is of slightly poorer quality than that found in the eastern fields and has only recently begun to be mined.

There are many small and a few large bituminous deposits scattered through and along the eastern margins of the Rocky Mountains. Extensive deposits in Wyoming and Montana have come into production in the last two decades. There are also several extensive fields of lignite (brown coal) in the northern Great Plains.

Scattered deposits of petroleum and natural gas are found throughout the Appalachian coal field. Southern Illinois and south-central Michigan produce some petroleum, as do scattered sites across the northern Great Plains and the northern Rockies.

Easily the most important petroleum fields, however, have been those in the southern plains, along the Gulf coast, and in southern California. One great arc of producing wells is located along the full length of the Texas and Louisiana coasts. Another slightly broken arc extends from central Kansas south through Oklahoma and westward across central Texas to New Mexico. Between and beyond these two large areas lie two more fields of great importance, the East Texas field and the Panhandle field in northwest Texas. Separate from these fields but also of major importance are those located in southern California. In the mid-1960s, exploitation of deposits of petroleum and natural gas was begun along the north Alaska slope.

Metamorphic rock is formed in a quite different manner than sedimentary rock. Under the tremendous pressure exerted through the gradual deformation of the earth's crust, the internal structure of previously formed rocks can be metamorphosed, or changed. So great is the pressure exerted over thousands of years and so great is the heat generated that the very molecular structure of the rock is altered. This transformation indicates why metallic minerals in economically extractable quantities are located most often in areas of metamorphic rock.

Many of the mining sites for early exploitation of the metallic minerals were located near the margins of the Canadian Shield. The pattern of mineral production follows a long arc extending from the North Atlantic and St. Lawrence River estuary across the Great Lakes and northward through Canada to the Arctic Ocean. The arc continues on both sides of Lake Superior: in northern Michigan, Wisconsin, and Minnesota with copper and iron.

A second zone of metamorphic rock is located along the eastern Appalachian Mountains. Copper and iron were important minerals found locally by New England colonists.

A third and extensive region of metallic minerals is formed by the western mountains. Scatter deposits of gold and silver, a few of them rich, drew prospectors and mining companies to isolated locations from south of the Mexican border to central Alaska. Of great industrial importance are the large deposits of copper, zinc, lead, molybdenum, and uranium found in this western region, as well as smaller deposits of tungsten, chromite, manganese, and other minerals.

It should not be assumed that America's industrial requirements are met fully by the tremendous variety of minerals found in these three zones of metamorphic rock. A few minerals needed by modern industry (for example, tin, manganese, and high-grade bauxite for aluminum) have not been located in America in sufficient quantities to satisfy domestic needs. In addition, the growth of industrial capacity has been matched by a growth in demand for many minerals. Nevertheless, few countries have equaled or even approached the original quantity and diversity of metallic minerals and mineral fuels located in the United States.

This abundance of minerals has been critical in assisting the development of the immense American manufacturing-industrial complex.

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Source: U.S. Department of State