The outer layer of Earth, which we call the crust, is only as thick, when compared to the remainder of the Earth, as a peach skin is to a peach, yet it is of supreme importance to us. We depend on it for fossil fuels and as a source of such diverse materials as the iron for automobiles, salt to flavor food, and gold for world trade. As the world population grows and the material requirements of modern society increase, the need to mine more extensively into Earth’s crust to locate additional supplies of useful minerals becomes more challenging.

Most of the energy and mineral resources used by humans are nonrenewable. Figure 33 shows the annual per capita consumption of several important mineral and energy resources. These data reflect an individual’s share of the materials required by industry to support the needs of our modern society—including a vast array of homes, cars, electronics, cosmetics, packaging, and other products and services.

Metallic Mineral Resources

Some of the most important accumulations of metals, including gold, silver, copper, platinum, and nickel, are produced by igneous and metamorphic processes that concentrate desirable materials to the extent that extraction is economically feasible (Table 1). Igneous processes that generate some metal deposits are straightforward. For example, as a large magma body cools, minerals that crystallize early and are heavy tend to settle to the lower portion of the magma chamber. This type of magmatic differentiation is particularly important in large basaltic magmas where chromite (ore of chromium), magnetite, and platinum are occasionally generated. Layers of chromite, along with other heavy minerals, are mined from such deposits in the Bushveld Igneous Complex in South Africa, which contains over 90 percent of the world’s known reserves of platinum group metals.

Igneous processes are also important in generating other types of mineral deposits. For example, as a granitic magma cools and crystallizes, the residual melt becomes enriched in rare elements and heavy metals, including gold and silver. Furthermore, because water and other volatile substances do not crystallize along with the bulk of the magma body, these fluids make up a high percentage of the melt during the final phase of solidification. Crystallization in a fluid-rich environment enhances the migration of ions and results in the formation of crystals several centimeters, or even a few meters, in length. The resulting rocks, called pegmatites, are composed of these unusually large crystals (Figure 34).

Most pegmatites are granitic in composition and consist of large crystals of quartz, feldspar, and muscovite. Feldspar is used in the production of ceramics, and muscovite is used for electrical insulation and glitter. In addition to these common silicates, some pegmatites contain semiprecious gems, such as beryl, topaz, and tourmaline. Moreover, minerals containing the elements lithium, gold, silver, uranium, and the rare earth elements are sometimes found in pegmatites. Most pegmatites are located within large igneous masses or as distinct sheet-like structures called dikes or veins that cut into the host rock surrounding a magma chamber (Figure 35).

Among the most important ore deposits are those generated from hydrothermal (hydro = water, therm = heat) solutions. Included in this group are the gold deposits of the Homestake Mine in South Dakota; the lead, zinc, and silver ores near Coeur d’Alene, Idaho; the silver deposits of the Comstock Lode in Nevada; and the copper ores of the Keweenaw Peninsula in Michigan.

The majority of hydrothermal deposits originate from hot, metal-rich fluids that are associated with cooling magma bodies. During solidification, liquids plus various metallic ions accumulate near the top of the magma chamber. Because these hot fluids are very mobile, they can migrate great distances through the surrounding host rock before they are eventually deposited. Some of this fluid moves along fractures or bedding planes, where it cools and precipitates the metallic ions to produce vein deposits (refer to Figure 35). Many of the most productive deposits of gold, silver, and mercury occur as hydrothermal vein deposits.

Another important type of accumulation generated by hydrothermal activity is called a disseminated deposit. Rather than being concentrated in narrow veins and dikes, these ores are distributed as tiny masses throughout the entire rock mass (refer to Figure 35). Much of the world’s copper is extracted from disseminated deposits, including the huge Bingham Canyon copper mine in Utah (Figure 36). Because these accumulations contain only 0.4 to 0.8 percent copper, between 125 and 250 metric tons of ore must be mined for every ton of metal recovered. The environmental impact of these large excavations, including the problems of waste disposal, is significant.

Nonmetallic Mineral Resources

Earth materials that are not used as fuels or processed for the metals they contain are referred to as nonmetallic mineral resources. Realize that use of the word mineral is very broad in this economic context and is quite different from geologists’ strict definition of “mineral” we discussed earlier this semester. Nonmetallic mineral resources are extracted and processed either to make use of the nonmetallic elements they contain or for the physical and chemical properties they possess (Table 2). Although these resources have diverse origins, many are sediments or sedimentary rocks. Because a large percentage of these materials are used in manufacturing or to make fertilizers—processes we rarely see—their importance is easy to underestimate.

The quantities of nonmetallic minerals used each year are enormous. Per capita consumption of nonfuel resources in the United States is over 18,000 kilograms (~40,000 pounds), of which more than 98 percent are nonmetallics. Nonmetallic mineral resources are commonly divided into two broad groups: building materials and industrial minerals. Some substances fall into both groups.

Building materials also include cut stone, aggregate (sand, gravel, and crushed rock), gypsum for plaster and wallboard, clay for tile and bricks, and cement, which is made from limestone and shale. Cement and aggregate go into the making of concrete, a material that is essential to practically all construction.

A wide variety of resources are classified as industrial minerals. Some are sources of elements or compounds used in the manufacture of chemicals. Others, including fluorite and limestone, are part of the steelmaking process; corundum and garnet are used as abrasives; and sylvite, a potassium-rich mineral, is used in the production of fertilizers. Deposits of industrial minerals are generally far less common than those of building materials.

Limestone, a rock formed of the nonmetallic mineral calcite, is perhaps the most versatile and widely used rock of all. As a building material, it is used not only as crushed rock and building stone but also in making cement. As an industrial mineral, limestone is an ingredient in the manufacture of steel and is used in agriculture to neutralize acidic soils.

Energy Resources

Earth’s tremendous industrialization over the past two centuries has been fueled by burning coal, petroleum, and natural gas. About 79 percent of the energy consumed in the United States today comes from these sources. Our reliance on fossil fuels is obvious. Although we are increasing the quantities of energy from alternative sources, such as solar, wind, geothermal, biomass, and hydroelectric, the U.S. Department of Energy projects that fossil fuels will remain a primary source of energy for decades to come.

Coal

Along with oil and natural gas, coal is commonly called a fossil fuel. This designation is appropriate because when coal is burned, energy from the Sun that was stored by plants many millions of years ago is being used, hence the actual burning of “fossils.” In the United States, coal fields are widespread and contain supplies that should last for hundreds of years (Figure 36).

Coal is plentiful, but its recovery and use present a number of challenges. Surface coal mining, which is costly, can turn the countryside into a scarred wasteland if careful reclamation is not carried out to restore the land. Currently, all U.S. surface coal mines are required to reclaim the land. Although underground coal mining does not scar the landscape to the same degree, it has been costly in terms of human life and health. Strong federal safety regulations have tremendously improved safety in U.S. coal mining. However, for those working in underground coal mines, collapsing roofs, gas explosions, and the required heavy equipment remain hazards.

In addition, burning coal produces emissions that adversely influence the environment and human health. Principal emissions resulting from coal combustion include the following:

• Sulfur dioxide (SO2), which contributes to acid rain and respiratory illnesses

• Nitrogen oxides (NOX), which contribute to smog and respiratory illnesses

• Particulate matter, which contributes to smog, haze, respiratory illnesses, and lung disease

• Carbon dioxide (CO2), the primary greenhouse gas produced from the burning of fossil fuels, which plays a significant role in the heating of our atmosphere

In the U.S., governmental regulations and policies have resulted in reduced harmful emissions from coal combustion and decreased pollutants that are released into waterways. Technologies continue to be developed and implemented to remove impurities from coal to make it more energy efficient and to reduce emissions from coal powerplants to alleviate pollution. But significant challenges in reducing harmful coal emissions remain for the United States and for governments globally.

Oil and Natural Gas

Together, oil and natural gas provided almost 70 percent of the energy consumed in the United States in 2022. In 2011, natural gas surpassed coal for the first time as a source of energy in the United States, and in 2016, coal became the most common fuel for electricity in the country. An important reason for this is the use of new technologies that have increased production from shale formations, such as hydraulic fracturing discussed later in this section.

Like coal, petroleum and natural gas are biological products derived from the remains of organisms that lived millions of years ago. However, while coal formed mostly from plant material that accumulated in swampy environments on land millions of years ago, petroleum and natural gas originated mainly from microscopic marine plankton in ancient seas millions of years ago. This organic material of microscopic marine plankton accumulated in marine sedimentary basins with mud, sand, and other sediments that protected it from oxidation. With increased temperature and burial, chemical reactions gradually transformed this organic matter into the liquid and gaseous hydrocarbons we call petroleum and natural gas.

Unlike the organic matter from which they form, petroleum and natural gas are mobile. These fluids are gradually squeezed from the compacting, mud-rich layers where they originate (called the source rock) into adjacent permeable beds, such as sandstone, where openings between sediment grains are larger. Because this occurs in a marine environment, the rock layers containing the oil and gas are saturated with saltwater. Oil and gas, being less dense than water, migrate upward through the water-filled pore spaces of the enclosing rocks.

A geologic environment that allows for economically significant amounts of oil and gas to accumulate underground is termed an oil trap. Several geologic structures can act as oil traps and have two basic features: a porous, permeable reservoir rock that can yield petroleum and natural gas in sufficient quantities to make drilling and extraction worthwhile; and a cap rock, such as shale, that is virtually impermeable to oil and gas and hence traps them in the reservoir rock. Figure 37 illustrates the following common oil and natural gas traps:

• Anticline. One of the simplest traps is an anticline, an up-arched series of sedimentary strata (refer to Figure 37A). The rising oil and gas collect at the top of the fold. Because of its lower density, natural gas collects above the oil. Both rest on the denser water that saturates the reservoir rock.

• Fault trap. When strata are displaced so that a dipping reservoir rock abuts an impermeable bed, a fault trap forms, as shown in Figure 37B. The upward migration of the oil and gas is halted where it encounters the fault.

• Salt dome. In the Gulf coastal plain region of the United States, important accumulations of oil occur in association with salt domes. Such areas have thick accumulations of sedimentary strata that include layers of rock salt. Salt occurring at great depths has been forced to rise in columns by the pressure of overlying beds. These rising salt columns gradually deform the overlying strata. Because oil and gas migrate to the highest level possible, they accumulate in the upturned sandstone beds adjacent to the salt column (refer to Figure 37C).

• Stratigraphic (pinchout) trap. A stratigraphic trap results primarily from the original pattern of sedimentation rather than from structural deformation. The stratigraphic trap illustrated in Figure 37D exists because a sloping bed of sandstone thins to the point of disappearance.

Regardless of the type of trap, when drilling punctures the lid created by the cap rock, the oil and natural gas, which are under pressure, migrate from the pore spaces of the reservoir rock to the drill pipe (well). On rare occasions, when fluid pressure is great, it may force oil up the drill hole to the surface, causing a “gusher” at the surface. Usually, however, a pump is required to extract the oil.

Hydraulic Fracturing

Some shale deposits contain significant reserves of oil and natural gas that cannot naturally leave because of the rock’s low permeability (ability to transmit fluids). The practice of hydraulic fracturing (often called fracking) shatters the shale, opening cracks through which the oil and natural gas can flow into wells and can then be brought to the surface. Figure 38 illustrates the process. The fracturing of the shale is initiated by pumping fluids into the rock at very high pressures. The fluid is mostly water but also includes other chemicals that aid in the fracturing process. Some of these chemicals may be toxic, and among several environmental concerns about fracking is that of fluids leaking into freshwater aquifers, which supply people with drinking water. The injection fluid also includes sand, so once fractures open in the shale, the sand grains can keep them propped open and permit the oil and gas to continue to flow.

Once the fracturing has been accomplished, the fracking fluid is brought back to the surface. This wastewater is often injected into deep disposal wells, and in some locations, these injections are known to trigger numerous minor earthquakes. Due to concerns about potential groundwater contamination and induced seismicity (earthquakes), hydraulic fracturing remains a controversial practice and its environmental effects remain a focus of continuing research.

• Igneous processes concentrate some economically important elements through both magmatic differentiation and the emplacement of pegmatites. Magmas may also release hydrothermal (hot-water) solutions that penetrate surrounding rock, carrying dissolved metals in them. The metal ores may be precipitated as fracture-filling deposits (veins) or may penetrate the surrounding strata, producing countless tiny deposits disseminated throughout the host rock.

• Earth materials that are not used as fuels or processed for the metals they contain are referred to as nonmetallic resources. The two broad groups of nonmetallic resources are building materials (such as gypsum, used for plaster) and industrial minerals (including sylvite, a potassium-rich mineral used to make fertilizers).

• Coal, oil, and natural gas are all fossil fuels. In each, the energy of ancient sunlight, captured by photosynthesis, is stored in the hydrocarbons of plants or other living things buried by sediments.

• Coal is formed from compressed plant fragments originally deposited in ancient swamps. Coal mining can be risky and environmentally damaging, and burning coal generates several kinds of pollution.

• Oil and natural gas are formed from the heated remains of marine plankton. Together, they account for more than 70 percent of U.S. energy use. Both oil and natural gas leave their source rock (typically shale) and migrate to an oil trap made up of other, more porous rocks called reservoir rock, covered by a suitable impermeable rock called cap rock.

• Hydraulic fracturing (or “fracking”) is a method of opening pore space in otherwise impermeable rocks, permitting natural gas to flow out into wells. Fracking is associated with several environmental concerns.