Rivers and streams transport water and sediment downslope to lakes and oceans. Along the way, sediment may be deposited, only to be eroded later during floods, and transported farther along the system. Dynamic processing of sediment in the ever-changing river environment separates grains according to density, size, shape, and resistance to weathering . Higher density and larger grains are found in the high-energy environment near the river channel. Lower density, smaller, and plate-shaped sediment grains are found in low-energy regions such as meander cutoffs and the floodplain , where water velocity is minimal or flow is absent.
This natural processing, which sorts sediments and minerals according to their physical properties, serves as a mechanism by which potentially valuable earth deposits are concentrated to a point where it is economically feasible to extract them for human use. These resources may be metallic, as in tin, gold and platinum placer deposits, or nonmetallic, such as deposits of sand and gravel. Sand and gravel, though not as spectacular as the precious metals, are among the most valuable mineral deposits.
The well-sorted sand and gravel deposits in ancient and modern-day river channels are important sources of aggregate materials. Sand and gravel are used in the construction industry for roadbed, concrete, and so on. The highest quality aggregate, used for making concrete, often is found near active river channels. High-quality aggregate may also be present in old river terraces where river channels once were located.
Instream mining extracts the sand and gravel from the modern channel system. The legal definition of instream mining varies from state to state, and in many states it is not allowed. Some states define instream mining as occurring within the limits of the 2-year floodplain (i.e., the area inundated by a flood having a 50-percent chance of occurrence in any year).
Off-channel mining extracts sand and gravel from older deposits often associated with floodplain terraces, which are remnants of an older floodplain level. Off-channel mining is regulated to protect floodplain resources within the 100-year floodplain.
Instream and off-channel mining can damage habitat for fish, and change the relationships between the channel and floodplain. Carefully designed and regulated mining may become a tool to help maintain or even increase the diversity of channel and floodplain habitat.
Diatoms are common unicellular algae whose different species live in freshwater, brackish water, and salt-water environments. Each species is distinguished by the shape and ornamentation of the frustule, the shell-like structure composed of silica. The frustules of dead diatoms accumulate with mineral and rock grains to form sediment known as diatomaceous earth. When lithified into a rock, diatomaceous earth is called diatomite. Although diatoms are widespread, relatively pure and thick deposits composed primarily of diatom frustules are rare because under normal conditions, the frustules are overwhelmed by other sediment.
Diatomite has many human uses, such as in the production of paint, paper, and plastics; as a nontoxic alternative to chemical pesticide dusts; and as a filtration media for liquids, including drinking water, fruit juices, and medicine.
The filtering efficiency of diatomite is controlled by the physical characteristics and relative abundance of the different diatom species that comprise the deposit. Each deposit has a different species grouping that depends on the aquatic environment in which it formed, and the time in the geologic past when it formed. Thus, a diatomite mine operator works closely with the client to optimize the properties of the excavated product.
In swamp environments, organic material composed mostly of plant remains may become submerged in oxygen-poor, stagnant water. The water protects the plant debris from oxidation and decomposition. The resulting sedimentary deposits may be organic-rich sediment, or in special cases, peat. Under average peat-forming conditions, only about 10 percent of the organic material produced in a swamp survives as peat. If peat is compacted and heated during burial, it becomes coal.
The poorly developed drainage systems in recently glaciated landscapes of the Northern Hemisphere are favorable settings for modern peat formation. In these settings, the swamps are relatively isolated from streams that carry mineral and rock debris. Similarly, coastal swamps are sites of peat formation.
Peat that has experienced little decomposition and is composed mostly of moss is used in gardening and agricultural production as peat moss. If peat is more highly decomposed and compressed, it can be used as a fuel after it is dried. However, the heat released is low relative to other common fuels such as wood. Yet new technologies that convert peat to methane gas by either bacterial digestion or by heating to 400°C to 500°C (752°F to 932°F) are changing the way peat is used.
Sea water contains almost 3.5 percent dissolved salt. When it evaporates, the dissolved solids form minerals such as halite, or rock salt (NaCl), gypsum (Ca 2 SO 4 · 2H 2 O), anhydrite (Ca 2 SO 4 ), and potash (KCl) salts. In the geologic past there have been a number of times when shallow and/or marginal seas have gone through periodic flooding and evaporation, and have led to the accumulation of a significant thickness of these materials.
Approximately 400 million years ago the evaporation of a shallow sea resulted in the accumulation of hundreds of meters of predominantly halite in the area now occupied by Michigan, Ohio, West Virginia, Pennsylvania, and New York. Additional deposits are found in India, Algeria, and China. Thick salt deposits in the Gulf Coast area have lead to the occurrence of salt domes and have been instrumental as petroleum traps. Halite is used primarily in cooking and as a food preservative, but also in the manufacture of soda ash for the glass industry and other sodium compounds.
The largest gypsum deposits are of Permian age. * Notable deposits are now found in Greece, Austria, Italy, France, Spain, England and Mexico. In the United States, deposits are found in Kentucky, Ohio, Michigan, South Dakota, and Utah. Gypsum commonly is mined to be used in plaster, Portland cement, paper filler, as a soil conditioner (to add sulfur) and in the manufacture of sulfuric acid. Wallboard is the largest single user of gypsum.
Included in some river sediments are minerals resistant to weathering and abrasion, and that have higher density than common rocks and minerals. Because of their higher density, these so-called "heavy minerals" become concentrated, whereas the lower density minerals are removed by the water current. Locally, these heavy minerals may become aggregated in placer deposits, and may attain concentrations of economic importance. Gold, tin, and platinum are examples of placer minerals.
Where rivers carry heavy minerals to the ocean, the minerals become concentrated on the beach by wave action. The high-density and resistant minerals are left behind as the waves winnow away the lower density and less resistant rocks and minerals. Gold, diamonds, garnet, chromium, tin, iron, and titanium are examples of heavy minerals that may be extracted from beach placer deposits. Nearshore underwater mining, and mining of uplifted marine terraces are important sources of these valuable mineral resources.
SEE ALSO Brines, Natural ; Fresh Water, Natural Composition of ; Fresh Water, Physics and Chemistry of ; Karst Hydrology ; Lakes: Physical Processes ; Mineral Resources from the Ocean ; Stream Channel Development ; Stream Hydrology .
Angier, Bradford. Looking for Gold: The Modern Prospector's Handbook. Mechanicsburg, PA: Stackpole Books, 1980.
Barnes, John W. Ores and Minerals: Introducing Economic Geology. Hoboken, NJ: John Wiley & Sons, Inc., 1991.
Evans, Anthony M. Introduction to Economic Geology. Malden, MA: Blackwell Publishers, 1997.
Prospecting for Gold in the United States. U.S. Geological Survey. <http://pubs.usgs.gov/gip/prospect2/prospectgip.html> .
Gold is a placer mineral that historically sparked mass migrations of people to sites of newly discovered deposits, often changing the course of cultural and economic development. Panning for gold mimics stream actions to separate grains of gold from other mineral and rock grains. To accomplish this separation, the panner takes advantage of the difference in the specific gravity of gold (19.3 grams per cubic centimeter) and common rocks and minerals (generally about 3 grams per cubic centimeter).
Sediment to be tested for gold is placed in the pan, which is tilted while the watery mixture is swirled. Swirling allows the lowdensity minerals to be washed out while the heavy minerals and gold remain behind. Continued careful swirling separates the heavy minerals from the gold, which can then be removed from the pan.
* See the frontmatter of this volume for a geologic timescale.
Comment about this article, ask questions, or add new information about this topic: