Ch02 Some countries poison soils Why should the leaders of countries today commit their governments and their people to the hard work and expense of a national programme of soil conservation? The answer is that soil takes many years to create, but it can be destroyed in almost no time at all. With the loss of soil goes man's ability to grow food crops and graze animals, to produce fibre and forests. It is not enough to describe the soil as a country's greatest source of wealth; it is more than that; it is a country's life. And in one country after another today, the soil is washing or blowing away. Soil is a complex mixture Soil covers most of the land surface of the earth in a thin layer, ranging from a few centimetres to several metres deep. It is composed of rock and mineral particles of many sizes mixed with water, air, and living things, both plant and animal, and their remains. As man measures time, soil formation is extremely slow. Where the climate is moist and warm, it takes thousands of years to form just a few centimetres of soil. In cold or dry climates, it takes even longer, or soil may not form at all.

While soil is technically a renewable resource, its slow rate of formation makes it practically irreplaceable. Soil is a dynamic mixture, forever changing as water comes and goes and plants and animals live and die. Wind, water, ice, and gravity move soil particles about, sometimes slowly, sometimes rapidly. But even though a soil changes, the layers of soil stay much the same during one human lifetime unless they are moved or scraped, or ploughed by man. Soil teems with life All soil is full of life, and good soils are teeming with it. Plants and animals help keep the soil fertile. Plant roots tunnel through the soil and break it up, and decaying plants form humus. Burrowing animals mix the soil; the excrete of animals contribute nutrients and improve soil structure. Besides the soil's more obvious inhabitants, which include rodents, insects, mites, slugs and snails, spiders, and earthworms, there are countless microscopic residents, some helpful to man and his crops, some harmful. Good soils seem to hold the greatest populations of bacteria. Almost without exception, bacteria are involved in basic enzyme transformations that make possible the growth of higher plants, including our food crops.

From man's point of view, bacteria may well be the most valuable of the life forms in soil. Chemical reactions occur in the soil as a result of exchange of positive ions, or cations. More exchanges take place in clay soils than in any other type. These chemical reactions are also essential to plant growth and development and are a good index of soil fertility. Only a fraction of land is arable Man's chief interest in soil is for agriculture, but not all soils are suitable for farming. The total land area of the world exceeds 13 billion hectares, but less than half can be used for agriculture, including grazing. A much smaller fraction - about 1.4 billion hectares - is presently suitable for growing crops. The rest of the land is either too wet or too dry, too shallow or too rocky. Some is toxic or deficient in the nutrients that plants require and some is permanently frozen . Europe, Central America, and North America have the highest proportion of soils suitable for farming, although a number of the more developed countries seem intent on paving over much of their best farmland with roads and buildings. The lowest proportions of arable soils are in North and Central Asia, South America, and Australia.

The single most serious drawback to farming additional land is lack of water. Erosion destroyed civilizations Civilizations began where farming was most productive. When farm productivity declined, usually as a result of soil mismanagement, civilizations also declined - and occasionally vanished entirely. Of the three requisites for a thriving civilization: fertile soil, a dependable water supply and relatively level land with reasonable rainfall which would not cause erosion, it is likely that the third factor was most important, and evidence is mounting that soil degradation has toppled civilizations as surely as military conquest. In countries bordering the Mediterranean, deforestation of slopes and the erosion that followed has created man-made deserts of once productive land. Ancient Romans ate well on produce from North African regions that are desert today. A recent study of the collapse in Guatemala around 900 AD of the 1700 year-old Mayan civilization suggests that it fell apart for similar reasons. Researchers have found evidence that population growth among the Mayans was followed by cutting trees on mountainsides to expand areas for farming. The soil erosion that resulted from growing crops on steeper and steeper slopes lowered soil productivity - both in the hills and in the valleys - to a point where the populations could no longer survive in that area.

Today only empty ruins remain. The same process of soil degradation which destroyed civilizations in the past are still at work today. Firstly, billions of tons of soil are being physically lost each year through accelerated erosion from the action of water and wind and by undesirable changes in soil structure. Secondly, many soils are being degraded by increases in their salt content, by waterlogging, or by pollution through the indiscriminate application of chemical and industrial wastes. Thirdly, many soils are losing the minerals and organic matter that make them fertile, and in most cases, these materials are not being replaced nearly as fast as they are being depleted. Finally, millions of hectares of good farmland are being lost each year to nonfarm purposes; they are being flooded for reservoirs or paved over for highways, airports, and parking lots. The result of all this mismanagement will be less productive agricultural land at a time when world population is growing and expectations are rising among people everywhere for a better life. The worst threat is erosion The most serious form of soil degradation is from accelerated erosion. Erosion is the washing or blowing away of surface soil, sometimes down to bedrock.

While some erosion takes place without the influence of man, the soil is lost so slowly that it is usually replaced through natural processes of decay and regeneration. Soil loss and soil creation of new soil stay in balance. What keeps soil in a natural state from eroding is vegetation. Undisturbed by man, soil is usually covered by a canopy of shrubs and trees, by dead and decaying leaves or by a thick mat of grass. Whatever the vegetation, it protects the soil when the rain falls or the wind blows. The leaves and branches of trees and the cushion of grass absorb the force of raindrops, and root systems of plants hold the soil together. Even in drought, the roots of native grasses, which extend several metres into the ground, help tie down the soil and keep it from blowing away. With its covering of vegetation stripped away, however, soil is as vulnerable to damage as a tortoise without its shell. Whether the plant cover is disturbed by cultivation, grazing, burning, or bulldozing, once the soil is laid bare to the erosive action of wind and water, the slow rate of natural erosion is greatly accelerated.

Losses of soil take place much faster than new soil can be created, and a kind of deficit spending begins with the topsoil. Bad farming encourages soil loss Unfortunately, many bad farming and forestry operations encourage erosion. Erosion accelerates when sloping land is ploughed and when grass is removed from semi-arid land to begin dryland farming. It accelerates when cattle, sheep and goats are allowed to overgraze and when hillside forests are felled or cut indiscriminately. While there are isolated instances of deserts being reclaimed by irrigation or of new forests being planted, man, in the majority of instances, degrades the soil when he begins agricultural operations. And his highest risk operations are conducted on cropland, which is particularly prone to the hazards of soil erosion, especially if farming systems leave the land bare for part of the year, exposed to wind and water. The mechanics of soil erosion are fairly well understood today by conservationists and by many farmers.

Erosion from water proceeds in three steps: (1) soil particles are loosened by the bomb-like impact of raindrops or the scouring action of runoff water; (2 ) the detached particles are moved down the slopes by flowing water; and (3) the soil particles are deposited at new locations, either on top of other soil at the bottom of the slope or in ponds or waterways. The soil washed downhill is usually the most fertile, containing most of the nutrients and organic matter required for normal plant growth. All other things being equal, the steeper the slope, the greater the soil erosion. Erosion is also more severe on long slopes than on short ones; the velocity of the water flow increases on long, unobstructed downhill stretches. Soil loss may be half again as great when the slope length is doubled. Also significant is the shape of the slope. A convex or bulging slope loses more soil than a uniform slope. A concave or dish-shaped slope loses less. Many erodible soils also seal off the surface pores of the soil as they travel downhill with the runoff water. This action further decreases the amount of water that can be absorbed by the soil and increases the water's velocity, causing even more erosion.

Rainfall energy varies Still another factor in soil erosion from water is the erosivity of the rain its intensity and duration. In many parts of Europe, where rains are relatively gentle, erosion is rarely severe. In most tropical countries and in parts of the United States, however, rains are much more intense and occasionally torrential. Much more rain falls per hour, and as rainfall intensity increases, the size of individual raindrops also increases. A tropical raindrop strikes unprotected soil with more force than raindrops in Europe, dislodging more soil. The flow of water down a slope is also greater, and the net result is more soil eroded and moved downhill. Time is also a factor in erosion. A hard continuous rain will dislodge more soil than several brief showers, particularly when the soils are relatively impermeable. Season is a factor, too. The monsoon rain in the Indian subcontinent keeps farmers from planting many soils, and the bare fields are subject to serious soil erosion. In the Corn Belt of the USA, spring rains are usually the heaviest of the year, striking the soil before seed can be planted or when seedlings can be easily washed out. Why some soils erode easily Another factor in water erosion is the character of the soil itself.

Some soils tend to erode easily from the action of rain and runoff; others are remarkably resistant, even in heavy downpours. The susceptibility of different kinds of soils to erosion under cultivation varies widely. Perhaps the most important factor is the relative ability of the soil to absorb rainfall rapidly. Certain soils of the tropics absorb rainfall so rapidly that there is little erosion, even on steep slopes. On the other hand, some erodible tropical soils require very little energy to disintegrate under the impact of raindrops. One reason for the instability of many tropical soils is the predominance of coarse particles, which are easily detached by the pounding action of the rain. The finer particles are then washed off the field with the runoff water. A number of the world's most erodible soils have a topsoil layer that is from 10 to 40 centimetres deep, underlain by a layer of subsoil that is barely permeable by water. After the upper layer of soil becomes saturated by rain, it begins to flow downhill, even on gentle slopes. What makes one soil subject to erosion and another relatively impervious is a complex matter. There is no single cause for erodibility.

But without question, the organic matter in the soil - decayed and decaying plant and animal matter - helps protect it from washing. Organic soils soak up water Organic matter in soil can absorb and store much more water than can inorganic fractions. It acts like a sponge, taking up water and releasing it as required by plants. It also helps bind soil particles into larger aggregates, or crumbs. Soils with this kind of structure are very resistant to erosion. Conversely, nearly all soils containing little or no organic matter are very susceptible to erosion. Besides absorbing water readily, a good cropland soil should be able to dry out or warm up quickly when the rain is over. It should hold enough moisture to supply the needs of a crop between rains, yet permit water to pass through the soil. A good soil will not stay too wet or too dry. Another factor in erosion from water is the crop that is being grown in the soil and the way that crop is being managed. Sloping land planted with trees or grass will erode less than the same land planted with maize or soybeans. Maize planted on terraces will suffer less erosion than maize planted in rows that march straight down the slope, inviting runoff water to rush downhill between the rows.

There are other, less obvious relationships between soil erosion and crop selection and management. Many soils can be planted with maize without much erosion risk if the maize crop is rotated with legumes and small grains. If maize is planted year after year, however, soil losses begin to mount. The basic factors then that contribute to soil erosion from water in rainfed agriculture appear to be similar the world over. For any particular plot of land, they include the degree of slope, the length of slope and its shape, the erosivity of the rain and inherent erodibility of the soil, and the mismanagement of the land by the farmer or herdsman. Much more remains to be learned, however, about the management of specific soils in tropical and subtropical areas to reduce the impact of these erosion factors. "Invisible" erosion takes toll There are several types of man-made erosion, all but the first clearly recognizable as trouble. The first - and most insidious - is sheet erosion, which is the more or less even removal of a thin layer or "sheet" of soil from a sloping field. It is insidious because the amount of soil seen to be removed is usually so small in any given year that a farmer often fails to notice that erosion is occurring.

Occasionally he becomes aware of sheet erosion only after he notices that a formerly buried object - a rock, the lower portion of a fence post, or root of a tree - is suddenly exposed. However, sheet erosion removes great quantities of topsoil. Even a very thin layer of soil, only slightly thicker than a piece of wrapping paper, when transported down a slope, can weigh several tons per hectare. It does not take many years or many rainstorms for losses from sheet erosion to become significant. A second variety of erosion is more evident to the farmer, and that is "rill" erosion. Sheet erosion occurs mainly when the surface of a field is smooth and the slope is uniform. But the surface of most fields is irregular. There are apt to be low places and high places; rough places and smooth places; and various kinds of soils, even in a 5 hectare field. When it rains, the soil erodes unevenly, and rainwater accumulates and flows into depressions, taking the path of least resistance as it moves downhill. The surface flow moves into small channels, or rills, which are cut into the soil several inches deep.

Rills are small enough to be erased easily with normal tillage methods, but left alone, they can become progressively wider and deeper until they cut into the subsoil and form gullies. Gullies climb uphill A gully always begins at the lower end of a slope and eats its way back uphill, where it creates a gully head with a sudden or steep fall. Eventually it will work its way to the top of the slope, growing deeper and wider with each rainstorm. The splash action of the falling water at the head of the gully undermines the lower part of the excavated earth wall, causing collapse of even more of the soil. Unlike a rill, a gully cannot be smoothed out with a plough or a disk. While a new gully may be narrow and 2 or 3 feet deep, older gullies can grow to enormous size - 40 feet deep and as much as 100 feet wide. The formation of gullies is frequently encouraged by man and his animals. Many gullies begin with stock trails, farm roads, and other regular or irregular pathways on sloping land.

Some large gullies develop tributaries, particularly at points where livestock habitually enter and leave a ravine. A recent study of the development in the 19th century of severe gully erosion at the head of a creek in New South Wales, Australia, revealed that it began during periods of cultivation and overgrazing and, not incidentally, during the years of the highest rabbit populations. These animals, like many insects, can speed up destruction of vegetation and soil erosion. Gullies are relentless destroyers of good farmland. They can cut up a field into small, odd-shaped parcels and restrict the free movement of animals and farm machinery. They are a menace to livestock; calves and other animals frequently fall in and are unable to escape. Gullies can also threaten nearby barns and other buildings, which may have to be moved before they are undermined. The stabilization and repair of gullies is the most costly of all erosion control work. Stopping a gully often requires extensive earthmoving and construction of dams or other measures. On the other hand, the formation of gullies can usually be prevented through good land use.

How erosion reduces yields For the farmer, and for the consumer as well, the worst thing about soil erosion is that it reduces crop yields and increases the costs of growing food and fibre. Firstly, erosion reduces the capacity of the soil to hold water and make that water available to plants. This subjects crops to more frequent and severe water stress. Secondly, erosion contributes to losses of plant nutrients, which wash away with the soil particles. Because subsoils generally contain fewer nutrients than topsoils, more fertilizer is needed to maintain crop yields. This, in turn, increases production costs. Moreover, the addition of fertilizer alone cannot compensate for all the nutrients lost when topsoil erodes. Thirdly, erosion reduces yields by degrading soil structure, increasing soil erodibility, surface sealing and crusting. Water infiltration is reduced, and seedlings have a harder time breaking through the soil crust. Fourthly, erosion reduces productivity because it does not remove topsoil uniformly over the surface of a field. Typically, parts of an eroded field still have several inches of topsoil left; other parts may be eroded down to the subsoil.

This makes it practically impossible for a farmer to manage the field properly, to apply fertilizers and chemicals uniformly and obtain uniform results. He is also unable to time his planting, since an eroded part of the field may be too wet when the rest of the field is dry and ready. Eroding soil affects water resources Damage from water erosion is not limited to the loss of productivity on the land where it occurs. The bulk of eroded soil from a hillside comes to rest a short distance away, at the foot of the slope or on a nearby flood plain, where it may bury crops or lower the fertility of bottomlands. A portion of the eroded soil is deposited in local drainage or irrigation ditches or runs into ponds, reservoirs, or tributary streams and rivers. Wherever it is deposited, it is unwelcome. Sediment-filled ditches have to be dug out again; ponds, lakes, and reservoirs either have to be dredged out or abandoned. Locally, sediment is an expensive nuisance . Damage also occurs downstream, sometimes at great distances from the farmland that originally contributed the sediment. Carried along by a river, sediment is dropped out as the waterway reaches flatter, lower reaches.

The sediment deposits raise the level of the riverbed and reduce the capacity of the channel to hold water. Riverbanks overtop more frequently, and valuable bottomland, often extremely productive, is damaged by flooding. Windblown soil endangers land Soil blown by wind is second only to erosion by water as a destroyer of agricultural land. It occurs most often in arid and semi-arid regions, but it can also happen in areas of seasonal rainfall. Wind erosion is a persistent hazard in the Sahara and Kalahari deserts of Africa; in Central Asia, particularly in the Steppes of the Soviet Union; in central Australia, and in the Great Plains of the United States, well known as the Dust Bowl of the 1930s. Windborne topsoil may be transported over very long distances and, like soil eroded by water, it is usually deposited where it is not wanted. Farmlands, fences, machinery, and buildings can be severely damaged by wind erosion, and sometimes they can be buried completely. Costs of rehabilitation can run so high that the land is abandoned.

The following conditions set the stage for erosion from wind: the soil is loose, dry, and finely divided; the soil surface is relatively smooth and plant cover is sparse; and the field is sufficiently large and the wind strong enough to initiate air movement. When the wind blows hard over a smooth field, at some point near the surface the wind velocity will be zero. Above that point there is a layer of smooth airflow, and above that, an area of turbulence. It is this turbulent airflow which causes soil particles to begin to move. Once movement is begun, the soil particles themselves abrade the soil surface and magnify the effect of the wind. In a severe storm, dust clouds rise hundreds of metres into the air, and on occasion travel hundreds, even thousands, of kilometres before the eroded soil falls on the land or into the ocean. The soil particles that are blown away are usually the finer ones; the coarse and heavy sand remains. If this process continues for long, the productivity of the damaged land gradually decreases. The physical causes of wind erosion are clearly different from those which allow soil to wash, except for one factor that is constant in all man-made soil erosion - the absence of vegetation to hold and cover the soil.

It is when trees, bushes, grasses, and other plants are removed from land that erosion occurs. Poor management degrades soil Soil does not have to be washed or blown away for its productivity to be lowered. Through improper soil and water management, a soil's properties may be altered so that its fertility is seriously reduced or lost for good. Excessive cultivation, for example, can wreck the structure of some soils so that they are no longer capable of holding enough moisture for growing plants. Salinization, or the accumulation of salts in the topsoil, can also have a deletrious effect on soil productivity and crop yields. In extreme cases, damage from salinization is so great that it is technically unfeasible or totally uneconomic to reverse the process. In general, salinization is caused by water and dissolved salts moving up in the soil through capillary action. While salinization is occasionally the result of natural soil-forming processes, it occurs most frequently in irrigated soils, where it is worsened by the high salt content of irrigation water. Salt-affected soils are found on every continent and nearly 7 percent of the land area of the world is affected.

Salinization is a serious problem in Australia, the Soviet Union, and the United States, and it is critical in countries of north Africa and the Near East. Soils degrade through waterlogging and loss of nutrients Waterlogged soils also deter agriculture in many countries, even in parts of the world where an excess of water is not usually thought of as a problem. Waterlogging interferes with agriculture in many countries; in Egypt, for example, where about one-third of the Nile Delta has a water table only 80 centimetres below the surface. Other countries with waterlogging from high water tables and runoff include Iran, Iraq, Somalia, parts of Syria, and Pakistan. Soil can also become degraded through loss of nutrients - chiefly nitrogen, phosphorus, and potassium - if these are not replenished to maintain soil fertility. Besides being lost through erosion, nutrients are also depleted by the crops themselves, particularly if the same crops are grown on the same land year after year. And in the humid tropics, many nutrients are leached during the intense rainstorms, especially on unprotected land. Without question, farming all over the world is removing more nutrients from the soil than are being put back. Soil compaction is still another destroyer of the soil.

Sometimes it results from repeated passes over the same field with heavy machinery, particularly when the field is wet. It can also result from the hooves of grazing animals pounding down the soil too often in the same area, as they do around the only waterhole for miles. Compaction is not easy to correct. Some countries poison soils Other forms of soil degradation occur in the more developed countries, but are rarely of concern to the developing ones - so far. Farmland is not only paved over by urbanization but is occasionally poisoned with chemicals. While pesticides and even fertilizers are sometimes suspected of causing soil impairment, the damage in most cases is not permanent. However, some apple orchards sprayed with arsenic compounds in the 1930s were reported as still unproductive 30 years later. In recent years, there has been a general movement in many developed countries against using the more persistent insecticides, including a chemical group that includes DDT and chlordane. Radioactive fallout, and Strontium 90 in particular, also caused public concern during the period of nuclear bomb tests.

Today a more serious problem in several highly industrialized countries is the indiscriminate dumping of chemical wastes, some of which are extremely toxic to plants, animals, and man, and the growing use of sewage sludge, some of which contains dangerous heavy metals which can be taken up by plants. For a developing nation, however, such problems are at present insignificant compared with the growing threat to their agricultural productivity from erosion, salinization, waterlogging, and general loss of fertility. Soil and Water Soil and Water There is an important link between soils and water, the quality of our water being closely linked to the quality of the soils. Water in the soil supplies plants and animals, but acts also to transport nutrients and other important substances from the soil to plants, microbes etc. However, too much water can have damaging consequences. For example excessive rainfall can result in problems like soil erosion, and can waterlog plants and vegetation thus depriving their roots of air. Thankfully, certain properties of soil can reduce the possibility of this happening. There are several stages in the interaction of water with soil.

Firstly, it enters the soil through a process known as infiltration – the higher the rate of infiltration, the more water is soaked up by the soil, and will be available to plants. Also, less water will run off the surface, eroding the soil, and washing away nutrients. Secondly, water is stored in the soil and released when required by plants. Following this, drainage (percolation) of the water occurs when there is too much for the soil to cope with – the excess water drains freely out of the soil, taking with it dissolved and suspended material in a process, known as leaching. Soils can vary greatly in their ability to perform these functions. Mineral Resources Mineral Resources Mineral Resources Almost all Earth materials are used by humans for something. We require metals for making machines, sands and gravels for making roads and buildings, sand for making computer chips, limestone and gypsum for making concrete, clays for making ceramics, gold, silver, copper and aluminum for making electric circuits, and diamonds and corundum (sapphire, ruby, emerald) for abrasives and jewelry. In this discussion, we hope to answer the following questions: What constitutes a mineral resource and an ore?

What determines whether or not a mineral sources is economical to exploit? By what processes do ores form? How are mineral resources found and exploited? What happens when a mineral resource become scarce as a result of human consumption? What are the adverse effects of exploiting mineral resource. Mineral resources can be divided into two major categories - Metallic and Nonmetallic. Metallic resources are things like Gold, Silver, Tin, Copper, Lead, Zinc, Iron, Nickel, Chromium, and Aluminum. Nonmetallic resources are things like sand, gravel, gypsum, halite, Uranium, dimension stone. A mineral resource is a volume of rock enriched in one or more useful materials. In this sense a mineral refers to a useful material, a definition that is different from the way we defined a mineral back in Chapter 5. Here the word mineral can be any substance that comes from the Earth. Finding and exploiting mineral resources requires the application of the principles of geology that you we have discussed or will discuss throughout this course. Some minerals are used as they are found in the ground, i.e. they require no further processing or very little processing. For example - gemstones, sand, gravel, and salt (halite). Most minerals must be processed before they are used.

For example: Iron is the found in abundance in minerals, but the process of extracting iron from different minerals varies in cost depending on the mineral. It is least costly to extract the iron from oxide minerals like hematite (Fe2O3), magnetite (Fe3O4), or limonite [Fe(OH)]. Although iron also occurs in olivines, pyroxenes, amphiboles, and biotite, the concentration of iron in these minerals is less, and cost of extraction is increased because strong bonds between iron, silicon, and oxygen must be broken. Aluminum is the third most abundant mineral in the Earth's crust. It occurs in the most common minerals of the crust - the feldspars (NaAlSi3O8, KalSi3O8, & CaAl2Si2O8, but the cost of extracting the Aluminum from these minerals is high. Thus, deposits containing the mineral gibbsite [Al(OH)3], are usually sought. This explains why recycling of Aluminum cans is cost effective, since the Aluminum in the cans does not have to be separated from oxygen or silicon.

Because such things as extraction costs, labor costs, and energy costs vary with time and from country to country, what constitutes an economically viable deposit of minerals varies considerably in time and place. In general, the higher the concentration of the substance, the more economical it is to mine. Thus we define an ore as a body of material from which one or more valuable substances can be extracted economically. An ore deposit will consist of ore minerals, that contain the valuable substance. Gangue minerals are minerals that occur in the deposit but do not contain the valuable substance. Since economics is what controls the grade or concentration of the substance in a deposit that makes the deposit profitable to mine, different substances require different concentrations to be profitable. But, the concentration that can be economically mined changes due to economic conditions such as demand for the substance and the cost of extraction. Examples: The copper concentration in copper ore deposits has shown changes throughout history. From 1880 to about 1960 the grade of copper ore showed a steady decrease from about 3% to less than 1%, mainly due to increased efficiency of mining.

From about 1960 to 1980 the grade increased to over 1% due to increasing costs of energy and an abundant supply produced by cheaper labor in other countries. Gold prices vary on a daily basis. When gold prices are high, old abandoned mines re-open, when the price drops, gold mines close. The cost of labor is currently so high in the U.S. that few gold mines can operate profitably, but in third world countries where labor costs are lower, gold mines that have ore concentrations well below those found in the U.S. can operate with a profit. For every substance we can determine the concentration necessary in a mineral deposit for profitable mining. By dividing this economical concentration by the average crustal abundance for that substance, we can determine a value called the concentration factor. The table below lists average crustal abundances and concentration factors for some of the important materials that are commonly sought. For example, Al, which has an average crustal abundance of 8%, has a concentration factor of 3 to 4. This means that an economic deposit of Aluminum must contain between 3 and 4 times the average crustal abundance, that is between 24 and 32% Aluminum, to be economical.

0.00016% 500 to 1000 Note that we will not likely ever run out of a useful substance, since we can always find deposits of any substance that have lower concentrations than are currently economical. If the supply of currently economical deposits is reduced, the price will increase and the concentration factor will increase. Origin of Mineral Resources Mineral deposits can be classified on the basis of the mechanism responsible for concentrating the valuable substance. Magmatic Ore Deposits - substances are concentrated within a body of igneous rock by magmatic processes like crystal fractionation and crystal settling. Magmatic process such as partial melting, crystal fractionation, or crystal settling in a magma chamber can concentrate ore minerals containing valuable substances by taking elements that were once widely dispersed in low concentrations in the magma and concentrating them in minerals that separate from the magma. Examples: Pegmatites - During fractional crystallization water and elements that do not enter the minerals separated from the magma by crystallization will end up as the last residue of the original magma.

This residue is rich in silica and water along with elements like the Rare Earth Elements (many of which are important for making phosphors in color television picture tubes), Lithium, Tantalum, Niobium, Boron, Beryllium, Gold, and Uranium. This residue is often injected into fractures surrounding the igneous intrusion and crystallizes as a rock called a pegmatite that characteristically consists of large crystals. Crystal Settling. As minerals crystallize from a magma body, heavy minerals may sink to the bottom of the magma chamber. Such heavy minerals as chromite, olivine, and ilmenite contain high concentrations of Chromium, Titanium, Platinum, Nickel, and Iron. These elements thus attain higher concentrations in the layers that form on the bottom of the magma chamber. Hydrothermal Ore Deposits - Concentration by hot aqueous (water-rich) fluids flowing through fractures and pore spaces in rocks. Hydrothermal deposits are produced when groundwater circulates to depth and heats up either by coming near a hot igneous body at depth or by circulating to great depth along the geothermal gradient. Such hot water can dissolve valuable substances throughout a large volume of rock.

As the hot water moves into cooler areas of the crust, the dissolved substances are precipitated from the hot water solution. If the cooling takes place rapidly, such as might occur in open fractures or upon reaching a body of cool surface water, then precipitation will take place over a limited area, resulting in a concentration of the substance attaining a higher value than was originally present in the rocks through which the water passed. Examples: Massive sulfide deposits at oceanic spreading centers. Hot fluids circulating above the magma chambers at oceanic ridges can scavenge elements like Sulfur, Copper, and Zinc from the rocks through which they pass. As these hot fluids migrate back toward the seafloor, they come in contact with cold groundwater or sea water and suddenly precipitate these metals as sulfide minerals like sphalerite (zinc sulfide) and chalcopyrite (Copper, Iron sulfide). Vein deposits surrounding igneous intrusions. Hot water circulating around igneous intrusions scavenges metals and silica from both the intrusions and the surrounding rock. When these fluids are injected into open fractures, they cool rapidly and precipitate mainly quartz, but also a variety of sulfide minerals, and sometimes gold, and silver within the veins of quartz.

Rich deposits of copper, zinc, lead, gold, silver, tin, mercury, and molybdenum result. Stratabound ore deposits in lake or oceanic sediments. When hot groundwater containing valuable metals scavenged along their flow paths enters unconsolidated sediments on the bottom of a lake or ocean, it may precipitate ore minerals in the pore spaces between grains in the sediment. Such minerals may contain high concentrations of lead, zinc, and copper, usually in sulfide minerals like galena (lead sulfide), sphalerite (zinc sulfide), and chalcopyrite (copper-iron sulfide). Since they are included within the sedimentary strata they are called stratabound mineral deposits. Sedimentary Ore Deposits - substances are concentrated by chemical precipitation from lake or sea water. Although clastic sedimentary processes can form mineral deposits, the term sedimentary mineral deposit is restricted to chemical sedimentation, where minerals containing valuable substances are precipitated directly out of water. Examples: Evaporite Deposits - Evaporation of lake water or sea water results in the loss of water and thus concentrates dissolved substances in the remaining water. When the water becomes saturated in such dissolved substance they precipitate from the water.

Deposits of halite (table salt), gypsum (used in plaster and wall board), borax (used in soap), and sylvite (potassium chloride, from which potassium is extracted to use in fertilizers) result from this process. Iron Formations - These deposits are of iron rich chert and a number of other iron bearing minerals that were deposited in basins within continental crust during the Proterozoic (2 billion years or older). They appear to be evaporite type deposits, but if so, the composition of sea water must have been drastically different than it is today. Placer Ore Deposits - substances are concentrated by flowing surface waters either in streams or along coastlines. The velocity of flowing water determines whether minerals are carried in suspension or deposited. When the velocity of the water slows, large minerals or minerals with a higher density are deposited. Heavy minerals like gold, diamond, and magnetite of the same size as a low density mineral like quartz will be deposited at a higher velocity than the quartz, thus the heavy minerals will be concentrated in areas where water current velocity is low. Mineral deposits formed in this way are called placer deposits.

They occur in any area where current velocity is low, such as in point bar deposits, between ripple marks, behind submerged bars, or in holes on the bottom of a stream. The California gold rush in 1849 began when someone discovered rich placer deposits of gold in streams draining the Sierra Nevada Mountains. The gold originally formed in hydrothermal veins, but it was eroded out of the veins and carried in streams where it was deposited in placer deposits. Residual Ore Deposits - substances are concentrated by chemical weathering processes. During chemical weathering and original body of rock is greatly reduced in volume by the process of leaching, which removes ions from the original rock. Elements that are not leached form the rock thus occur in higher concentration in the residual rock. The most important ore of Aluminum, bauxite, forms in tropical climates where high temperatures and high water throughput during chemical weathering produces highly leached lateritic soils rich in both iron and aluminum. Most bauxite deposits are relatively young because they form near the surface of the Earth and are easily removed by erosion acting over long periods of time. In addition, an existing mineral deposit can be turned in to a more highly concentrated mineral deposit by weathering in a process called secondary enrichment.

Mineral Deposits and Plate Tectonics Because different types of mineral deposits form in different environments, plate tectonics plays a critical role in the location of different geological environments. The diagram to the right shows the different mineral deposits that occur in different tectonic environments. Mineral Exploration and Production Ores are located by evidence of metal enrichment. Geologists look for hints in rocks exposed near the surface, for example, the enrichment process often results in discoloration of the soil and rock. When such hints are found, geophysical survey's involving measuring gravity, magnetism, or radioactivity are conducted. Geochemical surveys are conducted which analyze the composition of water, sediment, soil, rocks, and sometimes even plants and trees. Once it is determined that a valuable material could be present, the deposit is assessed by conducting core drilling to collect subsurface samples, followed by chemical analysis of the samples to determine the grade of the ore If the samples show promise of being economic to mine, then plans are made to determine how it will me mined. If the ore body is within 100 meters from the surface, open-pit mines, large excavations open to the air.are used to extract the ore before processing.

Open pit mines are less expensive and less dangerous than tunnel mines, although they do leave large scars on the land surface. If the ore body is deeper, or narrowly dispersed within the non-ore bearing rock tunneling is necessary to extract the ore from underground mines. Mine tunnels are linked to a vertical shaft, called and adit. Ores are removed from the walls of the tunnels by drilling and blasting, with the excavated ores being hauled to the surface from processing. Underground mines are both more expensive and dangerous than open pit mines and still leave scares on the landscape where non-ore bearing rock is discarded as tailings. . Global Mineral Needs Because the processes that form ores operate on geologic time scales, the most economic mineral resources are essentially nonrenewable.New deposits cannot be generated in human timescales. But, as mentioned previously, as the reserves of materials become depleted it is possible to find other sources that are more costly to exploit. Furthermore, mineral resources are not evenly distributed. Some countries are mineral-rich; some are mineral-poor. This is a particular issue for strategic mineral resources.

These strategic metals are those for which economical source do not exist in the U.S., must be imported from other potentially non-friendly nations, but are needed for highly specialized applications such as national security, defense, or aerospace applications. These metals include, Manganese, Cobalt, Platinum, and Chromium, all of which are stockpiled by the U.S. government in case supplies are cut off. How long current mineral resources will last depends on consumption rates and reserve amounts. Some mineral resources will run out soon, for example global resources of Pb, Zn, and Au? will likely run out in about 30 years. U.S. resources of Pt, Ni, Co, Mn, Cr less than 1 year. Thus, continued use of scarce minerals will require discovery of new sources, increase in price to make hard-to-obtain sources more profitable, increased efficiency, conservation, or recycling, substitution of new materials, or doing without. Environmental Issues Extraction and processing has large environmental impacts in terms of such things as air quality, surface water quality, groundwater quality, soils, vegetation, and aesthetics. Acid mine drainage is one example, Sulfide minerals newly exposed to Oxygen and water near the surface create sulfuric acid.

Rainwater falling on the mine tailings becomes acidified and can create toxic conditions in the runoff. This can mobilize potentially dangerous heavy metals and kill organisms in the streams draining the tailings. Questions on this material that could be asked on an exam. Define the following: (a) ore, (b) concentration factor, (c) secondary enrichment, (d) strategic metal, (e) acid mine drainage. Explain how each of the following types of ore deposits form and give examples of each (a) magmatic ore deposits, (b) hydrothermal ore deposits, (c) sedimentary ore deposits, (d) residual ore deposits, (e) placer ore deposits. What techniques are used to find ore deposits. Explain why such things seemingly common things such as sand, gravel, limestone, and gypsum can also be considered mineral resources. What are the adverse effects of the exploitation of mineral resources. What is leaching? definition and meaning - BusinessDictionary.com chemical oxygen... Use 'leaching' in a Sentence The corporation's lawyers argued that its toxic waste disposal practices were safe and environmentally friendly but conservationists produced evidence that clearly showed dangerous chemicals leaching out from contaminated soil beds.

18 people found this helpful In Chemistry class today, we went over leaching which made me and my friend bored because we hated everything about it. 16 people found this helpful When running an industrial plant you must be able to be good at leaching out the bad substances and getting rid of them effectively. 14 people found this helpful

Question: What is the name of the process by which substances are washed out of the soil?
Answer: Leaching