Stratigraphy: Making sense of chaos

Stratigraphy: Making sense of chaos

kacimo samy   10:42 م

What is Stratigraphy?

Stratigraphy- The branch of geology that seeks to understand the geometric relationships between different rock layers (called strata), and to interpret the history represented by these rock layers.

Public Domain Image by the US Dept. of Interior.

Contact- A boundary that separates different strata or rock units.

Steno's Laws of Stratigraphy

Image from J. P. Trap: berømte danske mænd og kvinder, 1868

Nicholas Steno (1638-1686) was a Danish-born pioneer of geology, and is considered to be the father of stratigraphy.

Nicholas Steno's observations of rocks layers suggested that geology is not totally chaotic.  Rather, the rock layers preserve a chronological record of Earth history and past life.

He developed three fundamental principles of stratigraphy, now known as Steno's Laws:

1) Law of Original Horizontality– Beds of sediment deposited in water form as horizontal (or nearly horizontal) layers due to gravitational settling.

2) Law of Superposition– In undisturbed strata, the oldest layer lies at the bottom and the youngest layer lies at the top.

3) Law of Lateral Continuity– Horizontal strata extend laterally until they thin to zero thickness (pinch out) at the edge of their basin of deposition.

Other Important Principles of Stratigraphy

4) Law of Cross-Cutting Relationships– An event that cuts across existing rock is younger than that disturbed rock.  This law was developed by Charles Lyell (1797-1875).

5) Principle of Inclusion– Fragments of rock that are contained (or included) within a host rock are older than the host rock.

Unconformities

Unconformity – A surface that represents a very significant gap in the geologic rock record (due to erosion or long periods of non-deposition).

There are 3 main types of unconformities:

1) Disconformity – A contact representing missing rock between sedimentary layers that are parallel to each other.  Since disconformities are parallel to bedding planes, they are difficult to see in nature.

2) Angular Unconformity – A contact in which younger strata overlie an erosional surface on tilted or folded rock layers.  This type of unconformity is easy to identify in nature.

Image provided by FCIT. Original image from Textbook of Geology by Sir Archibald Geikie (1893).

3) Nonconformity – A contact in which an erosion surface on plutonic or metamorphic rock has been covered by younger sedimentary or volcanic rock.

4) Paraconformity- A contact between parallel layers formed by extended periods of non-deposition (as opposed to being formed by erosion).  These are sometimes called "pseudo unconformities").

Unconformities VS Bedding Planes

Unconformities represent huge gaps in time!  The nonconformity between the Vishnu Schist and overlying sedimentary layers in the Grand Canyon represents 1.3 billion years of missing rock record.

Bedding planes, or planes separating adjacent sedimentary layers, also represent gaps in the rock record but on a much smaller scale than an unconformity.

Relative Age Dating

Relative age dating is a way to use geometric relationships between rock bodies to determine the sequence of geologic events in an area.  Relative dating is different from absolute dating in which specific dates are assigned to geologic events (we will discuss absolute dating techniques later).

Relative dating is based on the five principles of stratigraphy discussed above.

Historical Perspective on the Origin of Rocks: Werner's Concept of Neptunism

Abraham Werner (1749-1817), a German geologist, proposed that Earth’s crust originated in ocean water through the process of precipitation.  This idea became known as Neptunism, in reference to the Roman God of the sea.

Werner classified rocks into 4 categories, as shown in the diagram below:

Figure by RJR

1. Primitive rock (red)– Granite and metamorphic rock were precipitated from oceans.

2. Transition rock (light brown)– Next, fossil-rich sedimentary rocks were precipitated.  These rocks are tilted due to deposition on the non-horizontal surfaces of primitive rocks.  This aspect of Werner's model was useful for explaining the origin of tilted sedimentary rocks.

3. Secondary rock (dark brown)– Flat lying sedimentary rocks were eventually precipitated.  The secondary rocks were thought to include interlayered basalts, which Werner thought formed by combustion of buried coal layers.

4. Tertiary (or alluvial) rock (yellow)– Finally, after the ocean receded, recent erosion and deposition created a thin veneer of overlying sediment.

Today we know that Werner's basic assumption that granite precipitated from seawater is incorrect.  We also know that basalt is not the product of coal combustion.

Nevertheless, Werner's concept of Neptunism was influential because:

1) Werener was right that some sedimentary rocks, such as limestones, do precipitate from ocean water.

2) Werner was not a catastrophist and did not need to make his interpretation of rock layers consistent with scriptual teachings.

3) Werner’s relative age assignments represents an early attempt to determine Earth's sequential history.

Historical Perspective on the Origin of Rocks: Hutton's Concept of Plutonism

The Scottish geologist James Hutton (1726-1797) argued that granite and basalt by solidification within the earth (as opposed to precipitating in from oceanwater).  This idea is known as Plutonism, in reference to the God of the deep underworld.

This concept of plutonism was supported by basalt melting/cooling experiments Sir James Hall conducted in 1792.  These experiments showed that the basalts form by the solidification of liquid magma.

Hutton viewed tilted strata as having been initially deposited horizontally, and then were subsequently deformed (tilted and folded) by the forces of Earth's internal heat engine.  He would argue that these forces gave rise to mountains.

Furthermore, he suggested that the mountains eroded to produce the sedimentary rocks we find in the rock record.

Hutton viewed the earth continually recycling itself with a balance between destruction and rejuvenation.  Mountains are created, eroded, and reformed.

Hutton’s ideas were not well received by people in the early 1800’s because he was a poor writer, and because his science was anti-catastrophic and did not support the scriptures.

Mass Flow

Mass Flow

kacimo samy   11:48 م

What is Mass flow?

Mixtures of detritus and fluid that move under gravity are known collectively as mass flows, gravity flows or density currents. A number of different mechanisms are involved and all require a slope to provide the potential energy to drive the flow. This slope may be the surface over which the flow occurs, but a gravity flow will also move on a horizontal surface if it thins downflow, in which case the potential energy is provided by the difference in height between the tops of the upstream and the downstream parts of the flow.

Debris flows

Debris flows are dense, viscous mixtures of sediment and water in which the volume and mass of sediment exceeds that of water. A dense, viscous mixture of this sort will typically have a low Reynolds number so the flow is likely to be laminar. In the absence of turbulence no dynamic sorting of material into different sizes occurs during flow and the resulting deposit is very poorly sorted. Some sorting may develop by slow settling and locally there may be reverse grading produced by shear at the bed boundary. Material of any size from clay to large boulders may be present. Debris flows occur on land, principally in arid environments where water supply is sparse (such as some alluvial fans) and in submarine environments where they transport material down continental slopes and locally on some coarse-grained delta slopes. Deposition occurs when internal friction becomes too great and the flow ‘freezes’. There may be little change in the thickness of the deposit in a proximal to distal direction and the clast size distribution may be the same throughout the deposit. The deposits of debris flows on land are typically matrix-supported conglomerates although clast-supported deposits also occur if the relative proportion of large clasts is high in the sediment mixture. They are poorly sorted and show a chaotic fabric, i.e. there is usually no preferred orientation to the clasts, except within zones of shearing that may form at the base of the flow. When a debris flow travels through water it may partly mix with it and the top part of the flow may become dilute. The tops of subaqueous debris flows are therefore characterised by a gradation up into better sorted, graded sediment, which may have the characteristics of a turbidite.

Turbidity currents

Turbidity currents are gravity-driven turbid mixtures of sediment temporarily suspended in water. They are less dense mixtures than debris flows and with a relatively high Reynolds number are usually turbulent flows. The name is derived from their characteristics of being opaque mixtures of sediment and water (turbid) and not the turbulent flow. They flow down slopes or over a horizontal surface provided that the thickness of the flow is greater upflow than it is downflow. The deposit of a turbidity current is a turbidite. The sediment mixture may contain gravel, sand and mud in concentrations as little as a few parts per thousand or up to 10% by weight: at the high concentrations the flows may not be turbulent and are not always referred to as turbidity currents. The volumes of material involved in a single flow event can be anything up to tens of cubic kilometres, which is spread out by the flow and deposited as a layer a few millimetres to tens of metres thick. Turbidity currents, and hence turbidites, can occur in water anywhere that there is a supply of sediment and a slope. They are common in deep lakes, and may occur on continental shelves, but are most abundant in deep marine environments, where turbidites are the dominant clastic deposit. The association with deep marine environments may lead to the assumption that all turbidites are deep marine deposits, but they are not an indicator of depth as turbidity currents are a process that can occur in shallow water as well. Sediment that is initially in suspension in the turbidity current starts to come into contact with the underlying surface where it may come to a halt or move by rolling and suspension. In doing so it comes out of suspension and the density of the flow is reduced. Flow in a turbidity current is maintained by the density contrast between the sediment-water mix and the water, and if this contrast is reduced, the flow slows down. At the head of the flow turbulent mixing of the current with the water dilutes the turbidity current and also reduces the density contrast. As more sediment is deposited from the decelerating flow a deposit accumulates and the flow eventually comes to a halt when the flow has spread out as a thin, even sheet.

Low- and medium-density turbidity currents

The first material to be deposited from a turbidity current will be the coarsest as this will fall out of suspension first. Therefore a turbidite is characteristically normally graded. Other sedimentary structures within the graded bed reflect the changing processes that occur during the flow and these vary according to the density of the initial mixture. Low- to medium-density turbidity currents will ideally form a succession known as a Bouma sequence, named after the geologist who first described them. Five divisions are recognised within the Bouma sequence, referred to as ‘a’ to ‘e’ divisions and annotated Ta, Tb and so on. 

• Ta: This lowest part consists of poorly sorted, structureless sand: on the scoured base deposition occurs rapidly from suspension with reduced turbulence inhibiting the formation of bedforms. 
• Tb: Laminated sand characterises this layer, the grain size is normally finer than in ‘a’ and the material is better sorted: the parallel laminae are generated by the separation of grains in upper flow regime transport.
• Tc: Cross-laminated medium to fine sand, sometimes with climbing ripple lamination, form the middle division of the Bouma sequence: these characteristics indicate moderate flow velocities within the ripple bedform stability field and high sedimentation rates. Convolute lamination can also occur in this division. 
• Td: Fine sand and silt in this layer are the products of waning flow in the turbidity current: horizontal laminae may occur but the lamination is commonly less well defined than in the ‘b’ layer. 
• Te: The top part of the turbidite consists of finegrained sediment of silt and clay grade: it is deposited from suspension after the turbidity current has come to rest and is therefore a hemipelagic deposit. 

Turbidity currents are waning flows, that is, they decrease velocity through time as they deposit material, but this means that they also decrease velocity with distance from the source. There is therefore a decrease in the grain size deposited with distance. The lower parts of the Bouma sequence are only present in the more proximal parts of the flow. With distance the lower divisions are progressively lost as the flow carries only finer sediment and only the ‘c’ to ‘e’ or perhaps just ‘d’ and ‘e’ parts of the Bouma sequence are deposited. In the more proximal regions the flow turbulence may be strong enough to cause scouring and completely remove the upper parts of a previously deposited bed. The ‘d’ and ‘e’ divisions may therefore be absent due to this erosion and the eroded sediment may be incorporated into the overlying deposit as mud clasts. The complete Ta to Te sequence is therefore only likely to occur in certain parts of the deposit, and even there intermediate divisions may be absent due, for example, to rapid deposition preventing ripple formation in Tc. Complete Ta-e Bouma sequences are in fact rather rare.

High-density turbidity currents

Under conditions where there is a higher density of material in the mixture the processes in the flow and hence of the characteristics of the deposit are different from those described above. High-density turbidity currents have a bulk density of at least 1.1 g/cm 3. The turbidites deposited by these flows have a thicker coarse unit at their base, which can be divided into three divisions. Divisions S1 and S2 are traction deposits of coarse material, with the upper part, S2, representing the ‘freezing’ of the traction flow. Overlying this is a unit, S3, that is characterised by fluid-escape structures indicating rapid deposition of sediment. The upper part of the succession is more similar to the Bouma Sequence, with Tt equivalent to Tb and Tc and overlain by Td and Te: this upper part therefore reflects deposition from a lower density flow once most of the sediment had already been deposited in the ‘S’ division. The characteristics of high-density turbidites were described by Lowe, after whom the succession is sometimes named.

Grain flows

Avalanches are mechanisms of mass transport down a steep slope, which are also known as grain flows. Particles in a grain flow are kept apart in the fluid medium by repeated grain to grain collisions and grain flows rapidly ‘freeze’ as soon as the kinetic energy of the particles falls below a critical value. This mechanism is most effective in well-sorted material falling under gravity down a steep slope such as the slip face of an aeolian dune. When the particles in the flow are in temporary suspension there is a tendency for the finer grains to fall between the coarser ones, a process known as kinetic sieving, which results in a slight reverse grading in the layer once it is deposited. Although most common on a small scale in sands, grain flows may also occur in coarser, gravelly material in a steep subaqueous setting such as the foreset of a Gilbert-type delta.

Earth axis has changed

Earth axis has changed

kacimo samy   2:03 م

The axis of rotation of Earth is the one that determines the season of our planet. Earth axis as believed previously is the 23.5 degrees which is known from a long time but it's not any more the same as before but has shifted not gradually. There is no reason to include for the shift but evidence can provide the shift. Earth’s Axis has modified where the evidence for it is right in front of us. The season has drifted more ahead. Few years back it, the winter start was from September with intense in December but not any more. The winter now has shifted from September to November with intense in January and February and yet is dragged a lot to April and May. There will in all probability be a lot of weather over future month. Possibly more into March and April. North east is being pounded with historic levels of “Snow hurricanes”. The sun that ought to never be farther north than the tropic of Cancer in middle Mexico or farther south than the Tropic of Capricorn in middle Australia is currently considerably on the far side those points. The sun is currently regarding 2000 miles too way north within the summer shining in our northern windows and too way south within the winter shining in their southern facing windows at sunrise and sunset. 

The orbit round the sun had probably altered the direction of the angle  observations and measurements of the abnormal position of the sun due to a big shift of our axis in Dec 2004 is currently verified.

However, it's additionally considerably shifted once more within the past year for the primary time since 2004. You can measure it for yourself when you see the sun position is too far north of your home in June (northern hemisphere) when it should never be north of the Tropic of Cancer and too far south in the winter.

Many people don't seem to be tuned in to true. You can look into the sun position and verify the weather changes.

Coastal Processes

Coastal Processes

kacimo samy   5:01 ص

Waves 

Waves that batter the coast are generated by offshore storms, sometimes thousands of kilometres from the shoreline where they will expend their energy. Wind blowing over the water produces friction along the air-water boundary. Since the air is moving much faster than the water, the moving air transfers some of its energy to the water, resulting in waves. The waves, in turn, eventually expend their energy at the shoreline. 

The size of the waves produced depends on the following: 

• The velocity, or speed, of the wind. The greater the wind velocity, the larger the waves. 
• The duration of the wind. Storms of longer duration have more time to impart energy to the water, producing larger waves. 
• The distance that the wind blows across the surface, or fetch. The longer the fetch, the larger the waves.

Within the area of the storm, the ocean waves have a variety of sizes and shapes, but, as they move away from their place of origin, they become sorted out into groups of similar waves. These groups of waves may travel for long distances across the ocean and arrive at distant shores with very little energy loss. The important parameters are wave height(H), which is the difference in height between the waves trough and its peak, and wave length (L), the distance between successive peaks. The wave period (T) is the time in seconds for successive waves to pass a reference point. If you were floating with a life preserver in deep water and could record your motion as waves moved through your area, you would find that you bob up, down, forward, and back in a circular orbit, returning to about the same place. If you were below the surface with a breathing apparatus, you would still move in circles, but the circle would be smaller. That is, you would move up, down, forward, and back in a circular orbit that would remain in the same place while the waves travelled through. When waves enter shallow water at a depth of less than about one-half their wavelength (L), they feel bottom. The circular orbits change to become ellipses; the motion at the bottom may be a very narrow ellipse, or essentially horizontal, that is, forward and back. You may have experienced this phenomenon if you have stood or have swum in relatively shallow water on a beach and felt the water repeatedly push you toward the shore and then back out toward the sea. The wave groups generated by storms far out at sea are called swell.As the swell enters shallower and shallower water, transformations take place that eventually lead to the waves breaking on the shore. For deep-water conditions, there are equations to predict wave height, period, and velocity, based on the fetch, wind velocity, and length of time that the wind blows over the water. This information has important environmental consequences: By predicting the velocity and height of the waves, we can estimate when waves with a particular erosive capability generated by a distant storm will strike the shoreline. We have said that waves expend their energy when they reach the coastline. But just how much energy are we talking about? The amount is surprisingly large. For example, the energy expended on a 400 km (250 mi) length of open coastline by waves with a height of about 1 m (3.3 ft) over a given period of time is approximately equivalent to the energy produced by one average-sized nuclear power plant over the same time period. Wave energy is approximately proportional to the square of the wave height. Thus, if wave height increases to 2 m (6.6 ft), the wave energy increases by a factor 4. If wave height increases to 5 m (16 ft), which is typical for large storms, then the energy expended, or wave power, increases 52, or 25 times over that of waves with a height of 1 m (3.3 ft). 

When waves enter the coastal zone and shallow water, they impinge on the bottom and become steeper. Wave steepness is the ratio of wave height to wave length. Waves are unstable when the wave height is greater than about 10 percent (0.1) of the wave length. As waves move into shallow water, the wave period remains constant, but wave length and velocity decrease and wave height increases. The waves change shape from the rounded crests and troughs in deep water to peaked crests with relatively flat troughs in shallow water close to shore. Perhaps the most dramatic feature of waves entering shallow water is their rapid increase in height. The height of waves in shallow water, where they break, may be as much as twice their deep-water height. Waves near the shoreline, just outside the surf zone, reach a wave steepness that is unstable. The instability causes the waves to break and expend their energy on the shoreline. Although wave heights offshore are relatively constant, the local wave height may increase or decrease when the wave front reaches the near-shore environment. This change can be attributed to irregularities in the offshore topography and the shape of the coastline. The offshore topography is similar to that of the coastline. As a wave front approaches the coastline, the shape of the front changes and becomes more parallel to the coastline. This change occurs because, as the waves enter shallow water, they slow down first where the water is shallowest, that is, off the rocky point. The result is a bending, or refraction, of the wave front. Owing to the bending of the wave fronts by refraction, there is a convergence of the wave normals at the headland, or rocky point, and a divergence of the wave normals at the beaches, or embayments. Where wave normals converge, wave height increases; as a result, wave energy expenditure at the shoreline also increases. The long-term effect of greater energy expenditure on protruding areas is that wave erosion tends to straighten the shoreline. The total energy from waves reaching a coastline during a particular time interval may be fairly constant, but there may be considerable local variability of energy expenditure when the waves break on the shoreline. In addition, breaking waves may peak up quickly and plunge or surge; or they may gently spill, depending on local conditions, such as the steepness of the shoreline and the height and length of waves arriving at the shoreline from a distant storm. Plunging breakers tend to be highly erosive at the shoreline, whereas spilling breakers are more gentle and may facilitate the deposition of sand on beaches. The large plunging breakers that occur during storms cause much of the coastal erosion we observe.

Beach Form and Beach Processes 

A beach is a land form consisting of loose material, such as sand or gravel, that has accumulated by wave action at the shoreline. Beaches may be composed of a variety of loose material in the shore zone, the composition of which depends on the environment. For example, many Pacific island beaches include broken bits of shell and coral; Hawaiis black sand beaches are composed of volcanic rock; and grains of quartz and feldspar are found on the beaches of southern California. The landward extension of the beach terminates at a natural topographic and morphologic change, such as a sea cliff or a line of sand dunes. The berms are flat back shore areas on beaches formed by deposition of sediment as waves rush up and expend the last of their energy. Berms are where you will find people sunbathing. The beach face is the sloping portion of the beach below the berm, and the part of the beach face that is exposed by the up rush and backwash of waves is called the swash zone. The surf zone is that portion of the seashore environment where turbulent translational waves move toward the shore after the incoming waves break; the breaker zone is the area where the incoming waves become unstable, peak, and break. The long shore trough and long shore bar are an elongated depression and adjacent ridge of sand produced by wave action. A particular beach, especially if it is wide and gently sloping, may have a series of long shore bars, long shore troughs, and breaker zones.

Transport of Sand 

The sand on beaches is not static; wave action keeps the sand moving along the beach in the surf and swash zones. A long shore current is produced by incoming waves striking the coast at an angle. Because the waves strike the coast at an angle, a component of wave energy is directed along the shore. If waves arrive at a beach perfectly parallel to the beach, then no long shore current is generated. The long shore current is a stream of water flowing parallel to the shore in the surf zone. This current can be surprisingly strong. If you are swimming on a beach and wading in and out of the surf zone, you may notice that the longer you go in and out through the surf zone, the further away you are from where you started and left your beach towel and umbrella. As you move in and out through the surf and swash zone, the current will move you along the coast, and the sand is doing exactly the same thing. The process that transports sand along the beach, called long shore sediment transport, has two components: (1) Sand is transported along the coast with the long shore current in the surf zone; and (2) the up-and-back movement of beach sand in the swash zone causes the sand to move along the beach in a zigzag path. Most of the sand is transported in the surf zone by the long shore current. The direction of transport of sand along beaches in the United States is generally from the north to the south for beaches on both the East and West Coasts of the country. Although most of the transport is to the south, it can be variable and depends upon the wave action and in which direction they strike the shore. The amount of sand transported along a beach, whether we are talking about Long Island, New York, or Los Angeles, California, is surprisingly large, at several hundred thousand cubic meters of sediment per year. Having said this, the amount of sand transported on a given day or period of days is extremely variable. On many days, little sand is being transported, and on others the amount is much larger. Most of the sediment is transported during storms by the larger waves.

Rip Currents 

When a series of large waves arrives at a coastline and breaks on the beach, the water tends to pile up on the shore. The water does not return as it came in, along the entire shoreline, but is concentrated in narrow zones known as rip currents. Beach goers and lifeguards call them rip tides or undertow. They certainly are not tides, and they do not pull people under the water, but they can pull people offshore. In the United States, up to 200 people are killed and 20,000 people are rescued from rip currents each year. Therefore, rip currents constitute a serious coastal hazard to swimmers, killing more people in the United States on an annual basis than do hurricanes or earthquakes; the number of deaths caused by rip currents is equivalent to the number caused by river flooding. People drown in rip currents because they do not know how to swim or because they panic and fight the current by trying to swim directly back to shore. Winning a fight with a rip current is nearly impossible because the current can exceed 6 km per hour (4 mi per hour), a speed that even strong swimmers cannot maintain for long. A swimmer trying to fight a rip current soon becomes exhausted and may not have the energy to keep swimming. Fortunately, rip currents are usually relatively narrow, a few meters to a few tens of meters wide, and they dissipate outside of the surf zone, within tens to hundreds of meters offshore. To safely escape a rip current, a swimmer must first recognize the current and then swim parallel to the shore until he or she is outside the current. Only then should the swimmer attempt to swim back to shore. The key to survival is not to panic. When you swim in the ocean, watch the waves for a few minutes before entering the water and note the surf beat, the regularly arriving sets of small and larger waves. Rip currents can form quickly after the arrival of a set of large waves. They can be recognized as a relatively quiet area in the surf zone where fewer incoming waves break. You may see the current as a mass of water moving out through the surf zone. The water in the current may also be darker, carrying suspended sediment. Remember, if you do get caught in a rip current, do not panic; swim parallel to the shore until you are outside the current, then head back to the beach.

Geology of Mineral Resources

Geology of Mineral Resources

kacimo samy   6:43 ص

The geology of mineral resources is intimately related to the entire geologic cycle, and nearly all aspects and processes of the cycle are involved to a lesser or greater extent in producing local concentrations of useful materials.

Local Concentrations of Metals 

The term ore is sometimes used for those useful metallic minerals that can be mined at a profit, and locations where ore is found have anomalously high concentrations of these minerals. The concentration of metal necessary for a particular mineral to be classified as an ore varies with technology, economics, and politics. Before smelting (extraction of metal by heating) was invented, the only metal ores were those in which the metals appeared in their pure form; gold, for example, was originally obtained as a pure, or native, metal. Now gold mines extend deep beneath the surface, and the recovery process involves reducing tons of rock to ounces of gold. Although the rock contains only a minute amount of gold, we consider it a gold ore because we can extract the gold profitably. The concentration factor of a metal is the ratio of its necessary concentration for profitable mining (that is, of its concentration in ore) to its average concentration in the earth's crust. Aluminum has an average concentration of about 8 percent in the Earth's crust and needs to be found at concentrations of about 35 percent to be mined economically, giving it a concentration factor of about 4. Mercury, on the other hand, has an average concentration of only a tiny fraction of 1 percent and must have a concentration factor of about 10,000 to be mined economically. Nevertheless, mercury ores are common in certain regions, where they and other metallic ores are deposited. The percentage of a metal in ore (and thus the concentration factor) is subject to change as the demand for the metal changes.

Igneous Processes 

Most ore deposits, caused by igneous processes, result from an enrichment process that concentrates an economically desirable ore of metals, such as copper, nickel, or gold. In some cases, however, an entire igneous rock mass contains disseminated crystals that can be recovered economically. Perhaps the best-known example is the occurrence of diamond crystals, found in a coarse-grained igneous rock called kimberlite, which characteristically occurs as a pipe shaped body of rock that decreases in diameter with depth. Almost the entire kimberlite pipe is the ore deposit, and the diamond crystals are disseminated throughout the rock. Diamonds, which are composed of carbon, form at very high temperatures and pressures, perhaps at depths as great as 150 km well below the crust of the earth and into the mantle. Some kimberlite pipes in South Africa are believed to be as old as 2 billion years. Near the surface, diamonds are not stable over geologic time and will eventually change to graphite (the mineral in lead pencils). The transformation will not happen at surface temperature and pressure, and, as a result, diamonds are metastable, remaining beautiful and mysterious for periods of time of interest to humans. The fact that the kimberlite pipes are so old suggests that they must be intruded (moved upward) from deep diamond forming depth to near the surface relatively quickly. If this were not the case, the diamonds would have been transformed to graphite.

Crystal Settling 

More concentrated ore deposits can result from igneous processes called crystal settling that segregate crystals formed earlier from those formed later. For example, as magma cools, heavy minerals that crystallize early may slowly sink or settle toward the lower part of the magma chamber, where they form concentrated layers. Deposits of chromite (ore of chromium) have formed by this process.

Late Magmatic Processes and Hydrothermal Replacement 

Late magmatic processes occur after most of the magma has crystallized, and rare and heavy metalliferous materials in water- and gas-rich solutions remain. This late-stage metallic solution may be squeezed into fractures or settle into interstices (empty spaces) between earlier-formed crystals. Other late-stage solutions form coarse-grained igneous rock known as pegmatite, which is rich in feldspar, mica, and quartz, as well as certain rare minerals. Pegmatites have been extensively mined for feldspar, mica, spodumene (lithium mineral), and clay that forms from weathered feldspar. Hydrothermal (hot-water) mineral deposits are a common type of ore deposit. They originate from late-stage magmatic processes and give rise to a variety of mineralization, including gold, silver, copper, mercury, lead, zinc, and other metals, as well as many non-metallic minerals. The hydrothermal solutions that form ore deposits are mineralizing fluids that migrate through a host rock, crystallizing as veins or small dikes. The mineral material is either produced directly from the igneous parent rock or altered by metamorphic processes, as magmatic solutions intrude into the surrounding rock (alteration by metamorphic processes, called contact metamorphism, is discussed under Metamorphic Processes). Many hydrothermal deposits cannot be traced to a parent igneous rock mass, however, and their origin remains unknown. It is speculated that circulating groundwater, heated and enriched with minerals after contact with deeply buried magma, might be responsible for some of these deposits.
Two types of hydrothermal deposits can be recognized: cavity-filling and replacement. Cavity-filling deposits are formed when hydrothermal solutions migrate along openings in rocks (such as fracture systems, pore spaces, or bedding planes) and precipitate (crystallize) ore minerals. Replacement deposits form as hydrothermal solutions react with the host rock, forming a zone in which ore minerals precipitate from the mineralizing fluids and replace part of the host rock. Although replacement deposits are believed to dominate at higher temperatures and pressures than cavity-filling deposits, both may be found in close association as one grades into the other; that is, the filling of an open fracture by precipitation from hydrothermal solutions may occur simultaneously with replacement of the rock that lines the fracture.
Hydrothermal replacement processes are significant because, excluding some iron and non-metallic deposits, they have produced some of the worlds largest and most important mineral deposits. Some of these deposits result from a massive, nearly complete replacement of host rock with ore minerals that terminate abruptly; others form thin replacement zones along fissures; and still others form disseminated replacement deposits that may involve huge amounts of relatively low-grade ore.
The actual sequence of geologic events leading to the development of a hydrothermal ore deposit is usually complex. Consider, for example, the tremendous, disseminated copper deposits of northern Chile. The actual mineralization is thought to be related to igneous activity, faulting, and folding that occurred 60 million to 70 million years ago. The ore deposit is an elongated, tabular mass along a highly sheared (fractured) zone separating two types of granitic rock.
The concentration of copper results from a number of factors:

• A source igneous rock supplied the copper. 
• The fissure zone tapped the copper supply and facilitated movement of the mineralizing fluids. 
• The host rock was altered and fractured, preparing it for deposition and replacement processes that produced the ore. 
• The copper was leached and redeposited again by meteoric water, which further concentrated the ore.

Metamorphic Processes 

Contact Metamorphism 

Ore deposits are often found along the contact between igneous rocks and the surrounding rocks they intrude. This area is characterized by contact metamorphism, caused by the heat, pressure, and chemically active fluids of the cooling magma interacting with the surrounding rock, called country, or host rock. The width of the contact metamorphic zone varies with the type of country rock. The zone is usually thickest in limestone because limestone is more reactive: The release of carbon dioxide increases the mobility of reactants. The zone is generally thinnest for shale because the fine-grained texture retards the movement of hot, chemically active solutions, and the zone is intermediate for sandstone. As we have already mentioned, some of the mineral deposits that form in contact areas originate from the magmatic fluids and some from reactions of these fluids with the country rock.

Regional Metamorphism 

Metamorphism can also result from regional increase of temperature and pressure associated with deep burial of rocks or tectonic activity. This regional metamorphism can change the mineralogy and texture of the pre-existing rocks, producing ore deposits of asbestos, talc, graphite, and other valuable non-metallic deposits. Metamorphism has been suggested as a possible origin of some hydrothermal fluids. It is a particularly likely cause in high-temperature, high-pressure zones, where fluids might be produced and forced out into the surrounding rocks to form replacement or cavity-filling deposits. For example, the native copper found along the top of ancient basalt flows in the Michigan copper district was apparently produced by metamorphism and alteration of the basalt, which released the copper and other materials that produced the deposits.
Our discussion of igneous and metamorphic processes has focused primarily on ore deposits. However, igneous and metamorphic processes are also responsible for producing a good deal of stone used in the construction industry. Granite, basalt, marble (metamorphosed limestone), slate (metamorphosed shale), and quartzite (metamorphosed sandstone), along with other rocks, are quarried to produce crushed rock and dimension stone in the United States. Stone is used in many aspects of construction work; but many people are surprised to learn that, in total value, with the exception of iron and steel, the stone industry is one of the largest non-fuel mineral industries in the United States.

Sedimentary Processes 

Sedimentary processes are often significant in concentrating economically valuable materials in sufficient amounts for extraction. As sediments are transported, wind and running water help segregate the sediment by size, shape, and density. Thus, the best sand or sand and gravel deposits for construction purposes are those in which the finer materials have been removed by water or wind. Sand dunes, beach deposits, and deposits in stream channels are good examples.

Sand and Gravel 

The U.S. sand and gravel industry amounts to about $8.5 billion per year, and, by volume mined (about 1300 million tons in 2006), it is one of the largest non-fuel mineral industries in the United States. Currently, most sand and gravel are obtained from river channels and water-worked glacial deposits. The United States now produces more sand and gravel than it needs, but demand is increasing. Environmental restrictions on extraction are causing sand and gravel operations to move away from areas with high population density, and shortages of sand and gravel are expected to increase as zoning and land development restrict locations where they may be extracted. Extraction from river channels and active floodplains can cause degradation to the river environment, and objections to river extraction operations are becoming more common.

Placer Deposits 

Stream processes transport and sort all types of materials according to size and density. Therefore, if the bedrock in a river basin contains heavy metals such as gold, streams draining the basin may concentrate heavy metals to form placer deposits (ore formed by deposit of sediments) in areas where there is reduced turbulence or velocity of flow, such as between particles on riffles, in open crevices

or fractures at the bottoms of pools, or at the inside curves of bends. Placer mining of gold known as a poor man method because a miner needed only a shovel, a pan, and a strong back to work the stream side claim helped to stimulate settlement of California, Alaska, and other areas of the United States. Furthermore, the gold in California attracted miners who acquired the expertise necessary to locate and develop other resources in the western conterminous United States and Alaska. Placer deposits of gold and diamonds have also been concentrated by coastal processes, primarily wave action. Beach sands and near-shore deposits are mined in Africa and other places.

Evaporite Deposits 

Rivers and streams that empty into the oceans and lakes carry tremendous quantities of dissolved material derived from the weathering of rocks. From time to time, geologically speaking, a shallow marine basin may be isolated by tectonic activity (uplift) that restricts circulation and facilitates evaporation. In other cases, climatic variations during the ice ages produced large inland lakes with no outlets, which eventually dried up. In either case, as evaporation progresses, the dissolved materials precipitate, forming a wide variety of compounds, minerals, and rocks called evaporite deposits that have important commercial value.
Most evaporite deposits can be grouped into one of three types:
Marine evaporites (solids) potassium and sodium salts, calcium carbonate, gypsum, and anhydrite; non-marine evaporites (solids) sodium and calcium carbonate, sulphate, borate, nitrate, and limited iodine and strontium compounds; and brines (liquids derived from wells, thermal springs, inland salt lakes, and seawaters) bromine, iodine, calcium chloride, and magnesium. Heavy metals (such as copper, lead, and zinc) associated with brines and sediments in the Red Sea, Salton Sea, and other areas are important resources that may be exploited in the future. Extensive marine evaporite deposits exist in the United States. The major deposits are halite (common salt, NaCl), gypsum anhydrite , and inter-bedded limestone Limestone, gypsum, and anhydrite are present in nearly all marine evaporite basins, and halite and potassium minerals are found in a few. Evaporite materials are widely used in industry and agriculture. Marine evaporites can form stratified deposits that may extend for hundreds of kilometres, with a thickness of several thousand meters. The evaporites represent the product of evaporation of seawater in isolated shallow basins with restricted circulation. Within many marine evaporite basins, the different deposits are arranged in broad zones that reflect changes in salinity and other factors controlling the precipitation of evaporites; that is, different materials may be precipitated at the same time in different parts of the evaporite basin. Halite, for example, is precipitated in areas where the brine is more saline, and gypsum where it is less saline. Economic deposits of potassium evaporite minerals are relatively rare but may form from highly concentrated brines. Non-marine evaporite deposits form by evaporation of lakes in a closed basin. Tectonic activity, such as faulting, can produce an isolated basin with internal drainage and no outlet. However, to maintain a favourable environment for evaporite mineral precipitation, the tectonic activity must continue to uplift barriers across possible outlets or lower the basin floor faster than sediment can raise it. Even under these conditions, economic deposits of evaporites will not form unless sufficient dissolved salts have washed into the basin by surface run off from surrounding highlands. Finally, even if all favourable environmental criteria are present, including an isolated basin with sufficient run off and dissolved salts, valuable non-marine evaporates, such as sodium carbonate or borate, will not form unless the geology of the highlands surrounding the basin is also favourable and yields run off with sufficient quantities of the desired material in solution. Some evaporite beds are compressed by overlying rocks and mobilized, then pierce or intrude the overlying rocks. Intrusions of salt, called salt domes, are quite common in the Gulf Coast of the United States and are also found in north western Germany, Iran, and other areas. Salt domes in the Gulf Coast are economically important because:

• They are a good source for nearly pure salt. Some have extensive deposits of elemental sulphur. 
• Some have oil reserves on their flanks. 

Salt domes are also environmentally important as possible permanent disposal sites for radioactive waste, although, because salt domes tend to be mobile, their suitability as disposal sites for hazardous wastes must be seriously questioned. Evaporites from brine resources of the United States are substantial, assuring that no shortage is likely for a considerable period of time. But many evaporites will continue to have a place value because transportation of these mineral commodities increases their price, so continued discoveries of high-grade deposits closer to where they will be consumed remains an important goal.

Biological Processes 

Organisms are able to form many kinds of minerals, such as the various calcium and magnesium carbonate minerals in shells and calcium phosphate in bones. Some of these minerals cannot be formed inorganically in the biosphere. Thirty-one different biologically produced minerals have been identified. Minerals of biological origin contribute significantly to sedimentary deposits. 

An interesting example of mineral deposits produced by biological processes are phosphates associated with sedimentary marine deposition. Phosphorus-rich sedimentary rocks are fairly common in some of the western states, as well as in Tennessee, North Carolina, and Florida. The common phosphorus-bearing mineral in these rocks is apatite, a calcium phosphate associated with bones and teeth. Fish and other marine organisms extract the phosphate from seawater to form apatite, and the mineral deposit results from sedimentary accumulations of the phosphate-rich fish bones and teeth. The richest phosphate mine in the world, known as Bone Valley, is located about 40 km east of Tampa, Florida. The deposit is marine sedimentary rocks composed in part of fossils of marine animals that lived 10 million to 15 million years ago, when Bone Valley was the bottom of a shallow sea. That deposit has supplied as much as one-third of the worlds phosphate production. Another important source of phosphorus is guano (bird faeces), which accumulates where there are large colonies of nesting sea birds and a climate arid enough for the guano to dry to a rock like mass. Thus, the formation of one of the major sources of phosphorus depends upon unique biological and geographical conditions.

Weathering Processes 

Weathering is responsible for concentrating some materials to the point that they can be extracted at a profit. Weathering processes can produce residual ore deposits in the weathered material and provide secondary enrichment of low-grade ore.

Residual Ore Deposits 

Intensive weathering of rocks and soils can produce residual deposits of the less soluble materials, which may have economic value. For example, intensive weathering of some rocks forms a type of soil known as laterite (a residual soil derived from aluminium- and iron-rich igneous rocks). The weathering processes concentrate relatively insoluble hydrated oxides of aluminium and iron, while more soluble elements, such as silica, calcium, and sodium, are selectively removed by soil and biological processes. If sufficiently concentrated, residual aluminium oxide forms an aluminium ore known as bauxite. Important nickel and cobalt deposits are also found in laterite soils, developed from ferromagnesian-rich igneous rocks. Insoluble ore deposits, such as native gold, are generally residual, meaning that, unless they are removed by erosion, they accumulate in weathered rock and soil. Accumulation of the insoluble ore minerals is favoured where the parent rock is a relatively soluble material, such as limestone. Care must be taken in evaluating a residual weathered rock or soil deposit because the near-surface concentration may be a much higher grade than ore in the parent, un-weathered rocks.

Secondary Enrichment 

Weathering is also involved in secondary enrichment processes to produce sulphide ore deposits from low-grade primary ore. Near the surface, primary ore containing such minerals as iron, copper, and silver sulphides is in contact with slightly acid soil water in an oxygen-rich environment. As the sulphides are oxidized, they are dissolved, forming solutions rich in sulphuric acid and in silver and copper sulphate; these solutions migrate downward, producing a leached zone devoid of ore minerals. Below the leached zone, oxidation continues, as the sulphate solutions continue to move toward the groundwater table. Below the water table, if oxygen is no longer available, the solutions are deposited as sulphides, increasing the metal content of the primary ore as much as tenfold. In this way, low-grade primary ore is rendered more valuable, and high-grade primary ore is made even more attractive.

The presence of a residual iron oxide cap at the surface indicates the possibility of an enriched ore below, but it is not always conclusive. Of particular importance to the formation of a zone of secondary enrichment is the presence in the primary ore of iron sulphide (for example, pyrite). Without it, secondary enrichment seldom takes place, because iron sulphide in the presence of oxygen and water forms sulphuric acid, which is a necessary solvent. Another factor favouring development of a secondary-enrichment ore deposit is the primary ore being sufficiently permeable to allow water and solutions to migrate freely downward. Given a primary ore that meets these criteria, the reddish iron oxide cap probably does indicate that secondary enrichment has taken place.

Several disseminated copper deposits have become economically successful because of secondary enrichment, which concentrates dispersed metals. For example, secondary enrichment of a disseminated copper deposit at Miami, Arizona, increased the grade of the ore from less than 1 percent copper in the primary ore to as much as 5 percent in some localized zones of enrichment.

Other Minerals from the Sea 

Mineral resources in seawater or on the bottom of the ocean are vast and, in some cases, such as magnesium, nearly unlimited. In the United States, magnesium was first extracted from seawater in 1940. By 1972, one company in Texas produced 80 percent of our domestic magnesium, using seawater as its raw material source. In 1992, three companies in Texas, Utah, and Washington extracted magnesium, respectively, from seawater, lake brines, and dolomite (mineral composed of calcium and magnesium carbonate). The deep-ocean floor may eventually be the site of a next mineral rush. Identified deposits include massive sulphide deposits associated with hydrothermal vents, manganese oxide nodules, and cobalt-enriched manganese crusts.

Sulfide Deposits 

Massive sulphide deposits containing zinc, copper, iron, and trace amounts of silver are produced at divergent plate boundaries (oceanic ridges) by the forces of plate tectonics. Pressure created by several thousand meters of water at ridges forces cold seawater deep into numerous rock fractures, where it is heated by up-welling magma to temperatures as high as The pressure of the heated water produces vents known as black smokers, from which the hot, dark-coloured, mineral-rich water emerges as hot springs. Circulating seawater leaches the surrounding rocks, removing metals that are deposited when the mineral-rich water is ejected into the cold sea. Sulphide minerals precipitate near the vents, forming massive tower like formations, rich in metals. The hot vents are of particular biologic significance because they support a unique assemblage of animals, including giant clams, tube worms, and white crabs. Ecosystems including these animals base their existence on sulphide compounds extruded from black smokers, existing through a process called chemosynthesis, as opposed to photosynthesis, which supports all other known ecosystems on earth. The extent of sulphide mineral deposits along oceanic ridges is poorly known, and, although leases to some possible deposits are being considered, it seems unlikely that such deposits will be extracted at a profit in the near future. Certainly, potential environmental degradation, such as decreased water quality and sediment pollution, will have to be carefully evaluated prior to any mining activity. Study of the formation of massive sulphide deposits at oceanic ridges is helping geologists understand some of the mineral deposits on land. For example, massive sulphide deposits being mined in Cyprus are believed to have formed at an oceanic ridge and to have been later uplifted to the surface.

Manganese Oxide Nodules 

Manganese oxide nodules cover vast areas of the deep-ocean floor. They contain manganese (24 percent) and iron (14 percent), with secondary copper (1 percent), nickel (1 percent), and cobalt (0.25 percent). Nodules are found in the Atlantic Ocean off Florida, but the richest and most extensive accumulations occur in large areas of the north eastern, central, and southern Pacific, where they cover 20 to 50 percent of the ocean floor.

Manganese oxide nodules are usually discrete, but are welded together locally to form a continuous pavement. Although they are occasionally found buried in sediment, nodules are usually surficial deposits on the seabed. Their size varies from a few millimetres to a few tens of centimetres in diameter (many are marble to baseball sized). Composed primarily of concentric layers of manganese and iron oxides, mixed with a variety of other materials, each nodule formed around a nucleus of a broken nodule, a fragment of volcanic rock, or, sometimes, a fossil. The estimated rate of nodular growth is 1 to 4 mm per million years. The nodules are most abundant in those parts of the ocean where sediment accumulation is at a minimum, generally at depths of 5 to 7 km. 

The origin of the nodules is not well understood; presumably, they might form in several ways. The most probable theory is that they form from material weathered from the continents and transported by rivers to the oceans, where ocean currents carry the material to the deposition site in the deep-ocean basins. The minerals from which the nodules form may also derive from submarine volcanism, or may be released during physical and biochemical processes and reactions that occur near the water sediment interface during and after deposition of the sediments. Mining of manganese oxide nodules involves lifting the nodules off the bottom and up to the mining ship; this may be done by using suction or scraper equipment. Although mining of the nodules appears to be technologically feasible, production would be expensive compared to mining manganese on land. In addition, there are uncertainties concerning ownership of the nodules, and nodule mining would cause significant damage to the sea-floor and local water quality, raising environmental concerns.

Cobalt-enriched Manganese Crusts 

Oceanic crusts rich in cobalt and manganese are present in the mid- and south-west Pacific, on flanks of sea-mounts, volcanic ridges, and islands. Cobalt content varies with water depth; the maximum concentration of about 2.5 percent is found at water depths of 1 to 2.5 km. Thickness of the crust averages about 2 cm. The processes of formation are not well understood. Both the nature and the extent of the crusts, which also contain nickel, platinum, copper, and molybdenum, are being studied by U.S. Geological Survey scientists.

Water Pollutants

Water Pollutants

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Many different materials may pollute surface water or groundwater. We will focus on oxygen-demanding waste, pathogenic organisms, nutrients, oil, hazardous chemicals, heavy metals, radioactive materials, and sediment. 

Oxygen-Demanding Waste 

Dead organic matter in streams decays; that is, it is consumed by bacteria, which require oxygen. If there is enough bacterial activity, the oxygen in the water can be reduced to levels so low that fish and other organisms die. A stream without oxygen is a dead stream, devoid of fish and many organisms we value. The amount of oxygen used for bacterial decomposition is the biochemical oxygen demand (BOD), a commonly used measure in water quality management. The BOD is measured as milligrams per litre of oxygen consumed over five days at 20° C. A high BOD indicates a high level of decaying organic matter in the water.
Dead organic matter in streams and rivers comes from natural sources (for example, dead leaves from a forest), as well as from agriculture and urban sewage. Approximately 33 percent of all BOD in streams results from agricultural activities, but urban areas, particularly those with sewer systems that combine sewage and storm-water run off, may add considerable BOD to streams during floods, when sewers entering treatment plants can be overloaded and overflow into streams, producing pollution events.

Relationship between dissolved oxygen and biochemical oxygen demand (BOD) for a stream, following the input of sewage.

The threshold for water pollution is a dissolved oxygen content of less than 5 mg per litre (mg/l) of water. The diagram in Figure above illustrates the effect of BOD on dissolved oxygen content in a stream when raw sewage is introduced as a result of an accidental spill. Three zones are recognised. The pollution zone has a high BOD and a reduced dissolved oxygen content as initial decomposition of the waste begins. In the active decomposition zone, the dissolved oxygen content is at a minimum, owing to biochemical decomposition as the organic waste is transported downstream. In the recovery zone, the dissolved oxygen increases, and the BOD is reduced because most oxygen demanding organic waste from the input of sewage has decomposed and natural stream processes are replenishing the water with dissolved oxygen. All streams have some capability to degrade organic waste after it enters the stream. Problems result when the stream is overloaded with biochemical oxygen-demanding waste, overpowering the streams natural cleansing function.

Pathogenic Organisms 

Pathogenic (disease-causing) micro-organisms are important biological pollutants. Among the major water borne human diseases are cholera, typhoid infections, hepatitis, and dysentery. Because it is often difficult to monitor the pathogens directly, we use the count of human faecal coliform bacteria as a common measure of biological pollution and a standard measure of microbial pollution. These common and, usually, harmless bacteria are normal constituents of human intestines and are found in all human waste.
However, not all forms of faecal coliform bacteria are harmless. Escherichia coli (also known as E. coli 0157), a strain of E. coli bacteria, has been responsible for many human illnesses and deaths. E. coli 0157 produces strong toxins in humans that may lead to bloody diarrhea, dehydration, kidney failure, and death. In 1993, outbreaks of disease, apparently caused by E. coli 0157, occurred as a result of peoples eating contaminated meat at a popular fast-food restaurant. In 1998, E. coli apparently contaminated the water in a Georgia water park and a Wyoming towns water supply, causing illness and one death.
One of the worst outbreaks of E. coli bacterial infection in Canadian history unfolded in May 2000 in Walkerton, Ontario. It is believed that the likely cause of the contamination in Walkerton was the result of E. coli bacteria in cow manure that washed into the public water supply during heavy rains and flooding that occurred on May 12, 2000. The local Public Utility Commission was aware as early as May 18 that water from wells serving the town was contaminated, but they did not report this contamination immediately to health authorities. As a result, people were not advised to boil water until it was too late to avoid the outbreak of disease. By May 26, 5 people had died, over 20 were in the intensive care unit of the local hospital, and approximately 700 were ill with severe symptoms, including cramps, vomiting, and diarrhoea. The old and very young are most vulnerable to the ravages of the disease, which can damage the kidneys, and two of the first victims were a 2-year-old baby and an 82-year-old woman. Government officials finally took over management of the water supply, and bottled water was distributed. Tragically, before the outbreak was over, at least 7 people had died and over 1000 had been infected.
Authorities launched an investigation, focusing on why there was such a long delay between identifying the potential problem and warning people. Had there not been such a long delay, illnesses might have been avoided. We must remain vigilant in testing our waters and immediately report problems to public health authorities if any problems arise.
In the fall of 2006, E. coli 0157 was traced to farms in northern California. Contaminated spinach was shipped to 23 states. About 150 people became sick and one person died. In 2009, peanut butter was responsible for several hundred E. coli illnesses, with several deaths across the United States.
In the past, epidemics of water borne diseases have killed thousands of people in U.S. cities. Such epidemics have been largely eliminated by separating sewage water and drinking water and treating drinking water before consumption. Unfortunately, this is not the case worldwide, and, every year, several billion people, particularly in poor countries, are exposed to water borne diseases. For example, an epidemic of cholera occurred in South America in the 1990s. Although developing nations are more vulnerable, the risk of water borne diseases is a potential threat in all countries.
The threat of an outbreak of a water borne disease is exacerbated by disasters such as earthquakes, floods, and hurricanes; these events can damage sewer lines or cause them to overflow, resulting in contamination of water supplies. For example, after the 1994 North ridge earthquake, people in the San Fernando Valley of the Los Angeles Basin were advised to purify municipal water by boiling because of the threat of bacterial contamination.

Nutrients 

Relationship between land use and average nitrogen and phosphorus concentration in streams (in milligrams per liter).

Nutrients released by human activity may lead to water pollution. Two important nutrients that can cause problems are phosphorus and nitrogen, both of which are released from a variety of materials, including fertilizers, detergents, and the products of sewage-treatment plants. The concentration of phosphorus and nitrogen in streams is related to land use. Forested land has the lowest concentrations of phosphorus and nitrogen, while the highest concentrations are found in agricultural areas, such as fertilized farm fields and feed lots. Urban areas can also add phosphorus and nitrogen to local waters, particularly where waste water treatment plants discharge treated waters into rivers, lakes, or the ocean. These plants are effective in reducing organic pollutants and pathogens, but, without advanced treatment, nutrients pass through the system.
High human-caused concentrations of nitrogen and phosphorus in water often result in the process known as cultural eutrophication. Eutrophication (from the Greek for well fed ), a natural process, is characterized by a rapid increase in the abundance of plant life, particularly algae. Blooms of algae form thick mats that sometimes nearly cover the surface of the water in freshwater ponds and lakes. The algae block sunlight to plants below, and the plants eventually die. In addition, the algae consume oxygen as they decompose, thereby lowering the oxygen content of the water, and fish and aquatic animals may die as well.
Algae blooms from blue-green algae may produce toxins as part of their life cycle. Lakes in Wisconsin, Minnesota, Oregon, and other areas with blooms of blue-green algae turn pea green, and toxins that are produced have been responsible for deaths of dogs and other animals that drink the water. People who live near the lake have reported nauseating smells from the water, along with rashes, headaches, and sore throats. People have not been killed by the toxins because they generally avoid contact with the water.

Algae contaminated beaches in Hawaii (a) Ocean-front condominium on the island Maui, Hawaii. The brown line along the edge of the beach is an accumulation of marine algae (locally called seaweed). (b) On the beach itself, the algae pile up, sometimes to a depth of about 0.5 m (1.7 ft), and people using the beach avoid the areas of algae piles. (c) Condominium complexes often have small wastewater-treatment plants, such as the one shown here, inside the plant-covered fence, that provide primary and secondary treatment. After this treatment, the water is injected underground at a relatively shallow depth. The treatment does not remove nutrients such as phosphorus and nitrogen that apparently encourage the accelerated growth of marine algae in the nearshore environment.

In the marine environment, nutrients in near shore waters may cause blooms of seaweed, referred to as marine algae, to flourish. The marine algae become a nuisance when they are torn loose and accumulate on beaches. Algae may also damage or kill coral in tropical areas. For example, the island of Maui in the Hawaiian Islands has a cultural eutrophication problem resulting from nutrients entering the near shore environment from waste-disposal practices and agricultural run off. Beaches in some areas become fouled with algae that washes up on the shore, where it rots and creates a stench, providing a home for irritating insects that eventually drive away tourists.

Dead zone in Gulf of Mexico Area in the Gulf of Mexico in July 2001 with bottom water with less than 2 mg/L dissolved oxygen.

A serious and ongoing cultural eutrophication problem is occurring in the Gulf of Mexico, offshore of Louisiana. A so-called dead zone develops in the summer, over a large area about the size of New Jersey. Water in the zone has low concentrations of oxygen, killing shellfish and crabs, and blooms of algae occur. The cause of the cultural eutrophication is believed to be the Mississippi River. The Mississippi drains about 40 percent of the lower 48 states, and much of the land use in the drainage basin is agricultural. The nutrient believed to cause the problem is nitrogen, which is used in great amounts to fertilize fields. The problem will not be easy to solve, as long as agriculture continues to use tremendous amounts of fertilizer. Part of the solution will be modification of agricultural practices to use less nitrogen by using it more efficiently, so that less of the nutrient runs off the land into the river.

Oil

Oil spill from the Exxon Valdez in Alaska, 1989 (a) Aerial view of oil being offloaded from the leaking tanker Exxon Valdezon theleft to the smaller Exxon Baton Rougeon the right. Floating oil is clearly visible on the water.
(b) Attempting toclean oil from the coastal environment by scrubbing and spraying with hot water.

Oil discharged into surface water (rivers, lakes, and the ocean) has caused major pollution problems. The largest oil discharges have usually involved oil-tanker accidents at sea. For example, just after midnight on March 24, 1989, the oil tanker Exxon Valdez ran aground on Bligh Reef, 40 km (25 mi) south of Valdez, Alaska, in Prince William Sound. Crude oil poured out of the ruptured tanks of the vessel at a rate of approximately 20,000 barrels per hour. The Exxon Valdez was loaded with 1.2 million barrels of crude oil, and, of this, more than 250,000 barrels (11 million gallons) gushed from the hold of the 300-m (984-ft) tanker. The oil remaining in the Exxon Valdez was loaded into another tanker.

Mercury in the environment Input and changes of mercury in aquatic ecosystems. 

The oil spilled into what was considered one of the most pristine and ecologically rich marine environments of the world, and the accident is now known as the worst oil spill in the history of the United States. Short term impacts were very significant; commercial fisheries, sport fisheries, and tourism were disrupted. In addition, many sea birds and mammals were lost. Lessons learned from the Exxon Valdez spill have resulted in better management strategies for both the shipment of crude oil and emergency plans to minimize environmental degradation.

A large oil spill in 2006 was caused by the war in Lebanon, when a coastal power plant was bombed and over 100,000 barrels of fuel oil entered the Mediterranean Sea. Over half of Lebanon's tourist beaches were polluted, including a popular public beach visited by people from the capital city Beirut.

Toxic Substances

Many substances that enter surface water and groundwater are toxic to organisms. Three general categories of toxic substances synthetic organic chemicals, heavy metals, and radioactive waste will be discussed.

Synthetic Organic Chemicals 

Organic compounds are compounds of carbon that are produced naturally by living organisms or synthetically by industrial processes. Up to 100,000 new chemicals are now being used or have been used in the past. It is difficult to generalize concerning the environmental and health effects of synthetic organic compounds because there are so many of them and they have so many uses and produce so many different effects.

Selected Persistent Organic Pollutants (POPs).

Synthetic organic compounds have many uses in industrial processes, including pest control, pharmaceuticals, and food additives. Some of these compounds are called persistent organic pollutants, also known as POPs. Many of these chemicals were produced decades ago, before their harm to the environment was known, and a number have now been banned or restricted. Table above lists some of the common persistent organic pollutants and their uses. POPs have several general properties useful in defining them.First, they have a carbon-based structure and often contain reactive chlorine. Second, most are produced by human processes and, thus, are synthetic chemicals. Third, they persist in the environment, do not break down easily, are polluting and toxic, and tend to accumulate in living tissue. Fourth, they occur in a number of forms that allow them to be easily transported by water and wind, with sediment, for long distances.
A significant example of water polluter is the chemical MTBE (methyl tertbutyl ether). The Clean Air Act Amendments that were passed in 1990 required cities with air pollution problems to use what are known as oxygen additives in gasoline. MTBE is added to gasoline with the objective of increasing the oxygen level of the gasoline and decreasing emissions of carbon monoxide from gasoline-burning cars. It is used because MTBE is more economical than other additives, including alcohol. MTBE is very soluble in water and is a commonly detected volatile organic compound (VOC) in urban groundwater. It is hypothesized that the MTBE detected in shallow groundwater originates from three sources: urban storm-water runoff, leaking underground gasoline tanks, and leakage occurring at service stations when car tanks are being filled.

Pathways for chemical pollutants within the hydrologic cycle of the environment.

It is ironic that a gasoline additive intended to improve air quality contaminated the groundwater that was used as a source of drinking water for approximately 15 million people in California. In 1997, MTBE-polluted groundwater in Santa Monica, California, forced the city to stop pumping groundwater, eliminating approximately 50 percent of the total drinking water supply for the city. Concentrations of MTBE in Santa Monicas groundwater ranged from about 8 to 600 micrograms per litre. The Environmental Protection Agency has stated that concentrations of 20 to 40 of MTBE per litre of water are sufficient to cause objectionable taste and odour. MTBE in that concentration smells like turpentine or fresh paint and is nauseating to some people. Studies are under way concerning the toxicity of MTBE, and some researchers fear it is a carcinogenic chemical. As a result of the contamination, some states, such as California, have terminated the use of MTBE. Many other states followed and, by 2006, MTBE was all but phased out in the United States. However, MTBE remains a groundwater pollution problem that can contaminate surface water (see Putting Some Numbers on Water Pollution). Figure above illustrates some of the pathways of MTBE, as well as other volatile organic compounds in the hydrologic cycle of an urban area.

Heavy Metals 

Heavy metals, such as lead, mercury, zinc, cadmium, and arsenic, are dangerous pollutants that are often deposited with natural sediment in the bottoms of stream channels. If these metals are deposited on floodplains, they may become incorporated into plants, including food crops, and animals. Once the metal has dissolved in water used for agricultural or domestic use, heavy-metal poisoning can result.
As an example, consider mercury contamination of aquatic ecosystems. It has been known for decades that mercury is a significant pollutant of aquatic ecosystems, including ponds, lakes, rivers, and the ocean.
Perhaps the best-known case history of mercury toxicity comes from Minamata, Japan. Minamata is a coastal town on the island of Kyushu and was the site of a serious illness that was first recognized in the middle of the twentieth century.
It was first called the disease of the dancing cats because the illness was first observed in cats that seemingly went mad and ran in circles, foaming at the mouth. It was also noticed that birds flew into buildings or fell to the ground. People were subsequently affected, most being families of fishermen. Some of the first symptoms were fatigue, irritability, numbness in arms and legs, and headaches, as well as difficulty in swallowing. Some of the more severe symptoms included blurred vision, loss of hearing, and loss of muscular coordination. Some people complained of a metallic taste in their mouths and suffered from diarrhoea. By the time the disease ran its course, over 40 people died and over 100 were severely disabled. The people affected by the disease lived in a relatively small area, and their diet mostly came from fish harvested from Minamata Bay.
The disease was eventually traced to a vinyl chloride factory on Minamata Bay that used mercury in its production processes. Inorganic mercury was released as waste into the bay, and it was believed that the mercury would not get into the food chain. However, the inorganic mercury was converted by bacterial activity in Minamata Bay to methyl mercury. Methyl mercury readily passes through cell membranes and is transported throughout the body by red blood cells. It can enter and damage brain cells. The harmful effects of methyl mercury depend on a number of factors that include the amount of exposure and intake, the duration of the exposure, and the species affected. The effects of the mercury are often delayed from several weeks to months in people from the time of ingestion. Furthermore, if the intake of mercury ceases, some of the symptoms may disappear, but others are difficult to reverse.
The disease of the dancing cats eventually became known as Minamata disease, and nearly 800 people were officially recognized as having the disease but as many as several thousand may have been involved. The mercury pollution in the bay ceased in 1968. As recently as the 1990s, some of the people afflicted by the disease were still being compensated for damages.

 Fish may contain toxic metals People cooking and eating fish, here in the Fiji Islands, are taking in chemicals, including metals that the fish have in their tissue. Mercury is a potential problem with fish, such as tuna and swordfish. 

There are several natural sources of mercury, including input from volcanoes and erosion of natural mercury deposits. In most cases, however, we are most concerned with the input of mercury into the environment through processes such as burning coal, incinerating waste, and processing metals. Although the rates of mercury input into the environment by humans are poorly understood, it is believed that human activities have doubled or tripled the amount of mercury in the atmosphere, and it is increasing at about 1.5 percent per year. Deposition from the atmosphere through rainfall is the primary source of mercury in most aquatic ecosystems. Once ionic mercury is in surface water, it enters into complex cycles, during which a process known as methylation may occur. Bacterial activity changes the inorganic mercury to methyl mercury This process is important from an environmental viewpoint because methyl mercury is much more toxic than is ionic mercury. Furthermore, living things require longer periods of time to eliminate methyl mercury from their systems than they do to eliminate inorganic mercury. As the methyl mercury works its way through food chains, a process known as bio-magnification occurs, in which concentrations of methyl mercury increase in higher levels of the food chain. Thus, big fish in a pond contain higher concentrations of mercury than do the smaller fish and aquatic insects that the large fish feed upon. The input side of the mercury cycle shows the deposition of inorganic mercury through the formation of methyl mercury. On the output side of the cycle, mercury entering fish may be taken up by the organism that eats the fish (figure above). Sediment may also release mercury by a variety of processes, including re-suspension in the water; this can eventually result in the mercury entering the food chain or being released back into the atmosphere through volatilization, the process of converting a liquid or solid to a vapour.
Arsenic is an example of a highly toxic natural metal that is found in soil, rock, and water. There are many industrial and commercial uses of arsenic compounds, including the processing of glass, pesticides, and wood preservatives. Arsenic may enter our water supplies through a number of processes, including natural rain, snow melt, or groundwater flow. It may also be released with industrial waste water and agricultural processes. Finally, it may be released through the production of pesticides, the burning of fossil fuels, and as a by-product of mining.
Arsenic has been known as a deadly poison since ancient times, and, more recently, it has been recognized that elevated levels of arsenic in drinking water may cause a variety of health problems that affect organs such as the bladder, lung, and kidney. It may also cause disease to the central nervous system. Finally, arsenic is known to be a carcinogen (capable of causing or promoting cancer).
The occurrence of arsenic in drinking water is now recognized as a global problem. It certainly is not found in all water supplies, but it is found in many around the world. For example, arsenic in groundwater in Bangladesh has affected many millions of some of the poorest people on Earth. Ongoing research has the objective of identifying those locations where arsenic pollution occurs and of developing appropriate technology or methods to avoid or reduce the hazard of exposure to arsenic.

Radioactive Waste 

Radioactive waste in water may be a dangerous pollutant. Environmentalists are concerned about the possible effects of long-term exposure to low doses of radioactivity to people, other animals, and plants.

Sediment 

Sediment consists of unconsolidated rock and mineral fragments, the smallest of which range in size from sand particles to very small silt- and clay-sized particles. It is these small particles that cause most sediment pollution problems. Sediment is our greatest water pollutant by volume; it is clearly a resource out of place. It depletes soil, a land resource; can reduce the quality of the water resource it enters; and may deposit undesired materials on productive crop lands or on other useful land.

Thermal Pollution 

Thermal pollution is the artificial heating of waters, primarily by hot-water emission from industrial operations and power plants. Heated water causes several problems. First, heated water contains less oxygen than cooler water; even water only several degrees warmer than the surrounding water holds less oxygen. Second, warmer water favours different species than does cooler water and may increase the growth rates of undesirable organisms, including certain water plants and fish. In some cases, however, the warm water may attract and allow better survival of certain desirable fish species, particularly during the winter.