Segments of Environment - Details

Environmental Segments:

The environment consists of various segments such as atmosphere, hydrosphere, lithosphere and biosphere. Before explaining the chemistry that is taking place in these segments one by one, a brief out line about their importance will be discussed.
Atmosphere: The following points highlight the vital role played by atmosphere in the survival of life in this planet.
• The atmosphere is the protective blanket of gases which is surrounding the earth. It protects the earth from the hostile environment of outer space.
• It absorbs IR radiations emitted by the sun and reemitted from the earth and thus controls the temperature of the earth.
• It allows transmission of significant amounts of radiation only in the regions of 300 – 2500 nm (near UV, Visible, and near IR) and 0.01 – 40 meters (radio waves). i.e., it filters tissue damaging UV radiation below 300 nm.
• It acts as a source for CO2 for plant photosynthesis and O2 for respiration.
• It acts as a source for nitrogen for nitrogen fixing bacteria and ammonia producing plants.
• The atmosphere transports water from ocean to land.
Hydrosphere: The hydrosphere is a collective term given to all different forms of water. It includes all types of water resources such as oceans, seas, rivers, lakes, streams, reservoirs, glaciers and ground waters.
As can be seen, only 1% of the total water supply is available as fresh water in the form of rivers, lakes, streams and ground water for human consumption and other uses. The major problem with global water supply is it’s non-uniform distribution, since people in areas with low precipitation often consume more than people in regions with more rainfall.
Lithosphere:
• The lithosphere consists of upper mantle and the crust.
The crust is the earth’s outer skin that is accessible to human.
The crust consists of rocks and soil of which the latter is the important part of lithosphere.
Biosphere:
• The biosphere refers to the realm of living organisms and their interactions with the environment (VIZ: atmosphere, hydrosphere and lithosphere)
• The biosphere is very large and complex and is divided into smaller units called ecosystems.
• Plants, animals and microorganisms which live in a definite zone along with physical factors such as soil, water and air constitute an ecosystem.
• Within each ecosystems there are dynamic inter relationships between living forms and their physical environment

The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night (the diurnal temperature variation).
Atmospheric stratification describes the structure of the atmosphere, dividing it into distinct layers, each with specific characteristics such as temperature or composition. The atmosphere has a mass of about 5×1018 kg, three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. An altitude of 120 km (75 mi) is where atmospheric effects become noticeable during atmospheric reentry of spacecraft. The Kármán line, at 100 km (62 mi), also is often regarded as the boundary between atmosphere and outer space.
Air is the name given to atmosphere used in breathing and photosynthesis. Dry air contains roughly (by volume) 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.039% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1%. While air content and atmospheric pressure varies at different layers, air suitable for the survival of terrestrial plants and terrestrial animals is currently only known to be found in Earth's troposphere and artificial atmospheres.

Composition of Earth's atmosphere. The lower pie represents the trace gases which together compose 0.039% of the atmosphere. Values normalized for illustration. The numbers are from a variety of years (mainly 1987, with CO2 and methane from 2009) and do not represent any single source.
Air is mainly composed of nitrogen, oxygen, and argon, which together constitute the major gases of the atmosphere. The remaining gases are often referred to as trace gases, among which are the greenhouse gases such as water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Filtered air includes trace amounts of many other chemical compounds. Many natural substances may be present in tiny amounts in an unfiltered air sample, including dust, pollen and spores, sea spray, and volcanic ash. Various industrial pollutants also may be present, such as chlorine (elementary or in compounds), fluorine compounds, elemental mercury, and sulfur compounds such as sulfur dioxide [SO2].
In general, air pressure and density decrease in the atmosphere as height increases. However, temperature has a more complicated profile with altitude. Because the general pattern of this profile is constant and recognizable through means such as balloon soundings, temperature provides a useful metric to distinguish between atmospheric layers. In this way, Earth's atmosphere can be divided into five main layers. From highest to lowest, these layers are:
Exosphere
The outermost layer of Earth's atmosphere extends from the exobase upward. It is mainly composed of hydrogen and helium. The particles are so far apart that they can travel hundreds of kilometers without colliding with one another. Since the particles rarely collide, the atmosphere no longer behaves like a fluid. These free-moving particles follow ballistic trajectories and may migrate into and out of the magnetosphere or the solar wind.
Thermosphere
Temperature increases with height in the thermosphere from the mesopause up to the thermopause, then is constant with height. Unlike in the stratosphere, where the inversion is caused by absorption of radiation by ozone, in the thermosphere the inversion is a result of the extremely low density of molecules. The temperature of this layer can rise to 1,500 °C (2,700 °F), though the gas molecules are so far apart that temperature in the usual sense is not well defined. The air is so rarefied, that an individual molecule (of oxygen, for example) travels an average of 1 kilometer between collisions with other molecules. The International Space Station orbits in this layer, between 320 and 380 km (200 and 240 mi). Because of the relative infrequency of molecular collisions, air above the mesopause is poorly mixed compared to air below. While the composition from the troposphere to the mesosphere is fairly constant, above a certain point, air is poorly mixed and becomes compositionally stratified. The point dividing these two regions is known as the turbopause. The region below is the homosphere, and the region above is the heterosphere. The top of the thermosphere is the bottom of the exosphere, called the exobase. Its height varies with solar activity and ranges from about 350–800 km (220–500 mi; 1,100,000–2,600,000 ft).
Mesosphere
The mesosphere extends from the stratopause to 80–85 km (50–53 mi; 260,000–280,000 ft). It is the layer where most meteors burn up upon entering the atmosphere. Temperature decreases with height in the mesosphere. The mesopause, the temperature minimum that marks the top of the mesosphere, is the coldest place on Earth and has an average temperature around −85 °C (−120 °F; 190 K).  At the mesopause, temperatures may drop to −100 °C (−150 °F; 170 K). Due to the cold temperature of the mesosphere, water vapor is frozen, forming ice clouds (or Noctilucent clouds). A type of lightning referred to as either sprites or ELVES, form many miles above thunderclouds in the troposphere.
Stratosphere
The stratosphere extends from the tropopause to about 51 km (32 mi; 170,000 ft). Temperature increases with height due to increased absorption of ultraviolet radiation by the ozone layer, which restricts turbulence and mixing. While the temperature may be −60 °C (−76 °F; 210 K) at the tropopause, the top of the stratosphere is much warmer, and may be near freezing. The stratopause, which is the boundary between the stratosphere and mesosphere, typically is at 50 to 55 km (31 to 34 mi; 160,000 to 180,000 ft). The pressure here is 1/1000 sea level.
Troposphere
The troposphere begins at the surface and extends to between 9 km (30,000 ft) at the poles and 17 km (56,000 ft) at the equator, with some variation due to weather. The troposphere is mostly heated by transfer of energy from the surface, so on average the lowest part of the troposphere is warmest and temperature decreases with altitude. This promotes vertical mixing (hence the origin of its name in the Greek word  trope, meaning turn or overturn). The troposphere contains roughly 80% of the mass of the atmosphere. The tropopause is the boundary between the troposphere and stratosphere.
Other layers
Within the five principal layers determined by temperature are several layers determined by other properties.
•    The ozone layer is contained within the stratosphere. In this layer ozone concentrations are about 2 to 8 parts per million, which is much higher than in the lower atmosphere but still very small compared to the main components of the atmosphere. It is mainly located in the lower portion of the stratosphere from about 15–35 km (9.3–22 mi; 49,000–110,000 ft), though the thickness varies seasonally and geographically. About 90% of the ozone in our atmosphere is contained in the stratosphere.
•    The ionosphere, the part of the atmosphere that is ionized by solar radiation, stretches from 50 to 1,000 km (31 to 620 mi; 160,000 to 3,300,000 ft) and typically overlaps both the exosphere and the thermosphere. It forms the inner edge of the magnetosphere. It has practical importance because it influences, for example, radio propagation on the Earth. It is responsible for auroras.
•    The homosphere and heterosphere are defined by whether the atmospheric gases are well mixed. In the homosphere the chemical composition of the atmosphere does not depend on molecular weight because the gases are mixed by turbulence. The homosphere includes the troposphere, stratosphere, and mesosphere. Above the turbopause at about 100 km (62 mi; 330,000 ft) (essentially corresponding to the mesopause), the composition varies with altitude. This is because the distance that particles can move without colliding with one another is large compared with the size of motions that cause mixing. This allows the gases to stratify by molecular weight, with the heavier ones such as oxygen and nitrogen present only near the bottom of the heterosphere. The upper part of the heterosphere is composed almost completely of hydrogen, the lightest element.
•    The planetary boundary layer is the part of the troposphere that is nearest the Earth's surface and is directly affected by it, mainly through turbulent diffusion. During the day the planetary boundary layer usually is well-mixed, while at night it becomes stably stratified with weak or intermittent mixing. The depth of the planetary boundary layer ranges from as little as about 100 m on clear, calm nights to 3000 m or more during the afternoon in dry regions.
The average temperature of the atmosphere at the surface of Earth is 14 °C (57 °F; 287 K) or 15 °C (59 °F; 288 K), depending on the reference.

Composition of Earth’s Atmosphere:

Gas    Volume
Nitrogen (N2)
780,840 ppmv (78.084%)
Oxygen (O2)
209,460 ppmv (20.946%)
Argon (Ar)
9,340 ppmv (0.9340%)
Carbon dioxide (CO2)
390 ppmv (0.039%)
Neon (Ne)
18.18 ppmv (0.001818%)
Helium (He)
5.24 ppmv (0.000524%)
Methane (CH4)
1.79 ppmv (0.000179%)
Krypton (Kr)
1.14 ppmv (0.000114%)
Hydrogen (H2)
0.55 ppmv (0.000055%)
Nitrous oxide (N2O)
0.3 ppmv (0.00003%)
Carbon monoxide (CO)
0.1 ppmv (0.00001%)
Xenon (Xe)
0.09 ppmv (9×10−6%) (0.000009%)
Ozone (O3)
0.0 to 0.07 ppmv (0 to 7×10−6%)
Nitrogen dioxide (NO2)
0.02 ppmv (2×10−6%) (0.000002%)
Iodine (I2)
0.01 ppmv (1×10−6%) (0.000001%)
Ammonia (NH3)
trace
Not included in above dry atmosphere:
Water vapor (H2O)
~0.40% over full atmosphere, typically 1%-4% at surface

Physical properties
Pressure and thickness
The average atmospheric pressure at sea level is about 1 atmosphere (atm) = 101.3 kPa (kilopascals) = 14.7 psi (pounds per square inch) = 760 torr = 29.92 inches of mercury (symbol Hg). Total atmospheric mass is 5.1480×1018 kg (1.135×1019 lb), about 2.5% less than would be inferred from the average sea level pressure and the Earth's area of 51007.2 megahectares, this portion being displaced by the Earth's mountainous terrain. Atmospheric pressure is the total weight of the air above unit area at the point where the pressure is measured. Thus air pressure varies with location and weather.
If atmospheric density were to remain constant with height the atmosphere would terminate abruptly at 8.50 km (27,900 ft). Instead, density decreases with height, dropping by 50% at an altitude of about 5.6 km (18,000 ft). As a result the pressure decrease is approximately exponential with height, so that pressure decreases by a factor of two approximately every 5.6 km (18,000 ft) and by a factor of e = 2.718… approximately every 7.64 km (25,100 ft), the latter being the average scale height of Earth's atmosphere below 70 km (43 mi; 230,000 ft). However, because of changes in temperature, average molecular weight, and gravity throughout the atmospheric column, the dependence of atmospheric pressure on altitude is modeled by separate equations for each of the layers listed above. Even in the exosphere, the atmosphere is still present. This can be seen by the effects of atmospheric drag on satellites.
In summary, the equations of pressure by altitude in the above references can be used directly to estimate atmospheric thickness. However, the following published data are given for reference:
•    50% of the atmosphere by mass is below an altitude of 5.6 km (18,000 ft).
•    90% of the atmosphere by mass is below an altitude of 16 km (52,000 ft). The common altitude of commercial airliners is about 10 km (33,000 ft) and Mt. Everest's summit is 8,848 m (29,029 ft) above sea level.
•    99.99997% of the atmosphere by mass is below 100 km (62 mi; 330,000 ft), although in the rarefied region above this there are auroras and other atmospheric effects. The highest X-15 plane flight in 1963 reached an altitude of 108.0 km (354,300 ft).
Density and mass
The density of air at sea level is about 1.2  kg/m3 (1.2 g/L). Density is not measured directly but is calculated from measurements of temperature, pressure and humidity using the equation of state for air (a form of the ideal gas law). Atmospheric density decreases as the altitude increases. This variation can be approximately modeled using the barometric formula. More sophisticated models are used to predict orbital decay of satellites.
The average mass of the atmosphere is about 5 quadrillion (5×1015) tonnes or 1/1,200,000 the mass of Earth. According to the American National Center for Atmospheric Research, "The total mean mass of the atmosphere is 5.1480×1018 kg with an annual range due to water vapor of 1.2 or 1.5×1015 kg depending on whether surface pressure or water vapor data are used; somewhat smaller than the previous estimate. The mean mass of water vapor is estimated as 1.27×1016 kg and the dry air mass as 5.1352 ±0.0003×1018 kg."
Optical properties
Solar radiation (or sunlight) is the energy the Earth receives from the Sun. The Earth also emits radiation back into space, but at longer wavelengths that we cannot see. Part of the incoming and emitted radiation is absorbed or reflected by the atmosphere.
Scattering
When light passes through our atmosphere, photons interact with it through scattering. If the light does not interact with the atmosphere, it is called direct radiation and is what you see if you were to look directly at the Sun. Indirect radiation is light that has been scattered in the atmosphere. For example, on an overcast day when you cannot see your shadow there is no direct radiation reaching you, it has all been scattered. As another example, due to a phenomenon called Rayleigh scattering, shorter (blue) wavelengths scatter more easily than longer (red) wavelengths. This is why the sky looks blue, you are seeing scattered blue light. This is also why sunsets are red. Because the Sun is close to the horizon, the Sun's rays pass through more atmosphere than normal to reach your eye. Much of the blue light has been scattered out, leaving the red light in a sunset.
Absorption
Different molecules absorb different wavelengths of radiation. For example, O2 and O3 absorb almost all wavelengths shorter than 300 nanometers. Water (H2O) absorbs many wavelengths above 700 nm. When a molecule absorbs a photon, it increases the energy of the molecule. We can think of this as heating the atmosphere, but the atmosphere also cools by emitting radiation, as discussed below.
The combined absorption spectra of the gases in the atmosphere leave "windows" of low opacity, allowing the transmission of only certain bands of light. The optical window runs from around 300 nm (ultraviolet-C) up into the range humans can see, the visible spectrum (commonly called light), at roughly 400–700 nm and continues to the infrared to around 1100 nm. There are also infrared and radio windows that transmit some infrared and radio waves at longer wavelengths. For example, the radio window runs from about one centimeter to about eleven-meter waves.
Emission
Emission is the opposite of absorption, it is when an object emits radiation. Objects tend to emit amounts and wavelengths of radiation depending on their "black body" emission curves, therefore hotter objects tend to emit more radiation, with shorter wavelengths. Colder objects emit less radiation, with longer wavelengths. For example, the Sun is approximately 6,000 K (5,730 °C; 10,340 °F), its radiation peaks near 500 nm, and is visible to the human eye. The Earth is approximately 290 K (17 °C; 62 °F), so its radiation peaks near 10,000 nm, and is much too long to be visible to humans.
Because of its temperature, the atmosphere emits infrared radiation. For example, on clear nights the Earth's surface cools down faster than on cloudy nights. This is because clouds (H2O) are strong absorbers and emitters of infrared radiation. This is also why it becomes colder at night at higher elevations. The atmosphere acts as a "blanket" to limit the amount of radiation the Earth loses into space.
The greenhouse effect is directly related to this absorption and emission (or "blanket") effect. Some chemicals in the atmosphere absorb and emit infrared radiation, but do not interact with sunlight in the visible spectrum. Common examples of these chemicals are CO2 and H2O. If there are too much of these greenhouse gases, sunlight heats the Earth's surface, but the gases block the infrared radiation from exiting back to space. This imbalance causes the Earth to warm, and thus climate change.
Refractive index
The refractive index of air is close to, but just greater than 1. Systematic variations in refractive index can lead to the bending of light rays over long optical paths. One example is that, under some circumstances, observers onboard ships can see other vessels just over the horizon because light is refracted in the same direction as the curvature of the Earth's surface.
The refractive index of air depends on temperature, giving rise to refraction effects when the temperature gradient is large. An example of such effects is the mirage.

Atmospheric circulation is the large-scale movement of air through the troposphere, and the means (with ocean circulation) by which heat is distributed around the Earth. The large-scale structure of the atmospheric circulation varies from year to year, but the basic structure remains fairly constant as it is determined by the Earth's rotation rate and the difference in solar radiation between the equator and poles.
Evolution of Earth's atmosphere
Earliest atmosphere
The out gassings of the Earth were stripped away by solar winds early in the history of the planet until a steady state was established, the first atmosphere. Based on today's volcanic evidence, this atmosphere would have contained 60% hydrogen, 20% oxygen (mostly in the form of water vapor), 10% carbon dioxide, 5 to 7% hydrogen sulfide, and smaller amounts of nitrogen, carbon monoxide, free hydrogen, methane and inert gases.
A major rainfall led to the buildup of a vast ocean, enriching the other agents, first carbon dioxide and later nitrogen and inert gases. A major part of carbon dioxide exhalations were soon dissolved in water and built up carbonate sediments.
Second atmosphere
Water-related sediments have been found dating from as early as 3.8 billion years ago. About 3.4 billion years ago, nitrogen was the major part of the then stable "second atmosphere". An influence of life has to be taken into account rather soon in the history of the atmosphere, since hints of early life forms are to be found as early as 3.5 billion years ago. The fact that this is not perfectly in line with the - compared to today 30% lower - solar radiance of the early Sun has been described as the "faint young Sun paradox".
The geological record however shows a continually relatively warm surface during the complete early temperature record of the Earth with the exception of one cold glacial phase about 2.4 billion years ago. In the late Archaean eon an oxygen-containing atmosphere began to develop, apparently from photosynthesizing algae which have been found as stromatolite fossils from 2.7 billion years ago. The early basic carbon isotopy (isotope ratio proportions) is very much in line with what is found today, suggesting that the fundamental features of the carbon cycle were established as early as 4 billion years ago.

Third atmosphere
The accretion of continents about 3.5 billion years ago added plate tectonics, constantly rearranging the continents and also shaping long-term climate evolution by allowing the transfer of carbon dioxide to large land-based carbonate storages. Free oxygen did not exist until about 1.7 billion years ago and this can be seen with the development of the red beds and the end of the banded iron formations. This signifies a shift from a reducing atmosphere to an oxidising atmosphere. O2 showed major ups and downs until reaching a steady state of more than 15%.  The following time span was the Phanerozoic eon, during which oxygen-breathing metazoan life forms began to appear.

Hydrosphere

All the water in its various forms, present in all the components of Earth’s environment together constitutes the hydro¬sphere. Most of the water is present as liquid water on the Earth’s surface and some liquid water is present underground. Apart of total water is present as snow or ice on the Earth’s surface while a substantial part of water in Earth’s environment is also present as water vapor in the atmosphere. General fea¬tures of various parts of the Earth’s hydrosphere are as given below:
1. Oceans: Major part of water is present in the oceans of the Earth. Average depth of oceans is about 3.7 kilometers and about 1300 million cubic kilometers water is present in oceans.
2. Ice sheets: Substantial quantity of water, about 24 million cubic kilometers, is present as solid in the ice sheets of Earth. About 90% of the volume of such water is found in Antarctica.
3. Groundwater: About 24 million cubic kilometer water is present under the ground surface at depths of upto two kilometers.
4. Lakes and rivers: On the land surface, approximately 0.18 million cubic kilometer water is present in lakes while about 0.002 million cubic kilometer water is found in rivers.
5. Atmospheric moisture: The amount of water present as water vapor in the atmosphere is about 0.013 million cubic kilometer.
6. Biological water: In addition to above categories, about 0.001 million cubic kilometer water is contained in the bodies of living organisms.
The Earth did not have any hydrosphere in the beginning. It is thought that hydrosphere emerged as the result of processes taking place in the lithosphere. These processes released a substantial quantity of water vapor and juvenile waters during the geological history of Earth. Further, the amounts of water present in oceans, as ice and in atmosphere have fluctuated with appearance and disappearance of major periods of glaciations on the Earth. Palaeogeographic data indicates that the level of oceans on Earth had declined by more than 100 meters during the age of greatest glaciations during Quaternary period. It is estimated that if ice sheets present on Earth today were to melt completely, the level of oceans of Earth would rise by about 66 meters.
Most important feature of global environment of Earth is the hydrological cycle which determines the distribution of water on Earth’s surface and in the atmosphere. The water is evaporated from surface of water bodies like oceans, lakes, rivers etc. as water vapor into atmosphere. It is also absorbed by plants from soil and lost as vapor to atmosphere through transpiration. The water vapor condenses in the atmosphere to form precipitation and thus water is returned from the atmosphere to the surface of Earth. Cyclic movement of water in different components of the global environment is termed hydrological cycle.
OCEAN-WATER
Major portion of earth’s surface is covered with oceans. The water body in the oceans absorbs a large amount of solar radia¬tion and has far reaching impact on the heat balance of earth. The water in oceans moves up and down as well as from one place to other. These movements of oceanic water result in transfer of heat from one place to other and have fundamental influence on various components of water and energy balance of land and oceans. The distribution of water balance components has major role in creating and maintaining the climatic and weather conditions in different regions of earth. Therefore, a brief discussion of various aspects of ocean water, ocean currents and waves has been given below.
COMPOSITION OF SEA WATER
Sea water may be described as a brine i.e. the solution of dissolved salts which have accumulated over past periods of geological time from the inflow of runoff water from the land masses. On the land masses, the salts have been formed by the process of weathering of rocks in which weak acids corrode and dissolve the rocks forming various minerals. Due to evaporation of water from oceans, the concentration of salts in the sea water rises resulting in rise of salinity
The composition of sea water results in important properties which are important in understanding its role in Earth’s environment. One way to describe the composition of sea water is to state the principal ingredients that would be required to make an artificial brine approximately like sea water. These ingredi¬ents are listed below:
Ingradients in ocean water.

Salt    gm salt per 1000 gm water
Sodium chloride (NaCl)    23
Magnesium chloride (MgCl2)    5
Sodium sulfate (Na2SO4)    4
Calcium chloride (CaCl2)    1
Potassium chloride (KCl)    0.7
With other minor ingredients to total    34.5

Of the various elements combined in these salts, Chlorine alone makes up 55% by weight of all the dissolved matter and Sodium 31%. In addition to elements of above listed five salts, less abundant but important are Bromine, Carbon, Strontium, Boron, Silicon and Fluorine. At least some traces of half of the known elements can be found in sea water. Small amounts of all the gases of the atmosphere are also present in dissolved form in the sea water. Chief among these are Nitrogen, Oxygen, Carbon dioxide, hydrogen and argon.
SALINITY OF SEA WATER
Salinity is described as the proportion of dissolved salts to pure water. It is usually stated in units of parts per thousand by weight and is designated by a special symbol 0/00. The total figure 34.5 0/00 in the above table represents 3.45 percent. Salinity of sea water varies slightly from one place to other in the oceans. Where diluted by abundant rainfall, as in the equatorial oceans, the salinity may be between 34.5 and 35.0 0/00, whereas in the subtropical high-pressure belts, where extreme dryness prevails, the evaporation may increase the salin-ity of surface sea water upto 35.5 0/00.
DISSOLVED GASES
Most important gases dissolved in the oceanic water are oxygen and carbon dioxide. The quantity of oxygen dissolved in the ocean¬ic water changes within wide boundaries depending on the tempera¬ture, living activities and certain other factors. The concentra¬tion of carbon dioxide dissolved in sea water also changes but such change is has negligible importance since the overall quan¬tity of carbon dioxide dissolved in sea water is about sixty times more than its amount in atmosphere. Carbon dioxide in sea water is assimilated by autotrophic organisms during photosynthe¬sis and enters the organic matter cycle. Part of such assimilated carbon dioxide returns back to ocean water by respiration and after death and decomposition of living organisms but a substan¬tial part of such assimilated gas is deposited as carbonate sediments at the bottoms of oceans.


DENSITY OF WATER
The density of water is given in grams per cubic centimeter. The density of pure water is greatest at 4 degrees C. At this tempera¬ture, one cubic centimeter of water weighs exactly one gram i.e. its density is 1.000. However, the density of sea water ranges from 1.027 to 1.028. Two factors determine the density of sea water: salinity and temperature. Density increases with salinity and with low temperature upto -2 degrees C.
Density of sea water is of prime importance in circulation of ocean waters because slight density differences causes water to move. Where density of sea water increases by lowering of temperature of evaporation at the surface, the water tends to sink displacing less dense water below it. Just like convection wind systems, such vertical movements of sea water are also described as convectional currents.
OCEAN CURRENTS
Surface winds and density differences are two most important factors in generating and controlling ocean currents. Another factor affecting ocean current is the configuration of ocean basins and coasts.
Virtually all of the important surface currents of the oceans are set in motion by prevailing winds. Energy of winds is transferred to sea water by the frictional drag of the air blowing over the water surface. The Coriolis force impels the water drift toward the right of its path of motion in Northern hemisphere and, therefore, the current at the water surface is in the direction approximately 45 degrees to the right of the wind direc¬tion. Under the influence of winds, currents may tend to bank up the water close to the coast of a continent, in which case the force of gravity, tending to equalize the water level, will cause other currents to be set up.
Density differences in oceans arise from greater heating by insolation or greater cooling by radiation, in one place than another. Thus the surface water chilled in the arctic and polar seas sinks to the ocean floor and spreads equatorward displacing upward the warmer, less dense water. Density differences can also be set up due to salinity differences. A currents tends to flow from the area of low salinity to the area of higher salinity, but this flow is also deflected by Coriolis force through a right angle in Northern hemisphere so that the flow is actually paral¬lel with the slope of density gradient between the two places.
Configuration of ocean basins and coasts also affects the ocean currents. Currents initially set up by winds impinge upon a coast and are locally deflected to a different path or are confined in straits or gulfs.
The combined action of wind and density differences sets up the global oceanic circulation system including not only horizontal motions but vertical upswelling and down-sinking motions also. Oceanic currents of a shallow surface water zone have strong climatic influence upon overlaying layer of atmosphere and, hence, have been briefly discussed below.
GENERALIZED SCHEME OF OCEAN CURRENTS
There are a number of defined and permanent oceanic currents involving different oceans at the global scale. They have important impact on the climatic conditions of different regions of earth. It is now well recognized that oceanic circulation involves the complex motions of water masses of different temperatures and salinity characteristics. Important such movements have been briefly described below. However, this account does not take into account the movements of water masses at different depths.
1. Most striking features of generalized oceanic currents are the gyrals. These are circular movements of sea water around the subtropical highs, centered about 25 to 30 degrees S.
2. Two equatorial currents marks the belt of trade winds. Whereas the trade winds blow to the southwest and northwest obliquely across the parallels of latitudes, the water movement of this current flows the parallels. Thus the currents are turnd at an angle of about 45 degrees with the prevailing surface winds, because of the deflective force of the earth’s rotation.
3. The equatorial countercurrent separates the north and south equatorial currents and flows in opposite direction to them. It is well developed in the Pacific, Atlantic and Indian oceans.
4. Along the west sides of the oceans in low latitudes, the equatorial currents turn poleward forming warm currents parallel¬ing the coasts. Examples of such oceanic currents are Gulf Stream (Florida and Caribbean stream), Japan current (Kuroshio) and the Brazil current. These currents bring higher than average tempera¬tures along the respective coasts.
5. The west-wind drift is the slow eastward movement of oceanic water over the zone of westerlies. It covers a broad belt between 35degrees and 45degrees in the Northern hemisphere and between 30-35degrees to 70-75 degrees in Southern hemisphere where open ocean exists in the higher latitudes.
6. The west-wind drift, upon approaching the east side of the ocean is deflected both south and north along the coast. This results in equatorward flow of cool current produced by upwelling of colder water from greater depths. The Humboldt current (Peru current) off the coast of Chile and Peru, Benguela current off the southwest African coast, California current off the west coast of U.S.A. and Canaries current off the Spanish and North African coast are such currents. These currents bring colder than average temperatures along the respective coasts.
7. The North Atlantic current is a relatively warm current formed in the northern eastern Atlantic Ocean due to poleward deflection of west-wind drift. The current spreads around the British Isles, into the North Sea and along the Norwegian coast bringing about warming effect in summers alongwith it. This effect is more pronounced in winters.
8. In the Northern hemisphere, where the polar sea is largely landlocked, cold water flows equatorward along the west side of the large straits connecting the Arctic oceans with the Atlantic basin. Three such cold currents are Kamchatka current flowing southward along the Kamchatka Peninsula and Kurile Islands, Labrador current moving south from the Baffin Bay area through Davis Strait to reach the coast of Newfoundland, Nova Scotia and New England.
9. In both north Atlantic and Pacific oceans, the Icelandic and Aleutian lows coincide in a very rough manner with two centers of counterclockwise circulation involving the cold arctic currents and the west-wind drifts.
10. The Antarctic region has a relatively simple current scheme. It consists of a single Antarctic circumpolar current moving clockwise around the Antarctic continent in latitudes 50 to 60 degrees S where the expanse of open ocean exists. well.
OCEAN WAVES
Almost all the waves in the oceans that can be seen and felt, are produced by wind. The energy of moving air is transferred to water wave motion and this can, in turn, be expended upon the coasts of the lands causing the landforms of erosion and deposition. Thus ocean waves have important role in global energetics and coastal environments. A brief description of the waves in deep water, their growth and decay is given below.
The ocean waves generated by wind belong to a type known as progressive oscillatory waves since wave form travels through the water and causes an oscillatory water motion. Following terms are used in description of waves:
i.    Wave height: It is the vertical distance between the trough and the crest of the wave and is usually measured in feet or meters.
ii.    Wave length: It is the horizontal distance from trough to trough or crest to crest and is also stated in feet or meters.
iii.    Wave velocity: It is the speed at which wave advances through water and is given in feet or meter per second or in knots (nautical miles per hour).
iv.    Period: It is the time elapsed between successive passages of wave crests past a fixed point and is given in seconds.
In the progressive oscillatory wave a tiny particle, such as a drop of water or small floating object, completes one vertical circle, or orbit, with passage of each wave length. Particles move forward on the wave crest, backward in the wave trough. At the sea surface the orbit is of same diameter as the wave height, but dies out rapidly with depth.
In the long waves the water particles return to the same starting point at the completion of each orbit. Hence there is no net motion in the direction of the wind. Only the energy of the wave and its form are transmitted through the water. However, in case of steep, high waves the orbits are not perfect circles. The particle moves just a bit faster forward when on the crest than when it returns in the trough, so that at the end of each circuit the particle makes a slight advance. This produces a very slow surface drift in the direction in which waves are traveling. The rate of this drift is called mass transport velocity. Under favorable conditions, the flow may reach a velocity as high as two knots and will tend to raise the water level along a coast against which the waves are breaking. This motion is not the same as set up by wind friction.
Ocean waves are usually not simple parallel crests and troughs. Instead, they appear highly irregular in height and form because of the interference among several wave trains that are normally present. These trains are not only of different periods, but travel in slightly different directions, so as to intersect at many points. Where two wave crests intersect, the wave height is increased, forming a peak. Where two troughs intersect, the depression is accentuated.
Two forms are waves are usually recognized: wind waves and swell.
(a) Wind waves: These are waves that are being formed and active¬ly maintained by the wind. These grow through two mechanisms:
(1) The direct push of wind upon the windward slope of wave drives it forward, just as with floating object.
(2) The skin drag of air flowing over the water surface exerts a pull in the direction of wave motion. Over the wave crest, where drag is strongest, the orbital movement is supplemented adding energy to the wave. In the trough, which is protected, drag is weaker, hence does not counteract the reverse orbital movement as strongly as it is assisted on the crests. This results in a steady increase in the wave height and wave length to some maximum point possible under given wind strength. The wind waves commonly reach speeds much faster than the winds that produce and sustain them. This condition is possible only through the mechanism of skin drag.
The maximum height to which wind waves can grow is controlled by three factors.
(i) Wind velocity: It is obviously a major factor since this determines the amount of energy that can be supplied to the wave.
(ii) Duration of wind: This determines whether or not the waves the opportunity to grow to maximum size.
(iii) Available expanse of water (fetch): This is important because the waves travel as they grow. If waves are developed in a very large body of water over a period of many hours, so that neither duration nor fetch are limiting factors, the maximum wave height varies as the square of the wind velocity i.e.
Wave height (ft.) = 0.026 x Wind velocity2 (knots)s
This would represent the greatest waves to be expected.
Wind duration is important in the early stages of wave growth. Under strong winds, say 30 knots, waves will continue to grow for more than 32 hours although most rapid growth will be in the first 15 hours. Fetch may be an important limiting factor in small bays and straits but has no appreciable effect for water expanses greater than 1000 kilometers across.
(b) Swell: These consist of wind waves that have left the region where they were formed and are gradually dying out in a region of calm or lesser winds. As waves continue to grow, they not only increase their speed but also become longer i.e. their wave length increases. When they have passed beyond the region of strong winds that formed them, waves are transformed into a swell, consisting of very long, low waves of simple form and parallel, even crests. For each time that the swell has traveled a distance in nautical miles equivalent to its length in feet, the swell looses one-third of its height. The energy is lost by friction from air resistance.
SEISMIC SEA WAVES
When sudden displacements of large earth masses occur on the ocean floor, a series of waves is sent out across the ocean. The cause may be slippage along a fault, a volcanic eruption or a large submarine landslide. The waves thus produced are called the seismic sea waves or tsunami (Japanese). These waves are enormous in length (100 to 200 km) and the height of the waves upon reach¬ing the shore is observed to be as great as 50 meters in many cases or may even be upto 100 meters in rare instances. In the deep ocean, wave height is only a foot or two and because their length is much greater than the height, such waves may pass unnoticed by observes in a ship at sea. The period of such waves may be 10 to 30 minutes and the velocity of travel of the wave form may be 450 to 800 km per hour. Upon reaching the shallow water of a coastline, a seismic sea wave has the effect of caus¬ing an unusual rise of water level. The low areas are inundated and the wind waves which are superimposed upon them are able to break upon much higher grounds than normal.
SEA ICE ICEBERGS AND ICE ISLANDS
Large area in high latitudes of Arctic and Antarctic regions is characterized by presence of ice in various forms over the oceans. Sea ice, pack ice, icebergs and ice islands are important such forms of ice in these regions.
Sea ice is formed by direct freezing of ocean water. It begins to form when the surface water is cooled to temperatures of about -20C and is limited in thickness to about 5 meters be¬cause once the insulating layer of floating ice has been formed over the water, heat is supplied from the underlying water as rapidly as it is lost from upper surface. Surface zone of sea ice is composed of fresh water, the salt being excluded in the proc¬ess of freezing.
Pack ice is the name given to the ice that completely covers the sea surface. Under the force of wind and currents, pack ice breaks up into individual patches which are termed ice floes. Narrow strips of open water between such floes are called leads. Where ice floes are forcibly brought together by winds, the ice margins buckle and turn upward into pressure ridges resembling walls or irregular hummocks.
The North Polar Sea, which is surrounded by land masses, is normally covered by pack ice throughout the year, although open leads are numerous in the summer. The relatively warmer North Atlantic drift maintains an ice-free zone off the northern coast of Norway. In Antarctica, a vast ocean bounds the sea ice zone on the equatorward margin. Because the ice floes can drift freely north into warmer waters, the Antarctic ice pack does not spread beyond about 600S latitude in the cold season. In March, close to the end of the warm season, the ice margin shrinks to a narrow zone bordering the Antarctic continent.
Icebergs and ice islands differ from sea ice in origin and thickness. Icebergs are formed by breaking off or calving of the blocks from a valley glacier or tongue of an ice cap. These may be several hundred meters in thickness. Icebergs are only slight¬ly less dense than the sea water and so these float very low in the sea water, about 5/6th of the bulk lying below the water level. The ice in icebergs is fresh since it is formed of com¬pacted and re-crystallized snow. In the Northern hemisphere, icebergs are derived mostly from glacier tongues of the Greenland icecap. They drift slowly south with Labrador and Greenland currents and may find their way into North Atlantic sea in the vicinity of Grand Banks of Newfoundland. Icebergs of Antarctic region are distinctly different from those of arctic region. Whereas those of arctic region are irregular in shape and , therefore, present rather peaked outlines above sea water, the Antarctic icebergs are commonly tabular in form with flat tops and steep cliff-like sides. This is because the tabular icebergs are parts of ice shelves, the great, floating plate-like exten¬sions of the continental icecap. In dimensions, a large tabular iceberg of the Antarctic may be tens of kilometers broad and over 700 meters thick, with an ice wall rising 70-100 meters above sea level.
Ice islands of North Polar Sea are somewhat related to tabular icebergs of Antarctic in origin. These are huge plates of floating ice which may be 25 kilometers across and have an area of 300 to 400 square kilometers. The bordering ice cliff, 7 to 10 meters above sea level indicates an ice thickness of 70 meters or more. The few ice islands known are probably derived from a shelf of land-fast glacial ice attached to Ellesmere Island about 83degrees N latitude. The ice islands move slowly with the water drift of Polar Sea and a charting of their tracks reveals much about the circulation in that ocean.

Lithosphere

The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.
In the Earth, the lithosphere includes the crust and the uppermost mantle, which constitute the hard and rigid outer layer of the Earth. The lithosphere is underlain by the asthenosphere, the weaker, hotter, and deeper part of the upper mantle. The boundary between the lithosphere and the underlying asthenosphere is defined by a difference in response to stress: the lithosphere remains rigid for very long periods of geologic time in which it deforms elastically and through brittle failure, while the asthenosphere deforms viscously and accommodates strain through plastic deformation. The lithosphere is broken into tectonic plates. The uppermost part of the lithosphere that chemically reacts to the atmosphere, hydrosphere and biosphere through the soil forming process is called the pedosphere.
The concept of the lithosphere as Earth’s strong outer layer was developed by Joseph Barrell, who wrote a series of papers introducing the concept. The concept was based on the presence of significant gravity anomalies over continental crust, from which he inferred that there must exist a strong upper layer (which he called the lithosphere) above a weaker layer which could flow (which he called the asthenosphere). These ideas were expanded by the Harvard geologist Reginald Aldworth Daly in 1940 with his seminal work, Strength and structure of the Earth and have been broadly accepted by geologists and geophysicists. Although these ideas about lithosphere and asthenosphere were developed long before plate tectonic theory was articulated in the 1960s, the concepts that a strong lithosphere exists and that this rests on a weak asthenosphere are essential to that theory.
The lithosphere provides a conductive lid atop the convecting mantle; as such, it affects heat transport through the Earth.
There are two types of lithosphere:
•    Oceanic lithosphere, which is associated with Oceanic crust and exists in the ocean basins
•    Continental lithosphere, which is associated with Continental crust
The thickness of the lithosphere is considered to be the depth to the isotherm associated with the transition between brittle and viscous behavior. The temperature at which olivine begins to deform viscously (~1000°C) is often used to set this isotherm because olivine is generally the weakest mineral in the upper mantle. Oceanic lithosphere is typically about 50–100 km thick (but beneath the mid-ocean ridges is no thicker than the crust), while continental lithosphere has a range in thickness from about 40 km to perhaps 200 km; the upper ~30 to ~50 km of typical continental lithosphere is crust. The mantle part of the lithosphere consists largely of peridotite. The crust is distinguished from the upper mantle by the change in chemical composition that takes place at the Moho discontinuity.
Oceanic lithosphere
Oceanic lithosphere consists mainly of mafic crust and ultramafic mantle (peridotite) and is denser than continental lithosphere, for which the mantle is associated with crust made of felsic rocks. Oceanic lithosphere thickens as it ages and moves away from the mid-ocean ridge. This thickening occurs by conductive cooling, which converts hot asthenosphere into lithospheric mantle, and causes the oceanic lithosphere to become increasingly thick and dense with age. The thickness of the mantle part of the oceanic lithosphere can be approximated as a thermal boundary layer that thickens as the square root of time.
Here, h is the thickness of the oceanic mantle lithosphere, κ is the thermal diffusivity (approximately 10−6 m2/s), and t is time.
Oceanic lithosphere is less dense than asthenosphere for a few tens of millions of years, but after this becomes increasingly denser than asthenosphere. This is because the chemically-differentiated oceanic crust is lighter than asthenosphere, but due to thermal contraction, the mantle lithosphere is more dense than the asthenosphere. The gravitational instability of mature oceanic lithosphere has the effect that at subduction zones, oceanic lithosphere invariably sinks underneath the overriding lithosphere, which can be oceanic or continental. New oceanic lithosphere is constantly being produced at mid-ocean ridges and is recycled back to the mantle at subduction zones. As a result, oceanic lithosphere is much younger than continental lithosphere: the oldest oceanic lithosphere is about 170 million years old, while parts of the continental lithosphere are billions of years old. The oldest parts of continental lithosphere underlie cratons, and the mantle lithosphere there is thicker and less dense than typical; the relatively low density of such mantle "roots of cratons" helps to stabilize these regions.

The core is a layer rich in iron and nickel that is composed of two layers: the inner and outer cores. The inner core is theorized to be solid with a density of about 13 grams per cubic centimeter and a radius of about 1220 kilometers. The outer core is liquid and has a density of about 11 grams per cubic centimeter. It surrounds the inner core and has an average thickness of about 2250 kilometers.
The mantle is almost 2900 kilometers thick and comprises about 83% of the Earth's volume. It is composed of several different layers. The upper mantle exists from the base of the crust downward to a depth of about 670 kilometers. This region of the Earth's interior is thought to be composed of peridotite, an ultramafic rock made up of the minerals olivine and pyroxene. The top layer of the upper mantle, 100 to 200 kilometers below surface, is called the asthenosphere. Scientific studies suggest that this layer has physical properties that are different from the rest of the upper mantle. The rocks in this upper portion of the mantle are more rigid and brittle because of cooler temperatures and lower pressures. Below the upper mantle is the lower mantle that extends from 670 to 2900 kilometers below the Earth's surface. This layer is hot and plastic. The higher pressure in this layer causes the formation of minerals that are different from those of the upper mantle.
The lithosphere is a layer that includes the crust and the upper most portion of the asthenosphere. This layer is about 100 kilometers thick and has the ability to glide over the rest of the upper mantle. Because of increasing temperature and pressure, deeper portions of the lithosphere are capable of plastic flow over geologic time. The lithosphere is also the zone of earthquakes, mountain building, volcanoes, and continental drift.
The topmost part of the lithosphere consists of crust. This material is cool, rigid, and brittle. Two types of crust can be identified: oceanic crust and continental crust. Both of these types of crust are less dense than the rock found in the underlying upper mantle layer. Ocean crust is thin and measures between 5 to 10 kilometers thick. It is also composed of basalt and has a density of about 3.0 grams per cubic centimeter.
The continental crust is 20 to 70 kilometers thick and composed mainly of lighter granite.  The density of continental crust is about 2.7 grams per cubic centimeter. It is thinnest in areas like the Rift Valleys of East Africa and in an area known as the Basin and Range Province in the western United States (centered in Nevada this area is about 1500 kilometers wide and runs about 4000 kilometers North/South). Continental crust is thickest beneath mountain ranges and extends into the mantle. Both of these crust types are composed of numerous tectonic plates that float on top of the mantle. Convection currents within the mantle cause these plates to move slowly across the asthenosphere.