10A11 WHAT ARE MAPS?

Fundamentals of Maps

THE SPHERICAL earth is best represented bya three-dimensional model of the earth, called a 

globe. All parts of the earth can be shown on it in their true shape, area and location along with correct cardinal directions and distances. But it cannot be made large enough to include detailed surface features. Hence, maps are much more useful tool than globes. A map is a two-dimensional diagrammatic representation of the whole or part of the earth and its surface features both natural and cultural at a given scale on a flat surface. However, all that is shown on the map is not drawn to scale. Only distances and areas are drawn to scale. We are familiar with the maps of the earth surface. But it is possible to draw maps of other planets and even the sky when pertinent data becomes available to us. We already have crude maps of the Mars and the Moon. A map gives a picture of one or several of the elements of the earth’s surface and being creation of humans, it gives only those details, which its maker intends to give. Instead of showing the details in their true or visible shape and size, it uses symbols that may or may not have similarities with the shape and size of objects represented. Maps have unique advantage of showing objects or patterns that may be intangible or invisible.
For example, it may show political boundaries or rainfall pattern or crop distribution, which may not be marked on the ground. Thus, maps are basically symbolic drawing of visible as well as conceived locational and distributional patterns of whole or a part of the earth, the sky or any other heavenly body.

There are various ways by which the earth can be mapped:
(a) by freehand sketches and diagrams;
(b) by actual survey with the help of instruments like chain and tape, plane table, prismatic compass and theodolite etc.;
(c) by photographs (ground photographs/aerial photographs);
(d) by satellite and radar charts.
With the availability of high-speed computers and Global Positioning System
(GPS), digital mapping has emerged as an important tool of mapping.

The amount of information given in a map depends on:
• Scale;
• Projection;
• Conventional signs and symbols;
• Skill of the cartographer;
• Method of map making; and
• Requirement of the user.

Types of Maps
Maps are of different types. Each map is unique in its design, content and construction and hence, a type by itself. Maps are broadly classified on two bases: scale and purpose or content .
Based on the scale, there are two broad categories of maps:
• Large Scale; and
• Small Scale.

Large Scale : These maps represent small area of the earth on a large size of paper/cloth/plastic sheet with greater details. Examples of some of the large scale maps are:

(i) Cadastral Maps: The term cadastral is derived from French word cadastre’ meaning register of territorial property.The Cadastral maps are drawn to register the ownership of landed property by demarcating the boundaries of fields, buildings, etc. They are especially prepared by governments to realise land revenue and property taxes. The village maps of our country may be cited as an example of large scale maps. These maps are drawn on a very
large scale, varying from 16 cm to a km to 32 cm to a km so as to fill in all possible details. The city maps may also be included in this category.

(ii) Topographical Maps: These maps are also prepared on a fairly large scale. They are based on precise surveys conducted by the Survey of India, Dehradun. They show general surface features in detail both natural and cultural. Principal topographic features depicted on these maps are relief, drainage, swamps and lakes, forests, villages, towns, means of transport and communication like roads and railways, and canals. Indian toposheets are generally prepared on the scale of 1:50,000.

Small Scale : These maps represent large areas on a small sheet of paper. They have fewer details. Examples of small scale maps are Atlas and Wall maps. The maps included in this book are also small scale maps. They give only a general picture of the area represented.

(i) Wall Maps: These maps are generally drawn boldly so that they can be seen from a distance. They are used in classrooms and cater to a larger audience. These maps broadly show very large areas like world as a whole, hemispheres, continents, and countries, states and districts. The scale is smaller than that of topographical maps but larger than atlas maps.
(ii) Chorographical or Atlas Maps: The Atlas maps are drawn on a very small scale and give a highly generalised picture of the natural and cultural aspects such as the physical, climatic and economic conditions of different regions of the earth. Only a few atlases are prepared on a 1:1,000,000 scale like the ‘Times Atlas of the World’. The Registrar General of India brings out Census Atlas from time to time. National Atlas and Thematic Mapping Organisation (NATMO) is a well-known important organisation in our country that publishes all kinds of maps for various purposes depicting different parts of the country. Thematic maps are usually prepared on small scale highlighting specific themes such as relief, temperature, and political divisions. According to purpose or theme, maps could be broadly categorised as follow :
(a) Physical or Natural Maps(i) Orographic or relief maps represent features like mountains, plains, plateaus, drainage patterns, etc.
(ii) Bathymetric maps show the depth of the oceans and seas. They are also known as charts.
(iii) Geological maps represent rocks that form the crust of the earth, and their mode of occurrence and disposition.
(iv) Climate maps show average condition of temperature, pressure wind and
precipitation of the world or part of it over a long period of time.
(v) Natural vegetation maps show natural flora of an area or region.
(vi) Soil map exhibits various types of soils covering the area.
(vii) Weather maps denote the average condition of temperature, pressure, wind and precipitation over a short period, which may range from a day to a season.
(viii) Astronomical maps show the position of stars and planets in the sky.

(b) Cultural Maps
These maps show the man-made features or human aspects.
(i) Economic maps show distribution of important minerals, agricultural and
industrial products, and lines of transport and communication. They help in
assessing economic development and potential of the area covered by the map.
(ii) Political maps show boundaries between different countries and states within countries.
(iii) Historical maps show the past events and facts.
(iv) Social maps depict elements like language, caste, ethnic groups and religion.
(v) Land utilisation maps exhibit the character of land use.

(c) Military Maps
Maps used by Defence Services are called Military maps.
(i) General maps on a scale of 1:1,000,000 or more depict only the broad topographical features. They are used by the Defence Services for general planning purposes.
(ii) Maps having scales ranging from 1:1,000,000 to 1:500,000 are often
classified as strategic maps. These maps are used for planning concentrated
military action.
(iii) Maps with a scale of 1:500,000 or less are called tactical maps. They serve as guides to small units like battalions and patrol units prior to and during movements anywhere near the front line.
(iv) Photomap is an air photograph with strategic and tactical data superimposed on it.

All maps have some common elements. Location and distribution of various features and phenomena are depicted using distance, direction, and conventional signs and symbols.




Scale
Scale is the relationship between the distance on a map and the real distance on the earth’s surface. It may be expressed as a representative fraction (ratio), a line scale or a statement scale. It is an important element of a map because it gives relative picture of the ground reality. As you have read earlier, maps are generally classified into large scale and small scale. However, there is no universally accepted standard for classifying maps according to scale. What one considers to be large, may appear to be small or medium for others. The same person may consider a map to be of large scale for one purpose but of small scale for another purpose. As a result of this each specialised group of map users sets up its own standards for classification.
As a matter of convention, maps having a scale 1:50,000 upto are classified as large scale maps, those falling between 1:50,000 and 1:1,000,000 as medium scale maps and those having scales above 1:1,000,000 are treated as small scale maps. The million sheets of the Survey of India and the National Atlas of India are considered to be medium scale maps.
Methods of Expressing Scales

The scales can be expressed in three ways:

1. Statement: The scale may be indicated in the form of a written statement. For example 1cm on the map represents 1 km on the ground. The scale is written as 1 cm to 1 km. This means that 1 cm on the map corresponds to 1 km on the ground. Although it is simple to express in words, it is difficult for those who are not familiar with the unit of measurement used. Besides, the scale will not be the same when the original map is reduced or enlarged. As such, this method is not very useful.

2. Representative Fraction (R.F.): It is also called as numerical scale. It is expressed as a ratio of map distance and ground distance. For example 1:1,000,000 means one unit of distance on the map corresponds to 1,000,000 units of distance on the ground. The advantage of R.F. is that it can be used universally irrespective of the local unit of measurement of distance. The map
can be reduced or enlarged without changing the R.F.

3. Linear Scale or Graphical Scale: This scale is expressed as a horizontal or straight line. The base is calibrated to express visual equivalents of representative fraction or verbal scale. The bases are divided into a number of equal parts and are marked to show what these divisions represent on actual ground. The scale has the advantage that it remains true even after reduction or enlargement of the map. However, it is useful only to those who are familiar with the particular unit of measurement.

Distances on the map are smaller than the corresponding distances on actual ground. Scale is the means which enables us to reduce the whole or a part of the earth to a size which is not only convenient and handy but also logical and scientific. A general definition of scale is that it is a ratio between the distance on a map and the corresponding distance on the earth. For example if two points located 10 km apart are shown 1 cm apart on a map, then the scale of the map would be 1 cm to 10 km. It may also be converted into R.F as given below.
Suppose, 1 cm = 10 KM

MAP DISTANCE

_______________

GROUND DISTANCE

=

1 cm

_____

10 km

=

1 cm

____________

10 X 10,000 cm

= 1: 1,000,000

(Note: 1 km has 100,000 cm.)

Methods of Measuring Linear Distance
Linear distance on maps are of two types:

(i) Straight lines like roads, railway line, and canals;
(ii) Curved or Zigzag lines, showing streams, coastline etc.

(a) When the line is straight, the distance can be measured with the help of a divider.
Open a pair of dividers and place one of its legs at one end of the straight distance and the other leg on the other end on the map. Then lift the divider and place it on the calibrated bar scale to get the distance on the ground

(b) When the route is zigzag, place one end of a thread at the starting point and carry the thread along the line. After completing the distance stretch the thread and measure the length. It will give approximate distance.
It can also be measured with the help of a plain divider, as explained earlier, but the measurement will be less accurate. It can also be done with the help of a strip of paper.

(c) Rotameter is an instrument having a route measuring wheel. Distance between two points is measured by allowing the wheel of the rotameter to move along the route.
Direction
Direction has been defined as an imaginary straight line on the map or the ground showing the angular position of various maps with respect to a common base direction. The line pointing to the north is regarded as the zero direction or base direction line.
A map must have the base directions represented on it to enable the user to locate different features with respect to each other. North, south, east and west are the four major directions. These are also called cardinal points. In between cardinal points one may have several intermediate directions.


A rough estimation of direction of the true north can be obtained by the Means of an ordinary watch. In the northern hemisphere if the watch is held in the horizontal position and turned until its hour hand points to the sun, the line that bisects the angle made by the hour hand with the line joining 12’O clock through the centre of the watch will point to the south. A similar
exercise in the southern hemisphere will indicate the true north. This is also a crude method dependent upon the sun .
LegendsEvery map contains a legend or a key. It lists the features and the signs or symbols used in the map for showing these features. As you know various types of features or phenomena are represented on maps. They relate to both land and sea and are shown with the help of conventional signs and symbols. The signs and symbols include lines, icons, alphabets, shadings and colours. As a convention, specific colours show certain area features.
Review Questions1. Answer the following questions:
(i) What is a map? What are its advantages?
(ii) What is a scale? What purpose does it serve on a map?
(iii) What are the small scale maps? Give two examples of small scale maps.
(iv) What is a cadastral map?
(v) What are cardinal points?
2. Write short notes on:
(i) Wall maps (ii) Linear Scale (iii) Base direction line.
3. In how many ways the earth can be mapped and what factors determine the amount of information given in a map? Discuss.
4. Describe various types of maps and their uses.
5. What are the different ways of expressing scale? Explain the merits and limitations of each method giving suitable examples.
Land use Colour
Cultivated area Yellow
Forests Dark green
Grasslands Light green
Built up area Brown
Water features Blue
(Ponds, oceans, lakes, seas, rivers)
You will know about conventional signs in detail in the chapter on study of topographic maps.

G3 -oceans - will be edited- testing

The Hydrosphere

The surface of the Earth is made of land and water. All the water bodies (71%) on the Earth are together called the hydrosphere. The hydrosphere is often called the "water sphere" as it includes all the earth's water found in streams, lakes, the soil, groundwater, and in the air. The hydrosphere interacts with, and is influenced by, all the other earth spheres. The water of the hydrosphere is distributed among several different stores found in the other spheres. Water is held in oceans, lakes and streams at the surface of the earth. Water is found in vapor, liquid and solid states in the atmosphere. The biosphere serves as an interface between the spheres enabling water to move between the hydrosphere, lithosphere and atmosphere as is accomplished by plant transpiration. The hydrologic cycle traces the movement of water and energy between these various stores and spheres.

Distribution of water
The world's oceans contain 97% of the water in the hydrosphere, most of which is salt water. Ice caps, like that found covering Antarctica, and glaciers that occupy high alpine locations, compose a little less than 2% of all water found on earth. Seemingly a small amount, the water stored as ice in glaciers would have a great impact on the environment if it were to melt into a liquid. One fear is that global warming will cause the melting and collapse of large ice sheets resulting in sea level rise. Rising sea levels could devastate coastal cities, displace millions of people, and wreak havoc on freshwater systems and habitats.

Water beneath the surface comprises the next largest store of water. Groundwater and soil water together make up about .5% of all water (by volume). There is a difference between ground water and soil water. Soil water is the water held in pore spaces between soil particles. Soil pore spaces usually are partially void of water most of the time but fill with water after a rain storm. Groundwater, on the other hand, is found where earth materials are saturated throughout the year. That is, the pore spaces are always occupied with water. Both soil and groundwater are very important sources of water. Soil water is available for plants to extract and use. Groundwater is an important source of water for irrigation and drinking water supplies.
Above the surface water is found stored in streams, rivers and lakes. One might expect that given the large rivers that flow across the earth and the huge numbers of lakes that this store would be rather large. Instead, streams, rivers and lakes only comprise .02% of all water in the earth system. In the atmosphere, only about .0001 % of the water in the hydrosphere is found.

The abundance of water on Earth is a unique feature that clearly distinguishes our "Blue Planet" from others in the solar system. Not a drop of liquid water can be found anywhere else in the solar system. It is because the Earth has just the right mass, the right chemical composition, the right atmosphere, and is the right distance from the Sun (the "Goldilocks" principle) that permits water to exist mainly as a liquid. However, the range of surface temperatures and pressures of our planet permit water to exist in all three states: solid (ice), liquid (water), and gas (water vapor). Most of the water is contained in the oceans and the high heat capacity of this large volume of water (1.35 million cubic kilometers) buffers the Earth surface from large temperature changes such as those observed on the moon. Water is the universal solvent and the basis of all life on our Planet. It is an essential life-sustaining resource which led Benjamin Franklin to comment "When the well's dry, we know the worth of water."
The movement of water around, over, and through the Earth is called the water cycle, a key process of the hydrosphere.
A hydrosphere (Greek hydro means "water") in physical geography describes the collective mass of water found on, under, and over the surface of a planet.Contents
The Earth's hydrosphere consists of water in its liquid, solid, and gaseous forms: the ocean (which comprises the bulk of the hydrosphere), other surface waters including inland seas, lakes, and rivers; rain; underground water; ice (as in glaciers and snow); and atmospheric water vapor (as in clouds). The average depth of the oceans is 3,794 m (12,447 ft), more than five times the average height of the continents. The mass of the oceans is approximately 1.35 × 1018 tonnes, or about 1/4400 of the total mass of the Earth (ranges reported: 1.347 × 1021 to 1.4 × 1021 kg

The abundance of water on Earth is a unique feature that distinguishes our "Blue Planet" from others in the solar system. Approximately 70.8[citation needed] percent (97% of it being sea water and 3% fresh water[2]) of the Earth is covered by water and only 29.2 percent is landmass. Earth's solar orbit, vulcanism, gravity, greenhouse effect, magnetic field and oxygen-rich atmosphere seem to combine to make Earth a water planet.

Earth is actually beyond the outer edge of the orbits which would be warm enough to form liquid water. Without some form of a greenhouse effect, Earth's water would freeze. Paleontological evidence indicates that at one point after blue-green bacteria (Cyanobacteria) had colonized the oceans, the greenhouse effect failed, and Earth's oceans may have completely frozen over for 10 to 100 million years in what is called a snowball Earth event.

On other planets, such as Venus, gaseous water is destroyed (cracked) by solar ultraviolet radiation, and the hydrogen is ionized and blown away by the solar wind. This effect is slow, but inexorable. This is one hypothesis explaining why Venus has no water. Without hydrogen, the oxygen interacts with the surface and is bound up in solid minerals.

In the Earth's atmosphere, a tenuous layer of ozone within the stratosphere absorbs most of this energetic ultraviolet radiation high in the atmosphere, reducing the cracking effect. The ozone, too, can only be produced in an atmosphere with a large amount of free diatomic oxygen, and so also is dependent on the biosphere (plants). The magnetosphere also shields the ionosphere from direct scouring by the solar wind.

Finally, volcanism continuously emits water vapor from the interior. Earth's plate tectonics recycle carbon and water as limestone rocks are subducted into the mantle and volcanically released as gaseous carbon dioxide and steam. It is estimated that the minerals in the mantle may contain as much as 10 times the water as in all of the current oceans, though most of this trapped water will never be released.

The water cycle describes the methods of transport for water in the hydrosphere. This cycle includes water beneath the Earth's surface and in rocks (lithosphere), the water in plants and animals (biosphere), the water covering the surface of the planet in liquid and solid forms, and the water in the atmosphere in the form of water vapor, clouds, and precipitation. Movement of water within the hydrosphere is described by the hydrologic cycle. It is easy to see this motion in rivers and streams, but it is harder to tell that there is this motion in lakes and ponds.

The water in the oceans moves as it is of different temperature and salinity on different locations. Surface waters are also moved by winds, giving rise to surface ocean currents. Warm water is lighter or less dense than cold water which is more dense or heavier and salty water is also more dense than fresh water. The combination of the water's temperature and salinity determines whether it rises to the surface, sinks to the bottom, or stays at some intermediate depth.

There are several theories regarding the formation of the hydrosphere on the Earth. The planet contains proportionately more surface water than comparable bodies in the inner solar system. Outgassing of water from the interior of the Earth is not sufficient to explain the quantity of water.

A hypothesis that has gained popularity among scientists is that the early Earth was subjected to a period of bombardment by Comets and water-rich Asteroids. Much of the water on the surface today is thought to have originated from the outer parts of the solar system, such as from trans-Neptunian objects.

During the history of the Earth there have been a series of periods in which a significant portion of the hydrosphere was locked up in the form of glacial ice. It has even been hypothesized that during the Cryogenian period this sea ice extended all the way to the equator. (See Snowball Earth).

In all there are currently believed to have been four major ice ages during the Earth's history. The current ice age began about 40 million years ago, and gained in intensity during the Pleistocene. The most recent withdrawal of the ice sheets occurred only 10,000 years ago.

Life

All currently recognized forms of life rely on an active hydrosphere. The water cycle in the Earth's hydrosphere allows for the purification of salt water into fresh water. Evaporation and wetland swamps serve to remove a large portion of atmospheric pollutants from the atmosphere (ie. acid rain). Through this process the water cycle purifies the gaseous atmosphere. Although most life on the planet exists in the salt water oceans, humans are particularly interested in the hydrosphere because it provides the fresh water we depend upon.

The search for life in other celestial bodies in our solar system is focused on first locating water. The hydrosphere's of other planetary bodies is also the focus of research to find places that humans can inhabit without having to transport all their water with them.

Extinction

Scientists estimate that in approximately 4 × 109 years the Sun will have exhausted the supply of Hydrogen in its core and will evolve into a supergiant. The outer atmosphere will expand significantly and the planet Earth will lie within the photosphere. During this process the surface temperature will rise far above the boiling point of water, and all water on the Earth's surface will evaporate.

Other hydrospheres

A thick hydrosphere is thought to exist around the Jovian moon Europa. The outer layer of this hydrosphere is almost entirely frozen, but current models predict that there is an ocean up to 100 km in depth underneath the ice. This ocean remains in a liquid form due to tidal flexing of the moon in its orbit around Jupiter.

It has been suggested that the moon Ganymede may also possess a sub-surface ocean. However the ice covering is expected to be thicker on Ganymede than on Europa.

Q.) What is Guyot? Guyot is a flat topped seamount.It is estimated that more than 10,000 seamounts and guyots exist in the pacific ocean alone.

(Q.) What is salinity of ocean water?
Salinity is the term used to define the total content of dissolved salts in sea water. It is calculated as the amount of salt (in gm) dissolved in 1,000 gm (1 kg) of seawater.It is usually expressed as parts per thousand. Salinity is an important property of sea water. All waters in nature, whether rain water or ocean water, contain dissolved mineral salts.

(Q.) Name the largest barrier reef of the world. Where it is located?
The largest barrier reef of the world is the Great Barrier Reef which is located at the east coast ofAustralia.

Q.) What is continuous evapotranspiration? The continuous transfer of water into the gaseous state is referred to as the evapotranspiration

Q.) What is hydrography?
Hydrogaphy is the investigation of seas and other bodies of water including charting ocean surface, study of tides, currents etc.

Q.) What do you understand by the term 'Atoll'?
These are low islands found in the tropical oceans consisting of coral reefs surrounding a central depression.It may be a part of the sea (lagoon), or sometimes from enclosing a body of fresh or highly saline water.
Q.) What is continental rise?
The continental rise is an underwater feature found between the continental slope and the abyssal plain. The environment in the continental rise is quit unique, and many oceanographers study it extensively in the hopes of learning more about the ocean and geologic history.
Q.) What is continental slope?
Continental slope is the area which is extended from the continental shelf towards the bottom of the ocean.The gradient of the slope region varies between 2-50.The slope boundary indicates the end of the continents.

(Q.) What is continental shelf? Continental shelf is the extended margin of each continent occupied by relatively shallow seas and gulfs. It is the shallowest part of the ocean. The width of the continental shelves varies from one ocean to another.

Q.) What is submarine canyon? The submarine canyons are the deep gorges of the ocean bottom. They are sometimes found cutting across the continental shelves and slope, often extending from the mouths of large rivers.The Hudson canyon is the best known canyon in the world.
Q.) What is a submarine trench?
It is a long narrow and steep-sided depression of the ocean bottom. They are some 3-5 km deeper than the surrounding ocean floor. They occur at the bases of continental slopes and along island arcs and are associated with active volcanoes and strong earthquakes

(Q.) What do you understand by the term Reef? Reef isa line of rocks in the tidal zone of a coast submergedin high water but partially uncoveredin shallow water.

Q) Distinguish between Gulfs and Bays.

i. A part of ocean that penetrates into the land forms Gulf. Bays ae the waterbodies partially surrounded by land.

ii. Gulfs are formed by the downward movement of the earth's crust and bays are formed when the ovean ovrflows the coastline which has sunk or eroded because of the movement in the Earth's crust.

Q. Distinguish between sea mount and Guyot.
1. Sea mounts are the mountains on the sea floor. Guyots are the hills found on the sea floor
2. Sea mounts are above 1000 meters in height and Guyots are also above 1000 meters in height
3. Sea mounts have conical summits and Guyots have flat tops
4. Sea mounts are of volcanic originand guyots are also of volcanic origin.
5. Some of the sea mounts attain the height of 3000 metes and an ordinary guyot has a diameter of 20-25 km.

(Q.) What are the various components of water cycle? The various component ofwater cycle are as follows:1. Water storage in the oceans2. Water in the atmosphere3. Water storage in ice and snow4. Surface runoff5. Ground-water storage

(Q.) Distinguish between the submarine trenches and the canyon.
1. A submarine trench is a long narrow and steep-sided depression on the ocean bottom. Canyon is a deep gorge on the continental slope.
2. Depth of submarine trences varies from 7000 to 11000 meters. The canyon’s depth varies from 1800 to 2800 meters
3. Submarine trenches are almostwithout sediments. Coarse deposits are found on the canyons.
4. Submarine trenches are result ofendogenic forces. Canyons are found near river mouths and may have been the result of erosion

(Q.) Why do we call the earth the' blue planet'? Our planet earth is called as the blue planetbecause of the availability ofwater in abundance. The water body covers more than 70% of the surface of the earth and the rest of the surface is covered withland area. If we try to look at the hemisphere wise distribution ofsea water, it is seen that the northern hemispherecontains 43% and the southern hemispherecontains 57% of the total sea water.

Q.) Write a short note on ocean basin. Ocean basin is an extensive region of the basin floor, generally lying in a depth range of 2500 to 6000m. It covers about 76.2 % of the ocean areas. The ocean basin floor contains three types of features; abyssal plains, oceanic rise and sea mounts.

(Q.) Name the two major banks of the world? The two major banks of the world are:1. The Doggar bank (Northern sea)2. The Grand bank (Atlantic sea)

Q.) Write a short note on the Abyssal plain. Abyssal plain is an area beyond the continental slope. They are located at the depth of 3000 to 6000 meters. They cover about 40% of the ocean floor. The deposits of fine matter containing bones and shells of minute animals that live and die on the surface of sea are deposited on it. They are found in all the major oceans of the world.

Q.) What are the various processes by which the water moves through the water cycle?
The multiple processes under which the water cycle moves are listed under following heads:
1. Evaporation and Transpiration
2. Condensation
3. Precipitation
4. Infiltration
5. Runoff
Sub-surface runoff

Q.) Write a note on the mid-oceanic ridge. Mid-oceanic ridge is a great submarine mountain chain which is extended from the north of the Atlantic to the south of the Atlantic Ocean. The total length of the ridge is 64000 km and it also passes through the Indian Ocean towards the Australia and Antarctica and enters into the south Pacific Ocean.
On an average, the ridge has a width ranging between2000-24000 km. There are various abyssal plainsin the mid-oceanic ridge.

Q.) What is continental margin? Continental margin is the term which is commonly used for the area whichcontains continental shelf and continental slope together. Continental shelf is the area which is close to the continent and continental slope is the area which is next to the continental shelf.
So in other words continental margin is a combined term for continental shelf and continental slope.

Q.) What is Thermocline?
Thermocline is the layer of the ocean after which there is a rapid decrease of temperature in the ocean water. The boundary region after which the rapid decrease of temperature is seen is calledthermocline.Thermocline is a second layer of the ocean which is right below the first layer of the ocean and is characterisedby rapid decreasein temperature with increasing depth.The thermocline is 500-1,000m thick.


Q.) Write a note on the role of oceans in hydrological cycle.Draw a suitable diagram. Hydrological cycle is one of the most fundamental cycle that maintains life on the earth. Life on land sustains because of the transfer of water from the great ocean body to the land through the atmosphere. The hydrological cycle depends basically on the inter-relationship between the circulation of ocean waters and that of the atmosphere.

(Q.) Define temperature of the ocean water. Explain the distribution of temperature in the ocean water.
The temperature ofocean water varies from place to place. The temperature of ocean water at the pole ward area remainsvery low.On the other handocean temperature in the equatorial area remains high throughout the year because ofdirect heat from the sun and thus high amount ofinsolation is also recieved in this area.
Distribution of ocean temperature: The distribution of temperature in the oceans can be categorised intotwo categories:
1. Horizontal distribution
2. Vertical distribution
Horizontal distribution of temperature: The horizontal distribution of temperature is theamount of insolation received. The areas which are closer to the equator receive more insolation instead of the areas which are closer to the poles.

The decrease in the temperature is on 0.5º per latitude till the poles. The average temperature of the entire ocean is 17.2 ºC. The average annual temperature for the northern hemisphere is 19.4ºC and for the southern hemisphere it is 16.1ºC.
Vertical distribution of temperature: In the oceans vertical distribution of temperature is more important phenomenon in spite of the horizontal distribution of temperature. The temperature in the polar regions remains low while the middle latitude and the low latitude regions have high variation in the temperature ofocean water

The Vertical temperature distribution can be described inthree layer structure of the temperature in the oceans.
1. First layer: This layer within the tropics is present throughout the year but in mid latitudes it develops only during summer. This is the warm layer which is 20º cand 25º cwarm.
2. Second layer: This is known as the thermocline layer which is just below the first layer. The rapid decrease in temperature with the increase indepth is the major characteristic of this layer.
3. Third layer: This layer is very cold and extends up to the deep ocean floor. Most of the ocean water is contained in this layer.

Q.) Name the deepest trench of oceans.
(Ans) Mariana Trench in Pacific Ocean is the deepest trench of the world.It's maximum depth of about 11,033 meters.

(Q.) Name the major oceanic water bodies.
(Ans) The major oceanic water bodies are as follows:
1. Pacific ocean
2. Atlantic ocean
3. Indian ocean
4. Arctic ocean

Q. State two special features of the Pacific Ocean.

The Pacific Ocean is oval in shape and has greatest length from east to west.

Q.) Describe the bottom relief features of the ocean floor.
(Ans) The ocean floorhasvariety of features related to the topography of the oceans bottom. Its vast extended landformsare fragmented in the ocean bottom. The geographers have divided the oceanic part of the earth into four oceans, namely the Pacific ocean, the Atlantic ocean, the Indian ocean and the Arctic ocean. The various seas, bays, gulfs and other inlets are part of these four large oceans.

Classification of the ocean floor:
Broadly we can divide the ocean floor intofour major divisions:
The Continental Shelf
The Continental Slope
The Deep Sea Plain
The Oceanic Deeps
The Continental Shelf: The continental shelf is slightly inclined, inundated zone which structurally forms the border of the continent. It has a gentle slope with the gradient of 1º or even less and its maximum depth is normally 200 meters. It covers about 7.6% in the world’s ocean.


The Continental Slope: At the edge of the continental shelf, the seaward slope becomes steep all of a sudden. This partion with steep slope is known as continental slope. The gradient of the slope varies from 2º to 5º. It extends to a depth of about 3000 meters.
The Deep Sea Plain: The deep sea plains are also known as the Abyssal plains. The gradient of these plains are less than 1º. Their depth varies from 3000 to 6000 meters. These plains cover 40% of the total ocean floor.


Oceanic Deeps or Trenches: A trench is a long, narrow and steep sided depression on the ocean floor. Trenches are the deepest part of the oceans with the usual depth of 6000 meters. So far 57 trenches have been found, out of which 32 are in the Pacific ocean, 19 in the Atlantic ocean and 6in the Indian ocean. They occupy 1.2% area of the oceans.
Other features: In this category other minor bottom relief features are included. They are as follows:
1. Mid oceanic ridges
2. Abyssal plains
3. Submarine Canyons
4. Atolls

Q.) Define salinity. What are the factors that affect the distribution of salinity in the oceans?
(Ans) Salinity is the term used to define the total content of dissolved salts in sea water. It is estimated that one cubic kilometer of sea water contains 41 million tones of salt. The average salinity insea water is about 35 parts perthousand.
Factors Affecting Salinity: Salinity in water is mainly affected by the following factors:
1.Availability of fresh water:Fresh water comes fromriver and the melting of snow. The addition of fresh water leads to the decrease in percentage of salinity in the ocean water.2.Evaporation: Due to evaporation,fresh water of the water body islost and the remaining sediments enhance the salinityof the water.3.Ocean currents: The cold ocean currents which are generated because of the melting of the snow at the pole, carry fresh water and blow towards the equator and degrade the amount of salinity at the tropical areas.
Distribution ofsalinity in the sea water: The distribution of salinity in sea water can be defined in two ways:
Horizontal distribution
Vertical distribution

Horizontal distribution: The horizontal distribution of the salinity inocean can be divided into the following categories:
1. Salinity in the open seas – It is determined by the various factors such as the currents etc. The salinity is highest in the tropics of the cancer and Capricorn.
2. Salinity of partially enclosed sea – Partially enclosed seas like the Mediterranean, Black sea, Baltic seahave comparatively high amount of salinity.
3. Salinity in the enclosed sea – The enclosed seashave highest amount of salinity. No river orwater body merge into these seas so it is the main reason for the high amount of salinity in these areas.
2. Vertical distribution: The vertical distribution of the salinity in the ocean can be determined by depth. With the increase indepth, the amount of salinity increases. So the addition of fresh water or the high amount of evaporation does not affect the vertical distribution of salinity.

(Q.) What are the factors that influence temperature distribution of the ocean floor?
(Ans) Ocean temperature is the most important factor which influences various movements of the ocean current. The major factors which control the ocean temperature are as follows:
Latitudes: Latitudes are among the few major factors that control the temperature difference in the oceans. The temperature of the surface water decreases from equator towards the poles because the sun’s rays become more and more slanting towards the poles and thus the amount of isolation decreases towards the poles.
Uneven distribution of land and water: The oceans in the northern hemisphere receive more heat due to their contact with larger extent of land than the oceans in the southern hemisphere.
Prevailing winds: The winds blowing from land towards the ocean drive surface water away from the coasts resulting into upwelling of cold bottom water from below.
Ocean currents: The warm ocean currents keep up the temperature incold areas while the cold ocean current cools down the temperature inwarmer areas. So the ocean currents play an important role in terms of ocean temperature.
Other reasons: Local weather conditions also play an important role in the ocean temperature like local cyclones etc

(Q.) Distinguish between the features of the Pacific Ocean and the Indian Ocean.
The bottom relief of the Pacific and Indian ocean are not similar, though the basic characteristics of these oceans are same but still thereare enough differences. The differences between the Pacific and Indian ocean can be explained as follows:
Basis of differences between Pacific Ocean and Indian Ocean are:
Indian Ocean
1. Extension: The extension of the Pacific ocean is 165 million sq. km. The extension of the Indian ocean is 68.556 million sq. km.
2. Ridge: The mid oceanic ridge is not very extensive here. Some part of ridge is found in the south of the Pacific ocean. There is a broad submarine ridges found on the bottom of the Indian ocean
3. Ocean basin: The pacific ocean has a number of shallow basins. The major basinscan beeasily seen in the Pacific ocean. The mid oceanic ridge divides the ocean into many small basins. Six major basins can be identified in the Indian ocean.
4. Ocean deeps: Thereare 32 deeps in this ocean. The deepest among them is the Mariana Trench which is 11022 meter deep. The ocean deeps are almost absent from this ocean. Some small deeps are often seen in the Indian ocean.
5. Marginal seas: There are a few marginal seas in the Pacific ocean like South China sea etc. Some very important marginal seas are located such as Red sea, Persian, Arabian sea, Bay of Bengal etc.
6. Island: There are more than 20,000 islands in the Pacific ocean. There are some big islands like Madagascar and Sri Lanka.

Q.) Distinguish between the continental shelf and the continental slope.
The continental shelf and the continental slope are the areas which are very close to each other. For a layman, these areas are common but for a geographer, these two areashave very unique characteristics. The differences between the continental shelf and the continental slope can be seen as follows:
Continental Shelf
1. It joins shore line with continental slope.
2. Its slope is very gentle. It is about 2 meter per km.
3. It is a shallow water area. Its depth is less than 200 meters.
4. There are few canyons or deeps in the continental shelves.
5. They have large store of sand and gravel.They are of great use to man, particularly for fishing.

Continental Slope
1. It joins continental shelf with Abyssal Plain.
2. Its slope is very steep. The average gradient is about 4 degrees.
3. Its depth varies from 200 to 2000 meters.
4. Deep canyons are found on the continental slopes.
5. Very few land deposits are found on it.

G4-RIVERS

RIVERS

. 6.1 A river flows down the hill

Of all the geological processes, rivers may have the greatest impact on people. We depend upon rivers for drinking , irrigation, electricity travel etc. Fertile floodplains of the rivers have fostered human progress since the dawn of civilization. As the dominant agent of landscape alteration, rivers have shaped much of our physical environment. A river starts as a tiny trickle, or rill, on a slope. Rainfall, snowfall, a spring, or the melting of glacial ice may be its source. As it flows downhill it is joined by other trickles to make a brook. Several brooks add their waters to form a stream, which broadens into a creek. As the water continues its downward journey, it gains in volume and finally becomes a river.

A river does three different kinds of work. They are:

1. Erosion, 2. Transportation and 3. Deposition

1. Erosion: Rivers erode their channels in several different ways: a.by lifting loosely consolidated particles, b.by abrasion, and c.by solution activity. The last of these is by far the least significant. Although some erosion results from the solution of soluble bed rock and channel debris, most of the dissolved material in a river is contributed by the groundwater.

a. By lifting: When the flow of water in the river is turbulent, the water whirls and eddies. When an eddy is suffeciently strong, it can dislodge rock pieces and lift them into the moving water. In this manner the force of running water swiftly erodes lossely consolidated materials on the bed and banks of the river. Stronger the current, more effectively the stream will lift the rock pieces. In some instances water is forced into cracks and bedding planes with sufficient strength and moves pieces of rock from the bed and banks of the channel.

b. By abrasion: Observing a muddy river will reveal that currents of water can lift and carry debries. However, it is not obvious that a river is capable of eroding solid rock. Nevertheless, the solid particles carried by a river are capable of abrading a bedrock channel. The ceaseless bombardment of these rock pieces against the bed and banks of the river channel have created many steep sided gorges cut through solid rock. In addtion, the individual rock pieces are also abraded by the many impacts with the channel and with one another. Thus, the rock pieces by scraping, rubbing and bumping, erodes bedrock channel and the banks. Simultaneously these rock pieces also get reduced in their size and smoothend to form pebbles. This process of erosion is known as abrasion.

Abrading tools such as pebbles and sand particles that are swirling in fast-moving eddies, act as drill to bore holes on the river beds. In course of time, the rotational motion of pebbles and sand particles create rounded depressions several meter across and just as deep. Such large depressions are known as potholes. As the abrading particles wear down to nothing, they are replaced by new ones that continue to drill the river bed.

2. Transportation: The solid material carried by a river is called its load. A river transports its load in three different ways. They are:

a. in solution (dissolved load)

b. in suspension ( suspended load) and

c. by saltation (bed load).

a. In solution: Some of the material a river carries in solution, is acquired when running water dissolves bedrock channel. However, the greatest portion of the dissolved load transported by most rivers is supplied by groundwater. As water percolates through the ground, it first acquires soluable soil compounds. As the water seeps deeper through cracks and pores in the bedrock below, additional mineral matter is dissolved. Eventually much of mineral-rich water finds its way into rivers. The velocity of river flow, which is very important to the transportation of solid particles, has essentially no effect upon a river’s ability to carry its dissolved load. After material is in solution, it goes wherever the stream goes, regardless of velocity.

b. In Suspension: The major portion of the river load is carried in suspension. Usually only fine sand, silt and clay-sized particles can be carried this way, but during floods larger particles are carried in suspension as well. The finest materials such as silt and clay remain suspended in the water for most of their journey and sink very slowly. No. 6.2 Potholes

3.By saltation: A portion of a river’s bedload consists of solid rock fragments. They are too large to transport in suspension. So they are moved along the bottom by saltation, rolling, and sliding. Particles moving by saltation appears to jump or skip along the river bed. This occurs as particles are propelled upward by collisions or sucked upward by the current and then carried down stream a short distance until gravity pulls them back to the bed of the river. Particles that are too large or heavy to move by saltation, either rolled or pushed along the bottom,depending upon their size and shape. Unlike the suspended and dissolved loads, which are constantly in motion, the bed load is in motion only intermittently. The movement of bedload is determined by two factors. They are:

a. the competence and

b. the capcity of the river .

a. The Competence of the river: Competence is a measure of the maximum size of particles the river is capable of transporting. The river’s velocity determines its competence; the stronger the flow, the larger the particles it can carry in suspension or as bed load. It is a general rule that the competence of a river increases as the square of its velocity. Thus, if the velocity of a stream doubles, the impact force of the water increases four times; If the velocity triples, the force increases nine times, and so forth.

b. The capacity of the river: The maximum load of solid particles that a river can transport is termed its capacity. The capacity of a river is related to its discharge. The greater the amount of water flowing in a river, the greater the river’s capacity for hauling sediments. Hence, the large boulders that are often visible during a low-water stage and seem immovable, can be transported during floodstage because of stream’s increased velocity. With rising velocity the water becomes more turbulent, and larger and larger particles are moved by the river. In course of just a few days, or perhaps just a few hours, a stream in floodstage can erode and transport more sediments than it does during months of normal flow.

3. Deposition:

Whenever a river’s velocity subsides, its competence is reduced. As the velocity of a river diminishes, particles of sediment are deposited according to size. As the river flow drops below the criticle settling velocity of a certain particle size, sediment in that category begins to settle. Thus river transport provides a mechanism by which solid particles of various sizes are seperated. This process, called sorting, explains why particles of similar size are deposited together. The well sorted material typically deposited by a river is called alluvium. Many different depositional features are composed of alluvium. Alluvium deposits are found in three places. They are:

a.Foothill deposits

b. River bed deposits and

c.Plain deposits

a. Foothill deposits: When a river leaves a narrow valley in the mountainous terrain and comes out suddenly on to a broad, flat plain such as a plateau, or a valley floor , the velocity of the river is reduced. So, the river deposits its load in response to the abrupt drop in gradient, combined with the change from a narrow channel of a mountainous stream to the unconfined flow on the slopes of the plain. The sudden drop in velocity causes the river to dump its load of sediments quickly in a distinctive cone-or-fan-shaped accumulation at the foot hills. This accumulation is called foothill deposit or alluvial fans. The surface of the alluvial fan slopes outward in a broad arc from an apex at the mouth of the steep valley. Usually, coarse material is dropped near the apex of the fan, while finer material is carried toward the base of the deposit. One can see such alluvial fans in the Kambam valley of Madurai district.

2. River-bed deposits: In a river, volume of water flowing into is highly variable. During rainy season, the volume of water in a river is very high. In contrast, during dry season the volume of water in the same river is very low or completly dry.. When the volume of water in a river reduces, its velocity and the capcity to transport the coarser materials also reduces. The river deposits some of its materials on its bed. Such deposits are known as ‘river bed deposits’. River bed deposits are often composed of sand and gravel, the coarser components of a river’s load and are commonly referred to as bars. Such features, however, are only temporary, for, the material will be picked up again by the running water and be transported farther down stream.

3. Plain deposits: Before entering into plain, the river deposits all the larger particles and other smaller particles on the river bed at the foot hill . Only very fine particles are transported into the plain region. As the slope of the plain is very gentle, the velocity of the river is very much reduced. The forward motion of the river is very much arrested at the place where it enters the sea. As a result, finer particles are deposited near the mouth of the river. Such deposits are called as ‘plain deposits’. One form of plain deposits is known as Delta.Fig.No. 6.4 River bed deposits

Did you know?

Many large rivers have deltas that extend over thousands of squre kilometers. The delta of the Mississippi river is one such feature. It resulted from the accumulation of huge quantities of sediment derived from the vast region drained by the river and its tributaries. During the past 5000-6000 years, the mississipi river has built a series of seven subdeltas. The numbers in the figure indicate the order in which the subdeltas were deposited. The present bird foot delta (number7) represents the activity of the past 500 years.

Course of a river and its associated features: Nearly all rivers have an upper, middle, and lower course. Each level has its own characteristics.

The Upper Course: The upper course begins at higher elevations. Here the river is smaller and usually has a rapid, tumbling flow that cuts a narrow channel through rocky hills or mountains. Over thousands or millions of years the river erodes, or wears away, soil and rocks and carves a canyon or a deep, V-shaped valley. The type of formation it creates depends on the force of the river and the type of material it erodes.

V-shaped valley: A deep, steepwalled, V-shaped valley cut by a river through resistant rock is often called a canyon, from the Spanish word canon, meaning “tube.” Such valleys often occur in the upper courses of rivers, where the stream has a strong, swift current that digs its valley relatively rapidly. Smaller valleys of similar appearance are called gorges . The largest and most famous canyons have been cut through arid or semiarid lands by swift streams fed by rain or melting snow of moister regions. The walls remain steep and angular because they are not worn and softened by frequent rainfall and surface drainage.

The middle course: When the river descends to lower elevations, it runs more slowly over the gently sloping land lley

of its middle course.Its current no longer has the force to carry stones or gravel. This material drops to the riverbed, where it forms bars of sand or gravel or builds islands. These formations are continually changing shape as the river deposits or erodes material. The formations affect the river in turn by altering its course. River currents swing toward one bank or the other, gradually undercutting the banks and widening a V-shaped valley into a U shape.

As the river flows downstream it reaches the still gentler slope of its lower course. It drops more of its load than it did upstream and begins to build up its bed instead of tearing it down. The valley has been eroded into a wide plain. The river swings in great S-shaped curves, forming loops called meanders

Did you know?

The Grand Canyon, one of the most majestic natural wonders in the world, is nature’s handiwork. Nature’s greatest example of sculpture, the Grand Canyon in northern Arizona is the most spectacular canyon in the world. It is a 446-kilometer long gorge cut through high plateaus by the Colorado River. It is noted for its fantastic shapes and colors. Within the walls of the canyon stand imposing peaks, canyons, and ravines. In general, the color of the canyon is red, but each stratum or group of strata has a distinctive colour such as gray, delicate green or pink, and, in its depths, brown, gray, or violet. No other place on Earth compares with the kilometerdeep Grand Canyon for its record of geological events. Some of the canyon’s rocks date back about 4 billion years. The river’s speed and such “cutting tools” as sand, gravel, and mud account for its incredible cutting capacity.

Oxbow lakes: Meandering channels form where streams are flowing over a relatively flat landscape with a broad floodplain. When a river floods, it may cut across the narrow part of the loop, making a new, shorter channel. The loop is left as a lake known as an oxbow lake.

The lower course: At times heavy rain or melting snow rushes from the upper or middle course of a river into the shallow channel of the lower course. The river floods the surrounding country, leaving a thin layer of sediment. If the flooding is seasonal, in time the layers of sediment accumulate and build a broad, fertile floodplain.

Flood Plain: As its name implies, a floodplain is that part of a valley that is inundated during a flood. Most rivers are bordered by floodplains. Although some are impressive features that are many kilometers across, others are very modest, having widths of just a few meters.

Levees : Rivers that occupy valleys with broad, flat floors sometime create a landform called a natural levee that parallels the stream channel. Natural levees are built by successive floods over a period of many years. Levees are ridges found along the sides of the stream channel composed of sand or gravel. Levees are approximately one half to four times the channel width in diameter. The area behind the levee is characteristically poorly drained for the obvious reason that water cannot flow up the levee and into the river. Marshes called back swamps often result.Upon retreat of the flood waters, stream velocity is reduced causing the deposition of alluvium. Repeated flood cycles over time can result in the deposition of many successive layers of alluvial material. The well-sorted materials typically deposited by the stream is called alluvium, a general term applicable to any river-deposited sediments. Some of these features may be found within river channels, some occur on the valley floor adjacent to the channel, and some exist at the mouth of river.

So far we have learnt about how a river works and the different features found along the course of a river. Now Let us take river Cauvery that run accross Tamilnadu as an example and try to understand the different course of a river. River Cauvery is the only major river in Tamilnadu. This is an interstate river with many tributaries lying in Karnataka and Tamilnadu States. It has a catchment area of 80,290 Sq.kms., of which around 44,000 Sq.kms., is in Taminadu.

Upper course of river cauvery: River Cauvery originates from Coorg in Karnataka and it flows through the deep valleys of the mountain. The river cauvery is so narrow at its origin that a sheep could jump across it easily. The important tributaries that join the river Cauvery are the Hemavathi, the Lakshmanatheertha, and the Kabini. After crossing the mountainous region, cauvery river flows through Deccan plateau. The river enters Tamilnadu state at Hogenekal falls, in Dharmapuri district.

Middle course of river Cauvery: The tributaries such as Bhavani,Amaravathi, and Noyyal join the river cauvery while flowing through the Deccan plateau. As the volume in the cauvery increases, its bank becomes wider. Hence, The cauvery that flows in Tamilnadu is known as Aganda (wide) Cauvery.AUVERY BASIN

Lower course of river cauvery: The Aganda cauvery enters the the plain, and branches off into northern and southern branch near Thiruchirapalli district. The Coleroon, the northern branch is really the main distributary of the river and carries the bulk of floodwater to the sea. It flows in an eastern direction and before entering into sea forms a delta, little south of Portonova town. The southern branch, retains the name cauvery and branches off into Vennaru, Vettaru and Vadavaru. Eventually, before entering into sea at Kaveripattinam about 12 kms., north of Tranquebar forms a delta.

Points to remember:

1. The rotational motion of the sand and pebbles act as a drill to bore the holes. These smooth depressions are known as potholes.

2. A deep, steep-walled, V-shaped valley cut by a river through resistant rock is often called a canyon. Smaller valleys of similar appearance are called gorges .

3. The valley has been eroded into a wide plain . In such a the river swings in great S-shaped curves, forming loops called meanders.

4. When a river floods, it may cut across the narrow part of the loop, making a new, shorter channel. The loop is left as a lake known as an oxbow lake.

5. The materials that are deposited on the river bed is called river bed deposits. The materials that are deposited at the foot hills is known as foot hill deposits. The finest particles that are deposited on the plains, near the mouth of the river are called as plain deposits.

6. Alluvial fans develop when streams carrying a heavy load reduce their velocity as they emerge from mountainous terrain to a nearlyhorizontal plain.

7. The forward motion of the river is checked upon when a river enters an ocean. So, the river deposits its load of sediments near the mouth of the river. This is known as Delta.

Geologically, groundwater is also an important erosional agent. Besides groundwater is an important source for domestic use , irrigation and industrial use. In addtion, groundwater contamination due to human activities is a real and growing threat in many places.

Let us investigate all these aspects in the follwing lesson.

EXERCISES

I. Choose the correct answer:

1. Potholes are the product of

a) Corrosion b) solution c) suspension

2. The river that swings in great S-shaped curves, forming loops are called

a) meanders b) oxbow lakes c) levees

3. A deep, steep-walled, V-shaped valley cut by a river through resistant rock is often called a

a) Depression b) lakes c) canyon.

II. Fill in the blanks

1. When rivers erode their channel by lifting lossely consolidated particles by -------------- is called mechanical erosion.

2. The major portion of the river load is carried in ---------------.

3. A deep, steep-walled, V-shaped valley cut by a river through resistant rock is often called a ----------------.

4. The particles composing the ---------- is moved along the bottom by rolling, sliding and saltation.

5. The finest particles that are deposited on the plains, near the mouth of the river are called --------------- deposits.

IV. Answer briefly

1. How do rivers form?

2. Describe briefly about potholes?

3. Write a note on V shaped valley.

4. What are oxbow lakes.

5. Differentiate chemical and mechanical erosion?

G5-UNDERGROUND WATER

UNDERGROUND WATER

When rain falls on the landmass, some of the water runs off as streams and rivers, some evaporates, and the remaining water percolates into the ground. This groundwater provides water for domestic needs, for irrigating crops, and for industrial use. In some areas, however, overuse of this basic resource has resulted in depletion, land subsidence, saltwater intrusion, and increased pumping cost.

The amount of water that penetrates into the ground is controlled by several factors. They are: a.steepness of slope, b.nature of surface material, c.intensity and duration of rainfall, and d. type and amount of vegetation.

Heavy rain, falling upon steep slopes underlained by hard materials that prevents percolation, will obviously result in a high percentage of runoff. On the otherhand, if rain fall slowly, steadily and upon gentle slopes composed of permiable materials that could be easily penetrated, results in high percentage of percolation. Hence, the percolation and amount of groundwater that is stored depends upon the nature of the subsurface materials and quantity and duration of rainfall.

Nature of subsurface materials: The surface water seeps into the ground because the bedrock, sediments, and soil contain voids or openings. These openings are similar to those of a sponge and are often called pore spaces. The quantity of groundwater that can be stored depends on the number of pore spaces present in the material. The rock that consists of such pore spaces are called porus rock.

Porosity: The ratio between volume of pores to the total volume of rock is known as porosity. Let us understand this with an example. Clay have 50% porosity, whereas gravels have 20 percent porocity. In the case of clay, the ratio between pore spaces to the total volume is 0; that means the pore spaces occupy 50 percent and soild particles occupy the remaing 50 percent. Where as in gravel, the ratio between the pore spaces to the total volume is 20:80; that means the pore spaces occupy 20 percent and soild particles occupy the remaing 80 percent. So, sediments such as clay has high porosity where as sediments such as gravel has low porosity. Where sediments of various sizes are mixed, the porosity is reduced because the finer particles tend to fill the openings between the larger grains.

Most of the igneous and metomorphic rocks, as well as some sedimentary rocks have very low porosity. It is mainly because, they do not have much pore spaces. They become porus, only when there are cracks and fissures in these rocks.

Permiability: Groundwater moves by twisting and turning through the pore spaces that are present in a rock or sediment. A rock or sediment should have the ability to transmit the water through its pore spaces. The smaller the pore spaces, the slower the water moves. The ability or capcity of a rock or sediment that allow the water to perculate through its pore spaces is known as permiability.

Although porosity of clay is high, its pore spaces are so small that water is not allowed to percolate, whereas porosity of gravel is very low but water is allowed to perculate into the ground. Hence, clayey soils are non permiable and latetrite soils are permiable in nature.

Permeable rock strata or sediments such as sand or gravel that transmit groundwater freely are called aquifers. Impermiable layers composed of materials such as clay that hinders or prevents water movement are termed aquicludes.

So, porosity is not always a reliable guide to the amount of ground water that can be produced. The significant factor in determining the availability groundwater is the permiability.

Water Table: When rain falls on the surface, the soil retains a portion of it as moiture. The soil moiture is used as a source of water by plants. When the soil is saturated, the excess water penetrates downward until it reaches a zone, where pore spaces in sediment and rock are compeltly filled with water. This belt is called Saturation Zone. The water held in this zone is called groundwater. The upper limit of groundwater zone is known as the watertable. Extending upward from the watertable is the intermediate zone.

Although we cannot observe the watertable directly, its position can be mapped and studied in detail in areas where wells are numerous. The water level in wells coincides with the watertable of the groundwater. Such maps reveal that the level of water table fluctuates and changes now and then. A number of factors contribute to the fluctuations in the water table. For example, variations in rainfall and varition in permeability lead to uneven infiltration and thus to differences in water table level. The water table level may fluctuate considerably during the course of a year, dropping during dry seasons, and rising following periods of rain.

Erosional work of Groundwater: The primary erosional work carried out by groundwater is that of dissolving rock. Groundwater carries on , its rather unique and important role as an erosional agent in soluable rocks, especially in limestone regions. Although nearly insoluable in pure water, limestone is quiet easily dissolved in water containing carbon dioxide. Rainwater dissolves carbon dioxide from the air and decaying plants and becomes diluted carbomic acid. This infiltrates as groundwater and whenever groundwater comes in contact with limestone, the carbonic acid reacts with the calcite in the rocks to form calcium bicarbonate. It is a soluable material that is carried away in solution.

Caverns: Among the most spectacular results of groundwater’s erosional handiwork is the creation of limestone Caverns. Although most are relatively small, some have spectacualar dimensions. For example the Mammoth cave in Kentucky, USA is very famous cavern. It is the most extensive in the world, with more than 500 kilometers of interconnected passages.

Most caverns are created at or just below the water table in the zone of saturation. Here the groundwater follows the lines of weakness in the rock, such as joints and bedding planes. As time passes, the dissolving process slowly creates cavities and gradually enlarges them into caverns. The material that is dissolved by the groundwater is carried away and discharged into rivers.

The spectacular features that arouse the greatest curiosity for most cavern visitors are the stone formations and give some caverns a wonderland appearance. These features are created by the seemingly endless dripping of water over great span of time. These cave deposits, however, are also commonly called dripstones.

Perhaps the most familiar dripstones are stalactities. These icicle-like pendents hang from the ceiling of the cavern and form where water seeps through cracks above. When the water come in contact with air in the cave, some of the dissolved carbon dioxide escapes from the drop and calcite begins to precipitate. Deposition occurs as a ring around the edge of the water drop. As drop after drop follows, each leaves an infinitesimal trace of calcite behind, and a hollow limestone tube is created. Water then moves through the tube, remains suspended momentarily at the end, contributes a tiny ring of calcite, and falls to the cavern floor. The stalactite just described is appropriately called a Soda straw. Often the hollow tube of the soda straw becomes plugged or its supply of water increases. In either case, the water is forced to flow, and hence deposit takes place along the outside of the tube. As deposition continues, the stalactite takes on the more common conical shape.

The dripstones that form on the floor of a cavern and reach upward toward the ceiling are called stalagmites. The water supplying the calcite for stalagmite growth falls from the ceiling and splatters over the surface. As a result, stalagmites do not have a central tube and are usually more massive in appearence and rounded on their upper ends. Various dripstone features such as stalactite and stalgmites found in caverns are collectively called speleotherms.

Karst Topography: Some areas in the world, to a large extent, have been shaped by the dissolving power of groundwater. Such areas are said to exhibit karst topography. This term is derived from a plateau region located along the northeastern shore of the Adriatic Sea in the part of Yugoslavia called Slovenia where such topography is strikingly developed. The most common geologic setting for karst development is an area where limestone is present near the surface beneath a layer of soil. Karst areas characteristically exhibit an irregular terrain punctuated with many depressions, called sinkholes or sinks.

Sinkholes: Sinkholes commonly form in one or two ways. Some develop gradually over many years without any physical disturbance to the rock. The limestone immediately below the soil is dissolved by downward-seeping rain water that is freshly charged with carbon dioxide. With time, the bedrock surface is lowered and the fractures into which the water seeps are enlarged. grow in size, soil subsides into the widening voids, from which it is removed by groundwater flowing in the passages below. These depressions are usually deep and are characterized by relatively gentle slopes.

By contrast, sinkholes can also form suddenly and without warning when the roof of a cavern collapses under its own weight. Typically, the depressions created in this manner are steep-sided and deep. When they form in populous areas, they may represent a serious geologic hazard.

As many of our valuable natural resources, groundwater is beingexploited at an increasing rate. Thus, problems like saltwater intrution and groundwater contamination occurs at many places. Let us study in detail about how the underground water get contaminated.

Saltwater intrusion: In many coastal areas the groundwater resource is being threatened by encroachment of salt water . In order to understand this problem, we must examine the relationship between fresh groundwater and salt groundwater. Figure 7.6 is a diagrammatic cross section that illustrates this relationship in a coastal area underline by permeable homogeneous materials. Since fresh water is less dense than salt water, it floats on the salt water. Itforms a large, lens-shaped body that may extend to considerable depths below sea level. In such a situation, if the water table is 1 meter above sea level, the base of the fresh water body will extend to a depth of about 40 meters below sea level. Thus when excessive pumping lowers the water table by a certain amount, the bottom of the freshwater zone will rise by 40 times that amount. Therefore, if groundwater withdrawal continues to exceed recharge, the salt water intrudes into the fresh water zone. Deep wells and wells near the shore are usually the first to be affected.

Groundwater contamination: The pollution of groundwater is a serious matter, particularly in areas where aquifers supply a large quantity of water. A very common type of groundwater pollution is sewage. Its sources include an ever-increasing number of septic tanks as well as inadequate or broken sewer systems and barnyard wastes.

If groundwater is contaminated with bacteria from sewage, groundwater may become purified through natural processes. The harmful bacteria may be mechanically filtered out by the sediment through which the water perculates, destoyed by chemical oxidation, and/or assimilated by other organisms. In order for purification to occur, however, the aquifer must be of the correct composition. For example, extremely permeable aquifers such as highly fractured rocks, coarse gravel, or cavernous limestone have such large openings that contaminated groundwater may travel long distances without being cleansed.

Here , the water flows too rapidly and is not in contact with the surrounding material long enough for purification to occur. Although the contaminated water has travelled a long distance before reaching the well 1, the water moves too rapidly through the limestone to be purified. On the other hand, when water moves through sand or permiable sandstone, it can sometimes be purified within distances as short as a few tens of meters. The openings between sand grains are large enough to permit water movement, yet the movement of water is slow enough to allow ample time for its purification. As illustrated in figure no.7.8 the discharge from the septic tank percolates through the permeable sandstone, it is purified in a relatively short distance, before reaching the well 2.

Other sources of contamination: Other sources and types of contamination also threaten groundwater supplies. These include widely used substances such as fertilizers that are spread across the land surface, and pesticides. In addition, a wide array of chemicals and industrial materials may leak from pipelines, storage tanks and landfills. Some of these pollutants are classified as hazardous, meaning that they are highly toxic. As rainwater oozes through the refuse, it may dissolve a variety of organic and inorganic materials. If the leached material reaches the water table, it will mix with the groundwater and contaminate the supply. Since groundwater movement is usually slow, polluted water may go undetected for a considerable time.

Most contamination is discovered only after drinking water has been affected. By this time, the volume of polluted water may be very large, and even if the source of contamination is removed immediatly, the problem is not solved. Although the sources of groundwater contamination are numerous, the solutions are relatively few. Once the source of the problem has been identified and eliminated, the most common practice in dealing with contaminated aquifers is simply to abandon the water supply and allow the pollutants to be flushed away gradually. But the most effective solution to groundwater contamination is prevention.

Earlier, we have learnt that running water is a powerful agent of erosion. Infact, many of the present-day landscapes were modified by the widespread glaciers of the most recent ice age and still strongly reflect the handiwork of ice. Glaciers, of course, are not just a phenomenon of the geological past. As we shall see in the next lesson, they are still sculpturing and depositing in many regions today.

llPoints to Remember:

1 The surface water seeps into the ground because the bedrock, sediments, and soil contain voids or openings. These openings are similar to those of a sponge and are often called pore spaces

2. The quantity of groundwater that can be stored depends on the number of pore spaces present in the material. The rock that consists of such pore spaces are called porus rock.

3. The ratio between volume of pores to the total volume of rock is known as porosity. The ability or capcity of a rock or sediment that allow the water to perculate through its pore spaces is known as permiability.

4. Permeable rock strata or sediments such as sand or gravel that transmit groundwater freely are called aquifers. Impermiable layers composed of materials such as clay that hinders or prevent water movement are termed aquicludes.

5. Water that penetrates downward until it reaches a zone, where pore spaces in sediment and rock are completely filled with water is called Saturation Zone. The water held in this zone is called groundwater. The upper limit of groundwater zone is known as the watertable.

6. The groundwater follows the lines of weakness in the rock, such as joints and bedding planes. As time passes, the dissolving process slowly creates cavities and gradually enlarges them into caverns.

7. The features that are created by the seemingly endless dripping of water over great span of time. These cave deposits, however, are also commonly called dripstones.

8. Stalactities are icicle-like pendents hang from the ceiling of cavern and form where water seeps through cracks above. When the water is forced to flow, and hence deposit, along the outside of the tube and more common conical shape stalactites are created.

9. The term karst topography is derived from a plateau region called Slovenia where such topography is developed. It is located along the northeastern shore of the Adriatic Sea, Yugoslavia

10. Karst areas characteristically exhibit an irregular terrain punctuated with many depressions, called sinkholes or sinks. sinkholes are formed when the limestone immediately below the soil is dissolved by rain water and forms depressions. 1 dvering contaminated water