Geography

 

Table of content

• Weathering
• Mass Wasting
• Erosion and Deposition
• Soil Formation
• Landscape (Geological) Cycles
• Davis Cycle
• Penck Cycle

Weathering

 

Weathering is the general term applied to the combined action of all processes that cause rock to disintegrate physically and decompose chemically because of ex- posure near the Earth’s surface through the elements of weather. Among these elements temperature, rainfall, frost, fog and ice are the important ones. Weathering begins as soon as rocks come in contact with one or more than one elements of weather on the surface of the earth. In nature, generally both the disintegration and decomposition act together at the sametime and assist each other. We must remember that the weathered material (i.e. disintegrated and decomposed) lie in situ (i.e. at its original position). In this process no transportation or movement of material is involved other than its falling down under the force of gravity.

Weathering is the response of rocks to a changing environment. For example, plutonic rocks form under conditions at high pressures and temperatures. At the Earth’s surface they are not as stable as the conditions under which they formed. In response to the environmental change, they gradually weather (transform to more stable minerals).

Different types of Weathering are:-

1. Physical Weathering :-The mechanical breakup or disintegration of rock doesn’t change mineral makeup. It creates broken fragments or “detritus.” which are classified by size:

• Coarse-grained – Boulders, Cobbles, and Pebbles.
• Medium-grained – Sand
• Fine-grained – Silt and clay (mud).

Various process of Physical weathering are:-

• Development of Joints – Joints are regularly spaced fractures or cracks in rocks that show no offset across the fracture (fractures that show an offset are called faults).
• Crystal Growth – As water percolates through fractures and pore spaces it may contain ions that precipitate to form crystals. As these crystals grow they may exert an outward force that can expand or weaken rocks.
• Thermal Expansion – Although daily heating and cooling of rocks do not seem to have an effect, sudden exposure to high temperature, such as in a forest or grass fire may cause expansion and eventual breakage of rock. Campfire example.
• Root Wedging – Plant roots can extend into fractures and grow, causing expansion of the fracture. Growth of plants can break rock – look at the sidewalks of New Orleans for example.
• Animal Activity – Animals burrowing or moving through cracks can break rock.
• Frost Wedging – Upon freezing, there is an increase in the volume of the water (that’s why we use antifreeze in auto engines or why the pipes break in New Orleans during the rare freeze). As the water freezes it expands and
  exerts a force on its surroundings. Frost wedging is more prevalent at high altitudes where there may be many freeze-thaw cycles.

2. Chemcial weathering :-involves a chemical transformation of rock into one or more new compounds.  A group of weathering processes viz; solution , carnonation, hydration , oxidation and reduction acts on the roks to decompose, dissolve orreduce them to a fine clastic state through chemical reactions by oxygen ,surface /soil water and other acids. Water and air along with heat must be present to speed up all chemical reactions. Over and above the carbon dioxide present in the air, decomposition of plants and animals increases the quanitity of carbon dioxide underground . Chamical weathering involves four major processes:

• Oxidation is the process in which atmospheric oxygen reacts with the rock to produce oxides. The process is called oxidation. Greatest impact of this process is observed on ferrous minerals. Oxygen present in humid air reacts with iron grains in the rocks to form a yellow or red oxide of iron. This is called rusting of the iron. Rust decomposes rocks completely with passage of time.
• Carbonation is the process by which various types of carbonates are formed. Some of these carbonates are soluble in water. For example, when rain water con- taining carbon dioxide passes through pervious limestone rocks, the rock joints enlarge due to the action of carbonic acid. The joints enlarge in size and lime is removed in solution. This type of breakdown of rocks is called carbonation.
• Hydration is the process by which water is absorbed by the minerals of the rock. Due to the absorption of water by the rock, its volume increases and the grains lose their shape. Feldspar, for example, is changed into kaolin through hydration. Kaolin on Vindhyan Hills near Jabalpur has been formed in this manner.
• Solution is the process in which some of the minerals get dissolved in water. They are therefore removed in solution. Rock salt and gypsum are removed by this process.

3. Biotic weathering :- is a type of weathering that is caused by living organisms. Most often the culprit ofbiotic weathering are plant roots. These roots can extend downward, deep into rock cracks in search of water, and nutrients. In the process they act as a wedge, widening and extending the cracks.

Mass Wasting

 

Mass wasting is defined as the down slope movement of rock and regolith near the Earth’s surface mainly due to the force of gravity.   Mass movements are an important part of the erosional process, as it moves material from higher elevations to lower elevations where transporting agents like streams and glaciers can then pick up the material and move it to even lower elevations.   Mass movement processes are occurring continuously on all slopes; some act very slowly, others occur very suddenly, often with disastrous results.  Any perceptible down slope movement of rock or regolith is often referred to in general terms as a landslide.  Landslides, however, can be classified in a much more detailed way that reflects the mechanisms responsible for the movement and the velocity at which the movement occurs. Mass wasting can be classified as:-

 

• Slope Failures – a sudden failure of the slope resulting in transport of debris down hill by sliding, rolling, falling, or slumping.
• Sediment Flows – debris flows down hill mixed with water or air.

 

Erosion and Deposition

 

Soil erosion is the deterioration of soil by the physical movement of soil particles from a given site. Wind, water, ice, animals, and the use of tools by man are usually the main causes of soil erosion. It is a natural process which usually does not cause any major problems. It becomes a problem when human activity causes it to occur much faster than under normal conditions.The removal of soil at a greater rate than its replacement by natural agencies (water, wind etc.) is known as soil erosion.
Soil erosion is of four types which are as follows:-

• Wind Erosion :-Winds carry away vast quantity of fine soil particles and sand from deserts and spread it over adjoining cultivated land and thus destroy their fertility. This type of erosion is known as wind erosion. It takes place in and around all desert regions of the world. In India, over one lakh kilometers of land is under Thar Desert, spread over parts of Gujarat, Haryana, Punjab and Rajasthan states. These areas are subject to intense wind erosion.
• Sheet Erosion :-Water when moves as a sheet takes away thin layers of soil. This type of erosion is called sheet erosion. Such type of erosion is most common along the river beds and areas affected by floods. In the long run, the soil is com- pletely exhausted due to removal of top soil and becomes infertile.
• Rill Erosion :-The removal of surface material usually soil, by the action of running water. The processes create numerous tiny channels (rills) a few centimeters in depth, most of which carry water only during storms.
• Gully Erosion :-When water moves as a channel down the slope, it scoops out the soil and forms gullies which gradually multiply and in the long run spread over a wide area. This type of erosion is called gully erosion. The land thus dissected is called bad lands or ravines. In our country, the two rivers Chambal and Yamuna are famous for their ravines in U.P. and M.P. states.

Deposition / Sedimentation – occurs when sediment settles out as winds/water current die down, or as glaciers melt. When sediment is transported and deposited, it leaves clues to the mode of transport and deposition. For example, if the mode of transport is by sliding down a slope, the deposits that result are generally chaotic in nature, and show a wide variety of particle sizes. Grain size and the interrelationship between grains gives the resulting sediment texture. Thus, we can use the texture of the resulting deposits to give us clues to the mode of transport and deposition. Sorting – The degree of uniformity of grain size. Particles become sorted on the basis of density, because of the energy of the transporting medium. High energy currents can carry larger fragments. As the energy decreases, heavier particles are deposited and lighter fragments continue to be transported. This results in sorting due to density.

Soil Formation

 

Soil consists of rock and sediment that has been modified by physical and chemical interaction with organic material and rainwater, over time, to produce a substrate that can support the growth of plants.Soil is the uppermost layer of the land surface that plants use and depend on for nutrients, water and physical support.

Factors of soil formation are:-

• Parent material: soil formation depends on the mineral material, or organic material from which the soil is formed. Soils will carry the characteristics of its parent material such as color, texture, structure, mineral composition and so on. For example, if soils are formed from an area with large rocks (parent rocks) of red sandstone, the soils will also be red in color and have the same feel as its parent material.
• Time: Soils can take many years to form. Younger soils have some characteristics from their parent material, but as they age, the addition of organic matter, exposure to moisture and other environmental factors may change its features. With time, they settle and are buried deeper below the surface, taking time to transform. Eventually they may change from one soil type to another.
• Climate:Two important climatic components, temperature and precipitation are key. They determine how quickly weathering will be, and what kind of organic materials may be available on and inside of the soils. Moisture determines the chemical and biological reactions that will occur as the soils are formed. Warmer climate with more rainfall means more vegetative cover and more animal action. It also means more runoff, more percolation and more water erosion. They all help to determine the kind of soils in an area.
• Relief:i.e. the landscape position and the slopes it has. Steep, long slopes mean water will run down faster and potentially erode the surfaces of slopes. The effect will be poor soils on the slopes, and richer deposits at the foot of the slopes. Also, slopes may be exposed to more direct sunlight, which may dry out soil moisture and render it less fertile.
• Organisms:The source and richness of organic matter is down to the living things (plants and animals) that live on and in the soils. Plants in particular, provide lots of vegetative residue that are added to soils. Their roots also hold the soils and protect them from wind and water erosion. They shelter the soils from the sun and other environmental conditions, helping the soils to retain the needed moisture for chemical and biological reactions. Fungi, bacteria, insects, earthworms, and burrowing animals help with soil aeration. Worms help breakdown organic matter and aid decomposition. Animal droppings, dead insects and animals result in more decaying organic matter. Microorganisms also help with mineral and nutrient cycling and chemical reactions.

 

Davis Cycle

 

After the upliftment of landmass by the tectonic forces the process of denudation is started. The rivers, valleys and associated landforms passes through distinctive stages, provided that there has been no significant interference by earth movements or by changes of sea-level or climate. This idealized concept of landscape evolution was introduced to geomorphology more than sixty years ago by W.M. Davis, who referred to the whole sequence of stage as a Cycle of Erosion.

The basic goal of Davisian model of geographical cycle and general theory of landform development was to provide basis for a systematic descriptions and genetic classification of landforms. According to this concept a landscape has a definite life history, and as the processes of land structure operate on it the surface features are marked by several changes in its life time. Thus, the evolution of landscape passes through a cycle, and cycle follows a definite sequence of development.

The successive stage of developmental sequences can be divided into three parts and may be identified as youth, maturity and old age. Davis presentation of scheme was both vigorous and vivid and his colourful analogy of the human life and landscapes both passing through the stages of youth, maturity and old age caught the imagination of scientific world.

• Youth:The uplift is complete and has stopped. Immediately erosion of the uplifted block sets in. The streams follow initial irregularities available without adjusting to the structure. These are consequent streams. The floors of the valley suffer down cutting while the summits remain almost unaffected. Increased relief heralds the beginning of mature age
• Maturity:At this stage, the vertical erosion slows down and the horizontal action increases. A characteristic feature is the erosion of mountain tops at a faster rate than lowering of the valley floor. The coming closer of lines ‘A’ and ‘B’ indicates emergence of a gentle slope. The subsequent streams gain importance now.
• Old Age:A gentle gradient, accentuated by horizontal action and deposition, reduces the erosion intensity. A thick layer of sediment represents the earlier erosion activity. The landforms get mellowed—lines ‘A’ and ‘B’ run parallel to each other. Relicts of mountains or monad knocks are dotting the water divides and a featureless plain—peneplane is produced.

In order to understand the evolution of a particular landscape it is extremely important to know the stage of development. But the geographical structure and the nature of rocks also exert an important influence on the fashioning of landscapes is a function of structure, process and time (as called as stage by the followers of Davis). These three factors are called as ‘Trio of Davis’.

Structure :means lithological (rock types) and structural characteristics (folding, faulting, joints etc.) of rocks. Time was not only used in temporal context but it was also used as a process itself leading to an inevitable progression of change of landform. Process means the agent of denudation including both, weathering and erosion (running water in the case of geographical cycle).

Process:Implies the factors or agents responsible for weathering and erosion.

Time:Implies the stage at which the cycle is—youth, maturity or old age.

Penck Cycle 

 

According to German geomorphologist Walther Penck, the characteristics of landforms of a given region are related to the tectonic activity of that region. Contrary to the Davisian concept that “landscape is a function of structure, process and time (stage)”, Penck put forward his view that geomorphic forms are an expression of the phase and rate of uplift in relation to the rate of degradation, where it is assumed that interaction between the two factors, uplift and degradation, is continuous. According to Penck’s view the landforms observed at any given site give expression to the relation between the two factors of uplift and degradation that has been or is in effect, and not to a stage in a progressive sequence.

Penck proposed three types of valley slopes on the basis of erosional intensity acting on crustal movements.

1. Straight slope:Indicating uniform erosion intensity and a uniform development of landforms or ‘Gleichformige Entwickelung’ in German.
2. Convex slope:Indicating waxing erosion intensity and a waxing development of landforms or ‘Aufsteigende Entwickelung.
3. Concave slope:Indicating waning erosion intensity and a waning development of landforms or ‘Absteigende Entwickelung.’

Different Phases according to Penck are:-

(a) Phase of waxing rate of landform development (Aufsteigende Entwickelung)
Endogenetic forces cause the slow rise of the initial land surface (Primarumpf) but later on the upliftment is rapid.
In this phase, because of upliftment and the increase in the channel gradient and stream velocity rivers continue to degrade their valleys with accelerated rate of valley deepening.
The rate of upliftment is faster than the rate of down-cutting. It results in the formation of gorges and narrow V-shaped valleys. Since the upliftment of landmass far exceeds the valley deepening, the absolute height goes on increasing.
Altitude of the summit of interfluves and valley bottom continues to increase due to the faster rate of upliftment than that of the vertical erosion.
This phase is characterized by the maximum altitude and the maximum relief (relative heights of the valley floors).

(b) Phase of uniform development of land form (Gleichformige Entwickelung)
This phase may be divided into three sub-phases on the basis of upliftment and consequent degradation

(i) The first sub-phase is characterised by the continuance of accelerated rate of uplift. The absolute height continues to increase because the rate of upliftment is still greater than the rate of down-cutting.
The maximum altitude or the absolute relief is achieved, but relative relief remains unaffected because the rate of valley deepening is almost equal to the rate of lowering of the summits of stream interfluves.
The valley walls are steep. This is known as the phase of uniform development because of uniformity in the rate of valley deepening and lowering of divide summits.
(ii) In the second sub-phase the absolute relief neither increases nor decreases. This is due to the fact that rate of upliftment and the rate of erosion are the same. However, in this phase the absolute height and the relative relief’s are unchanged. So this may be called the phase of uniform development of landforms.
(iii) In this sub-phase there is no more upliftment of land.

(c) Phase of Wanning development of landscape (Absteigende Entwickelung)
The erosional processes dominate in this phase. The lateral erosion rather than vertical erosion is more important. There is progressive decrease in the height of the landforms. In other words, the absolute and the relative relief decline.
The valley side slope consists of two parts, the upper and the lower. The upper segment continues to have steep angle which is called as gravity slope.
The lower segment of the slope is called wash slope. The wash slope is composed of talus materials of lower inclination which is formed at the base of valley sides.
The later part of this phase is marked by the presence of inselbergs and a series of concave wash slopes.
This type of extensive surface produced at the fag end of absteigende entwickelung has been labelled is endrumpf which may be equivalent to peneplain as envisaged by Davis in his cycle concept. Thus, the cycle of landscape development as envisaged by Penck ends in endrumpf.

 

 

 

 

• It is the long term change in the statistical distribution of weather patterns over periods of time
• Though it has been happening naturally for millions of years, in recent years it has accelerated due to anthropogenic causes and has been causing global warming.
• UNFCCC defines climate change as – “a change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods”

 

 

Climatic Regions of India : Koeppen’s Classification
Climate Type	Climatic Region		Annual Rainfall in the Region
Amw (Monsoon type with shorter dry winter season)	Western coastal region, south of Mumbai		over 300 cm
As (Monsoon type with dry season in high sun period)	Coromandel coast = Coastal Tamil Nadu and adjoining areas of Andhra Pradesh	75 – 100 cm [wet winters, dry summers]
Aw (Tropical Savanah type)	Most parts of the peninsular plateau barring Coromandel and Malabar coastal strips	75 cm
BShw (Semi-arid Steppe type)	Some rain shadow areas of Western Ghats, large part of Rajasthan and contiguous areas of Haryana and Gujarat	12 to 25 cm
BWhw (Hot desert type)	Most of western Rajasthan	less than 12 cm
Cwg (Monsoon type with dry winters)	Most parts of the Ganga Plain, eastern Rajasthan, Assam and in Malwa Plateau	100 – 200 cm
Dfc (Cold, Humid winters type with shorter summer)	Sikkim, Arunachal Pradesh and parts of Assam	~200 cm
Et (Tundra Type)	Mountain areas of Uttarakhand The average temperature varies from 0 to 10°C	Rainfall varies from year to year.
E (Polar Type)	Higher areas of Jammu & Kashmir and Himachal Pradesh in which the temperature of the warmest month varies from 0° to 10°C	Precipitation occurs in the form of snow

 

 

 

 

 

Climatic Regions of India : Trewartha’s Classification

Climate Type	Climatic Region	Other Cliamatic Condtions
Am (Tropical Rain Forest)	Western coastal region, Sahayadris and parts of Assam	200 cm annual rainfall  & 18.2 C to 29 C temperature
Aw (Tropical Savanna)	Peninsular India except the semi arid zone	150 cm annual rainfall &  18 C-32 C temperature
Aw (Tropical Savanah type)	Most parts of the peninsular plateau barring Coromandel and Malabar coastal strips	75 cm
BS (Semi-arid Steppe type)	Runs southwards from central Maharashtra to Tamilnadu, Andhra Pradesh	40-75 cm annual rainfall & 20-32 C temperature
BSh (Tropical and subtropical Steppe)	Ranges from Punjab to Kutch	Annual temperature 35 C & 30-60 cm annual rainfall
BWh (Tropical Desert)	Western parts of Barmer, Jaiselmer and Bikaner and parts of Kutch	Annual Temperature 35 C & annual rainfall 25 cm
Caw (Humid Subtropical Climate with dry winters)	It ranges from Punjab to Assam	Rainfall from 100-150 cm
H (Mountain Climate)	Mountain areas of Himalayas including Jammu & Kashmir, Uttarakhand, Himachal Pradesh, Sikkim, Arunachal Pradesh	The average temperature varies from 0 to 10°C.

 

 

 

 

A tsunami is a very long-wavelength wave of water that is generated by sudden displacement of the seafloor or disruption of any body of standing water. Tsunami are sometimes called “seismic sea waves”, although they can be generated by mechanisms other than earthquakes.
Tsunami have also been called “tidal waves”, but this term should not be used because they are not in any way related to the tides of the Earth. Because tsunami occur suddenly, often without warning, they are extremely dangerous to coastal communities.

Tsunamis can be associated with earthquakes. Sometimes a large earthquake beneath the ocean floor will produce a tsunami, which is a series of large waves.

The rate at which a wave loses its energy is inversely related to its wavelength. Since a tsunami has a very large wavelength, it will lose little energy as it propagates. Thus, in very deep water, a tsunami will travel at high speeds with little loss of energy.

As a tsunami leaves the deep water of the open sea and arrives at the shallow waters near the coast, it undergoes a transformation. Since the velocity of the tsunami is also related to the water depth, as the depth of the water decreases, the velocity of the tsunami decreases. The change of total energy of the tsunami, however, remains constant.

Furthermore, the period of the wave remains the same, and thus more water is forced between the wave crests causing the height of the wave to increase. Because of this “shoaling” effect, a tsunami that was imperceptible in deep water may grow to have wave heights of several meters or more.

The main damage from tsunami comes from the destructive nature of the waves themselves. Secondary effects include the debris acting as projectiles which then run into other objects, erosion that can undermine the foundations of structures built along coastlines, and fires that result from disruption of gas and electrical lines. Tertiary effects include loss of crops and water and electrical systems which can lead to famine and disease.

 

 

 

 

An atmosphere is a layer of gases surrounding a planet or other material body, that is held in place by the gravity of that body. Many of the planets in this solar system have atmospheres, but none that we know of have an atmosphere quite like ours – one that can support life.

The air is a mixture of several gases. The air encompasses the earth from all sides. The air surrounding the Earth is called the atmosphere. The atmosphere is an integral part of our Earth. It is connected with the earth due to the gravitational force of the earth. It helps in stopping the ultra violet rays harmful for the life and maintain the suitable temperature necessary for life. The air is essential for the survival of all forms of life on the earth.

Composition of the atmosphere

 

The atmosphere is made up of different types of gases, water vapors and dust particles. The composition of the atmosphere is not static. It changes according to the time and place.

• Nitrogen N2  78%
• Oxygen O2 20.9%
• Argon Ar 9.34%
• Carbon dioxide CO2 3.84 %
• Neon
• Helium
• Methane
• Krypton
• Hydrogen
• Nitrous oxide
• Xenon
• Ozone

Water vapor is unique in that its concentration varies from 0-4% of the atmosphere depending on where you are and what time of the day it is.  In the cold, dry artic regions water vapor usually accounts for less than 1% of the atmosphere, while in humid, tropical regions water vapor can account for almost 4% of the atmosphere.  Water vapor content is very important in predicting weather.

Greenhouse gases whose percentages vary daily, seasonally, and annually have physical and chemical properties which make them interact with solar radiation and infrared light (heat) given off from the earth to affect the energy balance of the globe.

The atmosphere also change composition with height and can be divided into two layers. The lower layer is called the homosphere and has the composition we talked about earlier. It’s top is approximately the mesopause.

Above the homosphere lies the heterosphere, a layer in which the gases are stratified into four shells. The lowermost shell is dominated by molecular nitrogen (N2); next, a layer of atomic oxygen (O) is encountered, followed by a layer dominated by helium atoms (He), and finally, a layer consisting of hydrogen atoms (H).

Importance of various components of atmosphere are:-

(i) Oxygen is very important for the living beings.
(ii) Carbon dioxide is very useful for the plants.
(iii) Dust particles present in the atmosphere create suitable conditions for the precipitation.
(iv) The amount of water vapour in the atmosphere goes on changing and directly affects the plants and living beings.
(v) Ozone protects all kinds of life on the earth from the harmful ultra violet rays of the sun.

 

Structure  and stratification of the atmosphere

Variations of temperature, pressure and density are much larger in vertical directions than in horizontal. This strong vertical variations result in the atmosphere being stratified in layers that have small horizontal variability compare to the variations in the vertical.

The atmosphere can be divided into five layers according to the diversity of temperature and density.
(a) Troposphere :-It is the lowest layer of the atmosphere. The height of this layer is about 18 kms on the equator and 8 kms on the poles. The main reason of higher height at the equator is due to presence of hot convection currents that push the gases upward.
This is the most important layer of the atmosphere because all kinds of weather changes take place only in this layer. Due to these changes development of living world take place on the earth. The air never remains static in this layer. Therefore this layer is called changing sphere or troposphere.
The environmental temperature decreases with increasing height of atmosphere. It decreases at the rate of 1 C at the height of 165 metre. This is called Normal lapse rate.
The upper limit of the troposphere is called tropopause. This is a transitional zone. In this zone characteristics of both the troposphere and ionosphere are found.

(b) Stratosphere :-This layer lies above the troposphere and spread upto the height of 50 kms from the Earth’s surface. Its average extent 40 kms.
The temperature remains almost the same in the lower part of this layer upto the height of 20 kms. After this the temperature increases slowly with the increase in the height. The temperature increases due to the presence of ozone gas in the upper part of this layer.
Weather related incidents do not take place in this layer. The air blows horizontally here. Therefore this layer is considered ideal for flying of aircrafts.

(c) Mesosphere :-It spreads above the stratosphere upto the height of 80 kms. from the surface of the earth. It’s extent is 30 kms. Temperature goes on decreasing and drops upto – 100 C.

(d) Ionosphere :-The ionosphere lies from about 80-400 km in height and is electrically charged as short wave solar radiation ionizes the gas molecules. The electrical structure of the atmosphere is not uniform and is arranged into three layers, D, E, and F. Since the production of charged particles requires solar radiation, the thickness of each layer, particularly the D and E layers, changes from night to day. The layers weaken and disappear at night and reappear during the day. The F layer is present during both day and night. This change in height of the various electrically charged layers doesn’t effect the weather, but does effect radio signals.

The auroras also take place in the ionosphere since this is the electrically charged layer. The aurora borealis (northern lights) and aurora australis (southern lights) is closely correlated to solar flare activity.

(e) Exosphere:-This is the last layer of the atmosphere located above ionosphere and extends to beyond 400 km above the earth.  Gases are very sparse in this sphere due to the lack of gravitational force. Therefore, the density of air is very less here.

 

Earthquakes occur when energy stored in elastically strained rocks is suddenly released. This release of energy causes intense ground shaking in the area near the source of the earthquake and sends waves of elastic energy, called seismic waves, throughout the Earth. Earthquakes can be generated by bomb blasts, volcanic eruptions, sudden volume changes in minerals, and sudden slippage along faults. Earthquakes are definitely a geologic hazard for those living in earthquake prone areas, but the seismic waves generated by earthquakes are invaluable for studying the interior of the Earth.

The point within the earth where the fault rupture starts is called the focus or hypocenter. This is the exact location within the earth were seismic waves are generated by sudden release of stored elastic energy.

The epicenter is the point on the surface of the earth directly above the focus. Sometimes the media get these two terms confused.

Seismic waves are the vibrations from earthquakes that travel through the Earth; they are recorded on instruments called seismographs. Seismographs record a zig-zag trace that shows the varying amplitude of ground oscillations beneath the instrument. Sensitive seismographs, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world. The time, locations, and magnitude of an earthquake can be determined from the data recorded by seismograph stations.

 

Two of the most common methods used to measure earthquakes are the Richter scale and the moment magnitude scale.

The Richter scale is used to rate the magnitude of an earthquake, that is the amount of energy released during an earthquake.
The Richter scale doesn’t measure quake damage (which is done by Mercalli Scale) which is dependent on a variety of factors including population at the epicentre, terrain, depth, etc. An earthquake in a densely populated area which results in many deaths and considerable damage may have the same magnitude as a shock in a remote area that does nothing more than frightening the wildlife. Large-magnitude earthquakes that occur beneath the oceans may not even be felt by humans. Richter Scale of Earthquake Energy
The magnitude of an earthquake is determined using information gathered by a seismograph.
The Richter magnitude involves measuring the amplitude (height) of the largest recorded wave at a specific distance from the seismic source. Adjustments are included for the variation in the distance between the various seismographs and the epicentre of the earthquakes.
The Richter scale is a base-10 logarithmic scale, meaning that each order of magnitude is 10 times more intensive than the last one.

 

 

 

They are aggregates or physical mixture of one or more minerals. Minerals on the other hand are made up of two or more elements in a definite ratio. They have a definite chemical composition. Crust is made up of more than 2000 minerals, but out of these, 6 are the most abundant and contribute the maximum to this uppermost part of the earth. These are feldspar, quartz, pyroxenes, amphiboles, mica and olivine.
Rocks are of immense economic importance to us.
Rocks differ in their properties, size of particles and mode of formation. On the basis of mode of formation rocks may be grouped into three types:
(a) Igneous
(b) Sedimentary and
(c) Metamorphic

Igneous  Rocks

Igneous Rocks are formed by crystallization from a liquid, or magma. They include two types
Volcanic or extrusive igneous rocks form when the magma cools and crystallizes on the surface of the Earth
Intrusive or plutonic igneous rocks wherein the magma crystallizes at depth in the Earth.

Magma is a mixture of liquid rock, crystals, and gas. Characterized by a wide range of chemical compositions, with high temperature, and properties of a liquid.
On the basis of their mode of occurrence, igneous rocks can be classified as : extrusive or volcanic rocks and intrusive rocks.
(i) Extrusive igneous rocks are formed by cooling of lava on the earth’s surface. As lava cools very rapidly on coming out of the hot interior of the earth, the mineral crystals forming these rocks are very fine. These rocks are also called volcanic rocks. Gabbro and basalt are very common examples of such rocks. These rocks are found in volcanic areas. Deccan plateau’s regur soil in India is derived from lava.

(ii) Intrusive igneous rocks are formed when magma solidifies below the earth’s surface. The rate of cooling below the earth’ s surface is very slow which gives rise to formation of large crystals in the rocks. Deep seated intrusive rocks are termed as plutonic rocks and shallow depth intrusive rocks are termed as hypabyssal. Granite and dolerite are common examples of intru- sive rocks. From this point of view, therefore, igneous rocks can, in accor- dance with their mode of formation, be classified as (a) Plutonic, (b) Hyp- abyssal and (c) Volcanic rockmasses. The huge blocks of coarse granitic rocks are found both in the Himalaya and the Decean Plateau.

 

Sedimentary Rocks

Sedimentary rocks are formed by successive deposition of sediments. These sediments may be the debris eroded from any previously existing rock which may be igneous rock, metamorphic or old sedimentary rock. Sedimentary rocks have layered or stratified structure. The thickness of strata varies from few millimeters to several metres. So these rocks are also called stratified rocks. Generally, these rocks have some type of fossil between their strata. Fossil is the solid part or an impression of a prehistoric animal or plant embedded in strata of sedimentary rocks. Sedimentary rocks are widely spread on the earth surface but to a shallow depth.

The formation of sedimentary rocks involves five processes:

1. Weathering – The first step is transforming solid rock into smaller fragments or dissolved ions by physical and chemical weathering as discussed in the last lecture.
   2. Erosion – Erosion is actually many process which act together to lower the surface of the earth. In terms of producing sediment, erosion begins the transpiration process by moving the weathered products from their original location. This can take place by gravity (mass wasting events like landslides or rock falls), by running water. by wind, or by moving ice. Erosion overlaps with transpiration.
   3. Transportation – Sediment can be transported by sliding down slopes, being picked up by the wind, or by being carried by running water in streams, rivers, or ocean currents. The distance the sediment is transported and the energy of the transporting medium all leave clues in the final sediment that tell us something about the mode of transportation.
   4. Deposition – Sediment is deposited when the energy of the transporting medium becomes too low to continue the transport process. In other words, if the velocity of the transporting medium becomes too low to transport sediment, the sediment will fall out and become deposited. The final sediment thus reflects the energy of the transporting medium.
   5. Lithification (Diagenesis) – Lithification is the process that turns sediment into rock. The first stage of the process is compaction. Compaction occurs as the weight of the overlying material increases. Compaction forces the grains closer together, reducing pore space and eliminating some of the contained water. Some of this water may carry mineral components in solution, and these constituents may later precipitate as new minerals in the pore spaces. This causes cementation, which will then start to bind the individual .

Metamorphic Rocks

Metamorphic rocks are formed under the influence of heat or pressure on sedimentary or igneous rocks. Tremendous pressure and high temperature change the colour, hard- ness, structure and composition of all types of pre-existing rocks. The process which bring about the change is known as Metamorphism and the ultimate products, formed due to operation of such processes are defined as the Metamrphic rocks.
Metamorphism refers to the changes in mineral assemblage and texture that result from subjecting a rock to pressures and temperatures different from those under which the rock originally formed.
The original rock that has undergone metamorphism is called the protolith. Protolith can be any type of rock .

Metamorphism occurs because rocks undergo changes in temperature and pressure and may be subjected to differential stress and hydrothermal fluids. Metamorphism occurs because some minerals are stable only under certain conditions of pressure and temperature. When pressure and temperature change, chemical reactions occur to cause the minerals in the rock to change to an assemblage that is stable at the new pressure and temperature conditions. But, the process is complicated by such things as how the pressure is applied, the time over which the rock is subjected to the higher pressure and temperature, and whether or not there is a fluid phase present during metamorphism. Different types of metamorphic rocks are found all over the world. In India, marble is found in Rajasthan, Bihar and Madhya Pradesh, whereas slates are available in plenty in Orissa, Andhra Pradesh and Haryana. In Kangra and Kumaun regions ]of Himalaya, slates of different colours are found.