CHAPTER NO.5 VOLCANOES AND EARTHQUAKES

CHAPTER NO.5 VOLACANOES AND EARTHQUAKES

Volcano is an opening in the crust of the earth, connected by a conduit to an underlying magma chamber, from which molten lava,volcanic gases, steam and pyroclastic material are ejected. It 1s usually in the form of a peak, which may be cone shaped or dome shaped, according

to the character of the material ejected. The vulcauicity covers all those processes and mechanism which are related to the origin of magmas,

gases, and vapours, their ascent and appearance on the earth’s surface in

various forms.

Causes of Vulcanism : Knowledge and information about vulcanism was quite restricted to lava and some other molten materials prior to modern research. Now we can cite following reasons for volcanic activity :

1. Theory of Plate Tectonic : The volcanic eruptions are closely associated with sea-floor spreading plate tectonics and mountain building processes. The type of volcanic system depends on the type of plate-tectonic settings. Basaltic magma is generated at divergent plate boundaries by partial melting of rising asthenosphere. It is extruded mostly in quite fissure eruptions. Most of active volcanoes of the world are found in circum pacific belt and Mid Continental belt.

2. Increasing Temperature of Interior : There is gradual increase in temperature with increasing depth at the rate of 1°C per 32 meters. The main cause of this increase in temperature is the disintegration of radioactive elements deep within the earth. Origin of Magma because of lowering of melting point inside the Earth caused by reduction in the pressure due to splitting of plates and their movement in opposite directions.

3. Formation of Gases and Steam : Ongin of gases and vapour due to heating of water which reaches underground through percolation of rain water and snow-melt water. On being in contact with magma various elements turn into water vapours and steam resulting into 80 to 95

percent presence of vapours and steam. Other gases that cause erruptions

are carbondioxide, Hydrogen, Amonea, Sulphuroxide etc.

Effects of Folds:Sedimentary rocks get compressed due to orgenetic force in the

interior of earth causing folds. This situation gives chance to relief to rise

which results into releasing pressure over lava and softer parts of Earth,

finally encouraging volcanic eruptions.

Types of Volcanic Activity :During the eruption, a volcano may behave in several different ways. The primary factors on determining an eruption type are; the chemistry of its magma and its viscosity means magma’s thickness,

resistance to flow, or degree of fluidity.

(1) Hawaiian Type Eruption : It is an eruption in which great quantities of extremely fluid basic lava flow out from a fissure or a central vent to form atypical shield volcano. In this type of volcano,explosive activity is rare while its spread is extensive. This type of

volcanic eruptions are common of Hawai Islands in Pacific Ocean, for example, Kilavea volcano of the southern Hawaii island.

(2) Strombolian Eruption : In this type of volcanic eruption the basic lava (basaltic) is less fluid than that at Hawaiian type. Consequently,explosions are more common and more fragmental material is ejected. It is named after the Stromboli volcano on the Lipari island of north Sicily.Strombolian Volcano eruptionin Mediterranean Sea is also known as “Light house to the Mediterranean’.

(3) Vulcanian Eruption : Such volcano erupts with great force and intensity. In this type of eruption, lava surface solidifies rapidly because of its very high viscosity. The solidification results in a build-up of pressure beneath the lava crust and a continous series of violent explosion takes place, during which large quantities of Pyroclastic material are ejected violently from the vent. Large dark cauliflower like clouds take form over eruption..

(4) Pelinian Eruption : In a Peleean eruption the lava is extremely viscous. It has the characteristics in the formation of 'nuee ardeutes'(glowing cloud). This type of volcanic eruption is named from Mt. Pelee (West Indies) where extremely violent eruptions have occured. Besides another eruption known as Pelinian or Pelean eruption, is of great violence in which the explosion cloud towers to considerable height in the shape of Pine tree. These are most violent eruptions.

Types of Volcano

On the basis of material erupted the volcanoes may be classified into five

major types : .

(1) Basalt cones: Basalt cones are rare. They are likely to be low rather than high cones because of the fludity of basaltic lava. The Rangitoto(New Zealand) is a suitable example.

(2) Basalt Dome or Shield: The Hawaiian volcanoes are the excellent examples of basalt domes or shield volcanoes as are Mt. Etna and many of the volcanoes of Iceland. Basalt domes are formed where fluid basaltic lavats extruded. They often attain great height for example: Mauna Loa has an altitude of 4,219 meter.

(3) Cinder Cones or Ash Cone : These are usually of low height and are formed of volcanic dusts and ashes and pyroclastic matter (fragmental material). The formation of cinder cones is initiated due to accumulation of finer particles around volcanic vent in the form of tiny

mound, say “ant mount’ which varies in height from a few centimeter to afew meters. The volcanic cone of Mt. Jurullo of Mexico and Mt. Izalok of El Selvader are good examples.

(4) Composite Cone : Such Volcanoes are the heightest of all volcanic cones. These are formed due to accumulation of different layers of various volcanic materials and hence these are also called as strato cones. Most of the world’s larger volcanoes such as Fujiyama (Japan),Vesuviun (Italy), Cotopaxi (Ecuador), Mt. Shasta, Mt. Ravier (U.S.A),are typical examples of composite volcanoes.

(5) Acid Lava Cones : Such cones are formed due to Silica rich thick

lava. Such lavas have very low mobility and hence they are immediately cooled and solidified lavas around the volcanic vents. Such volcanos have steep heights. Stromboli volcano is example of this type volcano.

Depressed forms/Valcanic Topography :

(1) Volcanic Craters : The depression formed at the mouth of a volcanic

vent is called crater or a volcanic mouth, which is usually funnel shaped.

It is surrounded by very streep inward facing cliffs and may be several hundred meters in depth. Its floor may contain lava lake or may be composed of layers of ejected material. Itis formed by either a major eruption or the collapse of a volcanic cone (engulfment). St. Anna Crater of El. Selvador is its good example.

(2) Calderas : Generally, enlarged form of a crater is called caldera.There are two parallel concepts for the origin of caldera. According to first group of scientists acalderais an enlarged form of acrater and itis surrounded by steep walls from all sides. The calderais formed due to subsidence of acrater. This concept has been propounded by the U.S.Geological survey. The second group of scientists has opined that the

caldera are formed due to vicent and explosive eruptions of volcanoes.

The significant calderas of the world are,Lake Toba of Sumatra (50 km x 50 km), Aira (25 km x 24 km) in Japan, Crater Lake (10 km x 10 km) in U.S.A. etc. Smaller calderas housed in a big caldera are called nested calderas or grouped calderas.

Intrusive Topography :When gases and vapour are not very much strong during volcanic activity, the ascending magma do not erupt as lava rather these are

intruded in Viods below the crystal surface and after cooling and

solidification assume several interesting forms like: Batholith,

Laccolith, Phacolith, Lapolith, Sills, Dvkes etc.

1. Batholith : Batholiths are very deep and are composed of Granite,Quartz monozite or Diorite principally. Batholiths are dome shaped.Bolder batholiths and Idaho batholiths of U.S.A. and Aswan granite of Africa are examples.

2. Laccolith : Laccoliths are formed when megma solidifes in cracks of sediments and take convex shape. As hot lava solidifies between two straps of sediments, upper layer rises upwards.

3. Phacolith : Phacoliths are formed when magma is filled in antichines and syclines of folded mountains.

4. Lapolith : Lapolitho take shape when magma solidifies on cooling in concave layers of sedimentary rocks in the interions.

5. Sill : Sills are parallely solidified lava layers in intrusive sedimentary rocks.

6. Dyke: Longitudinally solidified magma tn rocks is known as dyke.

7. Geysers : Geyser, infact is a special type of hot spring which spouts

hot water and vapour from time to time. The word ‘geyser’ which means gusher or spouter. This word was used to indicate the spouting water of a hot spring of Iceland known as Great Geyser.Old faithful geyser of Yellow Stone National Park in the Wyoming (U.S.A.), is the good example. If may throw water upto 100 feet.There are around 1000 geysers world over. Kamchatka Valley in Russia and Manikaranin Himachal Pradesh are other examples of geysers.

8. Fumaroles : Fumaroles (Latin fumus, smoke) means such a vent through which there is emission of gases and water vapour. It appears from a distant place that there ts emission of enormous

volume of smokes from a particular centre. Thus, smoke or gas emitting vents are called Fumaroles. Fumaroles are often in the neighbourhood of volcanoes. A fumaroles field is an area of thermal springs and gas vents where magma or hot igneous rocks at shallow

depth are releasing gases or interacting with ground water. The gases thus emitted are Carbondioxide, Sulphurdioxide, Hydrogen Chloride, Hydrosulphur oxide etc. Numerous fumaroles are found in groups near Katmai Volcano of Alaska (U.S.A.). 'The valley of Ten Thousand Smokes' is a good example, which was formed during the 1912 eruption of Novaruptain Alaska (U.S.A.).

Classification on the basis of Periodicity of Eruption Volcanoes are divided into three types on the basis of periodicity of eruptions:

(1) Active Volcanoes are those which constantly eject volcanic lava,gases, ashes, and fragmental material. There are about 600 active volcanoes in the world, most of which being in the Pacific Ocean, around “Ring of Fire’ and in the Atlantic islands. Mt. St. Helen (U.S.A.),Stromboli and Etna (Mediterranean Sea) are the most significant volcanoes in this category. Stromboli volcano is known as ‘Light House of the Mediterranean’ because of continous emission of burning and

luminous incandescent gases. All these are known for more than 100 years.

(2) Dormant Volcanoes are those which become quiet after their eruption for some time and there is no indication for future eruption but suddenly they erupt very violently and cause enormous damage to human health and wealth. Visuvious volcano is the best example of dormant volcano which erupted in 79 A.D., thenit kept quiet upto 1631 A.D. when it suddenly exploded with great force destroying Popi and Harculanion cities. Mt. Pinatubo in Phillipines is another example of dormant volcano. People living around it, never knew about it prior to 1991.

Extinct Volcanoes are considered extinct when there is no indication of future explosion. The crater is filled with water and lakes are formed.Edinbourgh Castle of Scotland and Shipark of Nethersland are good examples of this type. It may be pointed out that no volcano can be declared permanently dead as no one knows, what is happening below

the ground surface!Volcanic Materials (ejected out)

(1) Vapours and Gases : Steam and vapours constitute 60% to 90%

(percent) of total gases discharged during a volcanic eruption. Gases

include carbon dioxide, nitrogen oxide, sulphur dioxide, hydrogen,carbom monoxide, sulphurate hydrogen, hydrochloric acid ete.

(2) Magma and Lava : Generally, molten rock materials are called magma below the earth surface while they are called lava when they come out the earth’s surface. Lava and magma are divided on the basis of silica percentage into two groups e.g. (1) Acidic Lava (higher

percentage of silica and (11) Basic lava (low percentage of silica).

(3) Fragmental or Pyroclastic materials : Pyroclastic material thrown during explosive type of eruption are grouped into three categories:]. Volcanic dust

(finest particals) ,live lava, tephra

2.(Volcanic ash - size upto 2mm), lapilli (of the size of peas)and 3. volcanic bombs (6 cm or more in size). The volcanic bombs can be the size of baseball or basket ball to giant size. Some time the volcanic bombs weigh

100 tonnes in weight and are thrown upto distance of 10 km.

World Distribution of Volcanoes :

Volcanoes occur in many regions of the world, including the islands of the oceans, the young mountain ranges and plateaus of the continents. Like earthquakes, the spatial distribution of volcanoes over the globe is well marked and well understood because volcanoes are found in well defined belt or zone.

If we took at the world distribution of the volcanoes it appeares that the volcanoes are associated with the weaker zones of the Earth’s crust and these are closely associated with seismic events say earthquakes. The weaker zones of the Earth are represented by folded mountains (Western Cordillera of North America, Andes, Mountains of

East Asia and East Indies) with the exceptions of the Alps and the

Himalayas, and fault zones of the continents and oceans. Volcanoes are

also associated with the meeting zones of continents and oceans.Following are the three main belts of volcanic eruptions.

Convergent Plate Margins or Boundary

The Circum Pacific Belt (Ring of Fire) also known as the “volcanic zones of the convergent oceanic Plate Margins’, includes the volcanoes of the eastern and western coastal areas of the Pacific Ocean,of island are and festoons off the east coast of Asia and at the volcanic

island scattered over the Pacific Ocean. This volcanic belt is also called

as the “Fire Gridle of the Pacific’ or the fire ring of the Pacific.

This belt begins from Erebus Mountain of Antarctica and runs northward through Andes and Rocky Mountains of South and North America to reach Alaska from where this belt turns towards eastern Asiatic coast to include the volcanoes of island arcs and festoons (e.g.Sakhalin, Kamchatka, Japan, Philippines etc.). The belt ultimately merges with the mid-continental belt in the East Indies. Most of the volcanoes are found in chains e.g. the volcanoes of the Aleutian island,Hawaii island, Japan etc., Cotopaxi is the highest volcanic mountain of the world (19,613 feet). The other significant volcanoes are Fujiyama

(Japan), Shasta, Rainer and Hood (Western Cordillera of North America),

a valley of ten thousand smokes (Alaska), Mt. St. Helens (Washington,

U.S.A.), Kilavea (Hawaii island), Mt. Taal, Pinatubo and Mayon of Philippines etc. Hekla and Heggafall volcanoes erupted in 1974 and 1973 respectively.

Intra-Plate Volcanoes :Besides the aforesaid zones of volcanoes, scattered volcanoes are also found in the inner parts of the continets. Such distributional patterns of volcanoes are called as interplate volcanoes, the mechanism of their eruption is not yet precisely known. The location of volcanoes ts in Antarctica, Indian Ocean, Medgaskar island etc.

EXCERCISE

1. Describe Volcano.

2. What are the causes of Vulcanism?

3. Onthe basis of type of eruption, classify the volcanoes ?

4. What is a Volcano Crater ? How is it formed ?

5. What is aCaldera ?

6. What is difference between Batholith and Laccolith ?

7. Differentiate between Geyser and Fumaroles ?

8. Inhow many types can we classify volcanoes on basis of perodicity

of eruption ? Describe.

9. Describe the volcanic material comes out of volcanoes ?

10. Describe in detasl the distribution of Volcanoes in the world.

Map work:

Mark distribution of volcanos on the world map.

Earthquakes are vibration of Earth caused by ruptures and sudden movement of rocks that have been strained beyond their elastic limits.Earthquakes are movements within the earth caused by natural or man-made stresses. In other words “an earthquake is motion at ground surface ranging from faint tremor to a wild motion capable of shaking building

apart”.

Earthquake is a form of energy of wave motion transmitted through the surface layer of the Earth. Earthquake has also been defined as a shock or series of shocks due to a sudden movement of crystal rocks,generated at a point known as ‘Focus’ or ‘Hypocentre’ with in the crust or the depth of the focus generally varies from about 10 km to 700 km below the surface of the Earth. The magnitude of the earthquake is

inversely proportional to the depth of focus. The largest earthquakes occurs at shallower depths in the Earth’s crust but smaller earthquakes can and do occur at the depth down to about 700 kilomeners.

The point where the shock waves (seismic waves) reach the surface is termed as Epicentre, around which lives of equal seismic intensiy can be drawn (isoseismal lines). The waves generated by an earthquake are recorded by an instrument called ‘seismography’ or

“seismometer’. The science of earthquakes is known as seismology.

Causes of Earthquakes :

The main causes of earthquakes are as under :

1. Volcanic Eruption

2. Rupture and sudden movements of rocks (folding and faulting)

3. Plate Tectonics

4. Anthropogenic factors (Man made reasons)

1. Volcanic Eruptions : Volcanic eruption is one of the main causes of

earthquakes. Volcanic earthquakes are caused by gas explosions or the up

doming and fissuring of volcanic structures. Such earthquakes occur

either simultaneously with eruption or more commonly in the period preceding an eruption. They are generally of shallow origin and their area of disturbance is relatively small and rarely exceeds a few hundred square kilometer. Their intensity may be high near the volcano. The violent eruption of Krakatoa Volcano (between Sumatra and Java island)

caused such a sever earthquake that its impact was experienced at Cape Horn, Chile (South America, 12,800 km away). The Krakatoa earthquake generated 30 to 40 meters high Tsunamis (sea waves), which killed over 36,000 people in the coastal areas of Jave and Sumatra. The ash cloud reached into the mesosphere, and sounds of blast were heard in Central Australia, the Philippines and even 4,800 km (3,000 miles) away in the

Indian ocean. Such volcanic eruptions always result into severe earthquakes of hazordous nature.

2. Faulting : A fracture in a rock along which there has been an observable amount of displacement is known as fault. Earthquake occur when movementof the earth takes place along a line of fracture called a fault. Fault can be found in rocks of all ages. Its effect can be maximum near the active Plate boundary.

The San Andreas fault of Californiais atypical example which led to earthquake in 1906. This fault passes about 60 km (36 miles) inland of Los Angeles, placing the densely populated Metropolitan Los Angeles region in great jeopardy.

Himalayan Earthquake & Destruction in Nepal

The Nepal Earth, which occured on April 25, 2015, and was measured 7.9 on the

Richter scale destroyed not only parts of Nepal, but also the plains of Northern Bihar in India. According to UN and Nepal Governments own estimates more than 8600 people were killed and over 21,000 injured in the tremors. over 5,30,000 houses were damaged. This includes over 20,000 school buildings.

Epicentre of the Earthquake was beneath Gorkha- Lamjung 83 Kilometer North-West of Karhmandu.Like other Himalayan quakes, the Nepal temblor is a dramatic

manifestation of the ongoing tectonic

convergence between the Indo-Australian and Asian tectonic plates that have built the Himalayas over the last SO million years. A product of millions of years of crustal shortening,the Himalayas are under immense tectonic stress and occasional temblors. The last 200 years in the region have seen four great earthquakes. But central Himalaya has been an exception,researchers warns, andis considered to be susceptible to great temblors. This earthquake was caused by thrust faulting “on or near the main frontal thrust” where the Indian plate is pushed under the Eurasian plate. The India plate is converging with the Eurasian plate at arate of 4.5-5 cm per year.

According to Professor Michael P. Searle, Professor of Earth Sciences at

Oxford University, one of the leading geologists working in the Himalayas, and the author of Colliding Continents: A Geological Exploration of the Himalaya,

Karakoram and Tibet. explains the nature of the quake and its relevance in a larger narrative of continents, oceans and an ever restless earth that fuels it all:

1.The Nepal earthquake was devastating due to many factors. The source of the quake was shallow and the fault plane extended right up to densely populated Kathmandu.

2. Nepal is situated on an ancient lakebed and hence the soil at Kathmandu valley is soft and “lquifies easily.” Unlike arocky terrain where the seismic waves travelling with great velocity can have very good transmission, a terrain with sediments can amplify the seismic waves, thereby amplifying the ground motion, thus transform the sediment into

a liquid like substance which would couse intense shaking by surface waves si miler to the earthquake that davastated Maxico city in 1985.’

3. The slip of 1 to3 metres recorded along the 160-km-long rupture showed strain built up over acentury.

4. Research implies that this segment has seen no great earthquakes in the last 700 years. Thus, the unspent accumulated slip needed to be released through this quake and will further be released through future quakes. This means that the segment, which includes parts of Uttarakhand, is capable of witnessing more damage.

5. As per latest information, which comes from Europe’s Sentinel-1A radar satellite.The devastating earthquake have also shrunk the height of the world’s tallest peak —Mount Everest — by about 2.5 cm. The radar images showed that some of the world’s tallest peak, according to the non profit UNAVCO, a geoscience research consortium.

Plate Tectonics :The plate boundaries are the primary location of earthquake and volcanic activity. The ‘Ring of fire’, surrounding the pacific basin,

named for the frequent incidence of volcanoes, is most evident. The

subduction edge of the Pacific Plate thrusts deep into the crust and upper

mantle, producing molten material that makes its way back towards the surface, causing active volcanoes along the ‘Pacific Rim’. These volcanic eruptions are the main cause of earthquakes. Such processes occur similarly at plate boundaries throughout the world.

The global patterns of Earth’s seismicity show a narrow belt of

shallow-focus earthquakes that coincides almost exactly with the crust of the oceanic ridge and marks the boundaries between divergent plates.

Anthropogenic Factors :Human over interaction with nature is also one of the main causes of the occurence of many of the earthquakes. The extraction of minerals,deep underground mining, blasting of rocks by dynamites for

construction of roads, dams and reservoirs, nuclear explosions, etc., lead to the occurence of earthquakes of various intensity and magnitudes.

Many of the earthquakes of the wold in the present century are the result

of construction of dams and reservoirs. The earthquake of 1931 in Greece

has been attributed to Marathon Dam construction in 1929. The Koyna

earthquake of 1967 in Satara distnct of Maharashtra (India) was due to Koyna reservoir constructed in 1962, which caused more than 180 deaths and 1500 other human casualties.

Magnitude of Earthquakes :The magnitude of Earthquake is most commonly assessed by Richter Scale which was devised by an eminent seismologist Charles Francis Richter in 1935 and then modified by Richter and his colleague Beno Gutenberg. This scale can be related to the energy released at the earthquake centre, and thus can be used as an estimate of the serverity of a particular earthquake. The Richter magnitude of an earthquake is determined from the Logarithm of the amplitude of waves recorded by seismographs. The Richter magnitude and energy release, as modified by Kanamori, have been given in table:

(1) Circum Pacific Belt :The most widespread and intense earthquake activity occurs along subduction zones at convergent plate boundaries. The Circum Pacific Belt extends in the west from Alaska to Kurile, Japan, Mariana and the Philippine trenches, beyond which it is divided into two branches, one going towards the Indonesian trench and other towards Keramac-Tonga trench to the north-west of New Zealand. On the eastern side of the Pacific, the earthquake zone follows the west South America.

The Mid-Atlantic Belt :This belt of earthquakes extends along the mid oceanic ridges and several islands near the ridges of the Atlantic ocean. The sea-floor spreading is the main cause of earthquakes in this belt. Earthquakes of

moderate to mild intensity with shallow focus (less than 70 km deep)are recorded in this belt. The Rift Valley of East Africa and the Red Sea are considered as an extension to this belt.

The Mid-Continental Belt :This earthquake belt extends along the Alpine

mountains system of Europe,North America, through Asia Minor, Caucasia, Iran,Afghanistan and Pakistan to the

Himalayan mountain system,including Tibet, The Pamir, Tien-Shan, Altai, and the mountains of China, Myanmar and Eastern Siberia. This zone is characterised by larger earthquakes of shallow origin and some of intermediate origin deep focus earthquakes are

almost absent in this belt.

Earthquakes in India :Most of the earthquakes in India occur in Himalayan Belt. This is aregion of marked instability and is characterised by several thrust planner. The north-western (including Baluchistan and Hindukush and the Pamirs) and the north eastern corners at the Himalayas are

particularly vulnerable as there are sharp changes in strikes and rocks

are under great stress. The Indo-Gangetic alluvial track is mostly

affected by earthquakes originating in the Himalayan belt, but changes appear to be still taking place at the botton of the Gangetic trough giving rise to the occasional earthquakes. The Sindh earthquake of 1819 and the Bihar earthquake of 1934 had their focii in this trough.

The Peninsular Indiais considered to be a comparatively stable block while earthquakes of Koena (1967) and Latur (1993) are examples of such activity in this region.

Consequences of Earthquakes:Throughout the histroy, earthquakes have done great damage to both human lives and property. The major consequence of the earthquakes are:

(1) Landslides : In the young fold mountains like Andes, Rockies, Alps

and the Himalayas, the earthquakes result into landslides which damage

the human settlements and disturb the transport system. Financial & Commercial distruction is at its most 1n mountainous regions because of

earthquakes.

(2) Loss of Human Lives : It has been estimated that on an average about 15,000 people are killed every year by earthquakes. There had been earthquakes of great magnitudes in the densely populated areas of the world in which over one lakh people lost their lives, such as in Shenshi 1556 China massive earthquake killed 8,30,000 people.

(3) Fire Incidence : Contact of line electrical wire and damage to blast

furnaces in factories and other fire related appliances cause devastating

fires. Consequently, more damage to life and property occurs from these fires. Control over such incidences also go beyond possible.

(4) Loss of Human Property : Earthquakes pose a significant threat to much of the world population. Earthquakes inflicts great damage to buildings, roads, railways, dams, bridges, etc. The damage to property is more serious in the areas of unconsolidated materials, such as alluvium,colluvium, and artificially filled grounds and depressions. The Gujarat earthquake in 2001 not only killed over 30,000 people, but also damaged property of more than Rs. 2,000 crores. A much bigger destruction has

been caused by earthquake in Nepal in 2015.

(5) Flash Floods : Many a times, under the impact of severe earthquakes, the dams and embankments develop fissures, which become the cause of flash floods and big harm to human lives and property.

(6) Tsunami : The seismic waves, travelling through the ocean and sea

water, result into high sea waves which are known as Tsunamis.

‘Tsunami’ is a Japanese term which was been universally adopted to describe a large seismically generated sea waves which is capable of

considerable destruction in certain coastal areas, especially where

submarine earthquakes occur. Although in the open ocean the wave height may be less than 1 meter it steepens to heights of 15 meter or more on entering shallow coastal water. The wave length in the open ocean is of the order of 100 to 150 km and rate of travel of a tsunami is between 640 to 960 km/hr (400 - 600 miles/hr). Tsunamis can also be generated by violent volcanic explosion at or below sea level. The Tsunami of 26th December, 2004 as a consequence of the Aceh (Sumatra) earthquake,killed about two lakh people in Indonesia, Thailand, Sn Lanka, Andaman & Nicobar Islands, Tamilnadu (India), Maldives, Somalia, and Myanmar.The loss of property and structure was enormous.

Regions with high risk of Tsunami, typically use “Tsunami Warning System’ to warn the population before the wave reaches land.

Earthquake Forecasting :Making a forecast about the occurences of an earthquake in a region, place and time is still difficult proposition. One approach for making predictions ts to examine the history of each plate boundary, and determine the frequency of earthquakes in the past. Setsmologist then construct the maps that provide an estimate of expected earthquake

activity.

The Chinese, on the basis of seismographic studies on animal

behaviour, made fairly accurate predictions in the seventies. Despite

animal behaviour and other phenomena, effective earthquake predictions, which could save many lives and property damage is providing to be an elusive goal.

EXERCISE

1. What do you mean by earthquake ?

2. Whatis ahypocenter ?

3. What is an epicentre ?

4. What is focus and epicentre ? Draw a diagram to explain.

5. = Whichinstrument is used to measure earthquakes ?

6. What are the causes of earthquakes ?

7. What ts principle of Plate Techtonic ?

8. How humans are responsible for generating earthquakes ?

9. What is Richter Scale ? How intensity of earthquake is measured?

10. What is Pacific “Ring of Fire’ ?

11. Describe the distribution of earthquakes world over.