The forces acting on the earth’s surface leads to the creation, destruction, recreation and maintenance of geo-materials on the earth’s crust. The forces are divided as Endo-genetic and Exo-genetic forces.
– Various agents of denudation like running water, rain, frost, sun, wind, glaciers and waves have constantly reshaped the face of the earth making it very varied and diverse. However, the plateau and plains have only modified these structures which have been modeled by the movements of the earth’s crust.
– Since the down of the geological time there have been about nine orogenic or mountain building movements, folding and fracturing the earth’s crust. Some of them occurred in Pre-Cambrian times between 600 – 3500 Million years ago.
– The three most recent orogenies are the:
• Caledonian (about 320 million years ago) raised the mountains of Scandinavia and Scotland in North America. These mountain ranges have been worn down by the agents of denudation and are no longer exhibit the striking forms as existed earlier.
• Hercynian (about 240 million years ago) raised the Ural Mountains, the Pennines and Welsh Highlands in Britain, the Harz Mountains in Germany, the Appalachians in America and the high plateaux of Siberia and China. These mountains have also been reduced in size by the various sculpturing forces.
• Alpine (about 30 million years ago) the recent and major orogenic movements of the earth, raised mountain ranges such as Alps, Himalayas, Andes and Rockies which are the young loftiest mountains and are most imposing.
– The above developments of the landforms are the results of Endo-genetic and Exo-genetic forces.
• The forces which originate inside the earth due to volcanism, diastrophism and massive crustal rearrangement are known as Endo-genetic Forces.
• The external forces which bring changes in the landforms through the process of erosion, mass wasting and weathering are known as Exo-genetic Forces.
The forces coming from within the earth are called as endo-genetic forces which cause two types of movements in the earth, viz:
a) Horizontal Movements: Side to side movements of the earth’s crust (Horizontal and Tangential), causes lot of disruptions in the horizontal layer of strata as they involve compression and tension of the pre-existing rocks.
b) Vertical Movements: Slow and widespread bringing changes in the horizontal rock strata. Originate from the centre of the earth causing large scale upliftment or subsidence of a part of the earth’s crust forming continents and plateaus. These movements motored by the endo-genetic forces introduce various types of vertical irregularities which give birth to numerous varieties of relief features on the earth’s surface, eg., mountains, plateaus, plains, lakes, faults, folds, etc.
The energies in the endo-genetic forces are mostly generated by radioactivity, rotational and tidal friction and primordial heat from the origin of the earth. This energy is due to geothermal gradients and heat flow from within induces diastrophism and volcanism in the lithosphere.
The endo-genetic forces and movements are divided, on the basis of intensity, into two major categories:
a) Diastrophic Forces: It is a general term applied to slow bending, folding, warping and fracturing of the earth’s crust which becomes discernible after thousands of years. From the point of view of areal distribution, Diastrophic forces can further be divided into two sub-groups:
I. Epeirogenetic Movements: A slow process causing movements of the earth’s crust resulting into the displacements of continents and ocean basins. It acts along the radius of the earth and hence also known as radial movements.
• Their direction may be towards (subsidence) or away (uplift) from the center. The formation of mid-oceanic ridges, ocean trenches, the Great Rift Valley of Africa can be attributed to these movements.
• Some of the beaches which have been elevated (uplifted) as much as 15 m to 30 m above the present sea level such as Kathiawar, Nellore and Thirunelveli coasts. Several places which were the flourishing sea ports about 1000 to 2000 years ago are now a few miles inland such as Coringa near the mouth of the Godavari, Kaveripattinam in the Kaveri delta and Korkai on the coast of Thirunelveli.
• An earthquake in 1819 submerged a part of Rann of Kachchh. Presence of peat and lignite beds below the sea level in Thirunelveli and the Sundarbans is an example of subsidence. The Andamans and Nicobars have been isolated from the Arakan coast by submergence of the intervening land.
• Trees have been found embedded in mud about 4 m below the low water mark on the east side of Bombay Island and on the Thirunelveli coast in Tamil Nadu. A part of the former town of Mahabalipuram near Chennai (Madras) and a large part of Gulf of Mannar and Palk Strait (shallow zone) have been submerged in geologically recent times.
II. Orogenetic Movements: It involves mountain building through severe folding and affecting long and narrow belts of the earth’s crust acting tangentially to the earth surface, as in plate tectonics. It can further be divided into Compressional and Tensional.
• Compressional forces result into folding (folds), crustal bending, local rise and subsidence because this type of force acts towards a point from two or more directions.
• Tensional forces create cracks, ruptures, features and faults, since this type of forces acts away from a point in two directions.
b) Sudden forces: These are the result of long period preparation deep within the earth. Only their cumulative effects on the earth’s surface are quick and sudden. Geologically, these sudden forces are termed as ‘constructive forces’ because these create certain relief features on the earth’s surface.
STRUCTURES PRODUCED BY ENDO-GENETIC FORCES
Endo-genetic forces create deformation in the earth’s crust. This distortion takes place more in sedimentary and metamorphic rocks than in igneous rocks as the former rocks are usually horizontal, a little deformation becomes easily discernible. The structures are discussed as follows:
(A) Folds: Fold is a bending of the Earth’s crust owing to compressional forces.
– According to shape, the folds are of many types:
o Symmetrical Folds—These are ordinary folds. The limbs of the folds are equally inclined on either side.
o Asymmetrical Fold—One of the limbs is more inclined than the other.
o Monoclinal Fold—In this fold, one limb makes a right angle with the surface but the other limb is ordinarily inclined.
o Isoclinal Fold—The two limbs are so much inclined in such a way that they appear equally inclined and parallel to each other.
o Recumbent Fold—In this fold the two limbs are so much inclined that they become horizontal.
o Overturned Fold—In this fold one limb is overturned over the other limb. The difference between the overturned and recumbent folds is that the overturned limbs are not horizontal like those of recumbent fold.
o Plunging Fold—If the axis of the fold is not parallel to the horizontal but makes an angle with it, it is known as Plunging Fold.
o Fan Fold—It is a great anticline which has many small anticlines and synclines. It is also known as Anticlinorium. A great syncline having many small anticlines and synclines is called Synclinorium.
o Open Fold—If the angle between the limbs of a fold is obtuse, the fold is called Open Fold.
o Closed Fold—If the angle between the limbs of a fold is acute, it is called Closed Fold.
(B) Fault: Fault is a rupture and fracture of rocks strata due to strain. Faults have also been defined as a surface along which a rock-body has broken and been displaced.
– Some of the important faults are as under.
• Normal Fault (Tension): The faults having mainly vertical movement are called normal faults. A normal faults results in a seep, straight cliff like feature.
• Reverse Fault (Compression): One side of the fault moves upward vertically in comparison to the other side.
• Strike-slip fault: A strike-slip fault is a fault on which the two blocks slide past one another. These faults are identified as either right-lateral or left lateral depending on whether the displacement of the far block is to the right or the left when viewed from either side. The San Andreas Fault in California is an example of a right lateral fault.
Strike-slip faults are either right-lateral or left-lateral. That means someone standing near the fault trace and looking across it would see the far side move to the right or to the left, respectively. The one in the picture is left-lateral.
Various types of landforms are being produced by faults:
• Rift Valley or Graben: When a block between two normal faults is depressed, the valley formed in the faults is known as Rift Valley or Graben.
• Horst or Block Mountain: When a block between two normal faults is raised up, the raised block with its plateau like surface and steep sloppy edge, is known as Block Mountain or an Horst
• Drag: When there is a movement in a fault, the rocks get bent. This bending gives direction to the movement. This bending of the rocks is called Drag.
Sudden Forces and Movement is caused by sudden Endo-genetic forces coming deep within the earth resulting sudden and rapid events with massive destructions at and below the earth’s surface. Such events like volcanic eruptions and earthquakes are called ‘Extreme Events’ which becomes disastrous when they occur in densely populated localities. These forces work very quickly and their results are seen within minutes.
Geologically these forces are termed as constructive forces as these create certain relief features on the earth’s surface.
For example, volcanic eruptions result in the formation of volcanic cones and mountains while fissure flows of lavas form extensive lava plateau and lava plains. Earthquakes create faults, fractures, lakes etc.
Formed when molten magma in the earth’s interior escapes through the crust by vents and fissures in the crust, accompanied by gases (like Hydrogen Sulphide, Sulphur Dioxide, Hydrogen Chloride, Carbon Dioxide), steam and pyroclastic materials. Over 1300 volcanoes exist on the earth out of which about 600 are active.
Causes of Volcanism:
• It is closely connected with crustal disturbances, particularly where there are zones of weakness due to deep faulting or mountain folding.
• Increasing temperature with increasing depth below the earth’s crust (1° per 32 km) where the interior of the earth can be expected to be in a semi-molten state, comprising solid, liquid and gaseous materials, collectively termed as Magma.
• Origins of gases and water vapour due to heating of water underground.
• Ascent of Magma forced by enormous volume of gases and water vapour.
• Movement and splitting of the major and minor plates of the earth – most of the active volcanoes are found along the plate margins (mid-oceanic ridges and the ocean trenches (Benioff Zone or Ring of Fire).
• The magma is heavily charged with gases such as carbon dioxide, sulphurated hydrogen and small proportions of nitrogen, chlorine and other volatile substances. The gases and vapour increases the mobility and explosiveness of the lavas which are emitted through the orifice or vent of a volcano during a volcanic eruption.
There are two types of lavas:
|Basic Lava Volcano||Acidic Lava Volcano|
|· Fluid lava flow covers larger areas gently-sloping volcano shield volcano||· Less fluid lava, moves very slowly solidifies quickly a steep-coned volcano|
|· E.g. Island of Hawaii, regarded as a single shield volcano with 3 active peaks Maua kea, Mauna Loa, Kilauea||· Most common composite volcano (alternate layers of hardened lava, ash & cinder). E.g. Mt Mayon, Mt Merapi, Mt Fuji, Mt Etna, Vesuvius|
Types of Volcanic Eruptions
Classification of Volcanoes on the basis of Periodicity:
– Active Volcanoes: When volcanic materials like lava, gases, ash, cinder, pumice etc. are ejected constantly from the vent. Most of the active volcanoes are found in the Circum-Pacific Belt which is known as the ‘Ring of Fire’.
• A few examples of active volcanoes are: Etna and Visuvius, Mount Pelee (Martinique), Mount Karmai (Alaska), Mount Saint Helens, Nevado Del Ru’z (Columbia), Mount Unzen (Japan), Mount Pinatubo (Philippines), Mount Redoubt (Alaska) and Mount Mayon (Philippines).
• The Stromboli volcano emits so much fire and incandescent gases that it is known as ‘the Light House of the Mediterranean Sea’.
– Dormant Volcanoes: Those that have been known to erupt and show signs of possible eruption in the future. These are not extinct. For example: The Vesuvius erupted in 79 AD, 1631, 1803, 1872, 1906, 1927, 1928, 1929.
• Violent eruptions of dormant volcanoes are generally preceded and accompanied by earthquakes, some of which have been very destructive. Example Mt. Kilimanjaro.
– Extinct Volcanoes: A volcano that was active in the geological past and no longer has any active vulcanicity. The Crater is filled with water.
• For example: St. Arthur’s Sea (Edinburgh) and the numerous Crater Lakes in the Andes and Rockies Mountains.
• Some of the volcanoes that are today dormant may become active. For example: Monte Somma which erupted 700 years back are now considered extinct by the inhabitants.
Volcanic Belts of the World:
a) The Circum Pacific Belt:
• Due to subduction of the Pacific plate below the Asiatic plate, the large number of volcanic eruptions are found circling Pacific Ocean known as Ring of Fire, which extends through the Andes of South America, Central America, Mexico, the Cascade Mountains of Western United States, the Aleutian Islands, Kamchatka, the Kuril Isles, Japan, the Philippines, Celebes, New Guinea, the Solomon Islands, New Caledonia and New Zealand where about 80 active volcanoes are found.
• The Circum-Pacific belt meets the mid- continental belt in the East Indies. This belt is characterised by high volcanic cones and volcanic mountains.
• The volcanoes of the Aleutian Island, Hawaii Island and Japan are found in Chains.
• Cotapaxi is the highest volcanic mountain (6035m) in the world. Other important volcanoes found in this belt are Fuziyama, Shasta, Rainer and Hood. Volcanic eruptions occur in this belt because of the subduction of the Pacific plate below the Asiatic plate.
b) The Mid-Continental Belt:
• Having various volcanoes of the Alpine Mountain Chain, Mediterranean Sea (Stromboli, Vesuvius, Etna etc.), volcanoes of the Aegean Sea, Mt. Ararat, Elburz and Hindukush. There are several volcanic free zones found along the Alps and the Himalayas, come under this belt. Kilimanjaro, Elgon, Birunga and Rungwe etc. are the volcanoes found in the Rift Valleys of Africa.
• In the region where the boundaries of Persia, Afghanistan, and Baluchistan meet, there are several volcanic cones of large size, and one or two of them emit steam and other gases. This region has also a few extinct volcanoes.
c) The Mid Atlantic Belt:
• It includes the volcanoes of the Mid-Atlantic Ridge which are associated with the Atlantic Ocean and are located either on swells or ridges rising from the sea floor or on or near the edge of the continent where it slopes abruptly into the deep oceanic basins.
• The volcanoes formed along the Mid-Atlantic Ridge actually represent the splitting zone of the American plate moving towards west and the Eurasian plate moving towards east representing the zones of crystal movement.
• In the splitting zone there is constant upwelling of Magma hence known as crustal weakness.
• Volcanoes in this belt are generally of fissure-eruption type such as Volcanoes of Lesser Antilles, Azores, and St. Helens etc.
|HOT – SPOT VOLCANOES
About five percent of volcanoes are not near the margins of tectonic plates. They are over especially hot places in the Earth’s interior called HOT SPOTS.
HOT SPOTS are created by mantle plumes – hot currents that rise all the way from the core through the mantle. When mantle plume come up under the crust, they burn their way through to become hot-spot volcanoes.
Famous hot-spot volcanoes include the Hawaiian island volcanoes and Réunion Island in the Indian Ocean.
Hot-spot volcanoes ooze runny lava that spreads out to create shield volcanoes. Lava from hot-spot volcanoes also creates plateaux, such as the Massif Central in France. The geysers, hot springs and bubbling mud pots of Yellowstone National Park, USA, indicate a hot spot below.
Volcanoes impact over society:
• Explosive volcanic eruptions pose short-term and long-term hazards.
• Lava flows and lahars can wipe out the flanks of mountainsides.
• Volcanic ash can blanket the landscape for miles, and ash clouds can disrupt aircraft travel, such as the incident in 1989 when ash from Alaska’s Redoubt volcano temporarily disabled a passenger airplane.
• On longer time scales, eruptions can inject massive quantities of ash into the atmosphere, greatly reducing the solar heating of the Earth and potentially interrupting the global food supply for several years.
• They provide valuable resource (gold, silver, iron, copper, tin, lead, zinc, molybdenum) – industrial material like building stones, sulphur, pumice, precious stones-diamonds, gems, ruby.
• Climatic change – At the occurrence of Pinatubo volcano in 1991, the global temperature fell by about 0.5°C.
• It is a sudden release of energy from the rocks of the earth’s interior through weak zones over the earth’s surface in the form of Kinetic Energy of wave motion causing vibrations (at times devastating) on the earth’s surface.
• It may cause change in contours, change in river course, tsunamis (seismic waves created in sea by an earthquake), shoreline changes, spectacular glacial surges (as in Alaska), landslides, soil creeps, mass wasting etc.
• The depth of the Focus or Hypocenter generally varies from about 10 km to 700 km below the earth’s surface.
Causes of Earthquakes:
• Volcanic Eruptions: Volcanic eruptions are the main cause of earthquake caused by gas explosions or the upcoming and fissuring of volcanic structures. For example: Karakota (1883), Cotopaxi, Chimborazo, Kilimanjaro, Fujiyama etc.
• Faulting (Displacement of Rocks): Earthquakes occur when movement of earth takes place along a line of fracture (FAULT). Examples: San Andres Fault of California (Los Angeles) and Earthquakes of 1994 at Northridge, California.
• Plate Tectonics: The 6 major and 9 minor plates of the earth crust are constantly moving at different rates. The boundaries of these plates are the primary location of earthquakes, example: the Ring of Fire. Shallow focus earthquakes occur on the Oceanic Ridges and in the Oceanic Trenches, deep focus earthquakes occur.
• Anthropogenic Factors (Human’s over Integration with Nature): Extraction of minerals and the dams built on time to time disturbing the earth’s balance – Marathon Dam (Greece) – 1929, Koyna (Maharashtra 1962), Hoover Dam (1935), Mangla Dam (Pakistan), Kariba Dam (Zambia), Manic Dam (Canada), Kurobe Dam (Japan).
Facts about earthquakes:
• Focus or hypocenter of the earthquakes is always within the mantle.
• The depth of the earthquake generally varies from depth about 10 to 700 km below the surface of earth. The earthquakes on the ocean ridges originate from a depth of 70 km
• The vertical point above the focus, where the earthquakes waves (seismic waves) reach the surface is termed as the epicenter, around which lines of equal seismic intensity may be drawn (isoseismic lines).
• The waves generated by an earthquake are recorded on a seismograph or seismometer.
• A massive 8.8 earthquake in Chile moved the city of Concepcion 10 feet to the west on February 27, 2010. This quake also shortened Earth’s day and slightly changed the rotation of the planet.
• Parkfield, California, is known as “The Earthquake Capital of the World” and has a bridge that spans two tectonic plates.
• More earthquakes happen in the Northern Hemisphere than in the Southern Hemisphere.
Magnitude of Earthquakes:
• The magnitude of earthquakes is most commonly assessed by the Richer Scale – designed by Charles F. Richter in 1935 and then modified in 1965 by Richter and Beno Gutenberg.
• The Richer Scale is related to the energy released at the earthquakes centre.
• A quake is considered major when it registers more than 7.0 on the moment magnitude scale. A magnitude of 3.0 or lower is nearly imperceptible.
• The scale has neither a fixed maximum nor a fixed minimum, but earthquakes rated as high as 8.4 on Richter Scale have been measured so far.
• The earthquakes of a magnitude of 2 are the smallest normally detected by human senses but instrument can detect as small as-3 on the scale.
• The Richter Scale is open ended and logarithmic, i.e. each whole number on the scale represents a 10-folds increase in the measured amplitude. Translated into energy, each whole number demonstrates 31.5 more energy than a 2, and 992 times more energy than a 1.
• The moment magnitude scale (MMS) replaced the 1930s-era Richter scale in the 1970s as the method of measuring the size of earthquakes in terms of energy released.
Earthquake Forecasting: Very little success has been achieved about the prediction of earthquakes.
• The Chinese on the basis of animal – behavior made certain accurate predictions. In 1975, the Chinese successfully predicted a major earthquake at Haicheng. But in 1976 there was complete destruction in the Tangshan earthquakes in which 2.5 lakh people died.
• The FDL method makes use of Fibonacci, Dual and Lucas numbers and has shown considerable success in predicting earthquake events locally as well as globally.
• The connection between past earthquakes and future earthquakes based on FDL numbers has also been reported with sample earthquakes since 1900. Using clustering methods it has been shown that significant earthquakes (>6.5R) can be predicted with very good accuracy window (+-1 day).
World Distribution of Major Earthquakes:
• The world distribution or earthquakes coincides very closely with that of volcanoes. Regions of greatest seismicity are Circum-Pacific areas, with the epicenters and the most frequent occurrences along the Pacific Ring of Fire.
a) Circum Pacific Belt or Ring of Fire surrounding the Pacific Ocean. It is a junction of continental and oceanic margins; it is a zone of young folded mountains; it is a zone of active volcanoes thus this belt accounts for the 65 per cent of the total earthquakes of the world.
b) Mid-Continental Belt representing Alpine-Himalayan chains of Eurasia and northern Africa and epicenters of east African fault zone. This belt represents the collision or subduction zones of continental plates. About 21 per cent of the total seismic events occur in this belt.
c) Mid-Atlantic Belt representing the earthquakes located along the mid-Atlantic Ridge and its offshoots. This belt records moderate and shallow focus earthquakes which are essentially caused due to creation of transform faults and fractures because of divergent movement of plates.
d) Other Areas: These include Northern Africa and Rift Valley areas of Red Sea and Dead Sea. In addition to these, the ocean ridges are also active earthquake zones.
Earthquakes in India:
The Indian subcontinent is divided into five seismic zones with respect to severity of the earthquakes. The zone classification has been done by the geologist and scientists as early as 1956 when a 3-Seismic Mapping Zone (Severe, Light and Minor Hazard) of India was produced.
• The Bureau of Indian Standard is the official agency for publishing the seismic hazard maps and codes. It has brought out versions of seismic zoning map: a six zone map in 1962, a seven zone map in 1966, and a five zone map 1970/1984.
• The Five Zonal map was created based on the values of maximum MM (Modified Mercalli) intensities recorded in various parts of the country, in historic times
I. Zone V – Earthquakes with magnitudes exceeding 7.0 on Richter Scale have been kept in zone V where country’s most powerful shock having intensities higher than IX occurs. The IS code assigns zone factor of 0.36 for Zone 5.
This region included the Andaman & Nicobar Island, all of North-Eastern India, Parts of North-Western Bihar, Eastern Section of Uttranchal, The Kangra Valley in Himachal Pradesh, Vicinity of Srinagar in Jammu & Kashmirand the Rann of Kutchh in Gujarat.
II. Zone-IV covers remaining parts of Jammu & Kashmir and Himachal Pradesh, Union Territory of Delhi, Sikkim, northern parts of Uttar Pradesh, Bihar and West Bengal, parts of Gujarat and small portions of Maharashtra near the west coast and Rajasthan.
It is known as the High Damage Risk Zone that suffers earthquakes of intensity MSK (Medvedev-Sponheuer-Karnik) VIII. The IS code assigns zone factor of 0.24 for Zone 4.
III. Zone-III comprises of Kerala, Goa, Lakshadweep islands, and remaining parts of Uttar Pradesh, Gujarat and West Bengal, parts of Punjab, Rajasthan, Madhya Pradesh, Bihar, Jharkhand, Chhattisgarh, Maharashtra, Orissa, Andhra Pradesh, Tamil Nadu and Karnataka.
This zone is classified as Moderate Damage Risk Zone which is liable to MSK VII intensities. The IS code assigns zone factor of 0.16 for Zone 3.
IV. Zone-II covers remaining parts of the country.
Further, as part of pre-disaster preparedness measure, Government of India has also completed seismic micro-zonation studies of some of the major cities in the country such as, Jabalpur, Guwahati, Bangalore, greater Bharuch in Gujarat, Jammu in J & K, Shillong in Meghalaya, Chennai in Tamilnadu and sikkim state.
These studies involving preparation of geological, geomorphological and land use maps followed by drilling, geological logging, standard penetration test and geophysical studies to demarcate the zones of least to most damage prone areas within the urban areas helps the respective town and country planning agencies to formulate perspective planning within the overall earthquake impact minimization efforts.
Some contemporary facts related to Earthquakes in India:
• In India, 22 states are prone to earthquakes of various intensities like severe, high, moderate and low, and urban explosion and densely populated cities with high-rise buildings makes the quake scenario worse.
• India is working on an ambitious project with scientist, geophysicists and seismologists of 27 countries to develop an early warning system for prediction of earthquake.
• Before an earthquake, some chemical changes under the earth’s surface and some physical displacements on the surface occur which will be studied by deep bore wells filled with sensors and other equipments placed in all seismically active regions in the country to sense these changes.
• India is also planning to launch a satellite by 2019 to track the changes that take place on earth’s surface before the tremor which will send images of surface displacement up to the accuracy of few centimeters.
• India has been conducting experiments in the Koyna river valley in the Western Ghats in Maharashtra.