physical geography

The Earth is an oblate spheroid and not a perfect sphere.This is because the earth has a rotational motion and the rotational speed increases as one goes from poles towards the equator. 

As the sunlight passes through the atmosphere, the rays are scattered by tiny particles of dust, pollen, soot and other minute particulate matters present there. However, when we look up, the sky appears blue during mid-day, because Blue light is scattered most.

Apparent solar time or true solar time is given by the daily apparent motion of the true, or observed, Sun. It is based on the apparent solar day, which is the interval between two successive returns of the Sun to the local meridian. Solar time can be crudely measured by a sundial.

The length of a solar day varies through the year, and the accumulated effect of these variations produces seasonal deviations of up to 16 minutes from the mean. The effect has two main causes. First, Earth's orbit is an ellipse, not a circle, so the Earth moves faster when it is nearest the Sun (perihelion) and slower when it is farthest from the Sun (aphelion) (see Kepler's laws of planetary motion). Second, due to Earth's axial tilt (known as the obliquity of the ecliptic), the Sun's annual motion is along a great circle (the ecliptic) that is tilted to Earth's celestial equator. When the Sun crosses the equator at both equinoxes, the Sun's daily shift (relative to the background stars) is at an angle to the equator, so the projection of this shift onto the equator is less than its average for the year; when the Sun is farthest from the equator at both solstices, the Sun's shift in position from one day to the next is parallel to the equator, so the projection onto the equator of this shift is larger than the average for the year (see tropical year). Consequently, apparent solar days are shorter in March and September than they are in June or December.

A hurricane acquires its spin from the coriolis effect. The diameter of the hurricane decreases as it moves away from low latitudes. The diameter of a hurricane is never below 150 KM. 

Lunar eclipse takes place when the Earth comes directly between the Sun and the Moon.Solar eclipse happens when the Moon comes directly between the Sun and the Earth.

In a cyclone, the area of low pressure is at the center surrounded by the areas of low pressure. In an anticyclone, the area of high pressure is surrounded by the areas of low pressure.
El Nino is characterized as : it occurs at irregular intervals, It carries warmer water , It carries less saline water,Its atmospheric equivalent is southern oscillation.

El Niño is defined by prolonged differences in Pacific Ocean Sea surface temperatures when compared with the average value. The accepted definition is a warming or cooling of at least 0.5 °C (0.9 °F) averaged over the east-central tropical Pacific Ocean. Typically, this anomaly happens at irregular intervals of 3–7 years and lasts nine months to two years.The average period length is 5 years. When this warming or cooling occurs for only seven to nine months, it is classified as El Niño/La Niña "conditions"; when it occurs for more than that period, it is classified as El Niño/La Niña "episodes".

The first signs of an El Niño are:

1. Rise in surface pressure over the Indian Ocean, Indonesia, and Australia
2. Fall in air pressure over Tahiti and the rest of the central and eastern Pacific Ocean
3. Trade winds in the south Pacific weaken or head east
4. Warm air rises near Peru, causing rain in the northern Peruvian deserts
5. Warm water spreads from the west Pacific and the Indian Ocean to the east Pacific. It takes the rain with it, causing extensive drought in the western Pacific and rainfall in the normally dry eastern Pacific.

El Niño's warm rush of nutrient-poor tropical water, heated by its eastward passage in the Equatorial Current, replaces the cold, nutrient-rich surface water of the Humboldt Current. When El Niño conditions last for many months, extensive ocean warming and the reduction in Easterly Trade winds limits upwelling of cold nutrient-rich deep water and its economic impact to local fishing for an international market can be serious.

Early stages and characteristics of El Niño

Although its causes are still being investigated, El Niño events begin when trade winds, part of the Walker circulation, falter for many months. A series of Kelvin waves—relatively warm subsurface waves of water a few centimetres high and hundreds of kilometres wide—cross the Pacific along the equator and create a pool of warm water near South America, where ocean temperatures are normally cold due to upwelling. The weakening of the winds can also create twin cyclones, another sign of a future El Niño. The Pacific Ocean is a heat reservoir that drives global wind patterns, and the resulting change in its temperature alters weather on a global scale.Rainfall shifts from the western Pacific toward the Americas, while Indonesia and India become drier.

Jacob Bjerknes in 1969 contributed to an understanding of ENSO by suggesting that an anomalously warm spot in the eastern Pacific can weaken the east-west temperature difference, disrupting trade winds that push warm water to the west. The result is increasingly warm water toward the east. Several mechanisms have been proposed through which warmth builds up in equatorial Pacific surface waters, and is then dispersed to lower depths by an El Niño event. The resulting cooler area then has to "recharge" warmth for several years before another event can take place.

While not a direct cause of El Niño, the Madden-Julian Oscillation, or MJO, propagates rainfall anomalies eastward around the global tropics in a cycle of 30–60 days, and may influence the speed of development and intensity of El Niño and La Niña in several ways.^[14] For example, westerly flows between MJO-induced areas of low pressure may cause cyclonic circulations north and south of the equator. When the circulations intensify, the westerly winds within the equatorial Pacific can further increase and shift eastward, playing a role in El Niño development.^[15] Madden-Julian activity can also produce eastward-propagating oceanic Kelvin waves, which may in turn be influenced by a developing El Niño, leading to a positive feedback loop.

Earthquakes
The earthquakes of the Mid-Continental Belt along the Alpine-Himalayan chains are caused due to collision of Eurasian plates , African and Indian plates.The earthquakes of western marginal areas of North and South Americas are caused because of subduction of Pacific plate beneath the American plate and the resultant tectonic forces whereas the earthquakes of the eastern margins of Asia are originated because of subduction of Pacific plate under Asiatic plate. Similarly the subduction of African plate below European plate and the subduction of Indian plate under Asiatic plate cause earthquakes of the midcontinental belt. The severe earthquake of Bhuj of Jan 26, 2001(Gujarat, India)  was caused due to reactivated subsurface faults due to subduction of Indian plate below Asiatic plate.



Continental Shelf 

The continental marginal areas submerged under oceanic water with average depth of 100 fathome (one fathom = 6 feet) and gently sloping (1 degree - 3 degree) towars the sea or the oceans are called Continental Shelves largely depends on the nature of reliefs of the coastal land i.e.,

(1) The shelves are narrow where high mountains are very close and parallel to the coat (e.g.,the Pacific continental shelf along the western coast of South America is narrow (16 Km) because of the presence of the Andes mountain and
(2) The shelves are wider where the coast lands are wide plains. Though the continental shelves are generally wider in front of river mouths but the shelf off the Mississippi mouth is exceptionally narrow. On an average, the width of continental shelves- is about 48 Km though Sheppard has taken 67 Km (42 miles) as average width. The Pacific continental Shelf of south America represents the example of narrow shelf (16 Km), the Atlantic continental shelf off the cast coast of North America represents the example of medium size shelves (96-120Km) and extensive shelves having width of a few hundred kilometers are found off the coast of East Indies, in the Artic Sea,China Sea, Adriatic Sea, Arafura Sea etc. Continental Shelves represent 8.6 % of the total area of the ocean basins. regionally these cover 13.3% , 5.7 % and 4.2 % of areal coverage of the Atlantic Ocean, the Pacific Ocean and the Indian Ocean respectively.

The maximum seaward limit of the continental shelves off the Indian coast is demarcated by 100 fathom contour. The continental shelves along the eastern and the western coasts of India are 50 Km and 150 Km wide respectively. The shelves are narrow(30-35 Km) off the mouths of the Ganga, the Mahanadi, the Godawari, the Krishna and the Cauvery rivers but these are wider off the estuaries of the Narmada, the Tapi and the Mahi rivers. The average slope of the continental shelves off the eastern Indian coast is about 21degree whereas it is 1 degree near Cape comorin and only 1degree near the Gulf of Combay.

Origin of Continental Shelves 

The Nature, composition, extension and depth of continental shelves are so varied that it becomes difficult to explain their exact mode of origin through a single mechanism and process. The following different views have been expressed by several authorities to explain the complex origin of continental shelves --

(1) Continental shelves are basically the extended form of continental platforms. Marine waves and currents erode the continental margins and thus form extensive platforms which receive deposits of sediments brought down by the rivers and sea waves.
These sediments are continuously consolidated under sea water and ultimately extensive continental shelves are formed. Thus, the continental shelves are the result of marine erosion and fluvial deposits.

(2) Continental shelves are formed through prolonged deposition of detritus (under sea water) brought by the rivers alone. Such type of continental shelves is formed only in those areas where sea conditions are calm so that prolonged sedimentation goes on uninterruptedly resulting into subsidence and thus allowing more and more sedimentation. Such continental shelves are constructional and are most extensive.

(3) Rising thermal convective currents from beneath the continents and the ocean basins converge along the continent-ocean boundary and descend. The resultant compressive force causes subsidence of the continental margins and thus, continental shelves are formed.

(4) Sometimes, parallel faults are created in the continental margins. This event causes subsidence of the marginal land areas and consequent submergence under sea water. Such submerged land areas become continental shelves, which are generally called as rectonically formed continental shelves.

(5) Continental Shelves are formed through marine erosion of the continental margins when there is negative change in sea level (Fall in sea level) either during ice ages or due to subsidence of oceanic floors. According  to R.A.Daly the sea level fell by 38 fathoms during Pleistocene Ice age, with the result the continental margins which were previously submerged became free from sea water.These exposed land areas were glacially eroded and extensive platforms were formed. Due to deglaciation the sea level rose again and these platforms were submerged under sea water and thus extensive continental shelves were formed. This concept of the origin of the continental shelves belongs to glacial control theory.

(6) The coastal lands are effectively eroded through abrasive work of strong sea waves and several sea cliffs are formed. These cliffs gradually but continuously recede towards the land due to basal erosion and consequent fall of their hanging crests and thus extensive wave- cut platfoems are formed. These platforms are submerged under sea water to form continental shelves.

(7) The submergence of continental margins due to tilting of land towards the sea results into the formation of continental shelves. This process also leads to the extension of existing continental shelves.

The continental shelves of India have been formed differently. The continental shelves off the Ganga, the Godawari, the Krishna and the Cauvery mouths have been formed through delta formation. The continental shelves from Midinapur to Madura are the result of sedimentation and consequent subsidence while the shelves of Andaman Nicobar, Lakshadweep, Gulf of Manar(Between India and Sri Lanka) are originated due to coral reefs. The Continental Shelves of western coast are due to faulting and consequent submergence.

Coral reefs

Coral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps are like tiny sea anemones, to which they are closely related. Unlike sea anemones, coral polyps secrete hard carbonate exoskeletons which support and protect their bodies. Reefs grow best in warm, shallow, clear, sunny and agitated waters.

Often called “rainforests of the sea”, coral reefs form some of the most diverse ecosystems on Earth. They occupy less than 0.1% of the world's ocean surface, about half the area of France, yet they provide a home for 25% of all marine species,including fish,mollusks, worms, crustaceans, echinoderms, sponges, tunicates and other cnidarians. Paradoxically, coral reefs flourish even though they are surrounded by ocean waters that provide few nutrients. They are most commonly found at shallow depths in tropical waters, butdeep water and cold water corals also exist on smaller scales in other areas.

Biology :

Live coral are small animals embedded in calcium carbonate shells. It is a mistake to think of coral as plants or rocks. Coral heads consist of accumulations of individual animals called polyps, arranged in diverse shapes.^[28] Polyps are usually tiny, but they can range in size from a pinhead to 12 inches (30 cm) across.

Reef-building or hermatypic corals live only in the photic zone (above 50 m), the depth to which sufficient sunlight penetrates the water, allowing photosynthesis to occur. Coral polyps do not photosynthesize, but have a symbiotic relationship with zooxanthellae; these organisms live within the tissues of polyps and provide organic nutrients that nourish the polyp. Because of this relationship, coral reefs grow much faster in clear water, which admits more sunlight. Without their symbionts, coral growth would be too slow for the corals to form significant reef structures. Corals get up to 90% of their nutrients from their symbionts.^[29]

Reefs grow as polyps and other organisms deposit calcium carbonate,^[30][31] the basis of coral, as a skeletal structure beneath and around themselves, pushing the coral head's top upwards and outwards.^[32] Waves, grazing fish (such asparrotfish), sea urchins, sponges, and other forces and organisms act as bioeroders, breaking down coral skeletons into fragments that settle into spaces in the reef structure or form sandy bottoms in associated reef lagoons. Many other organisms living in the reef community contribute skeletal calcium carbonate in the same manner.^[33] Coralline algae are important contributors to reef structure in those parts of the reef subjected to the greatest forces by waves (such as the reef front facing the open ocean). These algae strengthen the reef structure by depositing limestone in sheets over the reef surface.

The colonies of the one thousand coral species assume a characteristic shape such as wrinkled brains, cabbages, table tops, antlers, wire strands and pillars

Corals reproduce both sexually and asexually. An individual polyp uses both reproductive modes within its lifetime. Corals reproduce sexually by either internal or external fertilization. The reproductive cells are found on the mesentery membranes that radiate inward from the layer of tissue that lines the stomach cavity. Some mature adult corals are hermaphroditic; others are exclusively male or female. A few specieschange sex as they grow.

Internally fertilized eggs develop in the polyp for a period ranging from days to weeks. Subsequent development produces a tiny larva, known as a planula. Externally fertilized eggs develop during synchronized spawning. Polyps release eggs and sperm into the water en masse, simultaneously. Eggs disperse over a large area. The timing of spawning depends on time of year, water temperature, and tidal and lunar cycles. Spawning is most successful when there is little variation between high and low tide. The less water movement, the better the chance for fertilization. Ideal timing occurs in the spring. Release of eggs or planula usually occurs at night, and is sometimes in phase with the lunar cycle (three to six days after a full moon). The period from release to settlement lasts only a few days, but some planulae can survive afloat for several weeks. They are vulnerable to predation and environmental conditions. The lucky few planulae which successfully attach to substrate next confront competition for food and space