Fundamental Of Physical Geography Chapter 13 Water (Oceans)

Guyots:
i. It is a flat topped seamount.

ii. They show evidences of gradual subsidence through stages to become flat topped submerged mountains.

iii. It is estimated that more than 10,000 seamounts and guyots exist in the Pacific Ocean alone.

The Mariana Trench.

Seamount: 

i. It is a mountain with pointed summits, rising from the seafloor that does not reach the surface of the ocean.

ii. Seamounts are volcanic in origin.

iii. These can be 3,000-4,500 m tall. The Emperor seamount, an extension of the Hawaiian Islands in the Pacific Ocean, is a good example.

The forms of precipitation are :

i. rainfall 
ii. snowfall 
iii. hailstone

(iii) Prevailing wind: the winds blowing from the land towards the oceans drive warm surface water away form the coast resulting in the upwelling of cold water from below. It results into the longitudinal variation in the temperature. Contrary to this, the onshore winds pile up warm water near the coast and this raises the temperature.

(iv) Ocean currents: warm ocean currents raise the temperature in cold areas while the cold currents decrease the temperature in warm ocean areas. Gulf stream (warm current) raises the temperature near the eastern coast of North America and the West Coast of Europe while the Labrador current (cold current) lowers the temperature near the north-east coast of North America

The trenches are:
i. Pacific Ocean 

ii. Atlantic Ocean 

iii. Indian Ocean

Factors affecting ocean salinity are mentioned below:

The ocean floors can be divided into four major divisions:

(i) the Continental Shelf;

(ii) the Continental Slope;

(iii) the Deep Sea Plain;

(iv) the Oceanic Deeps.

Submarine Canyons:

i. These are deep valleys, some comparable to the Grand Canyon of the Colorado river.

ii. They are sometimes found cutting across the continental shelves and slopes, often extending from the mouths of large rivers.

iii. The Hudson Canyon is the best-known canyon in the world.

The features are:
i. The continental slope connects the continental shelf and the ocean basins.

ii. It begins where the bottom of the continental shelf sharply drops off into a steep slope.

iii. The gradient of the slope region varies between 2-5°.

iv. The depth of the slope region varies between 200 and 3,000 m.

v. The slope boundary indicates the end of the continents. Canyons and trenches are observed in this region.

It is a mountain with pointed summits, rising from the seafloor that does not reach the surface of the ocean. Seamounts are volcanic in origin. These can be 3,000-4,500 m tall. 

Atoll:

i. These are low islands found in the tropical oceans consisting of coral reefs surrounding a central depression.

ii. It may be a part of the sea (lagoon), or sometimes form enclosing a body of fresh, brackish, or highly saline water.

Formation : The submarine ridges are formed by the following processes :

(1) Large outpouring of basalt along-fissure line.

(2) Rising of the crust by the rising convection currents.

(3) Thickening and buckling of the basaltic crust by the downward movement of the convectional currents.

(4) Formation of new oceanic crusts.

1. Evaporation and precipitation : Where evaporation is high the salinity will be high in hot and dry regions. If somewhere rainfall is high the salinity will be low. Example, equatorial regions.

2. Fresh Water : If the source of fresh water is regular, the surface salinity will be low. Polar regions have the low salinity by the process of freezing and thawing of ice.

3. Wind is another factor responsible for the movement of water from one area to another also influences salinity.

4. The ocean currents which are moving laterally or vertically in different parts of the ocean contribute to the salinity variations.

Haloclines:

i. Haloclines are a type of chemocline, which means that the chemical makeup of the cline is what makes it different from the surrounding water. Specifically, a halocline has a different level of salinity than the rest of the body of water.

ii. Usually the halocline is the layer that has a much higher level of salinity than the layers above and below it. This high level of salinity also makes the layer denser.

iii. Haloclines are found in many areas around the world. They are common in the oceans, especially in colder regions, where cold water with a lower salinity ‘floats’ on top of the saltier, warmer layer. These layers are also common in areas where freshwater and saltwater come together.

Thermoclines:

i. Thermoclines are layers in which the temperature of the water changes at a much faster rate than in the surrounding water.

ii. The thermocline forms where the warm and cold layers slowly mix. As the sun sets or rises, the top layer heats and cools, causing the mixed layer to rapidly change in temperature.Thermoclines occur due to a variety of reasons, including seasonal changes, currents and weather.

iii. Strong waves, tides or wind may stir the layer, causing it to disappear. Some of these layers are more or less permanent, while others come and go depending on a number of factors.

Mid-Oceanic Ridges:

i. A mid-oceanic ridge is composed of two chains of mountains separated by a large depression.

ii. The mountain ranges can have peaks as high as 2,500 m and some even reach above the ocean’s surface.

iii. Iceland, a part of the mid - Atlantic Ridge, is an example.

There are five types of slopes :

(i) Fairly steep with the surface dissected by canyons.

(ii) Gentle slope with elongated hills and basins.

(iii) Faulted slopes.

(iv) Slopes with terraces and

(v) Slopes with sea mounts.

The Abyssal Plains : Beyond the continental slope are the Abyssal Plains. They are located at a depth of 3000 to 6000 metres. They cover about 40% of the ocean floor. The deposits of fine matter containing bones and shells of minute animals that live and die on the surface of sea are found. They are found in all the major oceans of the world.

Fig. 13.7 : Submarine features and their depths

(i) The small gorges extend down from the edges of continental shelves to continental slopes.

(ii) The canyons situated at the mouths of rivers.

(iii) The canyons of dendritic appearance.

ridge is about 14,000 km long and about 1500 metres high. The ridge has stretched features. The slopes on both sides are very gentle throughout the greater part of its length. It rises as a series of steps and becomes rugged near the crest.

The ridge, though under the sea level, has many peaks projected out of the deep water above the surface of the ocean. These peaks are in fact, the islands of the Mid-Atlantic. Examples include Pico Island of Azores and Cape Verde Islands. Bermuda is a coral island. Ascension, Tristanda Cunha, St. Helena and Gough are volcanic islands.

The salinity variation in the Pacific Ocean:
i. The salinity variation in the Pacific Ocean is mainly due to its shape and larger areal extent.
ii. Salinity decreases from 35 o/oo - 31 o/oo on the western parts of the northern hemisphere because of the influx of melted water from the Arctic region.
iii. In the same way, after 15° - 20° south, it decreases to 33 o/oo.

Continental shelf:
i. The continental shelf is the extended margin of each continent occupied by relatively shallow seas and gulfs. It is the shallowest part of the ocean showing an average gradient of 1° or even less.

ii. The shelf typically ends at a very steep slope, called the shelf break. The width of the continental shelves vary from one ocean to another.

iii. The average width of continental shelves is about 80 km. The shelves are almost absent or very narrow along some of the margins like the coasts of Chile, the west coast of Sumatra, etc.

iv. On the contrary, the Siberian shelf in the Arctic Ocean, the largest in the world, stretches to 1,500 km in width. The depth of the shelves also varies. It may be as shallow as 30 m in some areas while in some areas it is as deep as 600 m.

v. The continental shelves are covered with variable thicknesses of sediments brought down by rivers, glaciers, wind, from the land and distributed by waves and currents. Massive sedimentary deposits received over a long time by the continental shelves, become the source of fossil fuels.

Trenches : The only famous trench is the Lunda Trench which lies south of the island of Java and runs parallel to it.

Ridges : There are a number of broad submarine ridges on the floor of Indian Ocean. A prominent ridge runs from Kanyakumari to Antarctica. It is situated in the middle and divides the ocean into two basins on either side. It is called the Lakshadweep-Chagos Ridge in the north, the St. Paul Ridge in the middle and the Amsterdam St. Plateau in the south, where it widens out considerably. There are two minor and parallel ridges which run north-westward. These are known as the Sacotra-Chagos Ridge and the Seychelles Ridges. The Madagascar Ridge and Prince Edward Crozet Ridges are other ridges. The Carlsberg Ridge divides Arabian sea into two parts. The Andaman and Nicobar Ridge extends from the mouth of the Irrawaddy to the Nicobar Islands.

Basins : The chief basins in the Indian Ocean are Central Basin, Arabian Basin, South Indian Basin, Mascarene Basin, West Australian and South Australian Basins.

Islands : The Andaman and Nicobar, Sri Lanka, Madagascar and Zanzibar islands are the parts of the continental blocks. The Lakshadweep Island is a coral island. The Mauritius and Reunion islands to the east of Madagascar are the volcanic origin.

Continental Shelf : Continental shelf occurs all around the Atlantic Ocean but it varies in width. It is wider in the north than in the south.

Seas : There are numerous marginal seas on both Sides of the Atlantic Ocean. Most of them are located in shelves like Hudson Bay, the Baltic Sea and the North Sea.

Fig. 13.9 : Topographic Features of the Atlantic Ocean.

Ridge : Special feature of the Atlantic Ocean is the Mid-Atlantic Ridge. It extends from north to south parallel to the ‘S’ shape of the ocean itself. It divides the Atlantic into two deeper basins on either side. The ridge is about 1400 km long and about 4000 metres high. The slopes on both sides are very gentle. It rises as a series of steps.

Islands : The ridge has many peaks projecting themselves above the water surface. Such a island is Pico Island of Azores. Cape Verde Island of Bermuda is a coral island. Ascension, Tristan Dacunha, St. Helena, Gough are volcanic islands.

Trough and Trenches : The Atlantic Ocean lacks in trough and trenches. North Gayman and Puerto Rico are the two troughs and Ramanche and South Sandwich are the two trenches in the Atlantic Ocean.

1. Latitude : The temperature of surface water decreases from equator towards the poles because the sun's rays become more and more inclined and thus the amount of insolation decreases poleward.

2. Unequal distribution of land and water : The oceans in the Northern Hemisphere receive more heat due to their contact with larger extent of land than their counterparts in the Southern Hemisphere.

3. Prevailing winds : The winds blowing from the land towards the ocean drive surface water away from the coasts resulting into upwelling of cold bottom water from below.

4. Ocean Currents : Warm ocean currents increase the temperature in cold areas while cold currents decrease the temperature in warm ocean. Example : gulf stream, a warm current increases the temperature of eastern coast of North America and west coast of Europe.

5. Other factors are local weather conditions like storms, cyclones, etc.

(1) Land area predominates in the Northern Hemisphere while oceans occupy the major part of the Southern Hemisphere.

(2) Land masses of the Northern Hemisphere are triangular-shaped with their bases in the north and tapering to the south, while oceans make an extensive girdle in the Southern Hemisphere and taper to the north.

(3) Landforms are almost unbroken and ring round the Northern Hemisphere and project to the south from their ring in three parts of the continents : (1) The Americas, (2) Europe and Africa, (3) Asia.

(4) Land and ocean occupy antipodal positions. The Arctic Ocean encircles the North Pole and Antarctica continents, the South Pole. Similarly the great land masses of Europe, Asia and Africa are antipodal to the great oceanic areas of the South Pacific.

(1) The submarine canyons are the valleys-like cut deep into the continental shelves and slopes.

(2) They occur around all the coasts of the world.

(3) They are mostly associated with the mouth of the great rivers such as the Hudson, the Mississippi, the Indus, and the Ganga.

The best known canyons in the world occur in Bering Sea of Alaska. They are-Bering, Pribilof and Zhemchug canyon, Hudson canyon.

Formation of Submarine Canyons : They are formed in the following ways :

(i) By Erosive action of River : It is believed that submarine canyons are formed by sub-aerial processes and submarine processes in the Pliestocene period. During this period, the sea level went up and down upto the extent of 100 metres. In the processes of up and down of the sea level, shelves were exposed to river action and other submarine processes. Thus, the canyons were formed by the erosive action of rivers.

(ii) By Faulting : It is also believed that faulting of the coastal areas near the river mouths produced the similar effects and pushed a part of the shelf up. This part of the shelf also had been cut by the rivers in shape of canyons. This view is supported by the presence of faults or bendings along the coasts which have sub-merged the river-cut valleys to become the sub-marine canyons.

(iii) By Turbidity Currents : The turbidity currents eroded the edges of the shelves. This erosive action of the turbidity currents formed the sub-marine canyons.

(2) They occur around all the coasts of the world.

(3) They are mostly associated with the mouth of the great rivers such as the Hudson, the Mississippi, the Indus, and the Ganga.

The best known canyons in the world occur in Bering Sea of Alaska. They are-Bering, Pribilof and Zhemchug canyon, Hudson canyon.

The hydrosphere is the combined mass of water found on, under, and above the surface of a planet, minor planet or natural satellite.

1. Continental shelf, 2. Continental slope, 3. Submarine canyon, 4. Abyssal plains, 5. Submarine ridges, 6. Ocean deeps

Continental Shelf : The area lying between shoreline and the continental slope. Its seaward slope is very gentle and is covered with shallow water.  
Continental Slope : It is an area of steep slope extending just after the continental shelf upto a considerable depth from where a gentle sea plain takes its form. The extent of the slope area is usually between 200-2000 metres depth. But sometimes it may extend to 36000 metres from the mean sea level. The continental slope along many coasts of the world is furrowed by deep canyon-like trenches. Continental slope covers 8.5 per cent of the total ocean area.

There are five types of slopes :

(i) Fairly steep with the surface dissected by canyons.

(ii) Gentle slope with elongated hills and basins.

(iii) Faulted slopes.

(iv) Slopes with terraces and

(v) Slopes with sea mounts.

The Abyssal Plains : Beyond the continental slope are the Abyssal Plains. They are located at a depth of 3000 to 6000 metres. They cover about 40% of the ocean floor. The deposits of fine matter containing bones and shells of minute animals that live and die on the surface of sea are found. They are found in all the major oceans of the world.

Submarine features and their depths

Submarine Canyons : They are deep valleys which have sharply oceanic floors. They occur along all the coasts of the world. Submarine canyons are of three types :

(i) The small gorges extend down from the edges of continental shelves to continental slopes.

(ii) The canyons situated at the mouths of rivers.

(iii) The canyons of dendritic appearance.
Oceanic deeps or trenches are the deepest parts of the oceans. Trenches are steep-sided and narrow basins.