Introduction
Oceans are one of the most important relief features which is present on the earth’s crust. It regulates rainfall and droughts and also holds 97% of our planet's water. It absorbs CO2 and helps to keep the carbon cycle in balance. Apart from its environmental regulations, oceans are also the lifeline for billions of people. It provides them with livelihood in the form of large fisheries stock and other forms of food. Oceans are also a part of geopolitics as it is the reserve of billions of tonnes of minerals and a major transport mode. Hence, it becomes important to study the oceans.
Hydrological Cycle
Fig. Hydrological Cycle (Source: NOAA)
The hydrologic cycle is a continuous circulation of water in the Earth-Atmosphere system. It is actually the process depicting the water cycle which is the motion of the water from the ground to the atmosphere and back again. There are many processes which are involved in the hydrological cycle. These can be assessed as:
Evaporation
Transpiration
Condensation
Precipitation
Runoff
Evaporation
Evaporation is the process of change of state in a substance from a liquid to a gas. In meteorology, the process of change of water from liquid state to vapour state is termed as evaporation. The process of evaporation requires energy which may be fulfilled from the sun, the atmosphere, the Earth, or objects on the Earth such as humans and anthropogenic activities.
Transpiration
Transpiration is the process of evaporation of water from plants through stomata. Stomata are small openings which are found on the underside of leaves. These are connected to vascular plant tissues. The process of transpiration is a passive process which is largely controlled by the humidity of the atmosphere and the moisture content of the soil. Out of the total transpired water which is passed through a plant only 1% is used in the growth process of the plant. The remaining 99% of it is passed into the atmosphere.
Condensation
Condensation is the process by which the vapour is changed into a liquid state. In the atmosphere, the process of condensation results in the formation of clouds or dew. Condensation does not occur on one particular temperature but it occurs due to the difference between two temperatures: the air temperature and the dew point temperature.
Dew Point
The dew point is the temperature at which dew can be formed, it is the point at which air becomes saturated and cannot hold any more water vapour. Any additional cooling causes water vapour to condense. Foggy conditions often occur when air temperature and dew point are equal.
The process of condensation is opposite to the process of evaporation. As the water vapour has a higher energy level than that of liquid water, when condensation occurs, the excess energy in the form of heat energy is released. This release of heat aids in the formation of hurricanes and cyclones. Hence, the process of evaporation is an endothermic process while condensation is an exothermic process.
Precipitation
When tiny condensation particles, through collision and coalescence, grow too large for the rising air to support or to carry upward, and thus fall to the Earth in the form of rainfall, hail, snowfall etc. then this process is termed as precipitation. Precipitation is the primary source of freshwater on Earth. On average, the world receives about 38½" (980 mm) of precipitation each year over both the oceans and land masses.
Runoff
When there is excessive precipitation and the ground is saturated i.e. it cannot absorb any more water under it, then the water starts to flow on the surface. This is called runoff. Rivers and lakes are formed due to the process of runoff. Some of the runoff evaporates into the atmosphere due to different processes of heating, but most water in rivers and lakes returns to the oceans.
The evaporation of this runoff into the atmosphere begins the hydrologic cycle over again. Some of the water percolates into the soil and into the ground water only to be drawn into plants again for transpiration to take place. Hence, the runoff can be considered as the crucial phase of the hydrological cycle.
Ocean Bottom Relief
The ocean bottom relief is one of the most diverse features of the earth. It comprises the Continental Margins which is followed by the Deep Ocean Basin. Continental margins which consist of the Continental shelves, continental slope and submarine canyon. Deep Ocean Basin consists of the Abyssal Plains, Abyssal Hills, Oceanic ridges and Rises, Ocean deeps and trenches.
Continental Margin
Fig: Continental Margin
The continental margin consists of the continental shelf, continental slope, and continental rise. The continental shelf is the submerged top of a continent that has been shaped by the ocean, except where mountains occur at the edge of the continent.
Continental Shelves
The continental shelf is a shallow water area which surrounds the main continental land mass. Geologically, it is a part of the continent which is submerged under oceanic waters. It is a shallow area near the coast which is usually 100 fathoms deep. The continental shelf slopes gently towards the ocean at an average rate of 20 meters per kilometer with the angle of slope varying from 1º to 3º. Continental shelves are narrow submerged landforms which are very close and parallel to the coast.
Continental Shelves across the globe
South America: Off the west coast of South America where the Andes Mountains are very close and parallel to the coast, the shelf is only a few hundred meters wide. In contrast, the eastern coast of South America has very wide shelves which measure over hundreds of kilometers. The maximum width of 560 km is off the coast of Rio de La Plata.
North America: Similarly, there is a narrow shelf along the western coast of North America where the Rockies approach the coast. On the other shelf along the eastern and northern coasts of North America. There the shaping force was the process that formed the mountains. From the shoreline, the shelf slopes gently toward the sea and the continental shelf breaks typically at a depth of about 130 meters.
Pacific Ocean: On average, the continental shelf is about 70 kilometers wide. On much of the Pacific Coast, it is relatively narrow, only a few tens of kilometers across.
The widest shelves occur in the Arctic Ocean. Abroad continental shelf is the sculptured build-up of sediment shed by a continent.
Process of Formation
The process of formation of continental shelves has been explained by various processes. These are:
1) Upliftment due to Plate Tectonics
2) Pleistocene Glaciations
3) Subsidence
4) Deposition by the terrestrial river etc.
Economic significance of the continental shelves
Mineral Resources
Continental shelves are rich in mineral resources such as barite, aragonite, phosphorite, and numerous heavy metals gold, diamonds, platinum, and native copper. Poly-Metallic Nodules (PMN) are found most commonly on the continental shelves of the Indian as well as Atlantic Oceans.
Energy Resources
Energy Resources are most commonly found on the continental shelves. Energy resources in the form of petroleum, natural gases and methane hydrates are found on continental shelves. Some of the other forms of energy resources such as ‘Tidal Energy’ are also harnessed on continental shelves.
Biotic Resources
Continental shelves are rich in biotic resources. Due to their presence on the boundary of the two ecosystems (Ecotone), continental shelves have rich species diversity. Further, due to their presence in the Photic zone, phytoplankton and zooplankton are present in abundance which attracts fishes as well as other marine animals. The presence of minerals in abundance on continental shelves also helps these microorganisms to survive.
Shipping
When the continental shelves are shallow and wider it is difficult for the ships to navigate. Hence, it becomes impossible to build ports on the coasts. Hence, the determination of the shipping infrastructure is dependent on the nature of the continental shelves.
Other Economic activities
Algae culture and pisciculture are most commonly developed on the continental shelves and thus constitute a part of the blue economy. Shallower continental shelves are most favourable for such economic activities.
Shelf Break:
The shelf Break (also called the Submerged Offshore edge) of the shallow continental shelf is the point where the sea floor transitions to the continental slope.
Continental Slopes
The continental slope starts after the shelf break. It is characterised by an increase in the slope angle which ranges from 5º to 60º.
Length of continental shelves around the globe: The world’s combined continental slope has a total length of approximately 300,000 km (200,000 miles). It descends at an average angle of over 4° from the shelf break at the edge of the continental shelf to the beginning of the ocean basins at depths of 100 to 3,200 meters (330 to 10,500 feet). The variation of Continental slopes is diverse.
For example,
it is 40° near St. Helena, 30° off the Spanish coast, 62° near St. Paul, 5° to 15° near the Calicut coast (India) etc.
Continental slopes occupy only 8.5 per cent of the total area of the ocean basins but it varies from one ocean to the other.
It occupies 12.4 percent of the Atlantic Ocean basin, 7 percent of the Pacific Ocean basin and 6.5 per cent of the Indian Ocean.
The ocean basin of the Atlantic Ocean has the largest continental slope. It occupies around 12.4 % of the total ocean basin of the Atlantic Ocean.
Characteristics of the continental slopes
Gradient: The gradient of the slope is lowest off stable coasts without major rivers and highest off coasts with young mountain ranges and narrow continental shelves. Most Pacific slopes are steeper than Atlantic slopes (It must be noted that the Atlantic Ocean’s basin has wider slopes than the Pacific Ocean). Gradients are flattest in the Indian Ocean. It must be due to the impact of large rivers such as the Ganga and the Brahmaputra, which bring a huge amount of sediment load with them.
Location: About one-half of all continental slopes descend into deep-sea trenches or shallower depressions, and most of the remainder terminates in fans of marine sediment or continental rises. The transition from continental crust to oceanic crust usually occurs below the continental slope.
Physical features associated with continental slope: Continental slopes are usually indented by numerous submarine canyons and mounds. Some of the continental slopes are separated from the continental shelves by high rise mounds, sea mounts or plateaus of intermediate depths.
For Example, The Blake Plateau off the southeastern United States and the continental borderland off southern California are examples of continental slopes separated from continental shelves by plateaus of intermediate depth. Slopes off mountainous coastlines and narrow shelves often have outcrops of rock.
Economic significance of the Continental Slopes
Continental slopes are usually occupied with sediments and gravels and no significant mineral deposits are found. Physical features such as submarine canyons are associated with the continental slope.
Submarine Canyon
Fig. Submarine Canyon
Submarine Canyons are long, narrow and very deep valleys or trenches which are located on continental shelves and slopes with vertical walls. These physical features resemble similar to the continental canyons and are called Submarine canyons because of their location beneath the water. These physical features start from continental shelves and extend up to continental slopes. Unlike deep-sea trenches, which are found mostly on plate boundaries, undersea canyons are found along the slopes of most continental margins.
Characteristics of Submarine Canyons
The majority of these submarine canyons are V-shaped depressions which have steep, rocky walls which are thousands of meters high.
For Example, The Grand Bahama Canyon has the highest walls which rise nearly 5 km (3 miles) from the canyon floor.
Similarly, the walls of the Grand Canyon of the Colorado River measure about 1.6 km high.
Submarine canyons act as conduits to bring sand-sized sediments from the continental margins to the deep sea.
For Example, During low stands of sea levels, rivers empty directly into the heads of many Atlantic canyons. Sand and mud are carried down these systems, many times bypassing the slope-rise system and are carried directly out onto the abyssal plains of the ocean floor.
Deep Ocean Basin: Abyssal Plains, Abyssal Hills, Oceanic ridges and Rises, Ocean deeps and trenches
Abyssal Plains
Fig: Ocean Floor configuration having features of deep ocean basin
Abyssal plains are extensive plains that lie between the continental margins and mid-oceanic ridges. These are the areas where the continental sediments move beyond the margins and get deposited.
These areas are the smoothest and flattest surfaces on Earth which are covered with sediments such as silt and mud. These plains are extensively found in the Atlantic Ocean and unusual in the Pacific Ocean.
Characteristics of the Abyssal Plains
Abyssal plains are the exceptionally smooth and featureless plains of the deep ocean floor. They are considered as the levelest areas on the earth.
They cover a major segment of the ocean floors between the depths of 3000 m to 6000 m. The floor is covered by sediments.
The sediments which are formed from the remains of living things are called Oozes.
The ridges are either flat like a plateau or steep-sided mountains.
Abyssal Hills
Abyssal hills are small, topographically well-defined submarine hills which may rise from several meters to several hundred metres above the abyssal plain. These hills are located 3,000 to 6,000 metres (10,000 to 20,000 feet) deep below the water.
Abyssal hills are formed either by volcanism or block faulting. These are generally covered with marine sediments and are identical in composition and origin to the extrusive basaltic prominences on the upper flanks of mid-ocean ridges and rises. These underlie most of the ocean floor but are locally buried by accumulations of abyssal sediment.
For Example: In the Atlantic Ocean, long abyssal hill provinces parallel both flanks of the Mid-Atlantic Ridge along most of its length.
The Pacific Ocean has a smaller supply of continental sediment than the Atlantic Ocean, and numerous trenches and local rises separate the main ocean floor from the continents, preventing the seaward transport of sediment.
Between 80 and 85 percent of the Pacific abyssal floor is occupied by abyssal hills. Abyssal hills are also called Sea Knolls.
Abyssal hill provinces: Abyssal hill provinces are areas of abyssal seafloor occupied exclusively by abyssal hills which occur seaward of the smooth abyssal plains at the bases of continental rises.
Sea Mounts
Seamount is a large submarine volcanic mountain rising at least 1,000 m (3,300 feet) above the surrounding deep-sea floor. These are isolated steep submarine volcanic hills which are 2-100 m wide and less than 1000 m high. The sides of the sea mounts have steep slopes of 20º to 25º. Some of the peaks of the sea mounts emerge out of water and become islands.
For Example, Hawaii Island (North Pacific) and Azores Island (North Atlantic Ocean) are examples of such Islands.
Guyots
Guyot is an isolated submarine volcanic mountain with a flat summit of more than 200 metres (660 feet) below sea level. Such flat tops may have diameters greater than 10 km. These hills are also called table mounts.
Mid-Oceanic Ridges
These are interconnected chains of mountain systems within the ocean. It is the longest mountain chain on the surface of the earth which is submerged under the oceanic waters.
Characteristics of Mid-Oceanic Ridges
This system consists of a central rift system at the crest, a fractionated plateau and a flank zone all along its length.
The rift system at the crest is the zone of intense volcanic activity. This type of volcano is termed a mid-oceanic volcano.
The mid-oceanic ridge system constitutes 23% of the Earth’s surface. In the centre of the mid-oceanic ridge, there is a rift valley which is 30 to 50 kilometres wide. It dissects 1000 to 3000 meters deep into the ridge structure.
They are a group of underwater mountains created by the outpouring of magma. Ridges can be found rising from a depth of about 5 kilometres to a consistent depth of about 2.5 kilometres and can be hundreds of miles wide.
Physical Features found in mid-oceanic ridges
Volcanic Seamounts
These are underwater features which are found along mid-oceanic ridges and are extinct volcanoes which were once part of the volcanic chain of the ridge system.
Pillow lava or pillow basalt
Pillow basalt is a volcanic igneous rock that forms when lava of basaltic composition is erupted underwater. The rapid cooling of the lava by cold water on all sides forms the pillow-shaped bodies, which can then break open and extrude more of the hot lava from inside.
The rapid cooling also creates pillows that are composed of volcanic glass on the outside and that have very tiny, almost invisible crystals on the interior. Pillow basalt typically forms at volcanoes at mid-ocean ridges or at oceanic hot-spot volcanoes, such as those that formed the Hawaiian Islands. Basalt forms the crust of all the ocean basins and is therefore the most common rock in the Earth’s crust.
Fig: Pillow Basalt
Hydrothermal Vents
Hydrothermal vents are a unique ecosystem which is found in the rifts of the ocean floor in a mid-oceanic ridge system. This system is characterized by sudden changes in temperature from near 0ºC to around 400ºC within a short distance (Depth). This unique ecosystem consists of hundreds of new species which derive energy from the thermal energy emanating from such vents.
In these vents, microorganisms such as bacteria with the help of chemosynthesis convert the minerals found in these vents into usable form, providing food for other vent organisms. Hydrothermal vents are like geysers, or hot springs, on the ocean floor and are one of the ecosystems in which organisms draw their energy from other than sunlight.
Formation of Hydrothermal Vents
These vents are formed along mid-ocean ridges where tectonic plates spread apart causing the magma to rise which later cools to form new crust and volcanic mountain chains. Seawater circulates deep in the ocean’s crust and becomes superheated by hot magma.The minerals found along the vents are heated by this sea-water causing minerals to dissolve and rise toward the surface of the crust leading to the formation of a unique ecosystem after the chemosynthesis of these minerals by the bacteria.
Black smokers and White smokers
Black smokers and White smokers are different terms which are used for hydrothermal vents according to sulphur content in minerals emitted by these vents. While vents emitting the hottest and darkest plumes having high sulphur content are called black smokers, cooler plumes emitting light-coloured smoke which are rich in barium, calcium, and silicon are called white smokers.
White smokers are shorter and smaller than black smokers.
Note for UPSC Aspirants: For UPSC aspirants interested in exploring further, here are some keywords to guide your research:oceanic processes, hydrological cycle, bottom relief, economic significance, underwater features, continental shelves, slopes, submarine canyons, deep ocean basins, mineral resources, energy resources, biotic resources, shipping infrastructure, abyssal plains, hydrothermal vents
Comments