Mega relief Basics
There are various structures on the surface of the earth that give rise to various kinds of Landscapes. On a large scale, the landscapes of earth can be divided into three orders of relief called Megarelief. The Megareliefs include the largest landscapes by scale, from enormous ocean basins and continents down to local hills, spurs, cliffs, valleys, gorges and river terraces. Accordingly, there are three orders of relief as follows:
First Order of Relief
The broadest category of land forms includes huge continental platforms and ocean basins. Continental platforms are the masses of crust that exist above or near sea level, including the undersea continental shelves along the coastline. The ocean basins are entirely below the sea level. Approximately 71 percent of the earth is covered by water, with only about 29 percent of its surface appearing as continents and islands. The distribution of land and water in evidence today demonstrates a distinct water hemisphere and continental hemisphere.
Second Order of Relief
In the ocean basins, the second order of relief includes continental rises, slopes, abyssal plains, mid- ocean ridges, submarine canyons, and subduction trenches. Continental features that are classified in the second order of relief include continental masses, mountain masses, plateaus, plains and lowlands. A few examples are the Himalayas, Alps, Rocky Mountains, Andes, Tibetan plateau, plateau of Anatolia (Turkey), Indo-Gangetic plains, Siberian lowlands and the plains of Mississippi. The great rock cores (shields) that form the heart of each continental mass arc of this order.
Third Order of Relief
The third order of relief includes individual peaks, cliffs, valleys, hills, spurs, gorges, sand dunes, caves, moraines, cirques, ripples, beaches, etc. These features are identified as local landscapes.
Ocean Relief Features
Surface area of earth is 510,072,000 km². It comprises of 148,940,000 km² land (29.2 %) & 361,132,000 km² water (70.8 %). Relief features of oceans are quite different from those of the continents. Please note that much of the oceanic crust is less than 60 million years old, while the great bulk of the continental crust is of Proterozoic age—mostly over 1 billion years old. Thus, the young age of the oceanic crust is quite remarkable. Ocean is blue because water shows slightly blue color and that is because of the fact that it absorbs the Red photons of the light. Because the absorption which gives water its color is in the red end of the visible spectrum, one sees blue, the complementary color of red, when observing light that has passed through several meters of water.
- The deepest point in the ocean is the Mariana Trench, located in the Paciﬁc Ocean near the Northern Mariana Islands.
- Its maximum depth has been estimated to be 10,971 meters (plus or minus 11 meters).
- British naval vessel, Challenger II surveyed the trench in 1951 and named the deepest part of the trench, the “Challenger Deep”.
- In 1960, the Trieste successfully reached the bottom of the trench, manned by a crew of two men.
The ocean relief can be divided into various parts such as Continental Shelf, Continental Slope, Continental Rise or Foot, Deep Ocean basins, Abyssal plains & Abyssal Hills, Oceanic Trenches, Seamounts and Guyots.
Continental Shelf is the submerged edge of a continent. It is a gently sloping plain that extends into the Ocean. The typical gradient is less than 1°. Taken together, total area of the continental shelves is 18% of earth’s dry land area. The width of the continental shelf varies considerably; there are many places on earth where there is virtually no shelf at all. The largest continental shelf is the Siberian Shelf in the Arctic Ocean, which stretches to 1,500 kilometers in width. The average width of continental shelves is about 80 km. The depth of the shelf also varies, but is generally limited to water shallower than 150 m. Continental shelf is made up of Granite rock overlain by the sediments. Because of the gentle slope, the continental shelf is influenced by the changes in the sea level.
Continental Slope & Continental Rise
Continental slope is relatively steep descent from the shelf break to the deep sea floor. Inclination of the typical continental slope is around 4° and usually between 2° to 5°. Shelf break is almost constant all over the globe and is around 150 meters, except the Antarctica and Greenland continental slopes. The slope plunges down at least 1 kilometer and usually 2-3 kilometers. The Continental Rise connects the Continental slope to the deep sea or abyssal plain. Its width is around 100-1000 kilometers. Slope is gradual and around l/8th of the continental slope. The transition from continental to oceanic crust commonly occurs within the continental rise.
Deep Ocean basin & Oceanic Ridges
Deep Ocean Basin is the lowest layer in the ocean. The sea floor is like a covering of sediments over a basalt rock which may be up to 5 kilometers thick. Oceanic ridges or Mid-oceanic Ridges refer to the boundary between the diverging plates. A mid-ocean ridge (MOR) refers to an underwater mountain system that consists of various mountain ranges (chains), typically having a valley known as a rift that runs along its spine, formed by plate tectonics. The midoceanic ridge of submarine hills divides the basin in about half. Precisely in the center of the ridge, at its highest point, is a narrow trenchlike feature called the axial rift. The location and form of this rift suggest that the crust is being pulled apart along the line of the rift.
The oceanic ridges present a mountainous chain of young rocks which stretch around 65000 kilometers, i.e. 1.5 times of the earth’s circumference. Oceanic ridges are made up of basalt rocks, are geologically active as the new magma constantly emerging onto the ocean floor accumulates in the crust at and near rifts along the ridge axes. The adjacent graphic shows distribution of some Oceanic Ridges around the world.
Location of the important ridges are as follows:
- Aden Ridge: Gulf of Aden and Indian Ocean along the south-eastern coastline of the Arabian Peninsula.
- Explorer Ridge: Located 240 km west of Vancouver Island, British Columbia, Canada.
- Gorda Ridge: off the coast of Oregon and northern California north of Cape Mendocino
- Juan de Fuca Ridge: off the coasts of the state of Washington in the United States
- Cocos Ridge: Its is a Volcanic hotspot. Also known as Galapagos hotspot located in East Pacific Ocean responsible for the creation of the Galapagos Islands as well as three major seismic ridge systems. Carnegie, Cocos and Malpelso.
- Gakkel Ridge: located in the Arctic Ocean between Greenland and Siberia, and has a length of about 1,800 kilometers. It is slowest known spreading ridge on the earth.
- Pacific-Antarctic Ridge: located in South Pacific Ocean
- Southeast Indian Ridge: It is located in the Indian Ocean and separates the Indo Ocean plate from the Antarctic plate.
- Carlsberg Ridge: Located in the Indian Ocean.
Abyssal plain is flat, cold and sediment covered ocean floor. Abyssal plains are more extensive in Atlantic and Indian Oceans and less extensive in Pacific Ocean. Abyssal plain is found at an average depth between 3000 and 6000 meters. They are among the flattest, smoothest and least explored regions on earth.
An elongated through or deep in the ocean floor is called ocean trench. It is more or less a U shaped valley. Most of world’s trenches are in Pacific Ocean. Trenches are most active geological features on earth where great earthquakes are Tsunamis are born. Here is a brief info about important trenches:
- Mariana Trench: Mariana Trench is the deepest part of the world’s oceans. It is located in the western Pacific Ocean, east of the Mariana Islands. The trench is about 2,550 kilometers long but has a mean width of only 69 kilometers. The maximum known depth is 11.03 kilometers at the Vityaz-l Deep and about 10 91 kilometers at the Challenger Deep.
- Tonga Trench: Tonga Trench us located in South Pacific Ocean and is second deepest trench. Its deepest point is called Horizon Deep. It is Steepest Trench of the World.
- Puerto Rico Trench: Puerto Rico Trench is located on the boundary between the Caribbean Sea and the Atlantic Ocean. The trench is 800 kilometers long and has a maximum depth of 8,605 meters at Milwaukee Deep which is the deepest point in the Atlantic Ocean.
Seamounts & Guyots
Seamounts are elliptical projections from the sea floor which look like mountains and have a steep slope of around 22° to 24°.Half of the world’s total seamounts arc in Pacific Ocean.
Guyots are basically inactive volcanoes which are flat topped. Some of them are tall enough to approach or even penetrate the sea surface. Guyots are confined to Central Pacific Ocean.
A rift valley is linear-shaped lowland between highlands or mountain ranges created by the action of a geologic rift or fault in opposite or parallel. The result is the formation of a long steep sided, flat floored valley. World’s largest Fresh water lakes are typical rift valleys. Examples are Lake Baikal in Siberia, Lake Tanganyika, Lake Superior, Lake Vostok, Lake Nipissing and Lake Timiskaming. Jordan Rift Valley, which is lowest land elevation on earth is located in the Dead Sea and is 760 meters below the surface of the Mediterranean Sea. Gulf of Aqaba in the Red Sea is also a rift valley.
Important Lakes in Rift Valleys
- Lake Baikal: Lake Baikal, also known as “Pearl of Siberia” is located in Siberia and is second most voluminous lake in the world after the Caspian Sea. It is also world’s oldest and deepest lake. It’s a Rift valley, created by the Baikal Rift Zone, and a World Heritage site declared in 1996. Lake Baikal is home to Buryats, the largest ethnic minority group and a tribe in Siberia. It was referred as North Sea by ancient Chinese writers.
- Lake Tanganyika: After lake Baikal, Lake Tanganyika is second deepest lake in the world. It is world’s longest Lake spanning in 4 countries of Africa viz. Burundi, Tanzania, Congo and Zambia. This lake is a Rift Valley and largest rift lake in Africa.
- Lake Superior: Lake Superior is largest lake of North America, shared by Canada as well as USA. It is largest freshwater lake in the world by surface area if lake Michigan and lake Huron are NOT considered one.
- Lake Vostok: Lake Vostok is in Antarctica and is a Sub Glacial lake. It is located below the Vostok Station of Russia in Antarctica.
- Lake Nipissing: Lake Nipissing is located in Canada. It’s one of the shallowest lakes of Canada
Abyssal fans are also known as deep-sea fans, underwater deltas, and submarine fans. They are delta like structures formed at the deep sea surfaces. Abyssal plain is found at the depths between 3000 and 6000 meters. Abyssal plains cover more than 50% of Earth’s total surface. They are considered to be major reservoir of biodiversity.
Archipelago refers to a cluster of islands which are formed tectonically. This term was initially used for Aegean Islands located in the Aegean Sea between Greece and Turkey. Indonesia is often referred to as the world’s largest archipelago; however, this means that it is largest by area and not by number of islands. Indonesia has 17,500 islands which span more than 5000 km².
World’s largest archipelago by number of Islands is Archipelago Sea which is located Baltic Sea between the Gulf of Bothnia and the Gulf of Finland. It has 50,000 Islands.
Top 5 archipelagos in the world by number of Islands are as follows:
- Archipelago Sea (Finland) 50,000
- Canadian Arctic Archipelago 36.563
- Stockholm Archipelago 24,000
- Indonesian Archipelago 17.508
- Philippine Archipelago 7.107
Lagoon is a shallow body of sea water or brackish water separated from the sea by some form of barrier. The biggest lagoon in the world is located in New Caledonia, in southwest pacific. In India, Chilika Lake in Orissa and the Vembanad Lake in Kerala are both connected to the Bay of Bengal and the Arabian Sea respectively through a narrow channel and they are typical lagoons.
Coral reefs, which are also called as “rainforests of the sea”, are underwater reefs made by calcium carbonate secreted by Corals. Coral is the hard exoskeleton of the polyps. Coral Reefs grow best in warm, shallow, clear, sunny and agitated waters. Coral reefs are some of the most diverse ecosystems on earth which despite of covering less than 10% of world ocean surface (284,300 km²) provide home to 25% of marine species including fishes, molluscs etc. Coral Reefs are very fragile ecosystem and are susceptible to “Surface Temperature” of the oceans. They are threatened by the climate change, ocean acidification, blast fishing, cyanide fishing for aquarium fish, overuse of reef resources, and harmful land-use practices.
Some Observations on Coral Reliefs
- The Indian Ocean and Pacific Ocean region including the Red Sea, Indian Ocean, Southeast Asia and the Pacific account for 91.9% or total Coral reefs in the world.
- Southeast Asia accounts for 32.3%, while the Pacific including Australia accounts for 40.8%.
- Atlantic and Caribbean coral reefs only account for 7.6%.
- Largest Coral reef in the world is Great Barrier Reef. It is located in the Coral Sea, off the coast of Queensland in north-east Australia. It is composed of over 2,900 individual reefs and 900 islands stretching for over 2,600 kilometers. This reef can be seen from outer space and is the world’s biggest single structure made by living organisms. It is a World Heritage Site (1981). It is also a state icon of Queensland, made by Queensland National Trust. A large part is protected by the Great Barrier Reef Marine Park, established by Government of Australia through Great Barrier Reef Marine Park Act 1975.
- Belize Barrier Reef is world’s second largest Coral Reef which is a part of 900 kilometer Mesoamerican Barrier Reef System. It was described by Charles Darwin in 1842 as “the most remarkable reef in the West Indies”.
- Pulley Ridge is located off the coast of Florida, United States. It is deepest photosynthetic coral reef known so far.
- Raja Ampat Islands, largest marine national park in Indonesia are located in Indonesia and New Guinea and comprise 1,500 small islands .It is known for highest recorded marine biodiversity on Earth. It makes the Coral Triangle which is a triangular shaped area of the tropical marine waters of Indonesia, Malaysia, Papua New Guinea, Philippines, Solomon Islands and Timor-Leste. These waters contain at least 500 species of reef-building corals in each ecoregion. Coral Triangle as well as Raja Ampat Islands is considered to be the global epicenters of marine biodiversity. INWF considers the region as a top priority for marine conservation and has launched the Coral Triangle Program in 2007
Beach is the shoreline of an ocean, sea or lake which consists of loose particles such as sand, gravel, pebbles etc. They are formed as a result of wave action by which waves or currents move sand or other loose sediments.
- Cox’s Bazaar: Cox’s Bazaar sandy beach in Bangladesh’s Chittagong is considered to be world’s longest natural sandy beach. It has an unbroken length of 120 kilometers.
- Marina Beach: Marina Beach is located in India’s Chennai and is one of the largest beaches of India.
Ocean sediments are particles and fragments of dirt, dust, and other debris that have settled out of the water and accumulated on the ocean floor. The crucial importance of marine sediments is that they reveal much about Earth’s history. Marine sediments provide clues to past climates, movements of the ocean floor, ocean circulation patterns, and nutrient supplies for marine organisms. Marine sediments can also be helpful in ascertaining the timing of major extinctions, global climate change, and the movement of plates.
More than half of the rocks exposed on the continents are sedimentary rocks deposited in ancient ocean environments and uplifted onto land by plate tectonic processes. Even the tallest mountains on the continents far from any ocean contain marine fossils, which indicate that these rocks originated on the ocean ﬂoor in the geologic past. Mount Everest consists of limestone, which is a type of rock that originated as sea ﬂoor deposits. Over time, sediments can become lithified and turned to rock and form sedimentary rock. Particles of sediment come from worn pieces of rocks, as well as living organisms, minerals dissolved in water, and outer space.
There are several types of marine sediments such as:
- Lithogenous sediment
- Biogenous Sediment
- Hydrogenous sediment
This type of sediment is derived from pre-existing rock material that originates on the continents or islands from erosion, volcanic eruptions, or blown dust. The another term used for lithogenous sediment is terrigenous sediment. Obviously, the origin of Lithogenous Sediment begins as rocks on continents or islands. Over time, weathering agents such as water, temperature extremes, and chemical effects break rocks into smaller pieces.
Neritic Deposits versus Pelagic Deposits
Marine sedimentary deposits can also be categorized as either neritic or pelagic. Neritic deposits are found on continental shelves and in shallow water near islands; these deposits are generally coarse grained. Alternatively, pelagic deposits are found in the deep ocean basins and are typically ﬁne grained.
When rocks are in smaller pieces, they can be more easily eroded and transported. This eroded material is the basic component of which all lithogenous sediment is composed. Eroded material from the continents is carried to the oceans by streams, wind, glaciers, and gravity. The transported sediment can be deposited in many environments, including bays or lagoons near the ocean, as deltas at the mouths of rivers, along beaches at the shoreline, or further offshore across the continental margin. It can also be carried beyond the continental margin to the deep-ocean basin by turbidity currents. The greatest quantity of lithogenous material is found around the margins of the continents, where it is constantly moved by high-energy currents along the shoreline and in deeper turbidity currents. The majority of lithogenous deposits such as beach sands are composed primarily of quartz.
Biogenous Sediment / Ooze
Biogenous sediment is derived from the remains of hard parts of once living organisms.
Origin of Biogenous Sediment
Biogenous sediment begins as the hard parts (shells, bones, and teeth) of living organisms ranging from minute algae and protozoans to fish and whales. When organisms that produce hard parts die, their remains settle onto the ocean floor and can accumulate as biogenous sediment. Biogenous sediment can be classified as either macroscopic or microscopic. Macroscopic biogenous sediment is large enough to be seen without the aid of a microscope and includes shells, bones, and teeth of large organisms.
The microscopic organisms produce tiny shells called tests that begin to sink after the organisms die and continually rain down in great numbers onto the ocean floor. These microscopic tests can accumulate on the deep ocean floor and form deposits called ooze. Ooze resembles very fine grained, mushy material like mud. The organisms that contribute to biogenous sediment are chiefly algae and protozoans. Algae are primarily aquatic, eukaryotic, photosynthetic organisms, ranging in size from microscopic single cells to large organisms like giant kelp. Protozoans are any of a large group of single-celled, eukaryotic, usually microscopic organisms that are generally not photosynthetic.
The two most common chemical compounds in biogenous sediment are calcium carbonate (which forms the mineral calcite) and silica. Silica is often chemically combined with water to produce the hydrated form of silica, which is called opal.
Diatoms and Diatomaceous Earth
Most of the silica in biogenous ooze comes from microscopic algae called diatoms and protozoans called radiolarians. Because diatoms photosynthesize, they need strong sunlight and are found only within the upper, sunlit surface waters of the ocean. Most diatoms are free floating, or planktonic. The living organism builds a glass greenhouse out of silica as a protective covering and lives inside. Where diatoms are abundant at the ocean surface, thick deposits of diatom rich ooze can accumulate below on the ocean floor. When this ooze lithified, it becomes diatomaceous earth, which is a lightweight white rock composed of diatom tests and clay.
Hydrogenous sediment is derived from the dissolved material in water. Chemical reactions within seawater cause certain minerals to precipitate. Precipitation usually occurs when there is a change in conditions, such as a change in temperature or pressure or the addition of chemically active fluids.
Manganese nodules are rounded, hard lumps of manganese plus iron plus other metals typically 5 cms to 20 cms in diameter. When cut in half, they often reveal a layered structure formed by precipitation around a central nucleation object, which might be a piece of lithogenous sediment, coral, volcanic rock, a fish bone, or a shark s tooth. Manganese nodules are found on the deep-ocean floor. The major components of these nodules are manganese dioxide (around 30% by weight) and iron oxide (around 20%). Other accessory metals present in manganese nodules include copper, nickel and cobalt. Although the concentration of these accessory metals is usually less than 1%, they can exceed 2% by weight, which may make them attractive exploration targets in the future.
Phosphates occur abundantly as coatings on rocks and as nodules on the continental shelf and on banks at shallow depths. Concentrations of phosphates in such deposits indicate abundant biological activityin surface water above where they accumulate. Because phosphates are valuable as fertilizers, ancient marine phosphate deposits that have been uplifted onto land are extensively mined to supply agricultural needs.
Carbonates The two most important carbonate minerals in marine sediment are aragonite and calcite. Both are composed of calcium carbonate but aragonite has a different crystalline structure that is less stable and changes into calcite over time. Most carbonate deposits are biogenous in origin. However, hydrogenous carbonate deposits can precipitate directly from seawater in tropical climates to form aragonite crystals.
Deposits of metal Sulfides are associated with hydrothermal vents and black smokers along the mid-ocean ridge. These deposits contain iron, nickel, copper, zinc, silver, and other metals in varying proportions. Transported away from the mid-ocean ridge by sea floor spreading, these deposits can be found throughout the ocean floor and can even be uplifted onto continents.
Some important Facts on Diatoms
- Diatoms are microscopic single-celled photosynthetic organisms. Each one lives inside a protective silica test, most of which contain two halves that fit together very much similar to a shoebox and its lid.
- The fossil record indicates that diatoms have been on Earth since the Jurassic Period (180 million years ago), and at least 70,000 species of diatoms have been identified. The tests of diatoms are exquisitely ornamented with holes, ribs, and radiating spines unique to individual species.
- Diatoms live for a few days to as much as a week, can reproduce sexually or asexually, and occur individually or linked together into long communities. They are found in great abundance floating in the ocean and in certain freshwater lakes but can also be found in many diverse environments, such as on the undersides of polar ice, on the skins of whales, in soil, in thermal springs, and even on brick walls. When marine diatoms die, their tests rain down and accumulate on the sea floor as siliceous ooze.
- Hardened deposits of siliceous ooze, called diatomaceous earth, can be as much as 900 meters thick. Diatomaceous earth consists of billions of minute silica tests and has many unusual properties such as: It is lightweight and has an inert chemical composition. It is resistant to high temperatures, and it has excellent filtering properties. Due these properties, diatomaceous earth is used to produce a variety of common products such as filters, mild abrasives(in toothpaste, facial scrubs, matches, and household cleaning and polishing compounds) absorbents (for chemical spills, in cat litter, and as a soil conditioner) chemical carriers(in pharmaceuticals, paint, and even dynamite)
- Other products from diatomaceous earth include optical-quality glass (because of the pure silica content of diatoms) and space shuttle tiles (because they are lightweight and provide good insulation). Diatomaceous earth is also used as an additive in concrete, a filler in tires, an anticaking agent, a natural pesticide, and as building stone in the construction of houses.
- Apart from this, each living diatom contains a tiny droplet of oil. When diatoms die, their tests containing droplets of oil accumulate on the sea floor and thus they are the beginnings of petroleum deposits.
Resources from Ocean Sediments
Ocean beds are rich in potential mineral and organic resources. Much of these resources, however, are not easily accessible, so their recovery involves technological challenges and high cost.
- The main energy resources associated with marine sediments are petroleum and gas hydrates. The ancient remains of microscopic organisms, buried within marine sediments before they could decompose, are the source of today s petroleum (oil and natural gas) deposits. Petroleum products account for 95% of the economic value of the ocean beds. This mainly includes the oil produced from offshore regions. Today major offshore reserves exist in the Persian Gulf, in the Gulf of Mexico, off Southern California and in the North Sea.
- Gas hydrates are unusually compact chemical structures made of water and natural gas. They form only when high pressures squeeze chilled water and gas molecules into an icelike solid. Although hydrates can contain a variety of gases including carbon dioxide, hydrogen sulfide, and larger hydrocarbons such as ethane and propane; methane hydrates are by far the most common hydrates in nature. Gas hydrates occur beneath Arctic permafrost areas on land and under the ocean floor, where they were discovered in 1976. In deep-ocean sediments, where pressures are high and temperatures are low, water and natural gas combine in such a way that the gas is trapped inside a lattice like cage of water molecules.
Sand and Gravel
- The offshore sand and gravel industry is second in economic value only to the petroleum industry. These include the rock fragments that are washed out to sea and shells of marine organisms, is mined by offshore barges using a suction dredge. This material is primarily used as aggregate in concrete, as a fill material in grading projects, and on recreational beaches.
- When seawater evaporates, the salts increase in concentration until they can no longer remain dissolved, so they precipitate out of solution and form salt deposits. The most economically useful salts are gypsum and halite i.e, common salt.
Manganese Nodules and Crusts
- Manganese nodules are rounded, hard, golf- to tennis-ball-sized lumps of metals that contain significant concentrations of manganese, iron, and smaller concentrations of copper, nickel, and cobalt, all of which have a variety of economic uses.
Trends in Ocean Temperature
The temperature of the oceanic water is important for phytoplanktons as well as zooplanktons. The temperature of sea water also affects the climate of coastal lands and plants and animals. The study of both, surface and subsurface temperature of sea water is thus significant.
Measurement of Temperature
Standard type of thermometer is used to measure the surface temperature while reversing thermometers and thermographs are used to measure the subsurface temperature. These thermometers record the temperature up to the accuracy of ±O.02° centigrade. Layers of Temperature in tropics Oceans absorb more than 80% of the solar radiation and water which has highest specific heat is the remarkable capacity of storing the heat. The uppermost 10% of the oceans has more heat than the entire atmosphere of earth!
With respect to temperature, there are three layers in the oceans from surface to the bottom in the tropics as follows: The first layer represents the top-layer of warm, oceanic water and is 500m thick with temperature ranging between 20° and 25°C. This layer is present within the tropics throughout the year but it develops in mid· latitudes only during summer. The thermocline layer represents vertical zone of oceanic water below the first layer and is characterized by rapid rate of decrease of temperature with increasing depth. The third layer is very cold and extends upto the deep ocean floor. The polar areas have only one layer of cold water from surface (sea level) to the deep ocean floor.
The radiant energy transmitted from the photosphere of the sun in the form of electromagnetic short waves and received at the ocean surface is called insolation. Besides, some energy, though insignificant, is also received from below the bottom and through the compression of sea water. The amount of insolation to be received at the sea surface depends on the angle of sun’s rays, length of day, and distance of the earth from the sun and effects of the atmosphere. The mechanism of the heating and cooling of ocean water differs from the mechanism on land because besides horizontal and vertical movements of water, the evaporation is most active over the oceans.
Daily Range of Temperature
The difference of maximum and minimum temperature of a day (24 hours) is known as daily range of temperature. The daily range of temperature of surface water of the oceans is almost insignificant as it is around 1°C only. The daily range of temperature is usually 0.3°C in the low latitudes and 0.2° to 0.3°C in high latitudes. The diurnal range depends on the Conditions of sky (cloudy or clear sky), Stability or instability of air and Stratification of seawater. The heating and cooling of ocean water is rapid under clear sky (cloudless) and hence the diurnal range of temperature becomes a bit higher than under overcast sky and strong air circulation. The high density of water below surface water causes very little transfer of heat through conduction and hence the diurnal range of temperature becomes low.
Annual Range of Temperature
The maximum and minimum annual temperatures of ocean water are recorded in August and February respectively in the northern hemisphere. Usually, the average annual range of temperature of ocean water is -12°C but there is a lot of regional variation which is due to regional variation in insolation, nature of seas, prevailing winds, location of seas etc. Annual range of temperature is higher in the enclosed seas than in the open sea (Baltic Sea records annual range of temperature of 4.4°C or 40°F). The size of the oceans and the seas also affects annual range of temperature e.g., bigger the size, lower the annual range and vice versa. The Atlantic Ocean records relatively higher annual range of temperature than the Pacific Ocean.
Distribution Pattern of Temperature
The distributional pattern of temperature of ocean water is studied in two ways viz. Horizontal distribution (temperature of surface water) and Vertical distribution (from surface water to the bottom). Since the ocean has three dimensional shape, the depth of oceans, besides latitudes, is also taken into account in the study of temperature distribution. The following factors affect the distribution of temperature of ocean water.
- Latitudes: The temperature of surface water decreases from equator toward the poles because the sun’s rays become more and more slanting and thus the amount of insolation decreases pole ward accordingly. The temperature of surface water between 40°N and 40°S is lower than air temperature but it becomes higher than air temperature between 40°Latitude and the poles in both the hemispheres.
- Unequal distribution of land and water: The temperature of ocean water varies in the northern and the southern hemispheres because of dominance of land in the northern hemisphere and water in the southern hemisphere. As far as surface temperature is concerned, it has the following implications:
- 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 and thus the temperature of surface water is comparatively higher in the northern hemisphere than the southern hemisphere. The isotherms are not regular and do not follow latitudes in the northern hemisphere because of the existence of both warm and cold landmasses whereas they (isotherms) are regular and follow latitudes in the southern hemisphere because of the dominance of water. The temperature in the enclosed seas in low latitudes becomes higher because of the influence of surrounding land areas than the open seas e.g., the average annual temperature of surface water at the equator is 26.7°C whereas it is 37.8°C in the Red Sea and 34.4°C (94°F) in the Persian Gulf.
- Prevailing wind: Wind direction largely affects the distribution of temperature of ocean water. The winds blowing from the land towards the oceans and seas (i.e. offshore winds) drive warm surface water away from the coast resulting into upwelling of cold bottom water from below. Thus, the replacement of warm water by cold water introduces longitudinal variation in temperature. Contrary to this, the onshore winds pile up warm water near the coast and thus raise the temperature.
- Ocean currents: Surface temperatures of the oceans are controlled by warm and cold currents. Warm currents raise the temperature of the affected areas whereas cool currents lower down the temperature.
- Other factors: Other factors include the following: Submarine ridges Local weather conditions such as storms, cyclones, hurricanes, fog, cloudiness, evaporation and condensation
- Location and Shape of area: The enclosed seas in the low latitudes record relatively higher temperature than the open seas whereas the enclosed seas have lower temperature than the open seas in the high latitudes.
Horizontal Distribution of Temperature
Average temperature of surface water of the oceans is 26.7°C and the temperature gradually decreases from equator towards the poles. The rate of decrease of temperature with increasing latitudes is generally 0.5°C per latitude. The average temperatures become 22°C at 20° N and S latitudes, 14°C at 40° N and S latitude, and 0°C near the poles.
It has already been mentioned above that the oceans in the northern hemisphere record relatively higher average temperature than in the southern hemisphere. Please note that the highest temperature is not recorded at the equator rather it is a bit north of it.
Also we should note that the average annual temperature of all the oceans is 17.2°C. The average annual temperatures for the northern and southern hemispheres are 19.4°C and 16.1°C respectively. The variation of temperatures in the northern and southern hemispheres is because of unequal distribution of land and water as Northern hemisphere is made up of more land, while the southern hemisphere is made up of more oceans.
In Northern Atlantic, there is a very low decrease of temperature with increasing latitudes towards north. This is because of the Gulf Stream currents which are warm currents. However, in southern Atlantic, the decrease of temperature with increasing latitude is more pronounced. The table shows the variations of three major oceans:
Vertical Distribution of Temperature
The maximum temperature of the oceans is always on the surface because it directly receives the insolation. The heat is transmitted to the lower sections of the oceans through the mechanism of conduction. Solar rays very effectively penetrate up to 20m depth and they seldom go beyond 200m depth.
Consequently, the temperature decreases from the ocean surface with increasing depth but the rate of decrease of temperature with increasing depth is not uniform everywhere. The temperature falls very rapidly up to the depth of 200m and thereafter the rate of decrease of temperature is slowed down. On this basis, oceans are vertically divided into three zones as follows:
- Photic Zone or Euphotic Zone This is the upper layer of the ocean. The temperature is relatively constant and is 100 meters deep.
- Thermocline: Thermocline lies between 100-1000 meters. There is a steep fall in the temperature. The following graph shows the thermocline.
- Deep Zone Below 1000 meters is the deep zone. Here, the temperature is near zero °C. Please note that near bottom, the temperature of water never goes to 0°C. It is always 2-3°C.
- Sea temperature decreases with increasing depth but the rate of decrease of temperature is not uniform.
- The change in sea temperature below the depth of 1000m is negligible.
- The maximum change in temperature is between 100-1000 meters which is called Thermocline or Pycnocline.
- Diurnal and annual ranges of temperature cease after a depth of 30 feet and 600 feet respectively.
- The rate of decrease of temperature with increasing depth from equator towards the poles is not uniform.
- Though, the surface temperature of the oceans decreases from equator to the poles, the temperature at the ocean bottom is uniform at all latitudes. However, some studies have shown that the coldest bottom temperatures, just below — 0.25°C, occur at 60-70°S, near the Antarctic continent.
Salinity of Ocean
Water Salinity of the ocean water is between 3.3-3.7 percent. The maximum amount of salt is common salt i.e. Sodium Chloride, which is followed by Magnesium Chloride. The major salts are as follows:
Most of the salinity of the sea comes from the dissolved material that originates from land and was carried by the rain, running water, ground water, wind, sea waves, glaciers etc. Some of the salts come from the deeper layers of earth. Volcanic lava, dead organic matters also contribute in the Ocean salinity.
The salinity of the ocean water depends upon the following:
- Evaporation: Higher the rate of evaporation, higher is salinity. The Highest evaporation has been recorded along the tropic of Cancer and that is one of the reasons that region of Red Sea and Persian Gulf has one of the highest salinity. Another reason is that enclosed seas tend to have more salinity in their water.
- Temperature: There is a direct relationship between ocean temperature and salinity. So the warmer parts are more saline and frigid parts are less saline.
- Precipitation: Precipitation is inversely related to salinity. Higher is the precipitation, lower is the proportion of salinity. The equatorial region records highest rainfall and that it is why it has low salinity in comparison to those which are near to tropics.
- Influx of Freshwater: Low salinity will be found at the mouth of rivers. This salinity is minimum in the raining season.
- Atmospheric Pressure: High pressure areas have high salinity and vice versa.
- Circulation of Ocean water: Ocean currents play a major role in distribution of salinity.
Horizontal Distribution of Salinity of Ocean Water
As a general rule, the salinity of the oceans decreases on both sides from the tropic of Cancer. This is attributed to the high occurrence of precipitation on equator. Highest salinity of the seawater has been recorded between 20°N to 40°N. The average salinity of the Northern and Southern hemisphere is 3.5 and 3.4 % respectively. This also because of the fact that the Northern Hemisphere is land dominated.
Vertical Distribution of Salinity of Ocean Water
There is no definite trend in the vertical distribution of salinity in the oceans, so there are no generalizations. However, it has been noted that the salinity of the ocean increases with increasing depth in the higher latitudes and polar areas. In the middle latitudes also, the same trends is seen but ONLY up to a depth of 370 meters after that it decreases with increasing depth.