Hydrosphere
Ocean Bottom Topography
Modern technology and various expeditions have revealed that the ocean floor is not a monotonous plain as some have imagined, but rather a topographical complex containing many diverse landform features that are as varied as, if not more diverse than, those found on land. On the ocean floor, there are four major divisions that can be easily identified.
A. The Continental Shelf.
B. The Slope of the Continental Plate.
C. The Rise of the Continent.
D. The Abyssal Plain
• Oceanic ridges, hills, seamounts, guyots, oceanic trenches, submarine canyons, oceanic plateau, Bank, Shoal, fracture zones, island arcs, coral reefs, submerged volcanoes, and sea-scarps are just a few of the associated and minor landform features.
• The interaction of tectonic, volcanic, erosional, and depositional processes has resulted in a wide range of relief. The tectonic and volcanic phenomena are more significant processes at greater depths.
Continental Shelf
• The continental shelf is the area along each continent's extended margin that is occupied by relatively shallow seas and gulfs. It is the ocean's shallowest part, with an average gradient of 1° or even less. Typically, the shelf ends at a very steep slope known as the shelf break.
• The continental shelves vary in width from one ocean to the next. Continental shelves have an average width of 80 kilometers. Along some of the margins, such as the Chilean coasts and the west coast of Sumatra, the shelves are almost non-existent or extremely narrow.
• On the other hand, the world's largest shelf, the Siberian shelf, stretches 1,500 kilometers across the Arctic Ocean. The depth of the shelves varies as well; on average, they are nearly 200 meters below mean sea level. However, they can be as shallow as 30 meters in some areas and as deep as 600 meters in others.
• The continental shelves are covered in varying thicknesses of sediments carried down from the land by rivers, glaciers, wind, and waves and currents. Massive sedimentary deposits that the continental shelves have accumulated over time become a source of fossil fuels. Glaciated shelves, coral reef shelves, river shelves, shelf with dendritic valleys, and shelf along young mountain ranges are all examples of shelves.
• The shelves are extremely useful to humans because they provide almost all of the marine food we eat; they also provide the richest fishing grounds in the Entire Ocean, as well as potential sites for economic minerals. The continental shelves account for nearly 20% of global petroleum and natural gas production. They also provide raw materials for the construction industry, such as sand and gravel repositories.
• The term continental shelf was given a legal definition by the United Nations Convention on the Law of the Sea, which defined it as the stretch of seabed adjacent to the shores of a specific country. Because of the sunlight available in shallow waters, continental shelves teem with life.
Continental Slope
• The continental shelf and ocean basins are connected by the continental slope. It starts where the continental shelf's bottom abruptly drops off into a steep slope.
• The slope region has a gradient of 2-5 degrees. The slope region's depth ranges from 200 to 3,000 metres. The end of the continents is marked by the slope boundary. This area is known for its canyons and trenches.
• The world's combined continental slope stretches for about 300,000 kilometers (200,000 miles) and descends at an average angle of more than 4 degrees from the continental shelf break to the beginning of the ocean basins.
• The slope has the lowest gradient off stable coasts with few major rivers and the highest gradient off coasts with young mountain ranges and narrow continental shelves.
• The majority of Pacific slopes are steeper than those of the Atlantic. The Indian Ocean has the flattest gradients.
Continental Rise
• With depth, the continental slope gradually loses its steepness. The continental rise occurs when the slope reaches a value of between 0.5 and 1 degree. As the depth of the rise increases, it becomes nearly flat and merges with the abyssal plain.
• Continental rises are formed by three sedimentary processes: mass wasting, contour current deposition, and biogenic particle vertical settling. Clay minerals and fine-grained particles (primarily quartz, mica, and carbonate) swept off the continental shelf, wind-blown dust, organic detritus, and plankton tests make up these sediments.
Abyssal Plain
• The word abyssal plain is a combination of the words abyss and dismal. And when it comes to describing an abyssal plain, those two words might not be too far off. If the word "abyss" conjures up images of the ocean's depths, you've come to the right place.
• And the term "plain" usually refers to a large, flat area. When you combine those two elements, you get an abyssal plain, which refers to the vast, flat areas on the ocean floor. It's almost an understatement to say that it's flat. The abyssal plains are among the world's flattest terrains.
• For every 1,000 feet of distance, some elevations drop by less than a foot. Abyssal plains are generally as flat as a tabletop, with the exception of a few small hills known as abyssal hills. The depths range from 3,000 to 6,000 meters. Fine-grained sediments such as clay and silt cover these plains.
• Between the continental rise (the part of the underwater continental margin farthest from the shore) and the mid-ocean ridge (where under-ocean tectonic plates move away from each other, forming volcanoes that become underwater mountain ranges) or ocean trenches are abyssal plains (where one tectonic plate slides beneath another, in a process called subduction).
Minor Ocean Relief Features
OCEANIC RIDGES-The oceanic ridge system is a continuous underwater mountain range that has parts in all of the world's oceans. When magma rising between diverging plates of the lithosphere cools and forms a new layer of crust, the ridge system is formed.
• Mountain ranges can reach heights of up to 2,500 metres, and some even rise above the ocean's surface. Iceland, which is located on the mid-Atlantic Ridge, is an example.
Abyssal Hills, Sea Mounts, And Guyots—
• These are volcanically formed elevated features. A seamount is a submerged mountain or peak that rises more than 1,000 meters above the ocean floor. Seamounts are formed by volcanic activity. These can reach heights of 3,000-4,500 meters. A good example is the Emperor seamount, which is an extension of the Hawaiian Islands in the Pacific Ocean.
• Guyots are mountain ranges with a flat top. Guyots show signs of subsidence over time, eventually becoming flat-topped submerged mountains. Seamounts and guyots are extremely common in the Pacific Ocean, with an estimated 10,000 of them.
Ocean Trenches-
• Ocean trenches are deep depressions in the ocean's deepest parts [where old ocean crust from one tectonic plate is pushed beneath another, raising mountains, causing earthquakes, and forming volcanoes on the seafloor and on land]. Trenches in the world's "Hadal Zone," named after Hades, the Greek god of the underworld, reach depths of nearly 6,000 metres (nearly 20,000 feet).
• Subduction is a geophysical process in which two or more of Earth's tectonic plates collide, causing the older, denser plate to be pushed beneath the lighter plate and deep into the mantle, bending the seafloor and outermost crust (the lithosphere) and forming a steep, V-shaped depression. Trenches become dynamic geological features as a result of this process, accounting for a significant portion of Earth's seismic activity and frequently being the epicenter of large earthquakes, including some of the world’s largest.
• Subduction also causes molten crust to rise to the surface, forming mountain ridges and volcanic islands parallel to the trench. The Japanese Archipelago, the Aleutian Islands, and numerous other locations along the Pacific "Ring of Fire" show examples of these volcanic "arcs."
• Much of the Hadal zone is made up of vast submarine slopes and steep trench walls, where a diverse range of species live in unique habitats extending across a range of depths, many of which are new or unknown to science.
• Trenches are long, narrow, and very deep gorges that can be found all over the world, though the majority are in the Pacific Ocean. The Mariana Trench, which runs near the Mariana Islands and is 1,580 miles long and 43 miles wide, is the world's deepest trench. It is home to the Challenger Deep, which is the deepest part of the ocean at 10,911 meters (35,797 feet).
A Submarine Fracture Zone-
• It is a long, narrow, and mountainous submarine lineation that generally separates ocean-floor ridges with depth differences of up to 1.5 kilometers (0.9 mile).
• The largest fracture zones in the eastern Pacific are thousands of kilometers long, 100 to 200 kilometers wide (60 to 125 miles), and have vertical relief of several kilometers.
Oceanic Plateau-
• An oceanic plateau is a large, flat submarine region that rises well above the ambient seabed level. Some oceanic plateaus are undersea remnants of large igneous provinces, while others are made up of continental crust and form a step interrupting the continental slope.
Submerged Volcanoes-
• Submarine volcanoes are magma-erupting underwater vents or fissures in the Earth's surface. A large number of submarine volcanoes are found near mid-ocean ridges, which are areas of tectonic plate movement.
• The volcanoes at mid-ocean ridges alone are thought to be responsible for 75% of the world's magma output.
• Although the majority of submarine volcanoes are found deep within seas and oceans, some can be found in shallow water and can release material into the atmosphere during an eruption.
• Over one million submarine volcanoes are thought to exist, with 75 000 rising more than one kilometer above the seabed.
Sea Scarps-
• An escarpment is a section of land where the elevation abruptly changes. The terms escarpment and scarp are frequently interchanged. Although some sources distinguish the two terms, there is a technical distinction:
a. Escarpment refers to the boundary between two landforms,
b. Whereas scarp refers to a cliff or steep slope. A sea scarp is a submarine escarpment formed by faulting of the submarine bed.
Bank, Shoal, And Reef-
Erosional, depositional, and biological activity all contribute to the formation of these marine features. These are based on diastrophic characteristics. As a result, they are found at the tops of hills.
a. Banks- These are formed as a result of erosional and depositional activity in the sea. A bank is a low-lying elevation on the continental margins with a flat top. The water depth here is shallow, but sufficient for navigation. Famous examples include the Dogger Bank in the North Sea and the Grand Bank in the northwestern Atlantic off the coast of Newfoundland. The banks are home to some of the world's most productive fisheries.
b. Shoal—A shoal is a low-lying ridge with shallow depths. They are dangerous for navigation because they project out of the water at a moderate height.
c. A reef- It is an organic deposit formed by living or dead organisms that forms a mound or rocky elevation similar to a ridge. Coral reefs are common in the Pacific Ocean, where they are found alongside seamounts and guyots.
Island Archs-
• When a lithospheric slab is sub ducted, the edges reach a sufficiently hot depth, the slab melts. The sub ducting slab's hot, re-melted material rises and leaks into the crust, forming a series of volcanoes. These volcanoes can form a "island arc," which is a chain of islands. The Japanese islands, the Kuril Islands, and the Aleutian Islands of Alaska are examples of island arcs.
Submarine Canyons-
• Submarine Canyons are narrow, steep-sided valleys that cut into oceanic continental slopes and rises. Submarine canyons can form on continental slopes or on the continental shelf. On continental margins with extremely steep continental slopes or escarpments, they are uncommon.
• Submarine canyons get their name from the fact that they resemble canyons formed by rivers on land. Undersea canyons, unlike deep-sea trenches, are found along the slopes of most continental margins, where one tectonic plate slides beneath another.
Ocean Water Temperature
• For marine organisms such as plants (phytoplankton) and animals, the temperature of oceanic water is critical (zooplanktons). The climate of coastal lands, as well as the plants and animals that live there, is influenced by the temperature of sea water.
• The study of ocean temperature is crucial for determining the movement of large volumes of water (vertical and horizontal ocean currents), the type and distribution of marine organisms at various ocean depths, the climate of coastal lands, and so on.
• For recording ocean temperatures, three types of instruments are used: Standard thermometers are used to measure surface temperature; reversing thermometers are used to measure sub-surface temperature; and thermographs are used to measure both surface and sub-surface temperature. Nowadays, instead of the above-mentioned thermometers, automatic self-recording instruments are used.
Sources of Oceanic Heat
Oceans get their energy primarily from the sun. Aside from that, the ocean is heated by the ocean's own internal heat. The following processes heat the water in the ocean:
1. Because of vertical insolation, absorption of solar radiation is greatest at low latitudes and gradually decreases towards the poles. The solar insolation received by the ocean varies even within the same latitude due to factors such as currents and cloudiness.
2. The oceanic water is heated by convectional currents in the water body. The ocean water at great depths is heated faster than the upper water layers in the ocean because the earth's temperature rises with depth. As a result, a convectional oceanic circulation occurs at the ocean's bottom layers, causing heat to circulate in the water.
3. Kinetic energy is created by friction caused by surface wind and tidal currents, both of which increase stress on the water body. As a result, the ocean water is warmed.
4. Submarine volcanoes also release heat into the ocean.
The ocean water is cooled in the following ways
1. Solar energy is re-radiated as long wave radiation from the saltwater, resulting in back radiation from the sea surface.
2. Heat is exchanged between the sea and the atmosphere, but only if the sea water is colder or warmer than the air.
3. When seawater is warm and air stratification is unstable, evaporation occurs.
Factors Affecting Distribution Of Temperature In Oceans
The temperature distribution of ocean water is influenced by the following factors:
1. Latitudes: Because the sun's rays get increasingly slanted as they go from the equator to the poles, the temperature of surface water lowers. As a result, the amount of insolation decreases poleward.
• Surface water temperature is lower than air temperature between 40°N and 40°S, but it rises above air temperature between 40° latitude and the poles in both hemispheres.
2. Unequal distribution of land and water: Ocean water temperature differs in the northern and southern hemispheres due to land dominance in the former and water dominance in the latter.
• Because the oceans in the northern hemisphere are in contact with a wider area of land, they get more heat than their counterparts in the southern hemisphere, and so the temperature of surface water in the former is higher than in the latter.
3. Prevailing wind: The direction of the wind has a significant impact on the temperature distribution of ocean water.
• Warm surface water is driven away from the coast by winds flowing from the land towards the oceans and seas (e.g., offshore winds), resulting in upwelling of chilly bottom water from below. As a result, when warm water is replaced with cold water, temperature difference occurs along the length of the pipe.
• On the contrary, onshore winds build up warm water near the shoreline, raising the temperature.
• Trade winds, for example, cause low temperatures (in the tropics along the eastern margins of oceans or along the western coastal regions of continents) because they blow from the land towards the oceans, whereas they raise temperatures (in the western margins of oceans or along the eastern coastal regions of continents) due to their onshore position.
• Similarly, owing of the onshore position of the westerlies, the eastern margins of the oceans in the middle latitudes (western coasts of Europe and North America) have a warmer temperature than the western shores.
4. Ocean currents: Warm and cold currents regulate the temperature of the ocean's surface. Warm currents boost the temperature of the affected areas, whereas cool currents bring it down.
• The Gulf Stream, for example, elevates the temperature near the eastern and western coasts of North America and Europe.
• Kuroshiyo pulls warm water away from Asia's eastern coast, raising temperatures in Alaska. The temperature near the north-east coast of North America is lowered by the Labrador cool current.
• Similarly, the Kurile cool current causes the temperature on Siberia's eastern shore to drop.
• Warm currents boost temperature more in the northern hemisphere than in the southern hemisphere, as evidenced by the fact that the 5°C isotherm in the northern Atlantic Ocean reaches 70° latitude, whereas it only reaches 50° latitude in the southern Atlantic Ocean. This is due to the warmer Brazil current's more prominent impacts in the southern Atlantic Ocean.
5. Minor Factors: Minor factors include:
• Submarine ridges, Storms, cyclones, hurricanes, fog, cloudiness, evaporation, and condensation are examples of local weather conditions.
• The sea's location and shape.
Vertical Distribution Of Temperature
• As the Mediterranean Sea recorded higher temperatures than the Gulf of California, longitudinally more widespread seas in low latitudes have higher temperatures than latitudinal more extensive seas. In low latitudes, contained seas have a higher temperature than open seas, whereas in high latitudes, enclosed seas have a lower temperature than open seas (Baltic Sea has 0°C (32°F) and open seas have 4.4°C or 40°F).
• As the descent progresses, the temperature gradually drops. 90% of the solar heat is normally absorbed in the top 15.6 m (60 feet) of water. Only up to a depth of roughly 100 m does sea water temperature match to surface temperature, and as depth increases, temperature normally declines.
• From the surface to the bottom of tropical oceans and seas, three layers can be distinguished. The first layer is around 500 meters deep and has a temperature range of 20 to 25 degrees Celsius. This top layer is only present in mid-latitude locations during the summer. Just beneath the first layer lies the thermocline layer.
• It is distinguished by a rapid drop in temperature as depth increases. The third layer is extremely cold and extends all the way down to the ocean floor.
• In contrast to tropical oceans, only one layer of frigid water may be found in the Polar Regions. It stretches from the top to the bottom.
• Because the temperature of water drops with depth, some scientists have separated the oceans into two zones:
a. Photic or euphotic zone, which stretches from the upper surface to 200 m and receives sufficient sun insolation;
b. Aphotic zone spanning from 200 meters to the ocean's bottom; this zone receives insufficient sunlight.