Category: Ocean Floor Relief and Oceanic Oozes


Seventh Standard

Subject:-Geography

Topic:- Ocean Floor Relief & Oceanic Oozes

Subtopic:-Oceanic Sediment

Source:- Wikipedia

 

Oceanic sediment

                                    Marine sediment, any deposit of insoluble material, primarily rock and soil particles, transported from land areas to the ocean by wind, ice, and rivers, as well as the remains of marine organisms, products of submarine volcanism, chemical precipitates from seawater, and materials from outer space (e.g., meteorites) that accumulate on the seafloor. Although systematic study of deep-ocean sediments began with the HMS Challenger  expeditions between 1872 and 1876, intensive research was not undertaken until nearly 100 years later. Since 1968 American scientists, in collaboration with those from the United Kingdom, the Soviet Union, and various other countries, have recovered numerous sedimentary core samples from the Atlantic and Pacific oceans through the use of a specially instrumented deep-sea drilling vessel called the Glomar Challenger.

Marine sediments deposited near continents cover approximately 25 percent of the seafloor, but they probably account for roughly 90 percent by volume of all sediment deposits. Submarine canyons constitute the main route for sediment movement from continental shelves and slopes onto the deep seafloor. In most cases, an earthquake triggers a massive slumping and stirring of sedimentary material at the canyon head. Mixed with seawater, a dense liquid mass forms, giving rise to a density current that flows down the canyon at speeds of several tens of kilometres per hour. After reaching the base of the continental slope, the sediment-laden mass moves out onto the continental rise at the base of the slope. Deposits from turbidity currents (i.e., short-lived density currents caused by suspended sediment concentrations) can build outward for hundreds and sometimes thousands of kilometres across the ocean bottom. Large sediment-built plains commonly occur in the Atlantic Ocean, where turbidity currents flow from the base of a continent to the Mid-Atlantic Ridge.

Deposits produced by turbidity currents are called turbidites. Most of them consist of sands and silts, but a few are composed of gravels. Turbidites tend to have distinct boundaries between adjacent units. Each of these units is formed by a separate flow and often exhibits a systematic change in grain size from coarsest at the bottom to finest at the top. Turbidites characteristically contain the remains of shallow-water organisms mixed with deep-water varieties. The shallow-water organisms came from areas where the density current originated, whereas the deep-water forms existed in the area traversed by the current or where it finally deposited its load.

The sediments deposited on continental shelves and rises, frequently referred to as hemi pelagic sediments, ordinarily accumulate too rapidly to react chemically with seawater. In most cases, individual grains thus retain characteristics imparted to them in the area where they formed. As a rule, sediments deposited near coral reefs in shallow tropical waters contain abundant carbonate material. Calcareous, reef-derived muds, for example, occur around atolls at the north western end of the Hawaiian Island chain. Near volcanoes, sediments contain ash—e.g., silicate glass and fine volcanic-rock fragments.

Roughly 75 percent of the deep seafloor is covered by slowly accumulating deposits known as pelagic. Because of its great distance from the continents, the abyssal plain does not receive turbidity currents and their associated coarse-grained sediments. Moreover, since relatively little land-derived sediment consisting of silicate mineral and rock fragments reach the ocean bottom, deposits there show a predominance of biogenic constituents (i.e., the skeletal remains of marine organisms). In areas where surface waters are fertile, opal from diatoms (algae) and radiolarians (protozoan) and calcium carbonate from such organisms as foraminiferans, coccolithophorids, and pteropods are supplied to the sediment. If the biological constituents exceed 30 percent by volume, then the deep-ocean sediments are usually classified on the basis of their biogenic components. For example, a mud containing 30 percent by volume of foraminiferal tests (external hard parts) is called a foraminiferal mud or Ooze. When one genus dominates, it is frequently referred to by the generic name, such as Globigerina ooze. Diatomaceous and radiolarian muds are named on the same basis. Where biogenic constituents compose less than 30 percent of the total, the deposit is called a deep-sea clay, brown mud, or red clay.

Types of Marine Sediments                                                                                                                                            There are four basic types of marine sediments, all of which are grouped and ordered by the origin of their particles, the grain sizes, and where they are deposited. These four kinds include lithogenous, biogenous, hydrogenous, and cosmogenous. All of these are different from one another in some way but all share in common the tendency to collect along the floor of the oceans as a testament to many natural processes such as weathering, erosion, and collision.

1] Lithogenous sediments:- These are formed by the weathering process and are made up of small particles of weathered rocks and oceanic volcanoes. They are often formed together when metal and silicate ions bond. There are two types of lithogenous sediments; terrigenous and “red clay” and they are different because of the process behind their existences. For instance, terrigenous sediments are produced as a result of the weathering process of rocks above the water. These eroded particles are carried by the wind and other natural means to the oceans and are deposited at the bottom. Although it can be easily found in river beds, not much of this finds its way to the deep ocean. Red clay lithogenous sediment, on the other hand, is plentiful in the ocean. It is reddish-brown (hence the name) and is a combination of terrigenous material and volcanic ash. It is transported to the oceans by currents and wind and it settles in deep places along the ocean floor.

 2] Biogenous sediments:- They are formed from the insoluble remains of past life forms and parts such as bones and teeth. In many areas where the water is shallow, a majority of these sediments are the remains of shells or fragments from shelled sea creatures as well as corals. In the deep sea where there is no such a high concentration of these life forms, biogenous sediment is made from the microscopic shells that are deposited by tiny plants, animals, and plankton that live on the water’s surface and eventually make their way down to the ocean floor.

3] Hydrogenous sediments:- It is formed by precipitation of minerals from the ocean’s water or can be formed as a new mineral as a result of chemical reactions between the water of the ocean and sediments that already exist on the ocean floor. Chemically speaking, this is an interesting sedimentary process because of the reactions that take place. For instance, the water of earth’s oceans contains ions that have already been dissolved. When evaporation occurs and large amounts of these ions remain the area can become saturated with the leftovers from this process, salt.

4] Cosmogenous sediments:- They are extraterrestrial in nature and are generally like miniature meteorites. These sediments are the remains of impacts of large bodies of space material (such as comets and asteroids). They are comprised of silicates and mixtures of different metals and, as one might imagine, they are not incredibly common to find. This is rather surprising because there is a constant “rain of these materials that falls to earth daily.  The amounts of such sediments also leads researchers to wonder if these space-driven events might have been responsible for mass extinction and thus these sediments hold several possible keys to future understanding of ancient life on earth.

 

Seventh Standard

Subject:-Geography

Topic:- Ocean Floor Relief & Oceanic Oozes

Subtopic:-Continental Shelf

Source:- Wikipedia

 

Continental shelf

                                 The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas (known as shelf seas) and gulfs, and was almost similarly submerged during other interglacial periods.

The continental margin, between the continental shelf and the abyssal plain, comprises a steep continental slope followed by the flatter continental rise.  Sediment from the continent above cascades down the slope and accumulates as a pile of sediment at the base of the slope, called the continental rise. Extending as far as 500 km from the slope, it consists of thick sediments deposited by turbidity currents from the shelf and slope. The continental rise’s gradient is intermediate between the slope and the shelf, on the order of 0.5-1°.

Under the United Nations Convention on the Law of the Sea, the name continental shelf was given a legal definition as the stretch of the sea bed adjacent to the shores of a particular country to which it belongs. Such shores are also known as territorial waters.

Geographical distribution

The width of the continental shelf varies considerably – it is not uncommon for an area to have virtually no shelf at all, particularly where the forward edge of an advancing oceanic plate dives beneath continental crust in an off shoresubduction zone such as off the coast of Chile or the west coast of Sumatra. The largest shelf – the Siberian Shelf in the Arctic Ocean – stretches to 1,500 kilometers (930 mi) in width. The South China Sea lies over another extensive area of continental shelf, the Sunda Shelf, which joins Borneo, Sumatra, and Java to the Asian mainland. Other familiar bodies of water that overlie continental shelves are the North Sea and the Persian Gulf. The average width of continental shelves is about 80 km (50 mi). The depth of the shelf also varies, but is generally limited to water shallower than 150 m (490 ft). The slope of the shelf is usually quite low, on the order of 0.5°; vertical relief is also minimal, at less than 20 m (66 ft).

Though the continental shelf is treated as a physiographic province of the ocean, it is not part of the deep ocean basin proper, but the flooded margins of the continent. Passive continental margins such as most of the Atlantic coasts have wide and shallow shelves, made of thick sedimentary wedges derived from long erosion of a neighboring continent. Active continental margins have narrow, relatively steep shelves, due to frequent earthquakes that move sediment to the deep sea.

Topography

The shelf usually ends at a point of increasing slope (called the shelf break). The sea floor below the break is the continental slope. Below the slope is the continental rise, which finally merges into the deep ocean floor, the abyssal plain. The continental shelf and the slope are part of the continental margin.The shelf area is commonly subdivided into the inner continental shelf, mid continental shelf, and outer continental shelf, each with their specific geomorphology and marine biology.The character of the shelf changes dramatically at the shelf break, where the continental slope begins. With a few exceptions, the shelf break is located at a remarkably uniform depth of roughly 140 m (460 ft); this is likely a hallmark of past ice ages, when sea level was lower than it is now. The continental slope is much steeper than the shelf; the average angle is 3°, but it can be as low as 1° or as high as 10°.[9] The slope is often cut with submarine canyons. The physical mechanisms involved in forming these canyons were not well understood until the 1960s.

Sediments

The continental shelves are covered by terrigenous sediments; that is, those derived from erosion of the continents. However, little of the sediment is from current rivers; some 60-70% of the sediment on the world’s shelves is relict sediment, deposited during the last ice age, when sea level was 100–120 m lower than it is now.

Sediments usually become increasingly fine with distance from the coast; sand is limited to shallow, wave-agitated waters, while silt and clays are deposited in quieter, deep water far offshore. These shelf sediments accumulate at an average rate of 30 cm/1000 years, with a range from 15–40 cm. Though slow by human standards, this rate is much faster than that for deep-seapelagic sediments.

Biota

Continental shelves teem with life, because of the sunlight available in shallow waters, in contrast to the biotic desert of the oceans’ abyssal plain. The pelagic (water column) environment of the continental shelf constitutes the neritic zone, and the benthic (sea floor) province of the shelf is the sublittoral zone.

Though the shelves are usually fertile, if anoxic conditions prevail during sedimentation, the deposits may over geologic time become sources for fossil fuels.

Economic significance

The relatively accessible continental shelf is the best understood part of the ocean floor. Most commercial exploitation from the sea, such as metallic-ore, non-metallic ore, and hydrocarbon extraction, takes place on the continental shelf. Sovereign rights over their continental shelves up to a depth of 200 metres or to a distance where the depth of waters admitted of resource exploitation were claimed by the marine nations that signed the Convention on the Continental Shelf drawn up by the UN’s International Law Commission in 1958. This was partly superseded by the 1982 United Nations Convention on the Law of the Sea. which created the 200 nautical mile exclusive economic zone and extended continental shelf rights for states with physical continental shelves that extend beyond that distance.

The legal definition of a continental shelf differs significantly from the geological definition. UNCLOS states that the shelf extends to the limit of the continental margin, but no less than 200 nautical miles from the baseline. Thus inhabited volcanic islands such as the Canaries, which have no actual continental shelf, nonetheless have a legal continental shelf, whereas uninhabitable islands have no shelf.

 

Seventh Standard

Subject:-Geography

Topic:- Ocean Floor Relief & Oceanic Oozes

Subtopic:-Abyssal Plain

Source:- Wikipedia

 

 

Abyssal Plain

                               An abyssal plain is an underwater plain on the deep ocean floor, usually found at depths between 3000 and 6000 metres. Lying generally between the foot of a continental rise and a mid-ocean ridge, abyssal plains cover more than 50% of the Earth’s surface. They are among the flattest, smoothest and least explored regions on Earth. Abyssal plains are key geologic elements of oceanic basins (the other elements being an elevated mid-ocean ridge and flanking abyssal hills).

Abyssal plains were not recognized as distinct physiographic features of the sea floor until the late 1940s and, until very recently, none had been studied on a systematic basis. They are poorly preserved in the sedimentary record, because they tend to be consumed by the sub duction process. The creation of the abyssal plain is the end result of spreading of the seafloor (plate tectonics) and melting of the lower oceanic crust. Abyssal plains result from the blanketing of an originally uneven surface of oceanic crust by fine-grained sediments, mainly clay and silt. Much of this sediment is deposited by turbidity currents that have been channelled from the continental margins along submarine canyons down into deeper water. The remainder of the sediment is composed chiefly of pelagic sediments.

Owing in part to their vast size, abyssal plains are currently believed to be a major reservoir of biodiversity. The abyss also exerts significant influence upon ocean carbon cycling, dissolution of calcium carbonate and atmospheric CO2 concentrations over timescales of 100–1000 years. The structure and function of abyssal ecosystems are strongly influenced by the rate of flux of food to the seafloor and the composition of the material that settles. Factors such as climate change, fishing practices and ocean fertilization are expected to have a substantial effect on patterns of primary production in the euphotic zone. This will undoubtedly impact the flux of organic material to the abyss in a similar manner and thus have a profound effect on the structure, function and diversity of abyssal ecosystems.

Formation

Oceanic crust, which forms the bedrock of abyssal plains, is continuously being created at mid-ocean ridges (a type of divergent boundary) by a process known as decompression melting. Plume-related decompression melting of solid mantle is responsible for creating ocean islands like the Hawaiian islands, as well as the ocean crust at mid-ocean ridges. This phenomenon is also the most common explanation for flood basalts and oceanic plateaus (two types of large igneous provinces). Accretion occurs as mantle is added to the growing edges of a tectonic plate, usually associated with seafloor spreading. The age of oceanic crust is therefore a function of distance from the mid-ocean ridge. The youngest oceanic crust is at the mid-ocean ridges, and it becomes progressively older, cooler and denser as it migrates outwards from the mid-ocean ridges as part of the process called mantle convection.

Oceanic crust and tectonic plates are formed and move apart at mid-ocean ridges. Abyssal hills are formed by stretching of the oceanic lithosphere. Consumption or destruction of the oceanic lithosphere occurs at oceanic trenches (a type of convergent boundary, also known as a destructive plate boundary) by a process known as subduction. Oceanic trenches are found at places where the oceanic lithospheric slabs of two different plates meet, and the denser (older) slab begins to descend back into the mantle. At the consumption edge of the plate (the oceanic trench), the oceanic lithosphere has thermally contracted to become quite dense, and it sinks under its own weight in the process of subduction. The subduction process consumes older oceanic lithosphere, so oceanic crust is seldom more than 200 million years old. The overall process of repeated cycles of creation and destruction of oceanic crust is known as the Supercontinent cycle, first proposed by Canadian geophysicistand geologist John Tuzo Wilson.

The flat appearance of mature abyssal plains results from the blanketing of originally uneven surface of oceanic crust by fine-grained sediments, mainly clay and silt. Much of this sediment is deposited from turbidity currents that have been channeled from the continental margins along submarine canyons down into deeper water. The remainder of the sediment comprises chiefly dust (clay particles) blown out to sea from land, and the remains of small marine plants and animals which sink from the upper layer of the ocean, known as pelagic sediments. The total sediment deposition rate in remote areas is estimated at two to three centimeters per thousand years. Sediment-covered abyssal plains are less common in the Pacific Ocean than in other major ocean basins because sediments from turbidity currents are trapped in oceanic trenches that border the Pacific Ocean. During parts of the Messinian salinity crisis much of the Mediterranean Sea’s abyssal plain was an empty hot dry salt-floored sink.

Biodiversity

Though the plains were once  assumed to be vast, desert-like habitats, research over the past decade or so shows that they teem with a wide variety of microbial life. However, ecosystem structure and function at the deep seafloor have historically been very poorly studied because of the size and remoteness of the abyss (अत्यंत खोल विवर). Recent oceanographic expeditions conducted by an international group of scientists from the Census of Diversity of Abyssal Marine Life have found an extremely high level of biodiversity on abyssal plains, with up to 2000 species of bacteria, 250 species of protozoan, and 500 species of invertebrates  (worms, crustaceans and molluscs), typically found at single abyssal sites. New species make up more than 80% of the thousands of seafloor invertebrate species collected at any abyssal station, highlighting our heretofore poor understanding of abyssal diversity and evolution. Richer biodiversity is associated with areas of known phyto detritus input and higher organic carbon flux.

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