Buoyancy or upthrust, is an upward force exerted by a fluid that opposes the weight of an immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid, thus the pressure at the bottom of a column of fluid is greater than at the top of the column. The pressure at the bottom of an object submerged in a fluid is greater than at the top of the object; the pressure difference results in a net upward force on the object. The magnitude of the force is proportional to the pressure difference, is equivalent to the weight of the fluid that would otherwise occupy the volume of the object, i.e. the displaced fluid. For this reason, an object whose average density is greater than that of the fluid in which it is submerged tends to sink. If the object is less dense than the liquid, the force can keep the object afloat; this can occur only in a non-inertial reference frame, which either has a gravitational field or is accelerating due to a force other than gravity defining a "downward" direction.
The center of buoyancy of an object is the centroid of the displaced volume of fluid. Archimedes' principle is named after Archimedes of Syracuse, who first discovered this law in 212 B. C. For objects and sunken, in gases as well as liquids, Archimedes' principle may be stated thus in terms of forces: Any object, wholly or immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object — with the clarifications that for a sunken object the volume of displaced fluid is the volume of the object, for a floating object on a liquid, the weight of the displaced liquid is the weight of the object. More tersely: buoyancy = weight of displaced fluid. Archimedes' principle does not consider the surface tension acting on the body, but this additional force modifies only the amount of fluid displaced and the spatial distribution of the displacement, so the principle that buoyancy = weight of displaced fluid remains valid; the weight of the displaced fluid is directly proportional to the volume of the displaced fluid.
In simple terms, the principle states that the buoyancy force on an object is equal to the weight of the fluid displaced by the object, or the density of the fluid multiplied by the submerged volume times the gravitational acceleration, g. Thus, among submerged objects with equal masses, objects with greater volume have greater buoyancy; this is known as upthrust. Suppose a rock's weight is measured as 10 newtons when suspended by a string in a vacuum with gravity acting upon it. Suppose that when the rock is lowered into water, it displaces water of weight 3 newtons; the force it exerts on the string from which it hangs would be 10 newtons minus the 3 newtons of buoyancy force: 10 − 3 = 7 newtons. Buoyancy reduces the apparent weight of objects that have sunk to the sea floor, it is easier to lift an object up through the water than it is to pull it out of the water. Assuming Archimedes' principle to be reformulated as follows, apparent immersed weight = weight − weight of displaced fluid inserted into the quotient of weights, expanded by the mutual volume density density of fluid = weight weight of displaced fluid, yields the formula below.
The density of the immersed object relative to the density of the fluid can be calculated without measuring any volumes.: density of object density of fluid = weight weight − apparent immersed weight Example: If you drop wood into water, buoyancy will keep it afloat. Example: A helium balloon in a moving car. During a period of increasing speed, the air mass inside the car moves in the direction opposite to the car's acceleration; the balloon is pulled this way. However, because the balloon is buoyant relative to the air, it ends up being pushed "out of the way", will drift in the same direction as the car's acceleration. If the car slows down, the same balloon will begin to drift backward. For the same reason, as the car goes round a curve, the balloon will drift towards the inside of the curve; the equation to calculate the pressure inside a fluid in equilibrium is: f + div σ = 0 where f is the force density exerted by some outer field on the fluid, σ is the Cauchy stress tensor. In this case the stress tensor is proportional to the identity tensor: σ i j = − p δ i j.
Here δij is the Kronecker delta. Using this the above equation becomes: f = ∇ p. Assuming the outer force field is conservative, it can be written as the negative gradient of some scalar valued function: f = − ∇
The Cretaceous is a geologic period and system that spans 79 million years from the end of the Jurassic Period 145 million years ago to the beginning of the Paleogene Period 66 mya. It is the last period of the Mesozoic Era, the longest period of the Phanerozoic Eon; the Cretaceous Period is abbreviated K, for its German translation Kreide. The Cretaceous was a period with a warm climate, resulting in high eustatic sea levels that created numerous shallow inland seas; these oceans and seas were populated with now-extinct marine reptiles and rudists, while dinosaurs continued to dominate on land. During this time, new groups of mammals and birds, as well as flowering plants, appeared; the Cretaceous ended with the Cretaceous–Paleogene extinction event, a large mass extinction in which many groups, including non-avian dinosaurs and large marine reptiles died out. The end of the Cretaceous is defined by the abrupt Cretaceous–Paleogene boundary, a geologic signature associated with the mass extinction which lies between the Mesozoic and Cenozoic eras.
The Cretaceous as a separate period was first defined by Belgian geologist Jean d'Omalius d'Halloy in 1822, using strata in the Paris Basin and named for the extensive beds of chalk, found in the upper Cretaceous of Western Europe. The name Cretaceous was derived from Latin creta; the Cretaceous is divided into Early and Late Cretaceous epochs, or Lower and Upper Cretaceous series. In older literature the Cretaceous is sometimes divided into three series: Neocomian and Senonian. A subdivision in eleven stages, all originating from European stratigraphy, is now used worldwide. In many parts of the world, alternative local subdivisions are still in use; as with other older geologic periods, the rock beds of the Cretaceous are well identified but the exact age of the system's base is uncertain by a few million years. No great extinction or burst of diversity separates the Cretaceous from the Jurassic. However, the top of the system is defined, being placed at an iridium-rich layer found worldwide, believed to be associated with the Chicxulub impact crater, with its boundaries circumscribing parts of the Yucatán Peninsula and into the Gulf of Mexico.
This layer has been dated at 66.043 Ma. A 140 Ma age for the Jurassic-Cretaceous boundary instead of the accepted 145 Ma was proposed in 2014 based on a stratigraphic study of Vaca Muerta Formation in Neuquén Basin, Argentina. Víctor Ramos, one of the authors of the study proposing the 140 Ma boundary age sees the study as a "first step" toward formally changing the age in the International Union of Geological Sciences. From youngest to oldest, the subdivisions of the Cretaceous period are: Late Cretaceous Maastrichtian – Campanian – Santonian – Coniacian – Turonian – Cenomanian – Early Cretaceous Albian – Aptian – Barremian – Hauterivian – Valanginian – Berriasian – The high sea level and warm climate of the Cretaceous meant large areas of the continents were covered by warm, shallow seas, providing habitat for many marine organisms; the Cretaceous was named for the extensive chalk deposits of this age in Europe, but in many parts of the world, the deposits from the Cretaceous are of marine limestone, a rock type, formed under warm, shallow marine circumstances.
Due to the high sea level, there was extensive space for such sedimentation. Because of the young age and great thickness of the system, Cretaceous rocks are evident in many areas worldwide. Chalk is a rock type characteristic for the Cretaceous, it consists of coccoliths, microscopically small calcite skeletons of coccolithophores, a type of algae that prospered in the Cretaceous seas. In northwestern Europe, chalk deposits from the Upper Cretaceous are characteristic for the Chalk Group, which forms the white cliffs of Dover on the south coast of England and similar cliffs on the French Normandian coast; the group is found in England, northern France, the low countries, northern Germany, Denmark and in the subsurface of the southern part of the North Sea. Chalk is not consolidated and the Chalk Group still consists of loose sediments in many places; the group has other limestones and arenites. Among the fossils it contains are sea urchins, belemnites and sea reptiles such as Mosasaurus. In southern Europe, the Cretaceous is a marine system consisting of competent limestone beds or incompetent marls.
Because the Alpine mountain chains did not yet exist in the Cretaceous, these deposits formed on the southern edge of the European continental shelf, at the margin of the Tethys Ocean. Stagnation of deep sea currents in middle Cretaceous times caused anoxic conditions in the sea water leaving the deposited organic matter undecomposed. Half the worlds petroleum reserves were laid down at this time in the anoxic conditions of what would become the Persian Gulf and the Gulf of Mexico. In many places around the world, dark anoxic shales were formed during this interval; these shales are an important source rock for oil and gas, for example in the subsurface of the North Sea. During th
The Nautilaceae form one of five superfamilies that make up the Nautilida according to Bernard Kummel, the only one that survived past the Triassic. The Nautilaceae comprise six families: Nautilidae, Paracenoceratidae, Cymatoceratidae and Aturiidae. Shimanskiy separated the Paracenoceratidae and Pseudonautilidae from his near equivalent Nautilina and added them to the Lyroceratina, expanding the equivalent Clydonautilaceae and bringing it into the Jurassic; the Nautilaceae are represented by Nautilus and Allonautilus, genera included in the Nautilidae. Species in the Nautilaceae are smooth and involute with straight to sinuous sutures and a small siphuncle; some groups have sinuous plications or ribs The Nautilaceae began in the Late Triassic with Cenoceras, a golublar to discoidal genus derived from the Syringonautilidae and from Syringonautilus. Cenoceras, the earliest member of the Nautilaceae and Nautilidae, is the only nautiloid known to have crossed the upper Triassic boundary and the only one known from the Lower Jurassic All six families of the Nautilaceae, except for the Aturiidae, are derived from the Cenoceras complex in the Middle Jurassic or from Eutrephoceras which followed.
The Cenozoic Aturia seems sufficiently derived to warrant familial distinction from its source, the Hercoglossidae. Kummel B. 1964. Nautiloidea-Nautilida.
Mollusca is the second largest phylum of invertebrate animals. The members are known as mollusks. Around 85,000 extant species of molluscs are recognized; the number of fossil species is estimated between 100,000 additional species. Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms. Numerous molluscs live in freshwater and terrestrial habitats, they are diverse, not just in size and in anatomical structure, but in behaviour and in habitat. The phylum is divided into 8 or 9 taxonomic classes, of which two are extinct. Cephalopod molluscs, such as squid and octopus, are among the most neurologically advanced of all invertebrates—and either the giant squid or the colossal squid is the largest known invertebrate species; the gastropods are by far the most numerous molluscs and account for 80% of the total classified species. The three most universal features defining modern molluscs are a mantle with a significant cavity used for breathing and excretion, the presence of a radula, the structure of the nervous system.
Other than these common elements, molluscs express great morphological diversity, so many textbooks base their descriptions on a "hypothetical ancestral mollusc". This has a single, "limpet-like" shell on top, made of proteins and chitin reinforced with calcium carbonate, is secreted by a mantle covering the whole upper surface; the underside of the animal consists of a single muscular "foot". Although molluscs are coelomates, the coelom tends to be small; the main body cavity is a hemocoel. The "generalized" mollusc's feeding system consists of a rasping "tongue", the radula, a complex digestive system in which exuded mucus and microscopic, muscle-powered "hairs" called cilia play various important roles; the generalized mollusc has three in bivalves. The brain, in species that have one, encircles the esophagus. Most molluscs have eyes, all have sensors to detect chemicals and touch; the simplest type of molluscan reproductive system relies on external fertilization, but more complex variations occur.
All produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults. The coelomic cavity is reduced, they have kidney-like organs for excretion. Good evidence exists for the appearance of gastropods and bivalves in the Cambrian period, 541 to 485.4 million years ago. However, the evolutionary history both of molluscs' emergence from the ancestral Lophotrochozoa and of their diversification into the well-known living and fossil forms are still subjects of vigorous debate among scientists. Molluscs still are an important food source for anatomically modern humans. There is a risk of food poisoning from toxins which can accumulate in certain molluscs under specific conditions and because of this, many countries have regulations to reduce this risk. Molluscs have, for centuries been the source of important luxury goods, notably pearls, mother of pearl, Tyrian purple dye, sea silk, their shells have been used as money in some preindustrial societies. Mollusc species can represent hazards or pests for human activities.
The bite of the blue-ringed octopus is fatal, that of Octopus apollyon causes inflammation that can last for over a month. Stings from a few species of large tropical cone shells can kill, but their sophisticated, though produced, venoms have become important tools in neurological research. Schistosomiasis is transmitted to humans via water snail hosts, affects about 200 million people. Snails and slugs can be serious agricultural pests, accidental or deliberate introduction of some snail species into new environments has damaged some ecosystems; the words mollusc and mollusk are both derived from the French mollusque, which originated from the Latin molluscus, from mollis, soft. Molluscus was itself an adaptation of Aristotle's τὰ μαλάκια ta malákia, which he applied inter alia to cuttlefish; the scientific study of molluscs is accordingly called malacology. The name Molluscoida was used to denote a division of the animal kingdom containing the brachiopods and tunicates, the members of the three groups having been supposed to somewhat resemble the molluscs.
As it is now known these groups have no relation to molluscs, little to one another, the name Molluscoida has been abandoned. The most universal features of the body structure of molluscs are a mantle with a significant cavity used for breathing and excretion, the organization of the nervous system. Many have a calcareous shell. Molluscs have developed such a varied range of body structures, it is difficult to find synapomorphies to apply to all modern groups; the most general characteristic of molluscs is they are bilaterally symmetrical. The following are present in all modern molluscs: The dorsal part of the body wall is a mantle which secretes calcareous spicules, plates or shells, it overlaps the body with enough spare room to form a mantle cavity. The anus and genitals open into the mantle cavity. There are two pairs of main nerve cords. Other characteristics that appear in textbooks have significant exceptions: Estimates of accepted described living species of molluscs vary from 50,000 to a maximum of 120,000 species.
In 1969 David Nicol estimated the probable total number of living mollusc species at 107,000 of which were ab
Australasia comprises Australia, New Zealand, some neighbouring islands. It is used in a number of different contexts including geopolitically, physiographically, ecologically where the term covers several different but related regions. Charles de Brosses coined the term in Histoire des navigations, he derived it from the Latin for "south of Asia" and differentiated the area from Polynesia and the southeast Pacific. In Australia "Australasia" is considered to be Australia, New Zealand, New Guinea, the neighbouring islands of the Pacific, while in New Zealand it means Australia, New Zealand and former New Zealand dependencies. Richards, Kel. "Australasia". Wordwatch. ABC News Radio. Retrieved 2006-09-30. Media related to Australasia at Wikimedia Commons
North Africa is a region encompassing the northern portion of the African continent. There is no singularly accepted scope for the region, it is sometimes defined as stretching from the Atlantic shores of Morocco in the west, to Egypt's Suez Canal and the Red Sea in the east. Others have limited it to top North-Western countries like Algeria and Tunisia, a region, known by the French during colonial times as "Afrique du Nord" and is known by all Arabs as the Maghreb; the most accepted definition includes Algeria, Morocco, Tunisia and Egypt, the 6 countries that shape the top North of the African continent. Meanwhile, "North Africa" when used in the term North Africa and the Middle East refers only to the countries of the Maghreb and Libya. Egypt, being part of the Middle East, is considered separately, due to being both North African and Middle Eastern at the same time. North Africa includes a number of Spanish and Portuguese possessions, Plazas de soberanía, Ceuta and Melilla and the Canary Islands and Madeira.
The countries of North Africa share a common ethnic and linguistic identity, unique to this region. Northwest Africa has been inhabited by Berbers since the beginning of recorded history, while the eastern part of North Africa has been home to the Egyptians. Between the A. D. 600s and 1000s, Arabs from the Middle East swept across the region in a wave of Muslim conquest. These peoples, physically quite similar, formed a single population in many areas, as Berbers and Egyptians merged into Arabic and Muslim culture; this process of Arabization and Islamization has defined the cultural landscape of North Africa since. The distinction between North Africa, the Sahel and the rest of the continent is as follows: Nineteenth century European explorers, attracted by the accounts of Ancient geographers or Arab geographers of the classical period, followed the routes by the nomadic people of the vast "empty" space, they documented the names of the stopping places they discovered or rediscovered, described landscapes, took a few climate measurements and gathered rock samples.
A map began to fill in the white blotch. The Sahara and the Sahel entered the geographic corpus by way of naturalist explorers because aridity is the feature that circumscribes the boundaries of the ecumene; the map details included topographical relief and location of watering holes crucial to long crossings. The Arabic word "Sahel" and "Sahara" made its entry into the vocabulary of geography. Latitudinally, the "slopes" of the arid desert, devoid of continuous human habitation, descend in step-like fashion toward the northern and southern edges of the Mediterranean that opens to Europe and the Sahel that opens to "Trab al Sudan." Longitudinally, a uniform grid divides the central desert shrinks back toward the Atlantic Ocean and the Red Sea. The Sahara-Sahel is further divided into a total of twenty sub-areas: central, southern, eastern, etc. In this way, "standard" geography has determined aridity to be the boundary of the ecumene, it identifies settlements based on visible activity without regard for social or political organizations of space in vast, purportedly “empty” areas.
It gives only cursory acknowledgement to what makes Saharan geography, for that matter, world geography unique: mobility and the routes by which it flows. The Sahel or "African Transition Zone" has been affected by many formative epochs in North African history ranging from Ottoman occupation to the Arab-Berber control of the Andalus; as a result, many modern African nation-states that are included in the Sahel evidence cultural similarities and historical overlap with their North African neighbours. In the present day, North Africa is associated with West Asia in the realm of geopolitics to form a Middle East-North Africa region; the Islamic influence in the area is significant and North Africa is a major part of the Muslim world. Some researchers have postulated that North Africa rather than East Africa served as the exit point for the modern humans who first trekked out of the continent in the Out of Africa migration. North Africa has three main geographic features: the Sahara desert in the south, the Atlas Mountains in the west, the Nile River and delta in the east.
The Atlas Mountains extend across much of northern Algeria and Tunisia. These mountains are part of the fold mountain system that runs through much of Southern Europe, they recede to the south and east, becoming a steppe landscape before meeting the Sahara desert, which covers more than 75 percent of the region. The tallest peaks are in the High Atlas range in south-central Morocco, which has many snow-capped peaks. South of the Atlas Mountains is the dry and barren expanse of the Sahara desert, the largest sand desert in the world. In places the desert is cut by irregular watercourses called wadis—streams that flow only after rainfalls but are dry; the Sahara's major landforms include large seas of sand that sometimes form into huge dunes. The Sahara covers the southern part of Algeria and Tunisia, most of Libya. Only two regions of Libya are outside the desert: Tripolitania in the northwest and Cyrenaica in the northeast. Most of Egypt is desert, with the exception of the Nile River and the irrigated land along its banks.
The Nile Valley forms a narrow fertile thread. Sheltered valleys in the Atlas Mountains, the Nile Valley and Delta, the Mediterranean coast are the main sources of fertile farming land. A wide variety of valuable crops including ce
A lagoon is a shallow body of water separated from a larger body of water by barrier islands or reefs. Lagoons are divided into coastal lagoons and atoll lagoons, they have been identified as occurring on mixed-sand and gravel coastlines. There is an overlap between bodies of water classified as coastal lagoons and bodies of water classified as estuaries. Lagoons are common coastal features around many parts of the world. Lagoons are shallow elongated bodies of water separated from a larger body of water by a shallow or exposed shoal, coral reef, or similar feature; some authorities include fresh water bodies in the definition of "lagoon", while others explicitly restrict "lagoon" to bodies of water with some degree of salinity. The distinction between "lagoon" and "estuary" varies between authorities. Richard A. Davis Jr. restricts "lagoon" to bodies of water with little or no fresh water inflow, little or no tidal flow, calls any bay that receives a regular flow of fresh water an "estuary". Davis does state that the terms "lagoon" and "estuary" are "often loosely applied in scientific literature."
Timothy M. Kusky characterizes lagoons as being elongated parallel to the coast, while estuaries are drowned river valleys, elongated perpendicular to the coast; when used within the context of a distinctive portion of coral reef ecosystems, the term "lagoon" is synonymous with the term "back reef" or "backreef", more used by coral reef scientists to refer to the same area. Coastal lagoons are classified as inland bodies of water. Many lagoons do not include "lagoon" in their common names. Albemarle and Pamlico sounds in North Carolina, Great South Bay between Long Island and the barrier beaches of Fire Island in New York, Isle of Wight Bay, which separates Ocean City, Maryland from the rest of Worcester County, Banana River in Florida, Lake Illawarra in New South Wales, Montrose Basin in Scotland, Broad Water in Wales have all been classified as lagoons, despite their names. In England, The Fleet at Chesil Beach has been described as a lagoon. In Latin America, the term laguna in Spanish, which lagoon translates to, may be used for a small fresh water lake in a similar way a creek is considered a small river.
However, sometimes it is popularly used to describe a full-sized lake, such as Laguna Catemaco in Mexico, the third largest lake by area in the country. The brackish water lagoon may be thus explicitly identified as a "coastal lagoon". In Portuguese the same usage is found: lagoa may be a body of shallow sea water, or a small freshwater lake not linked to the sea. Lagoon is derived from the Italian laguna, which refers to the waters around Venice, the Lagoon of Venice. Laguna is attested in English by at least 1612, had been Anglicized to "lagune" by 1673. In 1697 William Dampier referred to a "Lake of Salt water" on the coast of Mexico. Captain James Cook described an island "of Oval form with a Lagoon in the middle" in 1769. Atoll lagoons form as coral reefs grow upwards while the islands that the reefs surround subside, until only the reefs remain above sea level. Unlike the lagoons that form shoreward of fringing reefs, atoll lagoons contain some deep portions. Coastal lagoons form along sloping coasts where barrier islands or reefs can develop off-shore, the sea-level is rising relative to the land along the shore.
Coastal lagoons do not form along steep or rocky coasts, or if the range of tides is more than 4 metres. Due to the gentle slope of the coast, coastal lagoons are shallow, they are sensitive to changes in sea level due to global warming. A relative drop in sea level may leave a lagoon dry, while a rise in sea level may let the sea breach or destroy barrier islands, leave reefs too deep under water to protect the lagoon. Coastal lagoons are young and dynamic, may be short-lived in geological terms. Coastal lagoons are common. In the United States, lagoons are found along more than 75 percent of the Gulf coasts. Coastal lagoons are connected to the open ocean by inlets between barrier islands; the number and size of the inlets, precipitation and inflow of fresh water all affect the nature of the lagoon. Lagoons with little or no interchange with the open ocean, little or no inflow of fresh water, high evaporation rates, such as Lake St. Lucia, in South Africa, may become saline. Lagoons with no connection to the open ocean and significant inflow of fresh water, such as the Lake Worth Lagoon in Florida in the middle of the 19th century, may be fresh.
On the other hand, lagoons with many wide inlets, such as the Wadden Sea, have strong tidal currents and mixing. Coastal lagoons tend to accumulate sediments from inflowing rivers, from runoff from the shores of the lagoon, from sediment carried into the lagoon through inlets by the tide. Large quantities of sediment may be be deposited in a lagoon when storm waves overwash barrier islands. Mangroves and marsh plants can facilitate the accumulation of sediment in a lagoon. Benthic organisms may destabilize sediments. River-mouth lagoons on mixed sand and gravel beaches form at the river-coast interface where a braided, although sometimes meandering, river interacts with a coastal environment, affected by longshore drift; the lagoons which form on the MSG coastlines are common on the east coast of the South Island of New Zealand and have long been referred to as hapua by the Māori. This classification differentiates hapua from similar lagoons located on the N