Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and gradually moves away from the ridge. Earlier theories of continental drift postulated; the idea that the seafloor itself moves as it expands from a central axis was proposed by Harry Hess from Princeton University in the 1960s. The theory is well accepted now, the phenomenon is known to be caused by convection currents in the asthenosphere, ductile, or plastic, the brittle lithosphere. Seafloor spreading helps explain continental drift in the theory of plate tectonics; when oceanic plates diverge, tensional stress causes fractures to occur in the lithosphere. The motivating force for seafloor spreading ridges is tectonic plate pull rather than magma pressure, although there is significant magma activity at spreading ridges. At a spreading center, basaltic magma rises up the fractures and cools on the ocean floor to form new seabed. Hydrothermal vents are common at spreading centers.
Older rocks will be found farther away from the spreading zone while younger rocks will be found nearer to the spreading zone. Additionally spreading rates determine if the ridge is fast, intermediate, or slow; as a general rule, fast ridges have spreading rates of more than 9 cm/year. Intermediate ridges have a spreading rate of 5–9 cm/year while slow spreading ridges have a rate less than 5 cm/year. Seafloor spreading occurs at spreading centers, distributed along the crests of mid-ocean ridges. Spreading centers end in overlapping spreading center offsets. A spreading center includes a seismically active plate boundary zone a few kilometers to tens of kilometers wide, a crustal accretion zone within the boundary zone where the ocean crust is youngest, an instantaneous plate boundary - a line within the crustal accretion zone demarcating the two separating plates. Within the crustal accretion zone is a 1-2 km-wide neovolcanic zone. In the general case, seafloor spreading starts as a rift in a continental land mass, similar to the Red Sea-East Africa Rift System today.
The process starts by heating at the base of the continental crust which causes it to become more plastic and less dense. Because less dense objects rise in relation to denser objects, the area being heated becomes a broad dome; as the crust bows upward, fractures occur that grow into rifts. The typical rift system consists of three rift arms at 120-degree angles; these areas can be found in several places across the world today. The separated margins of the continents evolve to form passive margins. Hess' theory was that new seafloor is formed when magma is forced upward toward the surface at a mid-ocean ridge. If spreading continues past the incipient stage described above, two of the rift arms will open while the third arm stops opening and becomes a'failed rift'; as the two active rifts continue to open the continental crust is attenuated as far as it will stretch. At this point basaltic oceanic crust begins to form between the separating continental fragments; when one of the rifts opens into the existing ocean, the rift system is flooded with seawater and becomes a new sea.
The Red Sea is an example of a new arm of the sea. The East African rift was thought to be a "failed" arm, opening somewhat more than the other two arms, but in 2005 the Ethiopian Afar Geophysical Lithospheric Experiment reported that in the Afar region, September 2005, a 60 km fissure opened as wide as eight meters. During this period of initial flooding the new sea is sensitive to changes in eustasy; as a result, the new sea will evaporate several times before the elevation of the rift valley has been lowered to the point that the sea becomes stable. During this period of evaporation large evaporite deposits will be made in the rift valley; these deposits have the potential to become hydrocarbon seals and are of particular interest to petroleum geologists. Seafloor spreading can stop during the process, but if it continues to the point that the continent is severed a new ocean basin is created; the Red Sea has not yet split Arabia from Africa, but a similar feature can be found on the other side of Africa that has broken free.
South America once fit into the area of the Niger Delta. The Niger River has formed in the failed rift arm of the triple junction; as new seafloor forms and spreads apart from the mid-ocean ridge it cools over time. Older seafloor is, colder than new seafloor, older oceanic basins deeper than new oceanic basins due to isostasy. If the diameter of the earth remains constant despite the production of new crust, a mechanism must exist by which crust is destroyed; the destruction of oceanic crust occurs at subduction zones where oceanic crust is forced under either continental crust or oceanic crust. Today, the Atlantic basin is spreading at the Mid-Atlantic Ridge. Only a small portion of the oceanic crust produced in the Atlantic is subducted. However, the plates making up the Pacific Ocean are experiencing subduction along many of their boundaries which causes the volcanic activity in what has been termed the Ring of Fire of the Pacific Ocean; the Pacific is home to one of the world's most active spreading centers with spreading rates of up to 13 cm/yr.
The Mid-Atlantic Ridge is a "textbook" slow-spreading center, while the East Pacific Rise is used as an example of fast spreading. Spreading centers at slow and in
Asia is Earth's largest and most populous continent, located in the Eastern and Northern Hemispheres. It shares the continental landmass of Eurasia with the continent of Europe and the continental landmass of Afro-Eurasia with both Europe and Africa. Asia covers an area of 44,579,000 square kilometres, about 30% of Earth's total land area and 8.7% of the Earth's total surface area. The continent, which has long been home to the majority of the human population, was the site of many of the first civilizations. Asia is notable for not only its overall large size and population, but dense and large settlements, as well as vast populated regions, its 4.5 billion people constitute 60% of the world's population. In general terms, Asia is bounded on the east by the Pacific Ocean, on the south by the Indian Ocean, on the north by the Arctic Ocean; the border of Asia with Europe is a historical and cultural construct, as there is no clear physical and geographical separation between them. It has moved since its first conception in classical antiquity.
The division of Eurasia into two continents reflects East–West cultural and ethnic differences, some of which vary on a spectrum rather than with a sharp dividing line. The most accepted boundaries place Asia to the east of the Suez Canal separating it from Africa. China and India alternated in being the largest economies in the world from 1 to 1800 CE. China was a major economic power and attracted many to the east, for many the legendary wealth and prosperity of the ancient culture of India personified Asia, attracting European commerce and colonialism; the accidental discovery of a trans-Atlantic route from Europe to America by Columbus while in search for a route to India demonstrates this deep fascination. The Silk Road became the main east–west trading route in the Asian hinterlands while the Straits of Malacca stood as a major sea route. Asia has exhibited economic dynamism as well as robust population growth during the 20th century, but overall population growth has since fallen. Asia was the birthplace of most of the world's mainstream religions including Hinduism, Judaism, Buddhism, Taoism, Islam, Sikhism, as well as many other religions.
Given its size and diversity, the concept of Asia—a name dating back to classical antiquity—may have more to do with human geography than physical geography. Asia varies across and within its regions with regard to ethnic groups, environments, historical ties and government systems, it has a mix of many different climates ranging from the equatorial south via the hot desert in the Middle East, temperate areas in the east and the continental centre to vast subarctic and polar areas in Siberia. The boundary between Asia and Africa is the Red Sea, the Gulf of Suez, the Suez Canal; this makes Egypt a transcontinental country, with the Sinai peninsula in Asia and the remainder of the country in Africa. The border between Asia and Europe was defined by European academics; the Don River became unsatisfactory to northern Europeans when Peter the Great, king of the Tsardom of Russia, defeating rival claims of Sweden and the Ottoman Empire to the eastern lands, armed resistance by the tribes of Siberia, synthesized a new Russian Empire extending to the Ural Mountains and beyond, founded in 1721.
The major geographical theorist of the empire was a former Swedish prisoner-of-war, taken at the Battle of Poltava in 1709 and assigned to Tobolsk, where he associated with Peter's Siberian official, Vasily Tatishchev, was allowed freedom to conduct geographical and anthropological studies in preparation for a future book. In Sweden, five years after Peter's death, in 1730 Philip Johan von Strahlenberg published a new atlas proposing the Urals as the border of Asia. Tatishchev announced; the latter had suggested the Emba River as the lower boundary. Over the next century various proposals were made until the Ural River prevailed in the mid-19th century; the border had been moved perforce from the Black Sea to the Caspian Sea into which the Ural River projects. The border between the Black Sea and the Caspian is placed along the crest of the Caucasus Mountains, although it is sometimes placed further north; the border between Asia and the region of Oceania is placed somewhere in the Malay Archipelago.
The Maluku Islands in Indonesia are considered to lie on the border of southeast Asia, with New Guinea, to the east of the islands, being wholly part of Oceania. The terms Southeast Asia and Oceania, devised in the 19th century, have had several vastly different geographic meanings since their inception; the chief factor in determining which islands of the Malay Archipelago are Asian has been the location of the colonial possessions of the various empires there. Lewis and Wigen assert, "The narrowing of'Southeast Asia' to its present boundaries was thus a gradual process." Geographical Asia is a cultural artifact of European conceptions of the world, beginning with the Ancient Greeks, being imposed onto other cultures, an imprecise concept causing endemic contention about what it means. Asia does not correspond to the cultural borders of its various types of constituents. From the time of Herodotus a minority of geographers have rejected the three-continent system on the grounds that there is no substantial physical separation between
Bodden are briny bodies of water forming lagoons, along the southwestern shores of the Baltic Sea in Germany's state of Mecklenburg-Vorpommern. These lagoons can be found around the island of Rügen and the Fischland-Darss-Zingst peninsula; some of them are protected reserves. They have a distinctive geological origin and are enclosed by peninsulae and islands, leaving only narrow connections to adjacent bodden or the open sea. Freshwater inflow from the mainland and saltwater inflow from the open sea, which depends on wind direction and force as well as the proximity of the bodden to the sea, result in fluctuating salt gradients and distinctive ecosystems. During the Littorina Sea transgression, an island archipelago was formed by the carving of narrow glacial basins and channels resulting from meltwater. Bodden were formed in a comparatively short period between offshore sandbars; these shallow glacial scoops were subjected to extensive sedimentation during the Holocene, resulting in lakes with depths of no more than 4–6 metres.
Thermal and saline stratification is unstable under these conditions, bodden have the typical dynamics of small bodies of water with a sea connection, a rapid filling and draining due to tidal and wind action, inflow of fresh water. The frequent movement of water can lead to a scouring effect, but can with heavy pollution show a tendency toward eutrophication. Due to erosion of cliffs and sedimentary deposition, the shape of the bodden coasts remains unstable. Sudden changes have been caused by stormfloods, which closed connections to the sea or opened new ones in the past. While bodden-type bays can be found in Mecklenburg and Denmark, the most typical bodden are located off the Pomeranian mainland between the mouth of the Recknitz river and the island of Usedom. Several adjacent bodden between the Fischland-Darß-Zingst peninsula, the northern and western peninsulae of Rügen and the Pomeranian mainland are grouped as Bodden chains: Darss-Zingst Bodden Chain, consisting of Saaler Bodden with Ribnitzer See and Permin, connected by Koppelstrom to Bodstedter Bodden with Redensee and Prerower Strom, connected by Meiningen to Barther Bodden with Zingster Strom Grabow West Rügen bodden chain, consisting of Kubitzer Bodden with Prohner Wiek and Gellenstrom Schaproder Bodden with Udarser Wiek and Koselower See Vitter Bodden, connected to the Baltic Sea by Libben North Rügen bodden chain, consisting of Wieker Bodden, connected by Rassower Strom to Schaproder Bodden and Breetzer Bodden with Neuendorfer Wiek, Breeger Bodden Lebbiner Bodden with Tetzitzer See Großer Jasmunder Bodden with Mittelsee and Spykerscher See, connected by a narrow ditch to Kleiner Jasmunder BoddenAnother bodden is the Bay of Greifswald, the northern parts of which constitute the Rügischer Bodden with Schoritzer Wiek, Having Inlet with Neuensiener See and Selliner See, Hagensche Wiek.
To the south, the Bay of Greifswald comprises Kooser See and Dänische Wieck. The Bay of Greifswald is connected to the West Rügen bodden chain by the Strelasund, a bodden-type strait with Glewitzer Wiek, Puddeminer Wiek and Deviner See; the bodden are important sanctuaries for many species of birds and are important resting places for migratory birds like cranes and geese. This was the reason for the establishment of the Western Pomerania Lagoon Area National Park, comprising most of the bodden between Darß and Rügen. Traditionally bodden have been good fishing areas, rich in mesolithic community sites, in particular the Pomeranian bodden of Rügen and Peenestrom. From these waters anglers land 10–15 kg pike. Harvesting the Sea, Farming the Forest by Marek Zvelebil, Lucyna Domańska, Robin Dennell YouTube - Angling in the Bodden
An orogeny is an event that leads to both structural deformation and compositional differentiation of the Earth's lithosphere at convergent plate margins. An orogen or orogenic belt develops when a continental plate crumples and is pushed upwards to form one or more mountain ranges. Orogeny is the primary mechanism; the word "orogeny" comes from Ancient Greek. Although it was used before him, the term was employed by the American geologist G. K. Gilbert in 1890 to describe the process of mountain building as distinguished from epeirogeny; the formation of an orogen can be accomplished by the tectonic processes such as oceanic subduction or continental subduction convergence of two or more continents for collisional orogeny). Orogeny produces long arcuate structures, known as orogenic belts. Orogenic belts consist of long parallel strips of rock exhibiting similar characteristics along the length of the belt. Although orogenic belts are associated with subduction zones, subduction tectonism may be ongoing or past processes.
The subducting tectonism would consume crust, thicken lithosphere, produce earthquake and volcanoes, build island arcs in many cases. Geologists attribute the arcuate structure to the rigidity of the descending plate, island arc cusps relate to tears in the descending lithosphere; these island arcs may be added to a continental margin during an accretionary orogeny. On the other hand, subduction zones may be reworked at a time due to lithospheric rifting, leading to amphibolite to granulite facies metamorphism of the thinned orogenic crust; the processes of orogeny can take tens of millions of years and build mountains from plains or from the seabed. The topographic height of orogenic mountains is related to the principle of isostasy, that is, a balance of the downward gravitational force upon an upthrust mountain range and the buoyant upward forces exerted by the dense underlying mantle. Rock formations that undergo orogeny are deformed and undergo metamorphism. Orogenic processes may push buried rocks to the surface.
Sea-bottom and near-shore material may cover all of the orogenic area. If the orogeny is due to two continents colliding high mountains can result. An orogenic event may be studied: as a tectonic structural event, as a geographical event, as a chronological event. Orogenic events: cause distinctive structural phenomena related to tectonic activity affect rocks and crust in particular regions, happen within a specific period In general, there are two main types of orogens at convergent plate margins: accretionary orogens, which were produced by subduction of one oceanic plate beneath one continental plate to result in either continental arc magmatism or the accretion of island arc terranes to continental margins. An orogeny produces an orogen, but a range-foreland basin system is only produced on passive plate margins; the foreland basin forms ahead of the orogen due to loading and resulting flexure of the lithosphere by the developing mountain belt. A typical foreland basin is subdivided into a wedge-top basin above the active orogenic wedge, the foredeep beyond the active front, a forebulge high of flexural origin and a back-bulge area beyond, although not all of these are present in all foreland-basin systems.
The basin migrates with the orogenic front and early deposited foreland basin sediments become progressively involved in folding and thrusting. Sediments deposited in the foreland basin are derived from the erosion of the uplifting rocks of the mountain range, although some sediments derive from the foreland; the fill of many such basins shows a change in time from deepwater marine through shallow water to continental sediments. Although orogeny involves plate tectonics, the tectonic forces result in a variety of associated phenomena, including crustal deformation, crustal thickening, crustal thinning and crustal melting as well as magmatism and mineralization. What happens in a specific orogen depends upon the strength and rheology of the continental lithosphere, how these properties change during orogenesis. In addition to orogeny, the orogen is subject to other processes, such as erosion; the sequence of repeated cycles of sedimentation and erosion, followed by burial and metamorphism, by crustal anatexis to form granitic batholiths and tectonic uplift to form mountain chains, is called the orogenic cycle.
For example, the Caledonian Orogeny refers to a series of tectonic events due to the continental collision of Laurentia with Eastern Avalonia and other former fragments of Gondwana in the Early Paleozoic. The Caledonian Orogen resulted from these events and various others that are part of its peculiar orogenic cycle. In summary, an orogeny is an episode of deformation and magmatism at convergent plate margins, during which many geological processes play a role at convergent plate margins; every orogeny has its own orogenic cycle, but composite orogenesis is common at convergent plate margins. Erosion represents a subsequent phase of the orogenic cycle. Erosion removes much of the mountains
An ice age is a long period of reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth is in the Quaternary glaciation, known in popular terminology as the Ice Age. Individual pulses of cold climate are termed "glacial periods", intermittent warm periods are called "interglacials", with both climatic pulses part of the Quaternary or other periods in Earth's history. In the terminology of glaciology, ice age implies the presence of extensive ice sheets in both northern and southern hemispheres. By this definition, we are in an interglacial period—the Holocene; the amount of heat trapping gases emitted into Earth's Oceans and atmosphere will prevent the next ice age, which otherwise would begin in around 50,000 years, more glacial cycles. In 1742, Pierre Martel, an engineer and geographer living in Geneva, visited the valley of Chamonix in the Alps of Savoy. Two years he published an account of his journey.
He reported that the inhabitants of that valley attributed the dispersal of erratic boulders to the glaciers, saying that they had once extended much farther. Similar explanations were reported from other regions of the Alps. In 1815 the carpenter and chamois hunter Jean-Pierre Perraudin explained erratic boulders in the Val de Bagnes in the Swiss canton of Valais as being due to glaciers extending further. An unknown woodcutter from Meiringen in the Bernese Oberland advocated a similar idea in a discussion with the Swiss-German geologist Jean de Charpentier in 1834. Comparable explanations are known from the Val de Ferret in the Valais and the Seeland in western Switzerland and in Goethe's scientific work; such explanations could be found in other parts of the world. When the Bavarian naturalist Ernst von Bibra visited the Chilean Andes in 1849–1850, the natives attributed fossil moraines to the former action of glaciers. Meanwhile, European scholars had begun to wonder. From the middle of the 18th century, some discussed ice as a means of transport.
The Swedish mining expert Daniel Tilas was, in 1742, the first person to suggest drifting sea ice in order to explain the presence of erratic boulders in the Scandinavian and Baltic regions. In 1795, the Scottish philosopher and gentleman naturalist, James Hutton, explained erratic boulders in the Alps by the action of glaciers. Two decades in 1818, the Swedish botanist Göran Wahlenberg published his theory of a glaciation of the Scandinavian peninsula, he regarded glaciation as a regional phenomenon. Only a few years the Danish-Norwegian geologist Jens Esmark argued a sequence of worldwide ice ages. In a paper published in 1824, Esmark proposed changes in climate as the cause of those glaciations, he attempted to show. During the following years, Esmark's ideas were discussed and taken over in parts by Swedish and German scientists. At the University of Edinburgh Robert Jameson seemed to be open to Esmark's ideas, as reviewed by Norwegian professor of glaciology Bjørn G. Andersen. Jameson's remarks about ancient glaciers in Scotland were most prompted by Esmark.
In Germany, Albrecht Reinhard Bernhardi, a geologist and professor of forestry at an academy in Dreissigacker, since incorporated in the southern Thuringian city of Meiningen, adopted Esmark's theory. In a paper published in 1832, Bernhardi speculated about former polar ice caps reaching as far as the temperate zones of the globe. In 1829, independently of these debates, the Swiss civil engineer Ignaz Venetz explained the dispersal of erratic boulders in the Alps, the nearby Jura Mountains, the North German Plain as being due to huge glaciers; when he read his paper before the Schweizerische Naturforschende Gesellschaft, most scientists remained sceptical. Venetz convinced his friend Jean de Charpentier. De Charpentier transformed Venetz's idea into a theory with a glaciation limited to the Alps, his thoughts resembled Wahlenberg's theory. In fact, both men shared the same volcanistic, or in de Charpentier's case rather plutonistic assumptions, about the Earth's history. In 1834, de Charpentier presented his paper before the Schweizerische Naturforschende Gesellschaft.
In the meantime, the German botanist Karl Friedrich Schimper was studying mosses which were growing on erratic boulders in the alpine upland of Bavaria. He began to wonder. During the summer of 1835 he made some excursions to the Bavarian Alps. Schimper came to the conclusion that ice must have been the means of transport for the boulders in the alpine upland. In the winter of 1835 to 1836 he held. Schimper assumed that there must have been global times of obliteration with a cold climate and frozen water. Schimper spent the summer months of 1836 at Devens, near Bex, in the Swiss Alps with his former university friend Louis Agassiz and Jean de Charpentier. Schimper, de Charpentier and Venetz convinced Agassiz that there had been a time of glaciation. During the winter of 1836/37, Agassiz and Schimper developed the theory of a sequence of glaciations, they drew upon the preceding works of Venetz, de Charpentier and on their own fieldwork. Agassiz appears to have been familiar with Bernhardi's paper at that time.
At the beginning of 1837, Schimper coined the term "ice age" for the period of the glaciers. In July 1837 Ag
A bay is a recessed, coastal body of water that directly connects to a larger main body of water, such as an ocean, a lake, or another bay. A large bay is called a gulf, sound, or bight. A cove is a type of smaller bay with narrow entrance. A fjord is a steep bay shaped by glacial activity. A bay can be the estuary of a river, such as the Chesapeake Bay, an estuary of the Susquehanna River. Bays may be nested within each other; some large bays, such as the Bay of Bengal and Hudson Bay, have varied marine geology. The land surrounding a bay reduces the strength of winds and blocks waves. Bays were significant in the history of human settlement because they provided safe places for fishing, they were important in the development of sea trade as the safe anchorage they provide encouraged their selection as ports. The United Nations Convention on the Law of the Sea called the Law of the Sea, defines a bay as a well-marked indentation whose penetration is in such proportion to the width of its mouth as to contain land-locked waters and constitute more than a mere curvature of the coast.
An indentation shall not, however, be regarded as a bay unless its area is as large as, or larger than, that of the semi-circle whose diameter is a line drawn across the mouth of that indentation. There are various ways; the largest bays have developed through plate tectonics. As the super-continent Pangaea broke up along curved and indented fault lines, the continents moved apart and left large bays. Bays form through coastal erosion by rivers and glaciers. A bay formed by a glacier is a fjord. Rias are characterised by more gradual slopes. Deposits of softer rocks erode more forming bays, while harder rocks erode less leaving headlands. Bay platform Great capes Headlands and bays
An atoll, sometimes called a coral atoll, is a ring-shaped coral reef including a coral rim that encircles a lagoon or completely. There may be coral cays on the rim; the coral of the atoll sits atop the rim of an extinct seamount or volcano which has eroded or subsided beneath the water. The lagoon forms over the volcanic crater or caldera while the higher rim remains above water or at shallow depths that permit the coral to grow and form the reefs. For the atoll to persist, continued erosion or subsidence must be at a rate slow enough to permit reef growth upward and outward to replace the lost height; the word atoll comes from the Dhivehi word atholhu. OED Its first recorded use in English was in 1625 as atollon. Charles Darwin recognized its indigenous origin and coined, in his The Structure and Distribution of Coral Reefs, the definition of atolls as "circular groups of coral islets", synonymous with "lagoon-island". More modern definitions of atoll describe them as "annular reefs enclosing a lagoon in which there are no promontories other than reefs and islets composed of reef detritus" or "in an morphological sense, a ring-shaped ribbon reef enclosing a lagoon".
Most of the world's atolls are in the Indian Ocean. The Atlantic Ocean has no large groups of atolls, other than eight atolls east of Nicaragua that belong to the Colombian department of San Andres and Providencia in the Caribbean. Reef-building corals will thrive only in warm tropical and subtropical waters of oceans and seas, therefore atolls are only found in the tropics and subtropics; the northernmost atoll of the world is Kure Atoll at 28°24′ N, along with other atolls of the Northwestern Hawaiian Islands. The southernmost atolls of the world are Elizabeth Reef at 29°58′ S, nearby Middleton Reef at 29°29′ S, in the Tasman Sea, both of which are part of the Coral Sea Islands Territory; the next southerly atoll is Ducie Island in the Pitcairn Islands Group, at 24°40′ S. Bermuda is sometimes claimed as the "northernmost atoll" at a latitude of 32°24′ N. At this latitude coral reefs would not develop without the warming waters of the Gulf Stream. However, Bermuda is termed a pseudo-atoll because its general form, while resembling that of an atoll, has a different mode of formation.
While there is no atoll directly on the equator, the closest atoll to the Equator is Aranuka of Kiribati, with its southern tip just 12 km north of the equator. In most cases, the land area of an atoll is small in comparison to the total area. Atoll islands are low lying, with their elevations less than 5 meters. Measured by total area, Lifou is the largest raised coral atoll of the world, followed by Rennell Island. More sources however list as the largest atoll in the world in terms of land area Kiritimati, a raised coral atoll, 160 km² main lagoon, 168 km² other lagoons; the remains of an ancient atoll as a hill in a limestone area is called a reef knoll. The second largest atoll by dry land area is Aldabra with 155 km²; the largest atoll in terms of island numbers is Huvadhu Atoll in the south of the Maldives with 255 islands. In 1842, Charles Darwin explained the creation of coral atolls in the southern Pacific Ocean based upon observations made during a five-year voyage aboard HMS Beagle from 1831 to 1836.
Accepted as correct, his explanation involved considering that several tropical island types—from high volcanic island, through barrier reef island, to atoll—represented a sequence of gradual subsidence of what started as an oceanic volcano. He reasoned that a fringing coral reef surrounding a volcanic island in the tropical sea will grow upward as the island subsides, becoming an "almost atoll", or barrier reef island, as typified by an island such as Aitutaki in the Cook Islands, Bora Bora and others in the Society Islands; the fringing reef becomes a barrier reef for the reason that the outer part of the reef maintains itself near sea level through biotic growth, while the inner part of the reef falls behind, becoming a lagoon because conditions are less favorable for the coral and calcareous algae responsible for most reef growth. In time, subsidence carries the old volcano below the ocean surface and the barrier reef remains. At this point, the island has become an atoll. Atolls are the product of the growth of tropical marine organisms, so these islands are only found in warm tropical waters.
Volcanic islands located beyond the warm water temperature requirements of hermatypic organisms become seamounts as they subside and are eroded away at the surface. An island, located where the ocean water temperatures are just sufficiently warm for upward reef growth to keep pace with the rate of subsidence is said to be at the Darwin Point. Islands in colder, more polar regions evolve toward guyots. Reginald Aldworth Daly offered a somewhat different explanation for atoll formation: islands worn away by erosion, by ocean waves and streams, during the last glacial stand of the sea of some 900 feet below present sea level developed as coral islands, or barrier reefs on a platform surrounding a volcanic island not worn away, a