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60-508: The Salomon Islands or Salomon Atoll is a small atoll of the Chagos Archipelago , British Indian Ocean Territory (BIOT). The atoll is located in the northeast of the Chagos Archipelago , between Blenheim Reef and Peros Banhos . The main islands in the group are Île Boddam, with the former main settlement, and a land area of 1.08 km (0.42 sq mi), and Île Anglaise (0.82 km, 0.32 sq mi), both on

120-508: A reef knoll refers to the elevated remains of an ancient atoll within a limestone region, appearing as a hill. The second largest atoll by dry land area is Aldabra , with 155 km (60 sq mi). Huvadhu Atoll , situated in the southern region of the Maldives, holds the distinction of being the largest atoll based on the sheer number of islands it comprises, with a total of 255 individual islands. In 1842, Charles Darwin explained

180-413: A "circular group of coral islets", synonymously 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 exclusively morphological sense, [as] a ring-shaped ribbon reef enclosing a lagoon". There are approximately 440 atolls in the world. Most of

240-519: A pioneer of the study of karst in Slovenia and a fellow of the Royal Society , London, introduced the word karst to European scholars in 1689 to describe the phenomenon of underground flows of rivers in his account of Lake Cerknica . Jovan Cvijić greatly advanced the knowledge of karst regions to the point where he became known as the "father of karst geomorphology". Primarily discussing

300-582: A proper noun, the Slovene form Grast was first attested in 1177. Ultimately, the word is of Mediterranean origin. It has also been suggested that the word may derive from the Proto-Indo-European root karra- 'rock'. The name may also be connected to the oronym Kar(u)sádios oros cited by Ptolemy , and perhaps also to Latin Carusardius . Johann Weikhard von Valvasor ,

360-401: A sport than a science and so the underground karst caves and their associated watercourses were, from a scientific perspective, understudied. Karst is most strongly developed in dense carbonate rock , such as limestone, that is thinly bedded and highly fractured . Karst is not typically well developed in chalk , because chalk is highly porous rather than dense, so the flow of groundwater

420-442: A very different origin of formation. In most cases, the land area of an atoll is very small in comparison to the total area. Atoll islands are low lying, with their elevations less than 5 metres (16 ft). Measured by total area, Lifou (1,146 km , 442 sq mi) is the largest raised coral atoll of the world, followed by Rennell Island (660 km , 250 sq mi). More sources, however, list Kiritimati as

480-525: A volcanic island in the tropical sea will grow upward as the island subsides (sinks), becoming an "almost atoll", or barrier reef island, as typified by an island such as Aitutaki in the Cook Islands, and 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

540-529: A well in a fracture trace or intersection of fracture traces increases the likelihood to encounter good water production. Voids in karst aquifers can be large enough to cause destructive collapse or subsidence of the ground surface that can initiate a catastrophic release of contaminants. Groundwater flow rate in karst aquifers is much more rapid than in porous aquifers. For example, in the Barton Springs Edwards aquifer, dye traces measured

600-505: Is a UNESCO World Heritage Site. Many karst-related terms derive from South Slavic languages , entering scientific vocabulary through early research in the Western Balkan Dinaric Alpine karst. The study of the various aspects of karst regions is called karstology. This includes biological, chemical, ecological, geomorphological, hydrogeological, hydrological, political, socio-economical, and other processes over

660-562: Is a development of karst observed in geological history and preserved within the rock sequence, effectively a fossil karst. There are for example palaeokarst surfaces exposed within the Clydach Valley Subgroup of the Carboniferous Limestone sequence of South Wales which developed as sub-aerial weathering of recently formed limestones took place during periods of non-deposition within the early part of

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720-538: Is a karst landscape which is developed beneath a cover of insoluble rocks. Typically this will involve a cover of sandstone overlying limestone strata undergoing solution. In the United Kingdom for example extensive doline fields have developed at Cefn yr Ystrad , Mynydd Llangatwg and Mynydd Llangynidr in South Wales across a cover of Twrch Sandstone which overlies concealed Carboniferous Limestone ,

780-584: Is a passage into the lagoon , named Baie de Salomon, on the Northern side, between Île Anglaise and Île de la Passe. The Salomon Islands are one of the favorite anchoring spots for itinerant yachtsmen passing through the Chagos, even though there are no proper moorings for yachts and a permit of the BIOT authorities is needed. Now uninhabited, the islands are overrun by low jungle between the coconut trees and it

840-611: Is characterized by features like poljes above and drainage systems with sinkholes and caves underground. There is some evidence that karst may occur in more weathering -resistant rocks such as quartzite given the right conditions. Subterranean drainage may limit surface water, with few to no rivers or lakes. In regions where the dissolved bedrock is covered (perhaps by debris) or confined by one or more superimposed non-soluble rock strata, distinctive karst features may occur only at subsurface levels and can be totally missing above ground. The study of paleokarst (buried karst in

900-468: Is developed in areas where salt is undergoing solution underground. It can lead to surface depressions and collapses which present a geo-hazard. Karst areas tend to have unique types of forests. The karst terrain is difficult for humans to traverse, so that their ecosystems are often relatively undisturbed. The soil tends to have a high pH, which encourages growth of unusual species of orchids, palms, mangroves, and other plants. Paleokarst or palaeokarst

960-636: Is frequently unseen until the roof of a cavern suddenly collapses. Such events have swallowed homes, cattle, cars, and farm machinery. In the United States, sudden collapse of such a cavern-sinkhole swallowed part of the collection of the National Corvette Museum in Bowling Green, Kentucky in 2014. The world's largest limestone karst is Australia's Nullarbor Plain . Slovenia has the world's highest risk of sinkholes, while

1020-563: Is hard to find the traces of the former settlements. This atoll was settled in the last half of the 18th century by coconut plantation workers from Isle de France (now Mauritius ). Little is known about the condition of the workers who were mostly of African origin. Most probably they lived in conditions close to slavery . The company exploiting the plantation was called the Chagos Agalega Company . The Salomon Islands were surveyed in 1837 by Commander Robert Moresby of

1080-399: Is not concentrated along fractures. Karst is also most strongly developed where the water table is relatively low, such as in uplands with entrenched valleys , and where rainfall is moderate to heavy. This contributes to rapid downward movement of groundwater, which promotes dissolution of the bedrock, whereas standing groundwater becomes saturated with carbonate minerals and ceases to dissolve

1140-621: Is the following: In very rare conditions, oxidation can play a role. Oxidation played a major role in the formation of ancient Lechuguilla Cave in the US state of New Mexico and is presently active in the Frasassi Caves of Italy. The oxidation of sulfides leading to the formation of sulfuric acid can also be one of the corrosion factors in karst formation. As oxygen (O 2 )-rich surface waters seep into deep anoxic karst systems, they bring oxygen, which reacts with sulfide present in

1200-544: The Royal Society of London carried out drilling on Funafuti atoll in Tuvalu for the purpose of investigating the formation of coral reefs . They wanted to determine whether traces of shallow water organisms could be found at depth in the coral of Pacific atolls. This investigation followed the work on the structure and distribution of coral reefs conducted by Charles Darwin in the Pacific. The first expedition in 1896

1260-664: The Thousand Islands , Taka Bonerate Islands , and atolls in the Raja Ampat Islands . 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, and therefore atolls are found only in

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1320-636: The range of the Dinaric Alps , stretching from the northeastern corner of Italy above the city of Trieste , across the Balkan peninsula along the coast of the eastern Adriatic to Kosovo and North Macedonia , where the massif of the Šar Mountains begins. The karst zone is at the northwesternmost section, described in early topographical research as a plateau between Italy and Slovenia . Languages preserving this form include Italian : Carso , German : Karst , and Albanian : karsti . In

1380-435: The stratigraphic column ) is important in petroleum geology because as much as 50% of the world's hydrocarbon reserves are hosted in carbonate rock , and much of this is found in porous karst systems. The English word karst was borrowed from German Karst in the late 19th century, which entered German usage much earlier, to describe a number of geological, geomorphological, and hydrological features found within

1440-519: The Equator is Aranuka of Kiribati. Its southern tip is just 13 km (8 mi) north of the Equator. Bermuda is sometimes claimed as the "northernmost atoll" at a latitude of 32°18′ 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

1500-706: The Indian Navy on HMS Benares . Moresby's survey produced the first detailed map of this atoll. The atoll was surveyed again in 1905 by Commander B.T. Sommerville on HMS  Sealark , who drew a more accurate map. Some of the Salomon Islands were inhabited by the Chagossians , but at the time that the British Government decided to empty the Chagos of local inhabitants, only Île Boddam was inhabited. Between 1967 and 1973, 500 inhabitants of

1560-638: The Salomon Islands were evicted by the British and their pets were killed in gas chambers by Sir Bruce Greatbatch in order to make way for an American military base . The evictees were resettled in Mauritius and Seychelles . Île Boddam had a jetty, shops, offices, a school, a church and a villa where the plantation manager lived. All these buildings are now hidden by thick jungle. There are wells on Boddam and Takamaka islands which are still used by yachtsmen to replenish their supplies. The individual islets of

1620-520: The antecedent karst model as they found that the morphology of modern atolls are independent of any influence of an underlying submerged and buried island and are not rooted to an initial fringing reef/barrier reef attached to a slowly subsiding volcanic edifice. In fact, the Neogene reefs underlying the studied modern atolls overlie and completely bury the subsided island are all non-atoll, flat-topped reefs. In fact, they found that atolls did not form doing

1680-465: The atoll are, starting in the North, clockwise: 5°19′00″S 72°15′36″E  /  5.31667°S 72.26000°E  / -5.31667; 72.26000 Atoll Two different, well-cited models, the subsidence model and the antecedent karst model, have been used to explain the development of atolls. According to Charles Darwin 's subsidence model, the formation of an atoll is explained by

1740-511: The bedrock. The carbonic acid that causes karst features is formed as rain passes through Earth's atmosphere picking up carbon dioxide (CO 2 ), which readily dissolves in the water. Once the rain reaches the ground, it may pass through soil that provides additional CO 2 produced by soil respiration . Some of the dissolved carbon dioxide reacts with the water to form a weak carbonic acid solution, which dissolves calcium carbonate . The primary reaction sequence in limestone dissolution

1800-450: The complexity of karst aquifers, and need to be supplemented with dye traces , measurement of spring discharges, and analysis of water chemistry. U.S. Geological Survey dye tracing has determined that conventional groundwater models that assume a uniform distribution of porosity are not applicable for karst aquifers. Linear alignment of surface features such as straight stream segments and sinkholes develop along fracture traces . Locating

1860-417: 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. Darwin's explanation suggests 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

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1920-436: The direct result of the interaction between subsidence and preferential karst dissolution that occurred in the interior of flat topped coral reefs during exposure during glacial lowstands of sea level. The elevated rims along an island created by this preferential karst dissolution become the sites of coral growth and islands of atolls when flooded during interglacial highstands. The research of A. W. Droxler and others supports

1980-561: The dolomitization of calcite and aragonite within them. They are the evaporative, seepage-reflux, mixing-zone, burial, and seawater models. Although the origin of replacement dolomites remains problematic and controversial, it is generally accepted that seawater was the source of magnesium for dolomitization and the fluid in which calcite was dolomitized to form the dolomites found within atolls. Various processes have been invoked to drive large amounts of seawater through an atoll in order for dolomitization to occur. In 1896, 1897 and 1898,

2040-449: The first step in the formation of an atoll is the development of a flat top, mound-like coral reef during the subsidence of an oceanic island of either volcanic or nonvolcanic origin below sea level. Then, when relative sea level drops below the level of the flat surface of coral reef, it is exposed to the atmosphere as a flat topped island which is dissolved by rainfall to form limestone karst . Because of hydrologic properties of this karst,

2100-557: The fissures. The enlarged fissures allow a larger quantity of water to enter which leads to a progressive enlargement of openings. Abundant small openings store a large quantity of water. The larger openings form a conduit system that drains the aquifer to springs. Characterization of karst aquifers requires field exploration to locate sinkholes, swallets , sinking streams , and springs in addition to studying geologic maps . Conventional hydrogeologic methods such as aquifer tests and potentiometric mapping are insufficient to characterize

2160-466: The global demand for drinkable water. Farming in karst areas must take into account the lack of surface water. The soils may be fertile enough, and rainfall may be adequate, but rainwater quickly moves through the crevices into the ground, sometimes leaving the surface soil parched between rains. The karst topography also poses peculiar difficulties for human inhabitants. Sinkholes can develop gradually as surface openings enlarge, but progressive erosion

2220-472: 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. As formulated by J. E. Hoffmeister, F. S. McNeil, E. G. Prudy, and others, the antecedent karst model argues that atolls are Pleistocene features that are

2280-456: The karst groundwater flow rates from 0.5 to 7 miles per day (0.8 to 11.3 km/d). The rapid groundwater flow rates make karst aquifers much more sensitive to groundwater contamination than porous aquifers. Groundwater in karst areas is also just as easily polluted as surface streams, because Karst formations are cavernous and highly permeable, resulting in reduced opportunity for contaminant filtration. Well water may also be unsafe as

2340-534: The karst regions of the Balkans, Cvijić's 1893 publication Das Karstphänomen describes landforms such as karren, dolines and poljes . In a 1918 publication, Cvijić proposed a cyclical model for karst landscape development. Karst hydrology emerged as a discipline in the late 1950s and the early 1960s in France. Previously, the activities of cave explorers, called speleologists , had been dismissed as more of

2400-535: The lagoon within them. The word atoll comes from the Dhivehi word atholhu ( އަތޮޅު , pronounced [ˈat̪oɭu] ). Dhivehi is an Indo-Aryan language spoken in the Maldives . The word's first recorded English use was in 1625 as atollon . Charles Darwin coined the term in his monograph, The Structure and Distribution of Coral Reefs . He recognized the word's indigenous origin and defined it as

2460-412: The largest atoll in the world in terms of land area. It is also a raised coral atoll (321 km , 124 sq mi land area; according to other sources even 575 km , 222 sq mi), 160 km (62 sq mi) main lagoon, 168 km (65 sq mi) other lagoons (according to other sources 319 km , 123 sq mi total lagoon size). The geological formation known as

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2520-562: The last-named locality having been declared a site of special scientific interest in respect of it. Kegelkarst is a type of tropical karst terrain with numerous cone-like hills, formed by cockpits, mogotes , and poljes and without strong fluvial erosion processes. This terrain is found in Cuba, Jamaica, Indonesia, Malaysia, the Philippines, Puerto Rico, southern China, Myanmar, Thailand, Laos and Vietnam. Salt karst (or 'halite karst')

2580-412: The local South Slavic languages , all variations of the word are derived from a Romanized Illyrian base (yielding Latin : carsus , Dalmatian : carsus ), later metathesized from the reconstructed form * korsъ into forms such as Slovene : kras and Serbo-Croatian : krš , kras , first attested in the 18th century, and the adjective form kraški in the 16th century. As

2640-647: The most dramatic of these formations can be seen in Thailand 's Phangnga Bay and at Halong Bay in Vietnam . Calcium carbonate dissolved into water may precipitate out where the water discharges some of its dissolved carbon dioxide. Rivers which emerge from springs may produce tufa terraces, consisting of layers of calcite deposited over extended periods of time. In caves, a variety of features collectively called speleothems are formed by deposition of calcium carbonate and other dissolved minerals. Interstratal karst

2700-422: The original island, and a lagoon has taken the place of the former volcano. The lagoon is not the former volcanic crater. For the atoll to persist, the coral reef must be maintained at the sea surface, with coral growth matching any relative change in sea level (sinking of the island or rising oceans). An alternative model for the origin of atolls is called the antecedent karst model. In the antecedent karst model,

2760-709: The period. Sedimentation resumed and further limestone strata were deposited on an irregular karst surface, the cycle recurring several times in connection with fluctuating sea levels over prolonged periods. Pseudokarsts are similar in form or appearance to karst features but are created by different mechanisms. Examples include lava caves and granite tors —for example, Labertouche Cave in Victoria, Australia —and paleocollapse features. Mud Caves are an example of pseudokarst. Karst formations have unique hydrology, resulting in many unusual features. A karst fenster (karst window) occurs when an underground stream emerges onto

2820-401: The rate of dissolution of the exposed coral is lowest along its rim and the rate of dissolution increases inward to its maximum at the center of the island. As a result, a saucer shaped island with a raised rim forms. When relative sea level submerges the island again, the rim provides a rocky core on which coral grow again to form the islands of an atoll and the flooded bottom of the saucer forms

2880-780: The river flows into a cave in a formation known as the Madison Limestone and then rises again 800 m ( 1 ⁄ 2  mi) down the canyon in a placid pool. A turlough is a unique type of seasonal lake found in Irish karst areas which are formed through the annual welling-up of water from the underground water system. Main Article Aquifer#Karst Karst aquifers typically develop in limestone . Surface water containing natural carbonic acid moves down into small fissures in limestone. This carbonic acid gradually dissolves limestone thereby enlarging

2940-405: The sinking of a volcanic island around which a coral fringing reef has formed. Over geologic time, the volcanic island becomes extinct and eroded as it subsides completely beneath the surface of the ocean. As the volcanic island subsides, the coral fringing reef becomes a barrier reef that is detached from the island. Eventually, reef and the small coral islets on top of it are all that is left of

3000-478: The subsidence of an island until MIS-11, Mid-Brunhes, long after the many the former islands had been completely submerged and buried by flat topped reefs during the Neogene. Atolls are the product of the growth of tropical marine organisms, and so these islands are found only in warm tropical waters. Volcanic islands located beyond the warm water temperature requirements of hermatypic (reef-building) organisms become seamounts as they subside, and are eroded away at

3060-649: The surface between layers of rock, cascades some distance, and then disappears back down, often into a sinkhole. Rivers in karst areas may disappear underground a number of times and spring up again in different places, even under a different name, like Ljubljanica , the "river of seven names". Another example of this is the Popo Agie River in Fremont County, Wyoming , where, at a site named "The Sinks" in Sinks Canyon State Park ,

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3120-415: The surface, complex underground drainage systems (such as karst aquifers ) and extensive caves and cavern systems may form. Erosion along limestone shores, notably in the tropics , produces karst topography that includes a sharp makatea surface above the normal reach of the sea, and undercuts that are mostly the result of biological activity or bioerosion at or a little above mean sea level . Some of

3180-696: The surface. An island that is 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 seamounts or guyots ; warmer, more equatorial islands evolve toward atolls, for example Kure Atoll . However, ancient atolls during the Mesozoic appear to exhibit different growth and evolution patterns. Coral atolls are important as sites where dolomitization of calcite occurs. Several models have been proposed for

3240-1002: The system ( pyrite or hydrogen sulfide ) to form sulfuric acid (H 2 SO 4 ). Sulfuric acid then reacts with calcium carbonate, causing increased erosion within the limestone formation. This chain of reactions is: This reaction chain forms gypsum . The karstification of a landscape may result in a variety of large- or small-scale features both on the surface and beneath. On exposed surfaces, small features may include solution flutes (or rillenkarren), runnels , limestone pavement (clints and grikes), kamenitzas collectively called karren or lapiez. Medium-sized surface features may include sinkholes or cenotes (closed basins), vertical shafts, foibe (inverted funnel shaped sinkholes), disappearing streams, and reappearing springs . Large-scale features may include limestone pavements , poljes , and karst valleys. Mature karst landscapes, where more bedrock has been removed than remains, may result in karst towers , or haystack/eggbox landscapes. Beneath

3300-756: The tropics and subtropics. The northernmost atoll in the world is Kure Atoll at 28°25′ N, along with other atolls of the Northwestern Hawaiian Islands . The southernmost atolls in the world are Elizabeth Reef at 29°57′ S, and nearby Middleton Reef at 29°27′ 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°41′ S. The atoll closest to

3360-461: The water may have run unimpeded from a sinkhole in a cattle pasture, bypassing the normal filtering that occurs in a porous aquifer . Sinkholes have often been used as farmstead or community trash dumps . Overloaded or malfunctioning septic tanks in karst landscapes may dump raw sewage directly into underground channels. Geologists are concerned with these negative effects of human activity on karst hydrology which, as of 2007 , supplied about 25% of

3420-515: The western Highland Rim in the eastern United States is at the second-highest risk of karst sinkholes. In Canada, Wood Buffalo National Park , Northwest Territories contains areas of karst sinkholes. Mexico hosts important karst regions in the Yucatán Peninsula and Chiapas . The West of Ireland is home to The Burren , a karst limestone area. The South China Karst in the provinces of Guizhou , Guangxi , and Yunnan provinces

3480-460: The western rim of the reef. There were smaller settlements of Chagossians in Fouquet (0.45 km, 0.17 sq mi) and Takamaka (0.48 km, 0.19 sq mi) Islands. Île de la Passe is 0.28 km (0.11 sq mi) in area, and Île Mapou 0.04 km (0.02 sq mi). The remaining islets are much smaller. The total land area is 3.56 km (1.37 sq mi). There

3540-874: The world's atolls are in the Pacific Ocean (with concentrations in the Caroline Islands , the Coral Sea Islands , the Marshall Islands , the Tuamotu Islands , Kiribati , Tokelau , and Tuvalu ) and the Indian Ocean (the Chagos Archipelago , Lakshadweep , the atolls of the Maldives , and the Outer Islands of Seychelles ). In addition, Indonesia also has several atolls spread across the archipelago, such as in

3600-520: Was led by Professor William Johnson Sollas of the University of Oxford . Geologists included Walter George Woolnough and Edgeworth David of the University of Sydney . Professor Edgeworth David led the expedition in 1897. The third expedition in 1898 was led by Alfred Edmund Finckh. Karst Karst ( / k ɑːr s t / ) is a topography formed from the dissolution of soluble carbonate rocks such as limestone and dolomite . It

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