Lake islands may form in numerous ways. They may occur through a build-up ofsedimentation asshoals, and become true islands through changes in the level of the lake. They may have been originally part of the lake's shore, and been separated from it byerosion, or they may have been left as pinnacles when the lake formed through a raising in the level of a river or other waterway (either naturally, or artificially through the damming of a river or lake). On creation of aglacial lake amoraine can form an island. They may also have formed throughearthquake,meteor, orvolcanic activity. In the latter case,crater or caldera islands exist, with new volcanic prominences in lakes formed in the craters of larger volcanoes. Other lake islands include ephemeral beds of floating vegetation, and islands artificially formed by human activity.
Lakes may sometimes form in the circular depressions ofvolcanic craters. These craters are typically circular or oval basins around the vent or vents from which magma erupts. A largevolcanic eruption sometimes results in the formation of acaldera, caused by the collapse of themagma chamber under the volcano. If enough magma is ejected, the emptied chamber is unable to support the weight of the volcano, and a roughly circular fracture, thering fault, develops around the edge of the chamber. The centre of the volcano within the ring fracture collapses, creating a ring-shaped depression. Long after the eruption, this caldera may fill with water to become a lake. If volcanic activity continues or restarts, the centre of the caldera may be uplifted in the form of aresurgent dome, to become a crater lake island. Though typically calderas are larger and deeper than craters and form in different ways, a distinction between the two is often ignored in non-technical circumstances and the termcrater lake is widely used for the lakes formed in both craters and calderas. The following is a list of large or notable crater lake islands:
Impact craters, formed by the collision of largemeteorites orcomets with the Earth, are relatively uncommon, and those which do exist are frequently heavily eroded or deeply buried. Several, however, do contain lakes. Where the impact crater iscomplex, a central peak emerges from the floor of the crater. If a lake is present, this central peak may break the water's surface as an island. In other cases, other geological processes may have caused only a ring-shapedannular lake to remain from an impact, with a large central island taking up the remaining area of the crater. The world's largest impact crater island (and the world's second-largest lake island of any kind) isRené-Levasseur Island, inLake Manicouagan, Canada. TheSanshan Islands ofLake Tai, China, are also examples of impact crater islands, as are the islands in Canada'sClearwater Lakes, and theSlate Islands of Lake Superior, also in Canada.Sollerön Island inSiljan Lake, Sweden, and an unnamed island in LakeKarakul, Tajikistan, was also formed by meteor impact.
The termfloating island is sometimes used for accumulations of vegetation free-floating within a body of water. Due to the lack of currents and tides, these are more frequently found in lakes than in rivers or the open sea. Peaty masses of vegetable matter from shallow lake floors may rise due to the accumulation of gases during decomposition, and will often float for a considerable time, becoming ephemeral islands until the gas has dissipated enough for the vegetation to return to the lake floor.[1]
Artificial or man-made islands are islands constructed by human activity rather than formed by natural means. They may be totally created by humans, enlarged from existing islands or reefs, formed by joining small existing islands, or cut from a mainland (for example, by cutting through the isthmus of a peninsula). Artificial islands have a long history, dating back to thecrannogs of prehistoric Britain and Ireland, and the traditional floatingUru islands ofLake Titicaca in South America. Notable early artificial islands include the Aztec city ofTenochtitlan, at the site of modernMexico City. Though technically caused by human activity, islands formed from hilltops by the deliberate flooding of valleys (such as in the creation ofhydroelectricity projects andreservoirs) are not normally regarded as artificial islands.
The artificial island of IJburg, Amsterdam, Netherlands
Artificial islands are built for numerous uses, ranging from flood protection to immigration or quarantine stations. Other uses for reclaimed artificial islands include expansion of living space or transportation centres in densely populated regions. Agricultural land has also been developed through reclamation ofpolders in the Netherlands and other low lying countries.
Notable modern examples of artificial lake islands include the Dutch polder ofFlevopolder inFlevoland, the island ofIJburg inAmsterdam, and Flamingo Island inKamfers Dam, South Africa. At 948 km2 (366 sq mi), Flevopolder, in the now-freshwater lakeIJsselmeer, is the largest man-made island in the world.
Other islands are lost by sinking below the lake surface, either by erosion, subsidence, or rising water level.Sunken Island inOtsego Lake is one example. Islands may also be lost by being artificially attached to the mainland, such asUrk in the formerZuider Zee.
There are few naturally occurring lake islands with an area in excess of 270 km2 (100 sq mi). Of these, five are located in the largeGreat Lakes ofNorth America, three are located in the largeAfrican Great Lakes, one is located in the largest lake inCentral America, one was formed by the world'sfourth largest meteorite impact, and one is located in thelargest (by volume) lake in the world.
Soisalo, a 1,638 km2 (632 sq mi) body of land in Finland that is surrounded by individual lakes (Kallavesi,Suvasvesi,Kermajärvi, Ruokovesi,Haukivesi andUnnukka) connected by creeks and rivers – rather than sitting within an individual lake – was suggested in a 1987 study as an island, due to being effectively "surrounded by water".[5] Other scientists rebut this claim,[5] noting that the waters surrounding Soisalo are not on the same level, with elevation differences up to 6 m (20 ft) between the surrounding lakes, and does not meet the criteria of a true island.[5]
Samosir, a 630 km2 (240 sq mi) body of land inLake Toba,Indonesia, is a peninsula that is technically surrounded by water only because a canal was built across it, effectively separating it from the mainland. For this reason, it is not a naturally occurring lake island.
The largest island in a lake on an island isSamosir (a peninsula that is technically "surrounded by water" only because a narrow canal was built across it) inDanau Toba onSumatra – 630 km2 (240 sq mi).[7]
The largest lake on an island in a lake isLake Manitou on Manitoulin Island in Lake Huron – 104 km2 (40 sq mi).[7]
Vanajavesi with its several lake islands in Finland
Vozrozhdeniya Island in theAral Sea,Kazakhstan andUzbekistan – 2,300 km2 (890 sq mi).[2] Originally only 200 square kilometres (77 sq mi), the island grew rapidly from the 1960s until mid-2001, as the shrinking of the Aral Sea caused the water to recede from the land around the original island, until the moment when that same process caused the expanded island to connect to the mainland. By 2014, what used to be an island had become merely a part of the extensiveAralkum Desert.
Sääminginsalo inSaimaa,Finland – 1,069 km2 (413 sq mi). Saimaa is sometimes referred to as a "lake system", and Sääminginsalo is surrounded by three separately named lakes (Haukivesi,Puruvesi andPihlajavesi) that are at the same level, and by an artificial canal,Raikuun kanava, built in the 1750s. Since it is only separated from other land by a canal, it is debatable whether Sääminginsalo can be considered an island.
ThePamvotida lake, next to the city ofIoannina,Greece - 19.4 km2 (7.5 sq mi), has an island with a village. The name of the village is "Nisos", which is the Greek word for island. The village has 219 permanent residents according to the 2011 census.[8] The size of the island is 0.52 km2 (0.20 sq mi).
Lake Taal on the island ofLuzon in thePhilippines featuresVolcano Island, which in turn has its own lake,Main Crater Lake. This lake features the amalgamation of cinder cones and craters known asVulcan Point, making it one of the few known third-order recursive lake-islands.
.jpg)
