High island

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Moorea, a high island of volcanic origin where the central island is still prominent. Moorea ISS006.jpg
Moorea, a high island of volcanic origin where the central island is still prominent.
Stromboli is one of the eight Aeolian Islands, a volcanic arc north of Sicily. DenglerSW-Stromboli-20040928-1230x800.jpg
Stromboli is one of the eight Aeolian Islands, a volcanic arc north of Sicily.

In geology (and sometimes in archaeology), a high island or volcanic island is an island of volcanic origin. The term can be used to distinguish such islands from low islands, which are formed from sedimentation or the uplifting of coral reefs [1] (which have often formed on sunken volcanos).

Geology The study of the composition, structure, physical properties, and history of Earths components, and the processes by which they are shaped.

Geology is an earth science concerned with the solid Earth, the rocks of which it is composed, and the processes by which they change over time. Geology can also include the study of the solid features of any terrestrial planet or natural satellite such as Mars or the Moon. Modern geology significantly overlaps all other earth sciences, including hydrology and the atmospheric sciences, and so is treated as one major aspect of integrated earth system science and planetary science.

Archaeology, or archeology, is the study of human activity through the recovery and analysis of material culture. The archaeological record consists of artifacts, architecture, biofacts or ecofacts and cultural landscapes. Archaeology can be considered both a social science and a branch of the humanities. In North America archaeology is a sub-field of anthropology, while in Europe it is often viewed as either a discipline in its own right or a sub-field of other disciplines.

Island Any piece of sub-continental land that is surrounded by water

An island or isle is any piece of sub-continental land that is surrounded by water. Very small islands such as emergent land features on atolls can be called islets, skerries, cays or keys. An island in a river or a lake island may be called an eyot or ait, and a small island off the coast may be called a holm. A grouping of geographically or geologically related islands is called an archipelago, such as the Philippines.

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Definition and origin

There are a number of "high islands" which rise no more than a few feet above sea level, often classified as "islets or rocks", while some "low islands", such as Makatea, Nauru, Niue, Henderson and Banaba, as uplifted coral islands, rise several hundred feet above sea level.

Sea level Average level for the surface of the ocean at any given geographical position on the planetary surface

Mean sea level (MSL) is an average level of the surface of one or more of Earth's bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datum – a standardised geodetic datum – that is used, for example, as a chart datum in cartography and marine navigation, or, in aviation, as the standard sea level at which atmospheric pressure is measured to calibrate altitude and, consequently, aircraft flight levels. A common and relatively straightforward mean sea-level standard is instead the midpoint between a mean low and mean high tide at a particular location.

Islet Very small island

An islet is a very small island.

Makatea island in French Polynesia

Makatea, or Mangaia-te-vai-tamae, is a raised coral atoll in the northwestern part of the Tuamotus, which is a part of the French overseas collectivity of French Polynesia. It is located 79 kilometres (49 mi) southwest from Rangiroa to the west of the Palliser group, which also is in French Polynesia. Makatea is surrounded by spectacular cliffs, rising to a plateau 80 metres (260 ft) above sea level. This island is 7.5 kilometres (4.7 mi) long, with a maximum width of 7 kilometres (4.3 mi) in the south. It is 24 square kilometres (9.3 sq mi) in area. Makatea is one of the only four islands of the Tuamotu Archipelago that do not take the form of a typical atoll.

The two types of islands are often found in proximity to each other, especially among the islands of the South Pacific Ocean, where low islands are found on the fringing reefs that surround most high islands. Volcanic islands normally arise above a so-called hotspot.

Fringing reef Type of coral reef

A fringing reef is one of the four main types of coral reef recognized by most coral reef scientists. It is distinguished from the other main types in that it has either an entirely shallow backreef zone (lagoon) or none at all. If a fringing reef grows directly from the shoreline the reef flat extends right to the beach and there is no backreef. In other cases, fringing reefs may grow hundreds of yards from shore and contain extensive backreef areas with numerous seagrass meadows and patch reefs.

Hotspot (geology) Volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the surrounding mantle

In geology, the places known as hotspots or hot spots are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the surrounding mantle. Their position on the Earth's surface is independent of tectonic plate boundaries. There are two hypotheses that attempt to explain their origins. One suggests that hotspots are due to mantle plumes that rise as thermal diapirs from the core–mantle boundary. The other hypothesis is that lithospheric extension permits the passive rising of melt from shallow depths. This hypothesis considers the term "hotspot" to be a misnomer, asserting that the mantle source beneath them is, in fact, not anomalously hot at all. Well-known examples include the Hawaii, Iceland and Yellowstone hotspots.

Habitability for humans

The differences in geology and topography between high and low islands mean a lot in terms of habitability for humans: high islands above a certain size usually have fresh groundwater, while low islands often do not. This hampers human settlement on many low islands.

Groundwater water located beneath the ground surface

Groundwater is the water present beneath Earth's surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock become completely saturated with water is called the water table. Groundwater is recharged from the surface; it may discharge from the surface naturally at springs and seeps, and can form oases or wetlands. Groundwater is also often withdrawn for agricultural, municipal, and industrial use by constructing and operating extraction wells. The study of the distribution and movement of groundwater is hydrogeology, also called groundwater hydrology.

See also

An archipelagic apron is a fan-shaped gently sloping region of sea floor found around oceanic islands, particularly in the south Pacific Ocean. The name was first proposed by Henry William Menard in 1956 because of the resemblance of the slope to an apron. While most such underwater sedimentary surfaces are generally smooth, some are rough in texture. The total volume occupied by an archipelagic apron is often many times that of the adjacent island. Hence, it is unlikely to have formed from sediments washed off the part of the island above sea level. Instead, archipelagic aprons most likely formed from debris avalanches and pyroclastic flows. The material is then reworked by turbidity currents to achieve their current form. Such aprons form about eight percent of the area of the pacific basin.

Atoll Ring-shaped coral reef, generally formed over a subsiding oceanic volcano, with a central lagoon and perhaps islands around the rim

An atoll, sometimes called a coral atoll, is a ring-shaped coral reef including a coral rim that encircles a lagoon partially or completely. There may be coral islands or cays on the rim. The coral of the atoll often sits atop the rim of an extinct seamount or volcano which has eroded or subsided partially 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.

Galápagos Islands Achipelago and protected area of Ecuador in the Pacific Ocean

The Galápagos Islands, part of the Republic of Ecuador, are an archipelago of volcanic islands distributed on either side of the equator in the Pacific Ocean surrounding the centre of the Western Hemisphere, 906 km (563 mi) west of continental Ecuador. The islands are known for their large number of endemic species and were studied by Charles Darwin during the second voyage of HMS Beagle. His observations and collections contributed to the inception of Darwin's theory of evolution by means of natural selection.

Related Research Articles

Geography of Nauru

Nauru is a tiny phosphate rock island nation located in the South Pacific Ocean south of the Marshall Islands in Oceania. It is only 53 kilometres (33 mi) south of the Equator at coordinates 0°32′S166°55′E. Nauru is one of the three great phosphate rock islands in the Pacific Ocean—the others are Banaba in Kiribati and Makatea in French Polynesia.

Geography of Tonga

Located in Oceania, Tonga is a small archipelago in the South Pacific Ocean, directly south of Samoa and about two-thirds of the way from Hawaii to New Zealand. It has 169 islands, 36 of them inhabited, are divided into three main groups – Vava'u, Ha'apai, and Tongatapu – and cover an 800-kilometre (500-mile)-long north-south line. The total size is just 747 km2 (288 sq mi). Due to the spread out islands it has the 40th largest Exclusive Economic Zone of 659,558 km2 (254,657 sq mi).

Guyot An isolated, flat-topped underwater volcano mountain

In marine geology, a guyot, also known as a tablemount, is an isolated underwater volcanic mountain (seamount) with a flat top more than 200 m (660 ft) below the surface of the sea. The diameters of these flat summits can exceed 10 km (6.2 mi). Guyots are most commonly found in the Pacific Ocean, but they have been identified in all the oceans except the Arctic Ocean.

Seamount A mountain rising from the ocean seafloor that does not reach to the waters surface

A seamount is a mountain rising from the ocean floor that does not reach to the water's surface, and thus is not an island, islet or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise abruptly and are usually found rising from the seafloor to 1,000–4,000 m (3,300–13,100 ft) in height. They are defined by oceanographers as independent features that rise to at least 1,000 m (3,281 ft) above the seafloor, characteristically of conical form. The peaks are often found hundreds to thousands of meters below the surface, and are therefore considered to be within the deep sea. During their evolution over geologic time, the largest seamounts may reach the sea surface where wave action erodes the summit to form a flat surface. After they have subsided and sunk below the sea surface such flat-top seamounts are called "guyots" or "tablemounts".

Zavodovski Island island

Zavodovski Island is an uninhabited volcanic island in the Traversay Islands subgroup of the South Sandwich Islands. It lies 350 kilometres (217 mi) southeast of South Georgia Island. It is the northernmost of the South Sandwich Islands and the nearest to South Georgia.

Geology of the United States Regional geology

The richly textured landscape of the United States is a product of the dueling forces of plate tectonics, weathering and erosion. Over the 4.5 billion-year history of our Earth, tectonic upheavals and colliding plates have raised great mountain ranges while the forces of erosion and weathering worked to tear them down. Even after many millions of years, records of Earth's great upheavals remain imprinted as textural variations and surface patterns that define distinctive landscapes or provinces.

National Park of American Samoa United States national park in American Samoa

The National Park of American Samoa is a national park in the United States territory of American Samoa, distributed across three islands: Tutuila, Ofu, and Ta‘ū. The park preserves and protects coral reefs, tropical rainforests, fruit bats, and the Samoan culture. Popular activities include hiking and snorkeling. Of the park's 13,500 acres (5,500 ha), 9,000 acres (3,600 ha) is land and 4,500 acres (1,800 ha) is coral reefs and ocean. The park is the only American National Park Service system unit south of the equator.

Hanauma Bay bay

Hanauma is a marine embayment formed within a tuff ring and located along the southeast coast of the Island of Oʻahu in the Hawaii Kai neighborhood of East Honolulu, in the Hawaiian Islands.

Evolution of Hawaiian volcanoes Processes of growth and erosion of the volcanoes of the Hawaiian islands

The fifteen volcanoes that make up the eight principal islands of Hawaii are the youngest in a chain of more than 129 volcanoes that stretch 5,800 kilometres (3,600 mi) across the North Pacific Ocean, called the Hawaiian-Emperor seamount chain. Hawaiʻi's volcanoes rise an average of 4,572 metres (15,000 ft) to reach sea level from their base. The largest, Mauna Loa, is 4,169 metres (13,678 ft) high. As shield volcanoes, they are built by accumulated lava flows, growing a few meters/feet at a time to form a broad and gently sloping shape.

Low island island of coral origin

A low island is, in geology, an island of coral origin. The term applies whether the island was formed as a result of sedimentation upon a coral reef or of the uplifting of such islands. The term is used to distinguish such islands from high islands, whose origins are volcanic.

The Hawaiian Trough, otherwise known as the Hawaiian Deep, is a moat-like depression of the seafloor surrounding the Hawaiian Islands. The weight from the Volcanic Island chain depresses the plastic Lithosphere that is already weakened by the underlying thermal hotspot, causing subsidence to occur. The location with the greatest rate of subsidence is directly above the hotspot with a rate of about 2.5 millimeters per year. The Hawaiian Trough is about 5500 meters deep. The subsiding lithosphere is balanced out and through the concept of isostasy a part of the crust surrounding the trough is levered upwards creating the Hawaiian Arch. The Hawaiian Arch extends about 200 meters above the surrounding ocean floor, and contains tilted coral reefs.

A coral island is a type of island formed from coral detritus and associated organic material. They occur in tropical and sub-tropical areas, typically as part of coral reefs which have grown to cover a far larger area under the sea.

<i>The Structure and Distribution of Coral Reefs</i> book by Charles Darwin

The Structure and Distribution of Coral Reefs, Being the first part of the geology of the voyage of the Beagle, under the command of Capt. Fitzroy, R.N. during the years 1832 to 1836, was published in 1842 as Charles Darwin's first monograph, and set out his theory of the formation of coral reefs and atolls. He conceived of the idea during the voyage of the Beagle while still in South America, before he had seen a coral island, and wrote it out as HMS Beagle crossed the Pacific Ocean, completing his draft by November 1835. At the time there was great scientific interest in the way that coral reefs formed, and Captain Robert FitzRoy's orders from the Admiralty included the investigation of an atoll as an important scientific aim of the voyage. FitzRoy chose to survey the Keeling Islands in the Indian Ocean. The results supported Darwin's theory that the various types of coral reefs and atolls could be explained by uplift and subsidence of vast areas of the Earth's crust under the oceans.

Geology of the Pacific Ocean

The Pacific Ocean evolved in the Mesozoic from the Panthalassic Ocean, which had formed when Rodinia rifted apart around 750 Ma. The first ocean floor which is part of the current Pacific Plate began 160 Ma to the west of the central Pacific and subsequently developed into the largest oceanic plate on Earth.

Madeira began to form more than 100 million years ago in the Early Cretaceous, although most of the island has formed in the last 66 million years of the Cenozoic, particularly in the Miocene and Pliocene. The island is an example of hotspot volcanism, with mainly mafic volcanic and igneous rocks, together with smaller deposits of limestone, lignite and other sediments that record its long-running uplift.

The geology of Guam formed as a result of mafic, felsic and intermediate composition volcanic rocks erupting below the ocean, building up the base of the island in the Eocene, between 33.9 and 56 million years ago. The island emerged above the water in the Eocene, although the volcanic crater collapsed. A second volcanic crater formed on the south of the island in the Oligocene and Miocene. In the shallow water, numerous limestone formations took shape, with thick alternating layers of volcanic material. The second crater collapsed and Guam went through a period in which it was almost entirely submerged, resembling a swampy atoll, until structural deformation slowly uplifted different parts of the island to their present topography. The process of uplift led to widespread erosion and clay formation, as well as the deposition of different types of limestone, reflecting different water depths.

Limalok A Cretaceous-Paleocene guyot in the Marshall Islands

Limalok is a Cretaceous-Paleocene guyot/tablemount in the southeastern Marshall Islands, one of a number of seamounts in the Pacific Ocean. It was probably formed by a volcanic hotspot in present-day French Polynesia. Limalok lies southeast of Mili Atoll and Knox Atoll, which rise above sea level, and is joined to each of them through a volcanic ridge. It is located at a depth of 1,255 metres (4,117 ft) and has a summit platform with an area of 636 square kilometres (246 sq mi).

References

  1. Murphy, Raymond E. (July 1949). ""High" and "Low" Islands in the Eastern Carolines". Geographical Review . American Geographical Society. 39 (3): 425–439. doi:10.2307/210643. JSTOR   210643.