Igneous rock, or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava; the magma can be crust. The melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition. Solidification into rock occurs either below the surface as intrusive rocks or on the surface as extrusive rocks. Igneous rock may form with crystallization to form granular, crystalline rocks, or without crystallization to form natural glasses. Igneous rocks occur in a wide range of geological settings: shields, orogens, large igneous provinces, extended crust and oceanic crust. Igneous and metamorphic rocks make up 90–95% of the top 16 km of the Earth's crust by volume. Igneous rocks form about 15% of the Earth's current land surface. Most of the Earth's oceanic crust is made of igneous rock. Igneous rocks are geologically important because: their minerals and global chemistry give information about the composition of the mantle, from which some igneous rocks are extracted, the temperature and pressure conditions that allowed this extraction, and/or of other pre-existing rock that melted.
In terms of modes of occurrence, igneous rocks can be either extrusive. Intrusive igneous rocks make up the majority of igneous rocks and are formed from magma that cools and solidifies within the crust of a planet, surrounded by pre-existing rock; the mineral grains in such rocks can be identified with the naked eye. Intrusive rocks can be classified according to the shape and size of the intrusive body and its relation to the other formations into which it intrudes. Typical intrusive formations are batholiths, laccoliths and dikes; when the magma solidifies within the earth's crust, it cools forming coarse textured rocks, such as granite, gabbro, or diorite. The central cores of major mountain ranges consist of intrusive igneous rocks granite; when exposed by erosion, these cores may occupy huge areas of the Earth's surface. Intrusive igneous rocks that form at depth within the crust are termed plutonic rocks and are coarse-grained. Intrusive igneous rocks that form near the surface are termed subvolcanic or hypabyssal rocks and they are medium-grained.
Hypabyssal rocks are less common than plutonic or volcanic rocks and form dikes, laccoliths, lopoliths, or phacoliths. Extrusive igneous rocks known as volcanic rocks, are formed at the crust's surface as a result of the partial melting of rocks within the mantle and crust. Extrusive solidify quicker than intrusive igneous rocks, they are formed by the cooling of molten magma on the earth's surface. The magma, brought to the surface through fissures or volcanic eruptions, solidifies at a faster rate. Hence such rocks are smooth and fine-grained. Basalt is lava plateaus; some kinds of basalt solidify to form long polygonal columns. The Giant's Causeway in Antrim, Northern Ireland is an example; the molten rock, with or without suspended crystals and gas bubbles, is called magma. It rises; when magma reaches the surface from beneath water or air, it is called lava. Eruptions of volcanoes into air are termed subaerial, whereas those occurring underneath the ocean are termed submarine. Black smokers and mid-ocean ridge basalt are examples of submarine volcanic activity.
The volume of extrusive rock erupted annually by volcanoes varies with plate tectonic setting. Extrusive rock is produced in the following proportions: divergent boundary: 73% convergent boundary: 15% hotspot: 12%. Magma that erupts from a volcano behaves according to its viscosity, determined by temperature, crystal content and the amount of silica. High-temperature magma, most of, basaltic in composition, behaves in a manner similar to thick oil and, as it cools, treacle. Long, thin basalt flows with pahoehoe surfaces are common. Intermediate composition magma, such as andesite, tends to form cinder cones of intermingled ash and lava, may have a viscosity similar to thick, cold molasses or rubber when erupted. Felsic magma, such as rhyolite, is erupted at low temperature and is up to 10,000 times as viscous as basalt. Volcanoes with rhyolitic magma erupt explosively, rhyolitic lava flows are of limited extent and have steep margins, because the magma is so viscous. Felsic and intermediate magmas that erupt do so violently, with explosions driven by the release of dissolved gases—typically water vapour, but carbon dioxide.
Explosively erupted pyroclastic material is called tephra and includes tuff and ignimbrite. Fine volcanic ash is erupted and forms ash tuff deposits, which ca
Poor Knights Islands
The Poor Knights Islands are a group of islands off the east coast of the Northland Region of the North Island of New Zealand. They lie 50 kilometres to the north-east of Whangarei, 22 kilometres offshore halfway between Bream Head and Cape Brett. Uninhabited since the 1820s, they are a nature reserve and popular underwater diving spot, with boat tours departing from Tutukaka; the Poor Knights Islands Marine Reserve surrounds the island. Beaglehole comments that the origin of the island name is not clear, speculates that the name could be related to the Poor Knights of Windsor, or, that the islands were named for their resemblance to Poor Knight's Pudding, a bread-based dish topped with egg and fried, popular at the time of discovery by Europeans; the chain consists of two large islands, several smaller islands. Aorangaia and Archway Island lie to the southwest of Aorangi Island, there is a group of smaller rocky islets between the two main islands, the largest of, Motu Kapiti; the Poor Knights Islands are the eroded remnants of a 4-million-year-old rhyolitic volcano, estimated to have been 1,000 metres tall and 25 kilometres in diameter.
Spring tide range for the islands is around 2 m, decreasing to a neap tide of around 1 m. The deep water around the island results in only moderate tidal currents; these are around the same magnitude as the prevailing shelf currents. In the general vicinity of the islands mean flows are around 0.2 run toward the southeast. A remarkable feature of the region is the large internal tides; these are a form of internal wave driven by the local tidal flow forcing the stratification against sloping areas of the shelf face. The surface manifestation of these waves can be seen from space; these waves generate brief localised accelerations. Internal wave amplitudes of around 100 m have been observed, generating flow speeds as great as 0.5 m/s. The islands are protected as a nature reserve and a permit is required to land or tie boats up. Permits are granted only for scientific research. A notable native plant of the islands is the spectacularly flowering Poor Knights lily, which has become a popular garden plant.
Feral pigs, which had roamed Aorangi since the departure of Māori in the 1820s, were exterminated in 1936. The islands have been identified as an Important Bird Area, by BirdLife International because they are home to a breeding population of about 200,000 pairs of Buller's shearwaters; the islands contain rock arches and sea caves, including Rikoriko Cave, the largest cave in the world by volume, with a cavern measuring 221,494 cubic metres and an opening large enough for small tour boats to enter. Rikoriko Cave measures 130 by 80 metres, with a ceiling height of 35 metres and extends 26 metres deep below water. Tawhiti Rahi contains the Northern Arch, Middle Arch, Maomao Arch, the latter being a popular diving location. Aorangaia Island's east-west rock arch resembles a long tunnel, while the aptly named Archway Island is bisected by two rock arches, with the larger Cathedral Arch about 40 metres tall; the islands were earlier inhabited by Māori of the Ngāti Wai tribe who grew crops and fished the surrounding sea.
The tribe traded with other Maori. A chief of the tribe named Tatua led his warriors on a fighting expedition to the Hauraki Gulf with Ngā Puhi chief Hongi Hika in the early 1820s. While they were away, a slave escaped the islands and travelled to Hokianga where he told Waikato, a chief of the Hikutu tribe, that the islands had been left undefended; as Waikato had been offended by Tatua some years previous, he and his warriors set out on three large canoes to attack the islands. They arrived at the islands one night in December 1823 and soon overpowered the islanders in the absence of their warriors. Many islanders jumped off the high cliffs to avoid being taken as slaves. Tatua's wife and daughter were captured and taken to the mainland where a distant relative recognised the wife and helped the two to escape. Tatua returned to the islands to find a scene of destruction. Only nine or ten people were left on the islands, including his five-year-old son, hidden in a cave during the attack; the islands were declared tapu and Tatua left with the survivors and went to Rawhiti in the Bay of Islands where he unexpectedly found his wife and daughter.
List of islands of New Zealand List of volcanoes in New Zealand Poor Knights Islands nonmarine fauna "Geology – New Zealand's Geological History", from An Encyclopaedia of New Zealand, edited by A. H. McLintock published in 1966. Te Ara - The Encyclopedia of New Zealand, updated 2006-09-26. Retrieved 2007-04-15
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
Basalt is a mafic extrusive igneous rock formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or near the surface of a terrestrial planet or a moon. More than 90% of all volcanic rock on Earth is basalt. Basalt lava has a low viscosity, due to its low silica content, resulting in rapid lava flows that can spread over great areas before cooling and solidification. Flood basalt describes the formation in a series of lava basalt flows. By definition, basalt is an aphanitic igneous rock with 45–53% silica and less than 10% feldspathoid by volume, where at least 65% of the rock is feldspar in the form of plagioclase; this is as per definition of the International Union of Geological Sciences classification scheme. It is the most common volcanic rock type on Earth, being a key component of oceanic crust as well as the principal volcanic rock in many mid-oceanic islands, including Iceland, the Faroe Islands, Réunion and the islands of Hawaiʻi. Basalt features a fine-grained or glassy matrix interspersed with visible mineral grains.
The average density is 3.0 g/cm3. Basalt is defined by its mineral content and texture, physical descriptions without mineralogical context may be unreliable in some circumstances. Basalt is grey to black in colour, but weathers to brown or rust-red due to oxidation of its mafic minerals into hematite and other iron oxides and hydroxides. Although characterized as "dark", basaltic rocks exhibit a wide range of shading due to regional geochemical processes. Due to weathering or high concentrations of plagioclase, some basalts can be quite light-coloured, superficially resembling andesite to untrained eyes. Basalt has a fine-grained mineral texture due to the molten rock cooling too for large mineral crystals to grow; these phenocrysts are of olivine or a calcium-rich plagioclase, which have the highest melting temperatures of the typical minerals that can crystallize from the melt. Basalt with a vesicular texture is called vesicular basalt, when the bulk of the rock is solid; this texture forms when dissolved gases come out of solution and form bubbles as the magma decompresses as it reaches the surface, yet are trapped as the erupted lava hardens before the gases can escape.
The term basalt is at times applied to shallow intrusive rocks with a composition typical of basalt, but rocks of this composition with a phaneritic groundmass are referred to as diabase or, when more coarse-grained, as gabbro. Gabbro is marketed commercially as "black granite." In the Hadean and early Proterozoic eras of Earth's history, the chemistry of erupted magmas was different from today's, due to immature crustal and asthenosphere differentiation. These ultramafic volcanic rocks, with silica contents below 45% are classified as komatiites; the word "basalt" is derived from Late Latin basaltes, a misspelling of Latin basanites "very hard stone", imported from Ancient Greek βασανίτης, from βάσανος and originated in Egyptian bauhun "slate". The modern petrological term basalt describing a particular composition of lava-derived rock originates from its use by Georgius Agricola in 1556 in his famous work of mining and mineralogy De re metallica, libri XII. Agricola applied "basalt" to the volcanic black rock of the Schloßberg at Stolpen, believing it to be the same as the "very hard stone" described by Pliny the Elder in Naturalis Historiae.
Tholeiitic basalt is rich in silica and poor in sodium. Included in this category are most basalts of the ocean floor, most large oceanic islands, continental flood basalts such as the Columbia River Plateau. High and low titanium basalts. Basalt rocks are in some cases classified after their titanium content in High-Ti and Low-Ti varieties. High-Ti and Low-Ti basalts have been distinguished in the Paraná and Etendeka traps and the Emeishan Traps. Mid-ocean ridge basalt is a tholeiitic basalt erupted only at ocean ridges and is characteristically low in incompatible elements. E-MORB, enriched MORB N-MORB, normal MORB D-MORB, depleted MORB High-alumina basalt may be silica-undersaturated or -oversaturated, it has greater than 17% alumina and is intermediate in composition between tholeiitic basalt and alkali basalt. Alkali basalt is poor in silica and rich in sodium, it may contain feldspathoids, alkali feldspar and phlogopite. Boninite is a high-magnesium form of basalt, erupted in back-arc basins, distinguished by its low titanium content and trace-element composition.
Ocean island basalt Lunar basalt The mineralogy of basalt is characterized by a preponderance of calcic plagioclase feldspar and pyroxene. Olivine can be a significant constituent. Accessory minerals present in minor amounts include iron oxides and iron-titanium oxides, such as magnetite and ilmenite; because of the presence of such oxide minerals, basalt can acquire strong magnetic signatures as it cools, paleomagnetic studies have made extensive use of basalt. In tholeiitic basalt and calcium-rich plagioclase are common phenocryst minerals. Olivine may be a phenocryst, when
New Zealand is a sovereign island country in the southwestern Pacific Ocean. The country geographically comprises two main landmasses—the North Island, the South Island —and around 600 smaller islands. New Zealand is situated some 2,000 kilometres east of Australia across the Tasman Sea and 1,000 kilometres south of the Pacific island areas of New Caledonia and Tonga; because of its remoteness, it was one of the last lands to be settled by humans. During its long period of isolation, New Zealand developed a distinct biodiversity of animal and plant life; the country's varied topography and its sharp mountain peaks, such as the Southern Alps, owe much to the tectonic uplift of land and volcanic eruptions. New Zealand's capital city is Wellington. Sometime between 1250 and 1300, Polynesians settled in the islands that were named New Zealand and developed a distinctive Māori culture. In 1642, Dutch explorer Abel Tasman became the first European to sight New Zealand. In 1840, representatives of the United Kingdom and Māori chiefs signed the Treaty of Waitangi, which declared British sovereignty over the islands.
In 1841, New Zealand became a colony within the British Empire and in 1907 it became a dominion. Today, the majority of New Zealand's population of 4.9 million is of European descent. Reflecting this, New Zealand's culture is derived from Māori and early British settlers, with recent broadening arising from increased immigration; the official languages are English, Māori, NZ Sign Language, with English being dominant. A developed country, New Zealand ranks in international comparisons of national performance, such as quality of life, education, protection of civil liberties, economic freedom. New Zealand underwent major economic changes during the 1980s, which transformed it from a protectionist to a liberalised free-trade economy; the service sector dominates the national economy, followed by the industrial sector, agriculture. Nationally, legislative authority is vested in an elected, unicameral Parliament, while executive political power is exercised by the Cabinet, led by the prime minister Jacinda Ardern.
Queen Elizabeth II is the country's monarch and is represented by a governor-general Dame Patsy Reddy. In addition, New Zealand is organised into 11 regional councils and 67 territorial authorities for local government purposes; the Realm of New Zealand includes Tokelau. New Zealand is a member of the United Nations, Commonwealth of Nations, ANZUS, Organisation for Economic Co-operation and Development, ASEAN Plus Six, Asia-Pacific Economic Cooperation, the Pacific Community and the Pacific Islands Forum. Dutch explorer Abel Tasman sighted New Zealand in 1642 and named it Staten Land "in honour of the States General", he wrote, "it is possible that this land joins to the Staten Land but it is uncertain", referring to a landmass of the same name at the southern tip of South America, discovered by Jacob Le Maire in 1616. In 1645, Dutch cartographers renamed the land Nova Zeelandia after the Dutch province of Zeeland. British explorer James Cook subsequently anglicised the name to New Zealand. Aotearoa is the current Māori name for New Zealand.
It is unknown whether Māori had a name for the whole country before the arrival of Europeans, with Aotearoa referring to just the North Island. Māori had several traditional names for the two main islands, including Te Ika-a-Māui for the North Island and Te Waipounamu or Te Waka o Aoraki for the South Island. Early European maps labelled the islands North and South. In 1830, maps began to use North and South to distinguish the two largest islands and by 1907 this was the accepted norm; the New Zealand Geographic Board discovered in 2009 that the names of the North Island and South Island had never been formalised, names and alternative names were formalised in 2013. This set the names as North Island or Te Ika-a-Māui, South Island or Te Waipounamu. For each island, either its English or Māori name can be used. New Zealand was one of the last major landmasses settled by humans. Radiocarbon dating, evidence of deforestation and mitochondrial DNA variability within Māori populations suggest New Zealand was first settled by Eastern Polynesians between 1250 and 1300, concluding a long series of voyages through the southern Pacific islands.
Over the centuries that followed, these settlers developed a distinct culture now known as Māori. The population was divided into iwi and hapū who would sometimes cooperate, sometimes compete and sometimes fight against each other. At some point a group of Māori migrated to Rēkohu, now known as the Chatham Islands, where they developed their distinct Moriori culture; the Moriori population was all but wiped out between 1835 and 1862 because of Taranaki Māori invasion and enslavement in the 1830s, although European diseases contributed. In 1862 only 101 survived, the last known full-blooded Moriori died in 1933; the first Europeans known to have reached New Zeala
In geology, a blowhole or marine geyser is formed as sea caves grow landwards and upwards into vertical shafts and expose themselves towards the surface, which can result in hydraulic compression of sea water, released through a port from the top of the blowhole. The geometry of the cave and blowhole along with tide levels and swell conditions determine the height of the spray. Blowholes are to occur in areas where there are crevices, such as lava tubes, in rock along the coast; these areas are located along fault lines and on islands. As powerful waves hit the coast, water rushes into these crevices and bursts out in a high pressured release, it is accompanied by a loud noise and wide spray, for this reason, blowholes are sites of tourism. Marine erosion on rocky coastlines produce blowholes, they are found at intersecting faults and on the windward sides of a coastline where they receive higher wave energy from the open ocean. The development of a blowhole is linked to the formation of a littoral cave.
These two elements make up the blowhole system. A blowhole system always contains three main features: a catchment entrance, a compression cavern and an expelling port; the arrangement and size of these three features determine the force of the air to water ratio, ejected from the port. The blowhole feature tends to occur in the most distal section of a littoral cave; as their name suggests, blowholes have the ability to move air rapidly. Strong reverse draughts in response to pressure changes in a connecting littoral cave can send wind speeds upwards of 70 km/h; the formation of a blowhole system begins. The main factors that contribute to littoral caves formation are wave dynamics and the parent material’s rock property. A parent material property such as susceptibility or resistance to weathering plays a major role in the development of caves. Littoral caves can be formed by one of two processes: caves made of limestone are produced by karst processes, caves made of igneous rock are produced by pseudokarst processes.
In time the littoral cave enlarges growing inland and vertically through weak joints in the parent material. As weathering continues the roof of the cave is exposed, the blowhole continues to enlarge the roof of the littoral cave is weaken and collapses; this creates a steep-wall inlet. Blowholes have the capacity to change the topography near their locations. Blowholes can erode the area surrounding the crevices to form larger sea caves. In some instances, the cave itself may collapse; this event may create shallow pools along the coast. A blowhole is the name of a rare geologic feature in which air is blown through a small hole at the surface due to pressure differences between a closed underground system and the surface; the blowholes of Wupatki National Monument are an example of such a phenomenon. It is estimated that the closed underground passages have a volume of at least seven billion cubic feet. Wind speeds can approach 30 miles per hour. Another well-known example of the blowhole is the natural entrance to the Wind Cave.
La Bufadora is a large blowhole located in the Punta Banda Peninsula of Mexico. It consists of a littoral cave with a thin opening that has a recurrence eruption interval of 13 -17 seconds, ejecting water up to 100 ft. above sea level
The sea, the world ocean or the ocean is the connected body of salty water that covers over 70 percent of the Earth's surface. It moderates the Earth's climate and has important roles in the water cycle, carbon cycle, nitrogen cycle, it has been travelled and explored since ancient times, while the scientific study of the sea—oceanography—dates broadly from the voyages of Captain James Cook to explore the Pacific Ocean between 1768 and 1779. The word "sea" is used to denote smaller landlocked sections of the ocean and certain large landlocked, saltwater lakes such as the Caspian Sea and the Dead Sea; the most abundant solid dissolved in sea water is sodium chloride. The water contains salts of magnesium and potassium, amongst many other elements, some in minute concentrations. Salinity varies being lower near the surface and the mouths of large rivers and higher in the depths of the ocean. Winds blowing over the surface of the sea produce waves. Winds create surface currents through friction, setting up slow but stable circulations of water throughout the oceans.
The directions of the circulation are governed by factors including the shapes of the continents and the rotation of the earth. Deep-sea currents, known as the global conveyor belt, carry cold water from near the poles to every ocean. Tides, the twice-daily rise and fall of sea levels, are caused by the rotation of the Earth and the gravitational effects of the orbiting Moon, to a lesser extent of the Sun. Tides may have a high range in bays or estuaries. Submarine earthquakes arising from tectonic plate movements under the oceans can lead to destructive tsunamis, as can volcanoes, huge landslides or the impact of large meteorites. A wide variety of organisms, including bacteria, algae, plants and animals, live in the sea, which offers a wide range of marine habitats and ecosystems, ranging vertically from the sunlit surface waters and the shoreline to the enormous depths and pressures of the cold, dark abyssal zone, in latitude from the cold waters under the Arctic ice to the colourful diversity of coral reefs in tropical regions.
Many of the major groups of organisms evolved in the sea and life may have started there. The sea provides substantial supplies of food for humans fish, but shellfish and seaweed, whether caught by fishermen or farmed underwater. Other human uses of the sea include trade, mineral extraction, power generation and leisure activities such as swimming and scuba diving. Many of these activities create marine pollution; the sea is important in human culture, with major appearances in literature at least since Homer's Odyssey, in marine art, in cinema, in theatre and in classical music. Symbolically, the sea appears as monsters such as Scylla in mythology and represents the unconscious mind in dream interpretation; the sea is the interconnected system of all the Earth's oceanic waters, including the Atlantic, Indian and Arctic Oceans. However, the word "sea" can be used for many specific, much smaller bodies of seawater, such as the North Sea or the Red Sea. There is no sharp distinction between seas and oceans, though seas are smaller, are partly or wholly bordered by land.
However, the Sargasso Sea has no coastline and lies within a circular current, the North Atlantic Gyre. Seas are larger than lakes and contain salt water, but the Sea of Galilee is a freshwater lake; the United Nations Convention on the Law of the Sea states that all of the ocean is "sea". Earth is the only known planet with seas of liquid water on its surface, although Mars possesses ice caps and similar planets in other solar systems may have oceans, it is still unclear where Earth's water came from, seen from space, our planet appears as a "blue marble" of its various forms: oceans, ice caps, clouds. Earth's 1,335,000,000 cubic kilometers of sea contain about 97.2 percent of its known water and cover more than 70 percent of its surface. Another 2.15% of Earth's water is frozen, found in the sea ice covering the Arctic Ocean, the ice cap covering Antarctica and its adjacent seas, various glaciers and surface deposits around the world. The remainder form underground reservoirs or various stages of the water cycle, containing the freshwater encountered and used by most terrestrial life: vapor in the air, the clouds it forms, the rain falling from them, the lakes and rivers spontaneously formed as its waters flow again and again to the sea.
The sea's dominance of the planet is such that the British author Arthur C. Clarke once noted that "Earth" would have been better named "Ocean"; the scientific study of water and Earth's water cycle is hydrology. The more recent study of the sea in particular is oceanography; this began as the study of the shape of the ocean's currents but has since expanded into a large and multidisciplinary field: it examines the properties of seawater. The subfield dealing with the sea's motion, its forces, the forces acting upon it is known as physical oceanography. Marine biology studies the plants and other organisms inhabiting marine ecosystems. Both are informed by chemical oceanography, which studies the behavior of elements and molecules within the oceans: at the moment, the ocean's role in the carbon cycle and carbon dioxide's role in the increasing acid