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154-463: Liquid water has been present on Earth for most of the history of the planet . The underwater environment is thought to be the place of the origin of life on Earth, and it remains the ecological region most critical to the support of life and the natural habitat of the majority of living organisms. Several branches of science are dedicated to the study of this environment or specific parts or aspects of it. A number of human activities are conducted in

308-606: A basin , that is surrounded by land, apart from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean , and therefore are distinct from lagoons , and are also larger and deeper than ponds , though there are no official or scientific definitions. Lakes can be contrasted with rivers or streams , which are usually flowing. Most lakes are fed and drained by rivers and streams. Natural lakes are generally found in mountainous areas, rift zones , and areas with ongoing glaciation . Other lakes are found in endorheic basins or along

462-603: A nucleus or membrane-bound organelles such as mitochondria or chloroplasts . Like modern cells, it used DNA as its genetic code, RNA for information transfer and protein synthesis , and enzymes to catalyze reactions . Some scientists believe that instead of a single organism being the last universal common ancestor, there were populations of organisms exchanging genes by lateral gene transfer . The Proterozoic eon lasted from 2.5 Ga to 538.8 Ma (million years) ago. In this time span, cratons grew into continents with modern sizes. The change to an oxygen-rich atmosphere

616-409: A body of water from 2 hectares (5 acres) to 8 hectares (20 acres). Pioneering animal ecologist Charles Elton regarded lakes as waterbodies of 40 hectares (99 acres) or more. The term lake is also used to describe a feature such as Lake Eyre , which is a dry basin most of the time but may become filled under seasonal conditions of heavy rainfall. In common usage, many lakes bear names ending with

770-592: A coarse resolution; particularly-strategic areas have been mapped in detail, to assist in navigating and detecting submarines, though the resulting maps may be classified. An ocean is a body of water that composes much of a planet 's hydrosphere . On Earth , an ocean is one of the major conventional divisions of the World Ocean . These are, in descending order by area, the Pacific , Atlantic , Indian , Southern (Antarctic), and Arctic Oceans. The word "ocean"

924-500: A comprehensive analysis of the origin of lakes and proposed what is a widely accepted classification of lakes according to their origin. This classification recognizes 11 major lake types that are divided into 76 subtypes. The 11 major lake types are: Tectonic lakes are lakes formed by the deformation and resulting lateral and vertical movements of the Earth's crust. These movements include faulting, tilting, folding, and warping. Some of

1078-400: A depth of only 10 metres (33 ft) (9.8 metres (32 ft) for sea water). Thus, at about 10 m below the surface, the water exerts twice the pressure (2 atmospheres or 200 kPa) as air at surface level. Any object immersed in water is subjected to a buoyant force that counters the force of gravity , appearing to make the object less heavy. If the overall density of the object exceeds

1232-538: A distance of 1  astronomical unit (AU), the distance of the Earth from the Sun, probably did not contribute any water to the Earth because the solar nebula was too hot for ice to form and the hydration of rocks by water vapor would have taken too long. The water must have been supplied by meteorites from the outer asteroid belt and some large planetary embryos from beyond 2.5 AU. Comets may also have contributed. Though most comets are today in orbits farther away from

1386-402: A form of organic lake. They form where a buildup of partly decomposed plant material in a wet environment leaves the vegetated surface below the water table for a sustained period of time. They are often low in nutrients and mildly acidic, with bottom waters low in dissolved oxygen. Artificial lakes or anthropogenic lakes are large waterbodies created by human activity . They can be formed by

1540-538: A higher perimeter to area ratio than other lake types. These form where sediment from a tributary blocks the main river. These form where sediment from the main river blocks a tributary, usually in the form of a levee . Lakes formed by other processes responsible for floodplain basin creation. During high floods they are flushed with river water. There are four types: 1. Confluent floodplain lake, 2. Contrafluent-confluent floodplain lake, 3. Contrafluent floodplain lake, 4. Profundal floodplain lake. A solution lake

1694-510: A hypolimnion; accordingly, very shallow lakes are excluded from this classification system. Based upon their thermal stratification, lakes are classified as either holomictic , with a uniform temperature and density from top to bottom at a given time of year, or meromictic , with layers of water of different temperature and density that do not intermix. The deepest layer of water in a meromictic lake does not contain any dissolved oxygen so there are no living aerobic organisms . Consequently,

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1848-428: A lake consists of a large area of standing water that occupies an extensive closed depression in limestone, it is also called a karst lake . Smaller solution lakes that consist of a body of standing water in a closed depression within a karst region are known as karst ponds. Limestone caves often contain pools of standing water, which are known as underground lakes . Classic examples of solution lakes are abundant in

2002-470: A large number of studies agree that small ponds are much more abundant than large lakes. For example, one widely cited study estimated that Earth has 304 million lakes and ponds, and that 91% of these are 1 hectare (2.5 acres) or less in area. Despite the overwhelming abundance of ponds, almost all of Earth's lake water is found in fewer than 100 large lakes; this is because lake volume scales superlinearly with lake area. Extraterrestrial lakes exist on

2156-505: A larger group of undersea systems known as unmanned underwater vehicles . ROVs are unoccupied, usually highly maneuverable, and operated by a crew either aboard a vessel/floating platform or on proximate land. They are linked to a host ship by a neutrally buoyant tether , or a load-carrying umbilical cable is used along with a tether management system (TMS). The umbilical cable contains a group of electrical conductors and fiber optics that carry electric power, video, and data signals between

2310-440: A minute amount of oxygen, the combined metabolism of many cells over a vast time transformed Earth's atmosphere to its current state. This was Earth's third atmosphere. Some oxygen was stimulated by solar ultraviolet radiation to form ozone , which collected in a layer near the upper part of the atmosphere. The ozone layer absorbed, and still absorbs, a significant amount of the ultraviolet radiation that once had passed through

2464-489: A natural outflow and lose water solely by evaporation or underground seepage, or both. These are termed endorheic lakes. Many lakes are artificial and are constructed for hydroelectric power generation, aesthetic purposes, recreational purposes, industrial use, agricultural use, or domestic water supply . The number of lakes on Earth is undetermined because most lakes and ponds are very small and do not appear on maps or satellite imagery . Despite this uncertainty,

2618-529: A planet-sized body named Theia is thought to have formed the Moon. Over time, the Earth cooled, causing the formation of a solid crust , and allowing liquid water on the surface. The Hadean eon represents the time before a reliable (fossil) record of life; it began with the formation of the planet and ended 4.0 billion years ago. The following Archean and Proterozoic eons produced the beginnings of life on Earth and its earliest evolution . The succeeding eon

2772-462: A potentially fatal condition, occurs when the human body's core temperature falls below 35 °C. Insulating the body's warmth from water is the main purpose of diving suits and exposure suits when used in water temperatures below 25 °C. Sound is transmitted about 4.3 times faster in water (1,484 m/s in fresh water) than in air (343 m/s). The human brain can determine the direction of sound in air by detecting small differences in

2926-416: A problem known as the faint young Sun paradox . Stars are known to get brighter as they age, and the Sun has become 30% brighter since its formation 4.5 billion years ago. Many models indicate that the early Earth should have been covered in ice. A likely solution is that there was enough carbon dioxide and methane to produce a greenhouse effect . The carbon dioxide would have been produced by volcanoes and

3080-430: A range of adverse effects, such as inert gas narcosis , and oxygen toxicity . Decompression must be controlled to avoid bubble formation in the tissues and the consequent symptoms of decompression sickness . With a few exceptions, the underwater environment tends to cool the unprotected human body. This heat loss will generally lead to hypothermia eventually. There are several classes of hazards to humans inherent to

3234-403: A result of meandering. The slow-moving river forms a sinuous shape as the outer side of bends are eroded away more rapidly than the inner side. Eventually a horseshoe bend is formed and the river cuts through the narrow neck. This new passage then forms the main passage for the river and the ends of the bend become silted up, thus forming a bow-shaped lake. Their crescent shape gives oxbow lakes

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3388-535: A result of the 1959 Hebgen Lake earthquake . Most landslide lakes disappear in the first few months after formation, but a landslide dam can burst suddenly at a later stage and threaten the population downstream when the lake water drains out. In 1911, an earthquake triggered a landslide that blocked a deep valley in the Pamir Mountains region of Tajikistan , forming the Sarez Lake . The Usoi Dam at

3542-486: A river from precipitation in a drainage basin from surface runoff and other sources such as groundwater recharge , springs , and the release of stored water in natural ice and snow. Potamology is the scientific study of rivers, while limnology is the study of inland waters in general. An aquifer is an underground layer of water -bearing permeable rock , rock fractures or unconsolidated materials ( gravel , sand , or silt ). The study of water flow in aquifers and

3696-614: A stream sinks"), outflow caves ("from which a stream emerges"), and through caves ("traversed by a stream"). A reservoir is, most commonly, an enlarged natural or artificial lake, pond or impoundment created using a dam or lock to store water. Reservoirs can be created in a number of ways, including controlling a watercourse that drains an existing body of water, interrupting a watercourse to form an embayment within it, through excavation, or building retaining walls or levees . Canals are artificial waterways which may have dams and locks that create reservoirs of low speed current flow. Water

3850-422: A variation in density because of thermal gradients. Stratification can also result from a density variation caused by gradients in salinity. In this case, the hypolimnion and epilimnion are separated not by a thermocline but by a halocline , which is sometimes referred to as a chemocline . Lakes are informally classified and named according to the seasonal variation in their lake level and volume. Some of

4004-443: Is sag ponds . Volcanic lakes are lakes that occupy either local depressions, e.g. craters and maars , or larger basins, e.g. calderas , created by volcanism . Crater lakes are formed in volcanic craters and calderas, which fill up with precipitation more rapidly than they empty via either evaporation, groundwater discharge, or a combination of both. Sometimes the latter are called caldera lakes, although often no distinction

4158-399: Is a transparent , tasteless , odorless , and nearly colorless chemical substance . Its chemical formula is H 2 O, meaning that each of its molecules contains one oxygen and two hydrogen atoms , connected by covalent bonds . Water is the name of the liquid state of H 2 O at standard ambient temperature and pressure . Water at the surface of the Earth moves continually through

4312-403: Is a lake occupying a basin formed by surface dissolution of bedrock. In areas underlain by soluble bedrock, its solution by precipitation and percolating water commonly produce cavities. These cavities frequently collapse to form sinkholes that form part of the local karst topography . Where groundwater lies near the grounds surface, a sinkhole will be filled water as a solution lake. If such

4466-419: Is a natural flowing watercourse , usually freshwater , flowing under the influence of gravity on ocean , lake , another river, or into the ground. Small rivers can be referred to using names such as stream , creek, brook, rivulet, and rill . There are no official definitions for the generic term river as applied to geographic features , Rivers are part of the hydrological cycle ; water generally collects in

4620-606: Is a small watercraft designed to operate underwater. The term submersible is often used to differentiate from other underwater vessels known as submarines , in that a submarine is a fully autonomous craft, capable of renewing its own power and breathing air, whereas a submersible is usually supported by a surface vessel, platform, shore team or sometimes a larger submarine. There are many types of submersibles, including both manned and unmanned craft, otherwise known as remotely operated vehicles or ROVs. Remotely operated underwater vehicles and autonomous underwater vehicles are part of

4774-722: Is an area filled with water, either natural or artificial, that is smaller than a lake . It may arise naturally in floodplains as part of a river system, or be a somewhat isolated depression (such as a kettle , vernal pool , or prairie pothole ). It may contain shallow water with marsh and aquatic plants and animals. Ponds are frequently man-made or expanded beyond their original depth and bounds. Among their many uses, ponds provide water for agriculture and livestock, aid in habitat restoration, serve as fish hatcheries, are components of landscape architecture, may store thermal energy as solar ponds , and treat wastewater as treatment ponds . Ponds may be fresh, saltwater , or brackish . A river

Underwater environment - Misplaced Pages Continue

4928-448: Is commonly added to expand the vehicle's capabilities. Autonomous underwater vehicles (AUVs) are robots that travel underwater without requiring input from an operator. Underwater gliders are a subclass of AUVs. History of Earth The natural history of Earth concerns the development of planet Earth from its formation to the present day. Nearly all branches of natural science have contributed to understanding of

5082-468: Is dammed behind an ice shelf that is attached to the coastline. They are mostly found in Antarctica. Fluvial (or riverine) lakes are lakes produced by running water. These lakes include plunge pool lakes , fluviatile dams and meander lakes. The most common type of fluvial lake is a crescent-shaped lake called an oxbow lake due to the distinctive curved shape. They can form in river valleys as

5236-534: Is inferred that a period of intense meteorite impacts, called the Late Heavy Bombardment , began about 4.1 Ga, and concluded around 3.8 Ga, at the end of the Hadean. In addition, volcanism was severe due to the large heat flow and geothermal gradient . Nevertheless, detrital zircon crystals dated to 4.4 Ga show evidence of having undergone contact with liquid water, suggesting that

5390-452: Is made. An example is Crater Lake in Oregon , in the caldera of Mount Mazama . The caldera was created in a massive volcanic eruption that led to the subsidence of Mount Mazama around 4860 BCE. Other volcanic lakes are created when either rivers or streams are dammed by lava flows or volcanic lahars . The basin which is now Malheur Lake , Oregon was created when a lava flow dammed

5544-467: Is normally about 0.5% of that at the surface. The euphotic depth is the depth at which light intensity falls to 1% of the value at the surface. This depth is dependent upon water clarity, being only a few metres underwater in a turbid estuary, but may reach up to 200 metres in the open ocean. At the euphotic depth, plants (such as phytoplankton ) have no net energy gain from photosynthesis and thus cannot grow. There are three layers of ocean temperature:

5698-467: Is often used interchangeably with "sea" in American English . Strictly speaking, a sea is a body of water (generally a division of the world ocean) partly or fully enclosed by land, though " the sea " refers also to the oceans. Saline water covers approximately 361,000,000 km (139,000,000 sq mi) and is customarily divided into several principal oceans and smaller seas, with

5852-467: Is possible. The earliest ribozymes may have been formed of simpler nucleic acids such as PNA , TNA or GNA , which would have been replaced later by RNA. Other pre-RNA replicators have been posited, including crystals and even quantum systems. In 2003 it was proposed that porous metal sulfide precipitates would assist RNA synthesis at about 100 °C (212 °F) and at ocean-bottom pressures near hydrothermal vents . In this hypothesis,

6006-716: Is the Phanerozoic , divided into three eras: the Palaeozoic , an era of arthropods, fishes, and the first life on land; the Mesozoic , which spanned the rise, reign, and climactic extinction of the non-avian dinosaurs; and the Cenozoic , which saw the rise of mammals. Recognizable humans emerged at most 2 million years ago, a vanishingly small period on the geological scale. The earliest undisputed evidence of life on Earth dates at least from 3.5 billion years ago, during

6160-429: Is unknown but is estimated to be at least 2 million. Finland has 168,000 lakes of 500 square metres (5,400 sq ft) in area, or larger, of which 57,000 are large (10,000 square metres (110,000 sq ft) or larger). Most lakes have at least one natural outflow in the form of a river or stream , which maintain a lake's average level by allowing the drainage of excess water. Some lakes do not have

6314-466: Is why, in ocean water, the downward convection of colder water is not blocked by an expansion of water as it becomes colder near the freezing point. The oceans' cold water near the freezing point continues to sink. So creatures that live at the bottom of cold oceans like the Arctic Ocean generally live in water 4 °C colder than at the bottom of frozen-over fresh water lakes and rivers. As

Underwater environment - Misplaced Pages Continue

6468-542: The Big Bang 13.8  Ga (billion years ago) and heavier elements ejected by supernovae . About 4.5  Ga , the nebula began a contraction that may have been triggered by the shock wave from a nearby supernova . A shock wave would have also made the nebula rotate. As the cloud began to accelerate, its angular momentum , gravity , and inertia flattened it into a protoplanetary disk perpendicular to its axis of rotation. Small perturbations due to collisions and

6622-720: The Eoarchean Era, after a geological crust started to solidify following the earlier molten Hadean eon. There are microbial mat fossils such as stromatolites found in 3.48 billion-year-old sandstone discovered in Western Australia . Other early physical evidence of a biogenic substance is graphite in 3.7 billion-year-old metasedimentary rocks discovered in southwestern Greenland as well as "remains of biotic life " found in 4.1 billion-year-old rocks in Western Australia. According to one of

6776-759: The Malheur River . Among all lake types, volcanic crater lakes most closely approximate a circular shape. Glacial lakes are lakes created by the direct action of glaciers and continental ice sheets. A wide variety of glacial processes create enclosed basins. As a result, there are a wide variety of different types of glacial lakes and it is often difficult to define clear-cut distinctions between different types of glacial lakes and lakes influenced by other activities. The general types of glacial lakes that have been recognized are lakes in direct contact with ice, glacially carved rock basins and depressions, morainic and outwash lakes, and glacial drift basins. Glacial lakes are

6930-554: The Proto-Indo-European root * leǵ- ('to leak, drain'). Cognates include Dutch laak ('lake, pond, ditch'), Middle Low German lāke ('water pooled in a riverbed, puddle') as in: de:Wolfslake , de:Butterlake , German Lache ('pool, puddle'), and Icelandic lækur ('slow flowing stream'). Also related are the English words leak and leach . There is considerable uncertainty about defining

7084-410: The age of the universe , by accretion from the solar nebula . Volcanic outgassing probably created the primordial atmosphere and then the ocean, but the early atmosphere contained almost no oxygen . Much of the Earth was molten because of frequent collisions with other bodies which led to extreme volcanism. While the Earth was in its earliest stage ( Early Earth ), a giant impact collision with

7238-721: The carbon cycle , and influences climate and weather patterns. The World Ocean is the habitat of 230,000 known species , but because much of it is unexplored, the number of species that exist in the ocean is much larger, possibly over two million. Freshwater ecosystems include lakes and ponds , rivers , streams , springs , aquifers , bogs , and wetlands . They have a lower salt content than marine ecosystems. Freshwater habitats can be classified by different factors, including temperature, light penetration, nutrients, and vegetation. Freshwater ecosystems can be divided into lentic ecosystems (still water) and lotic ecosystems (flowing water). Aquatic ecosystems are characterised by

7392-402: The colour spectrum is rapidly altered with increasing depth. White objects at the surface appear bluish underwater, and red objects appear dark, even black. Although light penetration will be less if water is turbid , in the very clear water of the open ocean less than 25% of the surface light reaches a depth of 10 m (33 feet). At 100 m (330 ft) the light present from the sun

7546-399: The density of water varies with temperature, with a maximum at +4 degrees Celsius, thermal stratification is an important physical characteristic of a lake that controls the fauna and flora , sedimentation, chemistry, and other aspects of individual lakes. First, the colder, denser water typically forms a layer near the bottom, which is called the hypolimnion . Second, normally overlying

7700-399: The middle ear with outside water pressure can cause pain, and the tympanic membrane (eardrum) can rupture at depths under 10 ft (3 m). The danger of pressure damage is greatest in shallow water because the ratio of pressure change is greatest near the surface of the water. The raised pressure also affects the solution of breathing gases in the tissues over time, and can lead to

7854-403: The relicts of the first continental crust, formed by partial melting in basalt. Earth is often described as having had three atmospheres. The first atmosphere, captured from the solar nebula, was composed of light ( atmophile ) elements from the solar nebula, mostly hydrogen and helium. A combination of the solar wind and Earth's heat would have driven off this atmosphere, as a result of which

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8008-451: The surface of sea water begins to freeze (at −1.9 °C for salinity 3.5%) the ice that forms is essentially salt-free, with about the same density as freshwater ice. This ice floats on the surface, and the salt that is "frozen out" adds to the salinity and density of the sea water just below it, in a process known as brine rejection . This denser salt water sinks by convection. This produces essentially freshwater ice at −1.9 °C on

8162-475: The surface layer , the thermocline , and the deep ocean . The average temperature of surface layer is about 17 °C. About 90% of ocean's water is below the thermocline in the deep ocean, where most of the water is below 4 °C. There are temperature anomalies at active volcanic sites and hydrothermal vents , where deep-water temperatures can significantly exceed 100 °C. Water conducts heat around 25 times more efficiently than air. Hypothermia ,

8316-416: The water cycle of evaporation , transpiration ( evapotranspiration ), condensation , precipitation , and runoff , usually reaching the sea. Water seldom exists in a pure form, it almost always contains dissolved substances, and usually other matter in suspension. The density of water is about 1 gram per cubic centimetre (62 lb/cu ft) The density varies with temperature, but not linearly: as

8470-481: The water cycle , the processes by which water moves around the Earth. Most lakes are freshwater and account for almost all the world's surface freshwater, but some are salt lakes with salinities even higher than that of seawater . Lakes vary significantly in surface area and volume of water. Lakes are typically larger and deeper than ponds , which are also water-filled basins on land, although there are no official definitions or scientific criteria distinguishing

8624-466: The Earth ) and a small metallic core. Second, the Earth and Moon have the same oxygen isotopic signature (relative abundance of the oxygen isotopes). Of the theories proposed to account for these phenomena, one is widely accepted: The giant impact hypothesis proposes that the Moon originated after a body the size of Mars (sometimes named Theia ) struck the proto-Earth a glancing blow. The collision released about 100 million times more energy than

8778-550: The Earth already had oceans or seas at that time. By the beginning of the Archean, the Earth had cooled significantly. Present life forms could not have survived at Earth's surface, because the Archean atmosphere lacked oxygen hence had no ozone layer to block ultraviolet light. Nevertheless, it is believed that primordial life began to evolve by the early Archean, with candidate fossils dated to around 3.5 Ga. Some scientists even speculate that life could have begun during

8932-597: The Earth by extraterrestrial objects (either meteorites or asteroids ). Examples of meteorite lakes are Lonar Lake in India, Lake El'gygytgyn in northeast Siberia, and the Pingualuit crater lake in Quebec, Canada. As in the cases of El'gygytgyn and Pingualuit, meteorite lakes can contain unique and scientifically valuable sedimentary deposits associated with long records of paleoclimatic changes. In addition to

9086-443: The Earth could have condensed into a single body within a couple of weeks. Under the influence of its own gravity, the ejected material became a more spherical body: the Moon. Mantle convection , the process that drives plate tectonics, is a result of heat flow from the Earth's interior to the Earth's surface. It involves the creation of rigid tectonic plates at mid-oceanic ridges . These plates are destroyed by subduction into

9240-829: The Earth depends directly or indirectly on photosynthesis. The most common form, oxygenic photosynthesis, turns carbon dioxide, water, and sunlight into food. It captures the energy of sunlight in energy-rich molecules such as ATP, which then provide the energy to make sugars. To supply the electrons in the circuit, hydrogen is stripped from water, leaving oxygen as a waste product. Some organisms, including purple bacteria and green sulfur bacteria , use an anoxygenic form of photosynthesis that uses alternatives to hydrogen stripped from water as electron donors ; examples are hydrogen sulfide, sulfur and iron. Such extremophile organisms are restricted to otherwise inhospitable environments such as hot springs and hydrothermal vents. The simpler anoxygenic form arose about 3.8 Ga, not long after

9394-530: The Earth's surface first solidified, totally disappeared from a combination of this fast Hadean plate tectonics and the intense impacts of the Late Heavy Bombardment. However, it is thought that it was basaltic in composition, like today's oceanic crust , because little crustal differentiation had yet taken place. The first larger pieces of continental crust , which is a product of differentiation of lighter elements during partial melting in

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9548-571: The Lusatian Lake District, Germany. In India, Sudarshana Lake is a historical artificial lake located in the semi-arid region of Girnar, Gujarat, originally constructed during the reign of Chandragupta Maurya. See: List of notable artificial lakes in the United States Meteorite lakes, also known as crater lakes (not to be confused with volcanic crater lakes ), are created by catastrophic impacts with

9702-515: The Moon is 4.53 ± 0.01 billion years old, formed at least 30 million years after the Solar System. New evidence suggests the Moon formed even later, 4.48 ± 0.02 Ga, or 70–110 million years after the start of the Solar System. Theories for the formation of the Moon must explain its late formation as well as the following facts. First, the Moon has a low density (3.3 times that of water, compared to 5.5 for

9856-418: The Sun than Neptune , computer simulations show that they were originally far more common in the inner parts of the Solar System. As the Earth cooled, clouds formed. Rain created the oceans. Recent evidence suggests the oceans may have begun forming as early as 4.4 Ga. By the start of the Archean eon, they already covered much of the Earth. This early formation has been difficult to explain because of

10010-466: The ability to use oxygen to increase their metabolism and obtain more energy from the same food. The natural evolution of the Sun made it progressively more luminous during the Archean and Proterozoic eons; the Sun's luminosity increases 6% every billion years. As a result, the Earth began to receive more heat from the Sun in the Proterozoic eon. However, the Earth did not get warmer. Instead,

10164-459: The advance of ice covers the rocks, slowing the reduction in carbon dioxide, but in the Cryogenian the weathering of Rodinia was able to continue unchecked until the ice advanced to the tropics. The process may have finally been reversed by the emission of carbon dioxide from volcanoes or the destabilization of methane gas hydrates . According to the alternative Slushball Earth theory, even at

10318-759: The aid of sparks to mimic the effect of lightning . Although atmospheric composition was probably different from that used by Miller and Urey, later experiments with more realistic compositions also managed to synthesize organic molecules. Computer simulations show that extraterrestrial organic molecules could have formed in the protoplanetary disk before the formation of the Earth. Additional complexity could have been reached from at least three possible starting points: self-replication , an organism's ability to produce offspring that are similar to itself; metabolism , its ability to feed and repair itself; and external cell membranes , which allow food to enter and waste products to leave, but exclude unwanted substances. Even

10472-401: The ambient pressure by using an atmospheric diving suit (ADS), which is a small one-person articulated anthropomorphic submersible which resembles a suit of armour , with elaborate pressure resisting joints to allow articulation while maintaining an internal pressure of one atmosphere. An ADS can be used for relatively deep dives of up to 2,300 feet (700 m) for many hours, and eliminates

10626-464: The angular momentum of other large debris created the means by which kilometer-sized protoplanets began to form, orbiting the nebular center. The center of the nebula, not having much angular momentum, collapsed rapidly, the compression heating it until nuclear fusion of hydrogen into helium began. After more contraction, a T Tauri star ignited and evolved into the Sun . Meanwhile, in the outer part of

10780-408: The appearance of life. The timing of oxygenic photosynthesis is more controversial; it had certainly appeared by about 2.4 Ga, but some researchers put it back as far as 3.2 Ga. The latter "probably increased global productivity by at least two or three orders of magnitude". Among the oldest remnants of oxygen-producing lifeforms are fossil stromatolites . At first, the released oxygen

10934-433: The atmosphere is now depleted of these elements compared to cosmic abundances. After the impact which created the Moon, the molten Earth released volatile gases; and later more gases were released by volcanoes , completing a second atmosphere rich in greenhouse gases but poor in oxygen. Finally, the third atmosphere, rich in oxygen, emerged when bacteria began to produce oxygen about 2.8 Ga. In early models for

11088-459: The atmosphere. It allowed cells to colonize the surface of the ocean and eventually the land: without the ozone layer, ultraviolet radiation bombarding land and sea would have caused unsustainable levels of mutation in exposed cells. Photosynthesis had another major impact. Oxygen was toxic; much life on Earth probably died out as its levels rose in what is known as the oxygen catastrophe . Resistant forms survived and thrived, and some developed

11242-866: The base of the valley has remained in place for more than 100 years but the terrain below the lake is in danger of a catastrophic flood if the dam were to fail during a future earthquake. Tal-y-llyn Lake in north Wales is a landslide lake dating back to the last glaciation in Wales some 20000 years ago. Aeolian lakes are produced by wind action . These lakes are found mainly in arid environments, although some aeolian lakes are relict landforms indicative of arid paleoclimates . Aeolian lakes consist of lake basins dammed by wind-blown sand; interdunal lakes that lie between well-oriented sand dunes ; and deflation basins formed by wind action under previously arid paleoenvironments. Moses Lake in Washington , United States,

11396-440: The bottom, thus keeping the temperature of the water at the bottom constant (see diagram). The density of sea water depends on the dissolved salt content as well as the temperature. Ice still floats in the oceans, otherwise they would freeze from the bottom up. The salt content lowers the freezing point by about 1.9 °C and lowers the temperature of the density maximum of water to the fresh water freezing point at 0 °C. This

11550-423: The breakage of hydrogen bonds due to heating allows water molecules to pack closer despite the increase in the thermal motion (which tends to expand a liquid), above 4 °C water expands as the temperature increases. Water near the boiling point is about 4% less dense than water at 4 °C (39 °F). The unusual density curve and lower density of ice than of water is vital to life—if water were most dense at

11704-709: The characterization of aquifers is called hydrogeology . If an impermeable layer overlies the aquifer, pressure could cause it to become a confined aquifer. Aquifers may be classified as porous or karst , where a porous aquifer contains the water in the spaces between the grains of a loose sediment or rock (typically sand or sandstone ), while a karst aquifer contains water mainly in relatively large voids in relatively impermeable rock, such as limestone or dolomite . Water filled caves can be classified as active and relict: active caves have water flowing through them; relict caves do not, though water may be retained in them. Types of active caves include inflow caves ("into which

11858-411: The coast to include offshore systems, such as the surface ocean , pelagic ocean waters, the deep sea , oceanic hydrothermal vents , and the sea floor . Marine ecosystems are characterized by the biological community of organisms that they are associated with and their physical environment . As the world ocean is the principal component of Earth's hydrosphere, it is integral to life , forms part of

12012-485: The concentration of methane could have decreased dramatically, enough to counter the effect of the increasing heat flow from the Sun. However, the term Snowball Earth is more commonly used to describe later extreme ice ages during the Cryogenian period. There were four periods, each lasting about 10 million years, between 750 and 580 million years ago, when the Earth is thought to have been covered with ice apart from

12166-915: The courses of mature rivers, where a river channel has widened over a basin formed by eroded floodplains and wetlands . Some lakes are found in caverns underground . Some parts of the world have many lakes formed by the chaotic drainage patterns left over from the last ice age . All lakes are temporary over long periods of time , as they will slowly fill in with sediments or spill out of the basin containing them. Artificially controlled lakes are known as reservoirs , and are usually constructed for industrial or agricultural use, for hydroelectric power generation, for supplying domestic drinking water , for ecological or recreational purposes, or for other human activities. The word lake comes from Middle English lake ('lake, pond, waterway'), from Old English lacu ('pond, pool, stream'), from Proto-Germanic * lakō ('pond, ditch, slow moving stream'), from

12320-506: The courses of mature rivers. In some parts of the world, there are many lakes because of chaotic drainage patterns left over from the last ice age . All lakes are temporary over geologic time scales, as they will slowly fill in with sediments or spill out of the basin containing them. Many lakes are artificial and are constructed for industrial or agricultural use, for hydro-electric power generation or domestic water supply, or for aesthetic, recreational purposes, or other activities. A pond

12474-518: The creation of lakes by the disruption of preexisting drainage networks, it also creates within arid regions endorheic basins that contain salt lakes (also called saline lakes). They form where there is no natural outlet, a high evaporation rate and the drainage surface of the water table has a higher-than-normal salt content. Examples of these salt lakes include Great Salt Lake and the Dead Sea . Another type of tectonic lake caused by faulting

12628-404: The density of water, the object sinks. If the overall density is less than the density of water, the object rises until it floats on the surface. With increasing depth underwater, sunlight is absorbed, and the amount of visible light diminishes. Because absorption is greater for long wavelengths (red end of the visible spectrum ) than for short wavelengths (blue end of the visible spectrum),

12782-622: The difference between lakes and ponds , and neither term has an internationally accepted definition across scientific disciplines or political boundaries. For example, limnologists have defined lakes as water bodies that are simply a larger version of a pond, which can have wave action on the shoreline or where wind-induced turbulence plays a major role in mixing the water column. None of these definitions completely excludes ponds and all are difficult to measure. For this reason, simple size-based definitions are increasingly used to separate ponds and lakes. Definitions for lake range in minimum sizes for

12936-422: The diver which limit the depths and duration possible in ambient pressure diving. Breath-hold endurance is a severe limitation, and breathing at high ambient pressure adds further complications, both directly and indirectly. Technological solutions have been developed which can greatly extend depth and duration of human ambient pressure dives, and allow useful work to be done underwater. A diver can be isolated from

13090-457: The early Hadean, as far back as 4.4 Ga, surviving the possible Late Heavy Bombardment period in hydrothermal vents below the Earth's surface. Earth's only natural satellite , the Moon, is larger relative to its planet than any other satellite in the Solar System. During the Apollo program , rocks from the Moon's surface were brought to Earth. Radiometric dating of these rocks shows that

13244-536: The exception of criterion 3, the others have been accepted or elaborated upon by other hydrology publications. The majority of lakes on Earth are freshwater , and most lie in the Northern Hemisphere at higher latitudes . Canada , with a deranged drainage system , has an estimated 31,752 lakes larger than 3 square kilometres (1.2 sq mi) in surface area. The total number of lakes in Canada

13398-497: The external membranes of cells may have been an essential first step. Experiments that simulated the conditions of the early Earth have reported the formation of lipids, and these can spontaneously form liposomes , double-walled "bubbles", and then reproduce themselves. Although they are not intrinsically information-carriers as nucleic acids are, they would be subject to natural selection for longevity and reproduction. Nucleic acids such as RNA might then have formed more easily within

13552-399: The face of ever-changing physical environments. The process of plate tectonics continues to shape the Earth's continents and oceans and the life they harbor. In geochronology , time is generally measured in mya (million years ago), each unit representing the period of approximately 1,000,000 years in the past. The history of Earth is divided into four great eons , starting 4,540 mya with

13706-540: The fifth timeline. Horizontal scale is Millions of years (above timelines) / Thousands of years (below timeline) The standard model for the formation of the Solar System (including the Earth ) is the solar nebula hypothesis . In this model, the Solar System formed from a large, rotating cloud of interstellar dust and gas called the solar nebula . It was composed of hydrogen and helium created shortly after

13860-530: The formation of the atmosphere and ocean, the second atmosphere was formed by outgassing of volatiles from the Earth's interior. Now it is considered likely that many of the volatiles were delivered during accretion by a process known as impact degassing in which incoming bodies vaporize on impact. The ocean and atmosphere would, therefore, have started to form even as the Earth formed. The new atmosphere probably contained water vapor , carbon dioxide, nitrogen, and smaller amounts of other gases. Planetesimals at

14014-442: The formation of the planet. Each eon saw the most significant changes in Earth's composition, climate and life. Each eon is subsequently divided into eras , which in turn are divided into periods , which are further divided into epochs . The history of the Earth can be organized chronologically according to the geologic time scale , which is split into intervals based on stratigraphic analysis. The following five timelines show

14168-454: The freezing point, then in winter the very cold water at the surface of lakes and other water bodies would sink, the lake could freeze from the bottom up, and all life in them would be killed. Furthermore, given that water is a good thermal insulator (due to its heat capacity), some frozen lakes might not completely thaw in summer. The layer of ice that floats on top insulates the water below. Water at about 4 °C (39 °F) also sinks to

14322-406: The geologic time scale to scale. The first shows the entire time from the formation of the Earth to the present, but this gives little space for the most recent eon. The second timeline shows an expanded view of the most recent eon. In a similar way, the most recent era is expanded in the third timeline, the most recent period is expanded in the fourth timeline, and the most recent epoch is expanded in

14476-522: The geological record suggests it cooled dramatically during the early Proterozoic. Glacial deposits found in South Africa date back to 2.2 Ga, at which time, based on paleomagnetic evidence, they must have been located near the equator. Thus, this glaciation, known as the Huronian glaciation , may have been global. Some scientists suggest this was so severe that the Earth was frozen over from

14630-518: The height of the ice ages there was still open water at the Equator. Lake A lake is an often naturally occurring, relatively large and fixed body of water on or near the Earth's surface. It is localized in a basin or interconnected basins surrounded by dry land . Lakes lie completely on land and are separate from the ocean , although they may be connected with the ocean by rivers . Lakes, as with other bodies of water , are part of

14784-485: The high ambient pressure is not much of a problem; but it is a problem for any gas-filled spaces like the mouth , ears , paranasal sinuses and lungs. This is because the gas in those spaces is much more compressible than the solids and liquids, and reduces in volume much more when under pressure and so does not provide those spaces with support against the higher outside pressure. Even at a depth of 8 ft (2.4 m) underwater, an inability to equalize air pressure in

14938-455: The highest mountains, and average temperatures were about −50 °C (−58 °F). The snowball may have been partly due to the location of the supercontinent Rodinia straddling the Equator . Carbon dioxide combines with rain to weather rocks to form carbonic acid, which is then washed out to sea, thus extracting the greenhouse gas from the atmosphere. When the continents are near the poles,

15092-422: The hypolimnion is a transition zone known as the metalimnion . Finally, overlying the metalimnion is a surface layer of warmer water with a lower density, called the epilimnion . This typical stratification sequence can vary widely, depending on the specific lake or the time of year, or a combination of both. The classification of lakes by thermal stratification presupposes lakes with sufficient depth to form

15246-907: The intentional damming of rivers and streams, rerouting of water to inundate a previously dry basin , or the deliberate filling of abandoned excavation pits by either precipitation runoff , ground water , or a combination of both. Artificial lakes may be used as storage reservoirs that provide drinking water for nearby settlements , to generate hydroelectricity , for flood management , for supplying agriculture or aquaculture , or to provide an aquatic sanctuary for parks and nature reserves . The Upper Silesian region of southern Poland contains an anthropogenic lake district consisting of more than 4,000 water bodies created by human activity. The diverse origins of these lakes include: reservoirs retained by dams, flooded mines, water bodies formed in subsidence basins and hollows, levee ponds, and residual water bodies following river regulation. Same for

15400-471: The karst regions at the Dalmatian coast of Croatia and within large parts of Florida . A landslide lake is created by the blockage of a river valley by either mudflows , rockslides , or screes . Such lakes are most common in mountainous regions. Although landslide lakes may be large and quite deep, they are typically short-lived. An example of a landslide lake is Quake Lake , which formed as

15554-454: The laboratory fall well short of the minimum complexity for a living organism. The first step in the emergence of life may have been chemical reactions that produced many of the simpler organic compounds, including nucleobases and amino acids , that are the building blocks of life. An experiment in 1952 by Stanley Miller and Harold Urey showed that such molecules could form in an atmosphere of water, methane, ammonia and hydrogen with

15708-657: The largest lakes on Earth are rift lakes occupying rift valleys, e.g. Central African Rift lakes and Lake Baikal . Other well-known tectonic lakes, Caspian Sea , the Sea of Aral , and other lakes from the Pontocaspian occupy basins that have been separated from the sea by the tectonic uplift of the sea floor above the ocean level. Often, the tectonic action of crustal extension has created an alternating series of parallel grabens and horsts that form elongate basins alternating with mountain ranges. Not only does this promote

15862-447: The largest of Earth 's aquatic ecosystems and are distinguished by waters that have a high salt content. Marine waters cover more than 70% of the surface of the Earth and account for more than 97% of Earth's water supply and 90% of habitable space on Earth. Marine ecosystems include nearshore systems, such as the salt marshes , mudflats , seagrass meadows , mangroves , rocky intertidal systems and coral reefs . They also extend from

16016-528: The layers of sediment at the bottom of a meromictic lake remain relatively undisturbed, which allows for the development of lacustrine deposits . In a holomictic lake, the uniformity of temperature and density allows the lake waters to completely mix. Based upon thermal stratification and frequency of turnover, holomictic lakes are divided into amictic lakes , cold monomictic lakes , dimictic lakes , warm monomictic lakes, polymictic lakes , and oligomictic lakes. Lake stratification does not always result from

16170-560: The level of a lake are controlled by the difference between the input and output compared to the total volume of the lake. Significant input sources are precipitation onto the lake, runoff carried by streams and channels from the lake's catchment area, groundwater channels and aquifers, and artificial sources from outside the catchment area. Output sources are evaporation from the lake, surface and groundwater flows, and any extraction of lake water by humans. As climate conditions and human water requirements vary, these will create fluctuations in

16324-424: The limitation on ambient lighting due to absorption by the water itself and by dissolved and suspended matter in the water column, and by the support provided by buoyancy. Nutrients usable by plants are dissolved in the water, making them easily available. However, the interaction of light absorption by water, matter and living organisms themselves leads to very different light and light spectrum conditions depending on

16478-465: The liposomes than they would have outside. Some clays , notably montmorillonite , have properties that make them plausible accelerators for the emergence of an RNA world: they grow by self-replication of their crystalline pattern, are subject to an analog of natural selection (as the clay "species" that grows fastest in a particular environment rapidly becomes dominant), and can catalyze the formation of RNA molecules. Although this idea has not become

16632-826: The lower crust, appeared at the end of the Hadean, about 4.0 Ga. What is left of these first small continents are called cratons . These pieces of late Hadean and early Archean crust form the cores around which today's continents grew. The oldest rocks on Earth are found in the North American craton of Canada . They are tonalites from about 4.0 Ga. They show traces of metamorphism by high temperature, but also sedimentary grains that have been rounded by erosion during transport by water, showing that rivers and seas existed then. Cratons consist primarily of two alternating types of terranes . The first are so-called greenstone belts , consisting of low-grade metamorphosed sedimentary rocks. These "greenstones" are similar to

16786-401: The main events of Earth's past, characterized by constant geological change and biological evolution . The geological time scale (GTS), as defined by international convention, depicts the large spans of time from the beginning of the Earth to the present, and its divisions chronicle some definitive events of Earth history. Earth formed around 4.54 billion years ago, approximately one-third

16940-481: The mainland; lakes cut off from larger lakes by a bar; or lakes divided by the meeting of two spits. Organic lakes are lakes created by the actions of plants and animals. On the whole they are relatively rare in occurrence and quite small in size. In addition, they typically have ephemeral features relative to the other types of lakes. The basins in which organic lakes occur are associated with beaver dams, coral lakes, or dams formed by vegetation. Peat lakes are

17094-804: The major phyla known today, and divided the Proterozoic Eon from the Cambrian Period of the Paleozoic Era. It is estimated that 99 percent of all species that ever lived on Earth, over five billion, have gone extinct . Estimates on the number of Earth's current species range from 10 million to 14 million, of which about 1.2 million are documented, but over 86 percent have not been described. The Earth's crust has constantly changed since its formation, as has life since its first appearance. Species continue to evolve , taking on new forms, splitting into daughter species, or going extinct in

17248-413: The majority of significant physiological dangers associated with deep diving; the occupant need not decompress, there is no need for special gas mixtures, nor is there danger of decompression sickness or nitrogen narcosis , and the diver is effectively isolated from most aquatic organisms. Divers do not even need to be skilled swimmers, but mobility and dexterity are significantly degraded. A submersible

17402-478: The mantle at subduction zones . During the early Archean (about 3.0 Ga) the mantle was much hotter than today, probably around 1,600 °C (2,910 °F), so convection in the mantle was faster. Although a process similar to present-day plate tectonics did occur, this would have gone faster too. It is likely that during the Hadean and Archean, subduction zones were more common, and therefore tectonic plates were smaller. The initial crust, which formed when

17556-452: The metabolism-first scenario is finding a way for organisms to evolve. Without the ability to replicate as individuals, aggregates of molecules would have "compositional genomes" (counts of molecular species in the aggregate) as the target of natural selection. However, a recent model shows that such a system is unable to evolve in response to natural selection. It has been suggested that double-walled "bubbles" of lipids like those that form

17710-559: The methane by early microbes. It is hypothesized that there also existed an organic haze created from the products of methane photolysis that caused an anti-greenhouse effect as well. Another greenhouse gas, ammonia , would have been ejected by volcanos but quickly destroyed by ultraviolet radiation. One of the reasons for interest in the early atmosphere and ocean is that they form the conditions under which life first arose. There are many models, but little consensus, on how life emerged from non-living chemicals; chemical systems created in

17864-424: The mode of origin, lakes have been named and classified according to various other important factors such as thermal stratification , oxygen saturation, seasonal variations in lake volume and water level, salinity of the water mass, relative seasonal permanence, degree of outflow, and so on. The names used by the lay public and in the scientific community for different types of lakes are often informally derived from

18018-485: The moon Titan , which orbits the planet Saturn . The shape of lakes on Titan is very similar to those on Earth. Lakes were formerly present on the surface of Mars, but are now dry lake beds . In 1957, G. Evelyn Hutchinson published a monograph titled A Treatise on Limnology , which is regarded as a landmark discussion and classification of all major lake types, their origin, morphometric characteristics, and distribution. Hutchinson presented in his publication

18172-456: The more accessible parts of the underwater environment. These include research, underwater diving for work or recreation, and underwater warfare with submarines. It is hostile to humans in many ways and often inaccessible, and therefore relatively little explored. Three quarters of the planet Earth are covered by water. Most of the planet's solid surface is abyssal plain , at depths between 4,000 and 5,500 metres (13,100 and 18,000 ft) below

18326-421: The more recent Chicxulub impact that is believed to have caused the extinction of the non-avian dinosaurs. It was enough to vaporize some of the Earth's outer layers and melt both bodies. A portion of the mantle material was ejected into orbit around the Earth. The giant impact hypothesis predicts that the Moon was depleted of metallic material, explaining its abnormal composition. The ejecta in orbit around

18480-415: The morphology of the lakes' physical characteristics or other factors. Also, different cultures and regions of the world have their own popular nomenclature. One important method of lake classification is on the basis of thermal stratification, which has a major influence on the animal and plant life inhabiting a lake, and the fate and distribution of dissolved and suspended material in the lake. For example,

18634-406: The most numerous lakes in the world. Most lakes in northern Europe and North America have been either influenced or created by the latest, but not last, glaciation, to have covered the region. Glacial lakes include proglacial lakes , subglacial lakes , finger lakes , and epishelf lakes. Epishelf lakes are highly stratified lakes in which a layer of freshwater, derived from ice and snow melt,

18788-791: The names include: Lakes may be informally classified and named according to the general chemistry of their water mass. Using this classification method, the lake types include: A paleolake (also palaeolake ) is a lake that existed in the past when hydrological conditions were different. Quaternary paleolakes can often be identified on the basis of relict lacustrine landforms, such as relict lake plains and coastal landforms that form recognizable relict shorelines called paleoshorelines . Paleolakes can also be recognized by characteristic sedimentary deposits that accumulated in them and any fossils that might be contained in these sediments. The paleoshorelines and sedimentary deposits of paleolakes provide evidence for prehistoric hydrological changes during

18942-499: The nebula gravity caused matter to condense around density perturbations and dust particles, and the rest of the protoplanetary disk began separating into rings. In a process known as runaway accretion , successively larger fragments of dust and debris clumped together to form planets. Earth formed in this manner about 4.54 billion years ago (with an uncertainty of 1%) and was largely completed within 10–20 million years. In June 2023, scientists reported evidence that

19096-482: The ocean covering approximately 71% of Earth's surface and 90% of the Earth's biosphere . The ocean contains 97% of Earth's water, and oceanographers have stated that less than 100% of the World Ocean has been explored. The total volume is approximately 1.35 billion cubic kilometers (320 million cu mi) with an average depth of nearly 3,700 meters (12,100 ft). A lake is an area filled with water, localized in

19250-567: The offspring in each generation were quite likely to have different genomes from those that their parents started with. RNA would later have been replaced by DNA, which is more stable and therefore can build longer genomes, expanding the range of capabilities a single organism can have. Ribozymes remain as the main components of ribosomes , the "protein factories" of modern cells. Although short, self-replicating RNA molecules have been artificially produced in laboratories, doubts have been raised about whether natural non-biological synthesis of RNA

19404-444: The oldest detrital zircon crystals in rocks to about 4.4 Ga, soon after the formation of the Earth's crust and the Earth itself. The giant impact hypothesis for the Moon's formation states that shortly after formation of an initial crust, the proto-Earth was impacted by a smaller protoplanet, which ejected part of the mantle and crust into space and created the Moon. From crater counts on other celestial bodies, it

19558-544: The operator and the TMS. Where used, the TMS then relays the signals and power for the ROV down the tether cable. Once at the ROV, the electric power is distributed between the components of the ROV. In high-power applications, most of the electric power drives a high-power electric motor which drives a hydraulic pump for propulsion and to power equipment. Most ROVs are equipped with at least a video camera and lights. Additional equipment

19712-562: The organic-rich deposits of pre-Quaternary paleolakes are important either for the thick deposits of oil shale and shale gas contained in them, or as source rocks of petroleum and natural gas . Although of significantly less economic importance, strata deposited along the shore of paleolakes sometimes contain coal seams . Lakes have numerous features in addition to lake type, such as drainage basin (also known as catchment area), inflow and outflow, nutrient content, dissolved oxygen , pollutants , pH , and sedimentation . Changes in

19866-484: The planet Earth may have formed in just three million years, much faster than the 10−100 million years thought earlier. Nonetheless, the solar wind of the newly formed T Tauri star cleared out most of the material in the disk that had not already condensed into larger bodies. The same process is expected to produce accretion disks around virtually all newly forming stars in the universe, some of which yield planets . The proto-Earth grew by accretion until its interior

20020-400: The poles to the equator, a hypothesis called Snowball Earth. The Huronian ice age might have been caused by the increased oxygen concentration in the atmosphere, which caused the decrease of methane (CH 4 ) in the atmosphere. Methane is a strong greenhouse gas, but with oxygen it reacts to form CO 2 , a less effective greenhouse gas. When free oxygen became available in the atmosphere,

20174-475: The presence of carbon monoxide and hydrogen sulfide with iron sulfide and nickel sulfide as catalysts. Most of the steps in their assembly required temperatures of about 100 °C (212 °F) and moderate pressures, although one stage required 250 °C (482 °F) and a pressure equivalent to that found under 7 kilometers (4.3 mi) of rock. Hence, self-sustaining synthesis of proteins could have occurred near hydrothermal vents. A difficulty with

20328-449: The progenitors of nucleotides , lipids and amino acids. It is believed that of this multiplicity of protocells, only one line survived. Current phylogenetic evidence suggests that the last universal ancestor (LUA) lived during the early Archean eon, perhaps 3.5 Ga or earlier. This LUA cell is the ancestor of all life on Earth today. It was probably a prokaryote , possessing a cell membrane and probably ribosomes, but lacking

20482-489: The proto-cells would be confined in the pores of the metal substrate until the later development of lipid membranes. Another long-standing hypothesis is that the first life was composed of protein molecules. Amino acids, the building blocks of proteins , are easily synthesized in plausible prebiotic conditions, as are small peptides ( polymers of amino acids) that make good catalysts. A series of experiments starting in 1997 showed that amino acids and peptides could form in

20636-575: The researchers, "If life arose relatively quickly on Earth … then it could be common in the universe ." Photosynthetic organisms appeared between 3.2 and 2.4 billion years ago and began enriching the atmosphere with oxygen. Life remained mostly small and microscopic until about 580 million years ago , when complex multicellular life arose, developed over time, and culminated in the Cambrian Explosion about 538.8 million years ago. This sudden diversification of life forms produced most of

20790-415: The respective ecosystem and its water depth. This affects photosynthesis and the ecology of plants and phytoplankton . Outside the euphotic zone, photosynthesis cannot occur and life must use other sources of energy than sunlight. Although a number of human activities are conducted underwater—such as research, underwater diving for work or recreation , and underwater warfare with submarines ,

20944-460: The scientific consensus, it still has active supporters. Research in 2003 reported that montmorillonite could also accelerate the conversion of fatty acids into "bubbles", and that the bubbles could encapsulate RNA attached to the clay. Bubbles can then grow by absorbing additional lipids and dividing. The formation of the earliest cells may have been aided by similar processes. A similar hypothesis presents self-replicating iron-rich clays as

21098-416: The sediments today found in oceanic trenches , above subduction zones. For this reason, greenstones are sometimes seen as evidence for subduction during the Archean. The second type is a complex of felsic magmatic rocks . These rocks are mostly tonalite, trondhjemite or granodiorite , types of rock similar in composition to granite (hence such terranes are called TTG-terranes). TTG-complexes are seen as

21252-605: The simplest members of the three modern domains of life use DNA to record their "recipes" and a complex array of RNA and protein molecules to "read" these instructions and use them for growth, maintenance, and self-replication. The discovery that a kind of RNA molecule called a ribozyme can catalyze both its own replication and the construction of proteins led to the hypothesis that earlier life-forms were based entirely on RNA. They could have formed an RNA world in which there were individuals but no species , as mutations and horizontal gene transfers would have meant that

21406-724: The surface of the oceans. The solid surface location on the planet closest to the center of the geoid is the Challenger Deep , located in the Mariana Trench at a depth of 10,924 metres (35,840 ft). There is a smaller part of the surface covered by bodies of fresh water and a large volume of underground water in aquifers. The underwater environment is hostile to humans in many ways and therefore little explored. It can be mapped by sonar , or more directly explored via manned, remotely operated, or autonomous submersibles . The ocean floors have been surveyed via sonar to at least

21560-529: The surface. On a large scale, the process of brine rejection and sinking cold salty water results in ocean currents forming to transport such water away from the Poles, leading to a global system of currents called the thermohaline circulation . The density of water causes ambient pressures that increase dramatically with depth. The atmospheric pressure at the surface is 14.7 pounds per square inch or around 100 kPa. A comparable hydrostatic pressure occurs at

21714-409: The surrounding environment. They used fermentation , the breakdown of more complex compounds into less complex compounds with less energy, and used the energy so liberated to grow and reproduce. Fermentation can only occur in an anaerobic (oxygen-free) environment. The evolution of photosynthesis made it possible for cells to derive energy from the Sun. Most of the life that covers the surface of

21868-455: The temperature increases, the density rises to a peak at 3.98 °C (39.16 °F) and then decreases; this is unusual. Regular, hexagonal ice is also less dense than liquid water—upon freezing, the density of water decreases by about 9%. These effects are due to the reduction of thermal motion with cooling, which allows water molecules to form more hydrogen bonds that prevent the molecules from coming close to each other. While below 4 °C

22022-404: The thermal stratification, as well as the degree and frequency of mixing, has a strong control over the distribution of oxygen within the lake. Professor F.-A. Forel , also referred to as the "Father of limnology", was the first scientist to classify lakes according to their thermal stratification. His system of classification was later modified and improved upon by Hutchinson and Löffler. As

22176-549: The time it takes for sound waves in air to reach each of the two ears. For these reasons, divers find it difficult to determine the direction of sound underwater. Some animals have adapted to this difference and many use sound to navigate underwater. An aquatic ecosystem is an ecosystem in a body of water . Communities of organisms that are dependent on each other and on their environment live in aquatic ecosystems. The two main types of aquatic ecosystems are marine ecosystems and freshwater ecosystems . Marine ecosystems are

22330-456: The times that they existed. There are two types of paleolake: Paleolakes are of scientific and economic importance. For example, Quaternary paleolakes in semidesert basins are important for two reasons: they played an extremely significant, if transient, role in shaping the floors and piedmonts of many basins; and their sediments contain enormous quantities of geologic and paleontologic information concerning past environments. In addition,

22484-641: The two. Lakes are also distinct from lagoons , which are generally shallow tidal pools dammed by sandbars or other material at coastal regions of oceans or large lakes. Most lakes are fed by springs , and both fed and drained by creeks and rivers , but some lakes are endorheic without any outflow, while volcanic lakes are filled directly by precipitation runoffs and do not have any inflow streams. Natural lakes are generally found in mountainous areas (i.e. alpine lakes ), dormant volcanic craters , rift zones and areas with ongoing glaciation . Other lakes are found in depressed landforms or along

22638-503: The underwater environment is hostile to humans in many ways and therefore little explored. An immediate obstacle to human activity under water is that human lungs cannot naturally function in this environment. Unlike the gills of fish , human lungs are adapted to the exchange of gases at atmospheric pressure . Any penetration into the underwater environment for more than a few minutes requires artificial aids to maintain life. For solid and liquid tissues like bone, muscle and blood,

22792-473: The underwater environment. In ambient pressure diving, the diver is directly exposed to the pressure of the surrounding water. The ambient pressure diver may dive on breath-hold, or use breathing apparatus for scuba diving or surface-supplied diving , and the saturation diving technique reduces the risk of decompression sickness (DCS) after long-duration deep dives. Immersion in water and exposure to cold water and high pressure have physiological effects on

22946-467: The word pond , and a lesser number of names ending with lake are, in quasi-technical fact, ponds. One textbook illustrates this point with the following: "In Newfoundland, for example, almost every lake is called a pond, whereas in Wisconsin, almost every pond is called a lake." One hydrology book proposes to define the term "lake" as a body of water with the following five characteristics: With

23100-466: Was a crucial development. Life developed from prokaryotes into eukaryotes and multicellular forms. The Proterozoic saw a couple of severe ice ages called Snowball Earths . After the last Snowball Earth about 600 Ma, the evolution of life on Earth accelerated. About 580 Ma, the Ediacaran biota formed the prelude for the Cambrian Explosion . The earliest cells absorbed energy and food from

23254-454: Was bound up with limestone , iron , and other minerals. The oxidized iron appears as red layers in geological strata called banded iron formations that formed in abundance during the Siderian period (between 2500 Ma and 2300 Ma). When most of the exposed readily reacting minerals were oxidized, oxygen finally began to accumulate in the atmosphere. Though each cell only produced

23408-403: Was hot enough to melt the heavy, siderophile metals . Having higher densities than the silicates, these metals sank. This so-called iron catastrophe resulted in the separation of a primitive mantle and a (metallic) core only 10 million years after the Earth began to form, producing the layered structure of Earth and setting up the formation of Earth's magnetic field . J.A. Jacobs

23562-617: Was originally a shallow natural lake and an example of a lake basin dammed by wind-blown sand. China's Badain Jaran Desert is a unique landscape of megadunes and elongated interdunal aeolian lakes, particularly concentrated in the southeastern margin of the desert. Shoreline lakes are generally lakes created by blockage of estuaries or by the uneven accretion of beach ridges by longshore and other currents. They include maritime coastal lakes, ordinarily in drowned estuaries; lakes enclosed by two tombolos or spits connecting an island to

23716-563: Was the first to suggest that Earth's inner core —a solid center distinct from the liquid outer core —is freezing and growing out of the liquid outer core due to the gradual cooling of Earth's interior (about 100 degrees Celsius per billion years ). The first eon in Earth's history, the Hadean , begins with the Earth's formation and is followed by the Archean eon at 3.8 Ga. The oldest rocks found on Earth date to about 4.0 Ga, and

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