Wi-Fi or WiFi is a technology for wireless local area networking with devices based on the IEEE802.11 standards. Wi-Fi is a trademark of the Wi-Fi Alliance, which restricts the use of the term Wi-Fi Certified to products that successfully complete interoperability certification testing. Devices that can use Wi-Fi technology include personal computers, video-game consoles, digital cameras, tablet computers, digital audio players, Wi-Fi compatible devices can connect to the Internet via a WLAN network and a wireless access point. Such an access point has a range of about 20 meters indoors, hotspot coverage can be as small as a single room with walls that block radio waves, or as large as many square kilometres achieved by using multiple overlapping access points. Wi-Fi most commonly uses the 2.4 gigahertz UHF and 5 gigahertz SHF ISM radio bands, having no physical connections, it is more vulnerable to attack than wired connections, such as Ethernet. In 1971, ALOHAnet connected the Hawaiian Islands with a UHF wireless packet network, ALOHAnet and the ALOHA protocol were early forerunners to Ethernet, and the IEEE802.11 protocols, respectively. A1985 ruling by the U. S.
Federal Communications Commission released the ISM band for unlicensed use and these frequency bands are the same ones used by equipment such as microwave ovens and are subject to interference. In 1991, NCR Corporation with AT&T Corporation invented the precursor to 802.11, the first wireless products were under the name WaveLAN. They are the credited with inventing Wi-Fi. In 1992 and 1996, CSIRO obtained patents for a method used in Wi-Fi to unsmear the signal. The first version of the 802.11 protocol was released in 1997 and this was updated in 1999 with 802. 11b to permit 11 Mbit/s link speeds, and this proved to be popular. In 1999, the Wi-Fi Alliance formed as an association to hold the Wi-Fi trademark under which most products are sold. Wi-Fi uses a number of patents held by many different organizations. In April 2009,14 technology companies agreed to pay CSIRO $1 billion for infringements on CSIRO patents and this led to Australia labeling Wi-Fi as an Australian invention, though this has been the subject of some controversy.
In 2016, the local area network Test Bed was chosen as Australias contribution to the exhibition A History of the World in 100 Objects held in the National Museum of Australia. The name Wi-Fi, commercially used at least as early as August 1999, was coined by the brand-consulting firm Interbrand, the Wi-Fi Alliance had hired Interbrand to create a name that was a little catchier than IEEE802. 11b Direct Sequence. Phil Belanger, a member of the Wi-Fi Alliance who presided over the selection of the name Wi-Fi, has stated that Interbrand invented Wi-Fi as a pun upon the word hi-fi. Interbrand created the Wi-Fi logo, the yin-yang Wi-Fi logo indicates the certification of a product for interoperability
In aircraft, an ejection seat or ejector seat is a system designed to rescue the pilot or other crew of an aircraft in an emergency. In most designs, the seat is propelled out of the aircraft by a charge or rocket motor. The concept of an escape crew capsule has been tried. Once clear of the aircraft, the ejection seat deploys a parachute, Ejection seats are common on certain types of military aircraft. A bungee-assisted escape from an aircraft took place in 1910, in 1916 Everard Calthrop, an early inventor of parachutes, patented an ejector seat using compressed air. The modern layout for a seat was first proposed by Romanian inventor Anastase Dragomir in the late 1920s. The design, featuring a parachuted cell, was tested on 25 August 1929 at the Paris-Orly Airport near Paris and in October 1929 at Băneasa. Dragomir patented his catapult-able cockpit at the French Patent Office, the design was perfected during World War II. The first ejection seats were developed independently during World War II by Heinkel, early models were powered by compressed air and the first aircraft to be fitted with such a system was the Heinkel He 280 prototype jet-engined fighter in 1940.
At 2,400 m, Schenk found he had no control, jettisoned his towline, the He 280 was never put into production status and the first operational type built anywhere, to provide ejection seats for the crew was the Heinkel He 219 Uhu night fighter in 1942. In Sweden, a version using compressed air was tested in 1941, a gunpowder ejection seat was developed by Bofors and tested in 1943 for the Saab 21. The first test in the air was on a Saab 17 on 27 February 1944, and the first real use occurred by Lt. Bengt Johansson on 29 July 1946 after a mid-air collision between a J21 and a J22. As the first operational military jet in late 1944 to ever feature one, the lightweight Heinkel He 162A Spatz featured a new type of ejection seat, in this system, the seat rode on wheels set between two pipes running up the back of the cockpit. When lowered into position, caps at the top of the seat fitted over the pipes to close them, basically identical to shotgun shells, were placed in the bottom of the pipes, facing upward.
When fired, the gases would fill the pipes, popping the caps off the end, after World War II, the need for such systems became pressing, as aircraft speeds were getting ever higher, and it was not long before the sound barrier was broken. Manual escape at such speeds would be impossible, the United States Army Air Forces experimented with downward-ejecting systems operated by a spring, but it was the work of Sir James Martin and his company Martin-Baker that was to prove crucial. The first live flight test of the Martin-Baker system took place on 24 July 1946, shortly afterward, on 17 August 1946, 1st Sgt. Larry Lambert was the first live U. S. ejectee, Lynch demonstrated the ejection seat at the Daily Express Air Pageant in 1948, ejecting from a Meteor. W.52 experimental flying wing
An adverse event will not rise to the level of a disaster if it occurs in an area without vulnerable population. In a vulnerable area, such as Nepal during the 2015 earthquake, a landslide is described as an outward and downward slope movement of an abundance of slope-forming materials including rock, artificial, or even a combination of these things. During World War I, an estimated 40,000 to 80,000 soldiers died as a result of avalanches during the campaign in the Alps at the Austrian-Italian front. Many of the avalanches were caused by artillery fire, an earthquake is the result of a sudden release of energy in the Earths crust that creates seismic waves. At the Earths surface earthquakes manifest themselves by vibration, earthquakes are caused by slippage within geological faults. The underground point of origin of the earthquake is called the seismic focus, the point directly above the focus on the surface is called the epicenter. Earthquakes by themselves rarely kill people or wildlife and it is usually the secondary events that they trigger such as building collapse, fires and volcanoes.
Many of these could possibly be avoided by better construction, safety systems, when natural erosion or human mining makes the ground too weak to support the structures built on it, the ground can collapse and produce a sinkhole. Volcanoes can cause destruction and consequent disaster in several ways. The effects include the eruption itself that may cause harm following the explosion of the volcano or falling rocks. Second, lava may be produced during the eruption of a volcano, volcanic ash generally meaning the cooled ash – may form a cloud, and settle thickly in nearby locations. When mixed with water forms a concrete-like material. In sufficient quantity ash may cause roofs to collapse under its weight, since the ash has the consistency of ground glass it causes abrasion damage to moving parts such as engines. It is believed that Pompeii was destroyed by a pyroclastic flow, a lahar is a volcanic mudflow or landslide. The 1953 Tangiwai disaster was caused by a lahar, as was the 1985 Armero tragedy in which the town of Armero was buried, a specific type of volcano is the supervolcano.
It killed three-quarters of all plant life in the northern hemisphere, the main danger from a supervolcano is the immense cloud of ash, which has a disastrous global effect on climate and temperature for many years. It is a violent and destructive change either in quality of water or in distribution or movement of water on land below the surface or in atmosphere. A flood is an overflow of water that submerges land, the EU Floods Directive defines a flood as a temporary covering by water of land which is usually not covered by water
A low-pressure area, low, or depression is a region where the atmospheric pressure is lower than that of surrounding locations. Low-pressure systems form under areas of wind divergence that occur in the levels of the troposphere. The formation process of an area is known as cyclogenesis. Within the field of meteorology, atmospheric divergence aloft occurs in two areas, the first area is on the east side of upper troughs, which form half of a Rossby wave within the Westerlies. A second area of wind divergence aloft occurs ahead of embedded shortwave troughs, diverging winds aloft ahead of these troughs cause atmospheric lift within the troposphere below, which lowers surface pressures as upward motion partially counteracts the force of gravity. Thermal lows form due to localized heating caused by greater sunshine over deserts, since localized areas of warm air are less dense than their surroundings, this warmer air rises, which lowers atmospheric pressure near that portion of the Earths surface.
Large-scale thermal lows over continents help drive monsoon circulations, low-pressure areas can form due to organized thunderstorm activity over warm water. When this occurs over the tropics in concert with the Intertropical Convergence Zone, Monsoon troughs reach their northerly extent in August and their southerly extent in February. When a convective low acquires a well-hot circulation in the tropics it is termed a tropical cyclone, Tropical cyclones can form during any month of the year globally, but can occur in either the northern or southern hemisphere during November. Since clouds reflect sunlight, incoming solar radiation decreases, which causes lower temperatures during the day. At night the effect of clouds on outgoing longwave radiation, such as heat energy from the surface. The stronger the area of low pressure, the stronger the winds experienced in its vicinity, low-pressure systems are most frequently located over the Tibetan Plateau and in the lee of the Rocky mountains. In Europe, recurring low-pressure weather systems are known as depressions.
Cyclogenesis is the development and strengthening of cyclonic circulations, or low-pressure areas, cyclogenesis is the opposite of cyclolysis, and has an anticyclonic equivalent which deals with the formation of high-pressure areas—anticyclogenesis. Cyclogenesis is a term for several different processes, all of which result in the development of some sort of cyclone. Meteorologists use the term cyclone where circular pressure systems flow in the direction of the Earths rotation, the largest low-pressure systems are cold-core polar cyclones and extratropical cyclones which lie on the synoptic scale. Warm-core cyclones such as cyclones and polar lows lie within the smaller mesoscale. Subtropical cyclones are of intermediate size, cyclogenesis can occur at various scales, from the microscale to the synoptic scale
Road speed limits are used in most countries to set the maximum speed at which road vehicles may legally travel on particular stretches of road. Speed limits may be variable and in some places speeds are unlimited, Speed limits are normally indicated on a traffic sign. Speed limits are set by the legislative bodies of nations or provincial governments. The first maximum speed limit was the 10 mph limit introduced in the United Kingdom in 1861, the highest posted speed limit in the world is 140 km/h, which applies to some roads in Poland and Bulgaria, similarly Texas posts 85 mph on one 40-mile long toll road. However, some roads have no speed limit for certain classes of vehicles, best known are Germanys less congested Autobahns, where automobile drivers have no mandated maximum speed. Measurements from the German state of Brandenburg in 2006 showed average speeds of 142 km/h on a 6-lane section of autobahn in free-flowing conditions, rural roads on the Isle of Man and the Indian states of Andhra Pradesh and Telangana, lack speed limits.
Speed limits are set to attempt to cap road traffic speed. It is often done with an intention to improve traffic safety. Speed limits may be set in an attempt to reduce the impact of road traffic. Some cities have reduced limits to as little as 30 km/h for both safety and efficiency reasons, however, it has been shown that in some circumstances changing a speed limit has little effect on the average speed of cars. In situations where the road speed is considered too high by governments. For some classes of vehicle, speed limiters may be mandated to enforce compliance, since their introduction, speed limits have been opposed by some motoring advocacy groups. The United Kingdom Stage Carriage Act 1832 first introduced the offense of endangering the safety of a passenger or person by furious driving. The Locomotives on Highways Act 1896, which raised the limit to 14 mph is celebrated to this day by the annual London to Brighton Veteran Car Run. The first person to be convicted of speeding is believed to be Walter Arnold of East Peckham, Kent and he was fined 1 shilling plus costs.
In Australia, during the early 20th century, there were reported for furious driving offences. One conviction in 1905 cited furiously driving 20 mph when passing a tram traveling at half that speed. Most jurisdictions use the metric speed unit of kilometers per hour for speed limits, while some, primarily the United States, Australia followed the United Kingdom system before changing to the metric system in the 1970s
A pandemic is an epidemic of infectious disease that has spread through human populations across a large region, for instance multiple continents, or even worldwide. A widespread endemic disease that is stable in terms of how people are getting sick from it is not a pandemic. Further, flu pandemics generally exclude recurrences of seasonal flu, throughout history, there have been a number of pandemics, such as smallpox and tuberculosis. One of the most devastating pandemics was the Black Death, killing over 75 million people in 1350, the most recent pandemics include the HIV pandemic as well as the 1918 and 2009 H1N1 pandemics. A pandemic is an epidemic occurring on a scale which crosses international boundaries, the World Health Organization has a six-stage classification that describes the process by which a novel influenza virus moves from the first few infections in humans through to a pandemic. A disease or condition is not a pandemic merely because it is widespread or kills many people, for instance, cancer is responsible for many deaths but is not considered a pandemic because the disease is not infectious or contagious.
Then you might ask yourself, What is a global outbreak, global outbreak means that we see both spread of the agent … and we see disease activities in addition to the spread of the virus. The 2009 revision, including definitions of a pandemic and the leading to its declaration, were finalized in February 2009. The pandemic H1N12009 virus was neither on the horizon at that time nor mentioned in the document, all versions of this document refer to influenza. The phases are defined by the spread of the disease and mortality are not mentioned in the current WHO definition, HIV originated in Africa, and spread to the United States via Haiti between 1966 and 1972. AIDS is currently a pandemic, with rates as high as 25% in southern and eastern Africa. In 2006, the HIV prevalence rate among pregnant women in South Africa was 29. 1%, infection rates are rising again in Asia and the Americas. The AIDS death toll in Africa may reach 90–100 million by 2025, there have been a number of significant pandemics recorded in human history, generally zoonoses which came about with domestication of animals, such as influenza and tuberculosis.
There have been a number of particularly significant epidemics that deserve mention above the mere destruction of cities, possibly typhoid fever killed a quarter of the Athenian troops, and a quarter of the population over four years. This disease fatally weakened the dominance of Athens, but the sheer virulence of the disease prevented its wider spread, the exact cause of the plague was unknown for many years. In January 2006, researchers from the University of Athens analyzed teeth recovered from a mass grave underneath the city, possibly smallpox brought to the Italian peninsula by soldiers returning from the Near East, it killed a quarter of those infected, and up to five million in all. At the height of an outbreak, the Plague of Cyprian. Plague of Justinian, from 541 to 750, was the first recorded outbreak of the bubonic plague and it started in Egypt, and reached Constantinople the following spring, killing 10,000 a day at its height, and perhaps 40% of the citys inhabitants
Nuclear warfare is a military conflict or political strategy in which nuclear weaponry is used to inflict damage on the enemy. In contrast to conventional warfare, nuclear warfare can produce destruction in a much shorter time-frame, some activists had claimed in the 1980s that with this potential nuclear winter side-effect of a nuclear war, almost every human on Earth could starve to death. So far, two nuclear weapons have been used in the course of warfare, both by the United States near the end of World War II, on August 6,1945, a uranium gun-type device was detonated over the Japanese city of Hiroshima. Three days later, on August 9, a plutonium device was detonated over the Japanese city of Nagasaki. These two bombings resulted in the deaths of approximately 120,000 people, in 1974, and in 1998, two countries that were openly hostile toward each other, developed nuclear weapons. Israel and North Korea are thought to have developed stocks of nuclear weapons, the Israeli government has never admitted to having nuclear weapons, although it is known to have constructed the reactor and reprocessing plant necessary for building nuclear weapons.
Nuclear weapons have been detonated on over 2,000 occasions for testing purposes, after the collapse of the Soviet Union in 1991 and the resultant end of the Cold War, the threat of a major nuclear war between the two nuclear superpowers was generally thought to have declined. Since then, concern over nuclear weapons has shifted to the prevention of localized nuclear conflicts resulting from nuclear proliferation, the possibility of using nuclear weapons in war is usually divided into two subgroups, each with different effects and potentially fought with different types of nuclear armaments. The first, a nuclear war, refers to a small-scale use of nuclear weapons by two belligerents. This term could apply to any use of nuclear weapons that may involve military or civilian targets. The second, a nuclear war, could consist of large numbers of nuclear weapons used in an attack aimed at an entire country, including military, economic. Such an attack would almost certainly destroy the economic and military infrastructure of the target nation.
Some Cold War strategists such as Henry Kissinger argued that a nuclear war could be possible between two heavily armed superpowers. Some predict, that a war could potentially escalate into a full-scale nuclear war. Even the most optimistic predictions of the effects of a nuclear exchange foresee the death of many millions of victims within a very short period of time. However, such predictions, assuming total war with nuclear arsenals at Cold War highs, have not been without criticism. The authors of the study estimated that as much as five tons of soot could be released, producing a cooling of several degrees over large areas of North America. The cooling would last for years and could be catastrophic, according to the researchers, either a limited or full-scale nuclear exchange could occur during an accidental nuclear war, in which the use of nuclear weapons is triggered unintentionally
Interstate 45 is an interstate highway located entirely within the U. S. state of Texas. While most interstate routes ending in five are cross-country north-south routes, I-45 is comparatively short and it connects the cities of Dallas and Houston, continuing southeast from Houston to Galveston over the Galveston Causeway to the Gulf of Mexico. I-45 replaced US75 over its length, although portions of US75 remained parallel to I-45 until its elimination south of downtown Dallas in 1987. At the south end of I-45, State Highway 87 continues into downtown Galveston, the north end is at Interstate 30 in downtown Dallas, where US75 used the Good-Latimer Expressway. A short continuation, known by traffic reporters as the I-45 overhead, signed as part of US75, traffic can use Spur 366 to connect to Interstate 35E at the north end of I-345. The portion of I-45 between downtown Houston and Galveston is known to Houston residents as the Gulf Freeway, I-45 and I-345 in the Dallas area, north of the interchanges with Interstate 20 and State Highway 310, is the Julius Schepps Freeway.
The Gulf Freeway and North Freeway both include reversible high-occupancy vehicle lanes for buses and other vehicles to and from downtown Houston. U. S. Highway 190 joins I-45 for 26 miles from Huntsville, Texas to Madisonville, U. S. Highway 287 joins I-45 for 18 miles from Corsicana, Texas to Ennis, Texas. US287 signs are posted from the northern end of Business Loop 45 in Corsicana to the Ellis County line. Interstate 45 gained notoriety during Hurricane Rita in 2005, thousands of Houston area evacuees jammed the roadway trying to leave. As a result, the became a parking lot. Gas stations ran dry and hundreds of peoples cars simply ran empty, four-hour drives suddenly became 24-hour drives. At just 284.913 miles, I-45 is the shortest of the interstates. The stretch of I-45 connecting Galveston with Houston is known as the Gulf Freeway and it was the first freeway built in Texas—opened in stages beginning on October 1,1948, up to a full completion to Galveston in 1952, as part of U. S. Highway 75.
At the north end, it connects to the North Freeway via the short Pierce Elevated, completed in 1967, after several interchanges, I-45 crosses the Galveston Causeway and passes Tiki Island. Old U. S. Highway 75 south of junction was upgraded on the spot. The Gulf Freeway generally parallels State Highway 3 about 1 mile to the west, bypassing La Marque and South Houston. It includes interchanges with other freeways, the Emmett F. Lowry Expressway, NASA Road 1 Bypass
A lifeboat is a small, rigid or inflatable boat carried for emergency evacuation in the event of a disaster aboard a ship. Lifeboat drills are required by law on larger commercial ships, in the military, a lifeboat may double as a whaleboat, dinghy, or gig. The ships tenders of cruise ships often double as lifeboats, recreational sailors usually carry inflatable life rafts, though a few prefer small proactive lifeboats that are harder to sink and can be sailed to safety. Inflatable lifeboats may be equipped with auto-inflation canisters or mechanical pumps, a quick release and pressure release mechanism is fitted on ships so that the canister or pump automatically inflates the lifeboat, and the lifeboat breaks free of the sinking vessel. Commercial aircraft are required to carry auto-inflating life rafts in case of an emergency water landing. Ship-launched lifeboats are lowered from davits on a deck, and are hard to sink in normal circumstances. The cover serves as protection from sun and rain, can be used to collect rainwater, lifeboats have oars and mirrors for signaling, first aid supplies, and food and water for several days.
Modern lifeboats carry an Emergency Position-Indicating Radio Beacon and either a radar reflector or Search, during the Age of Sail, the ships boats were often used as lifeboats in case of emergency. In March 1870, answering a question at the House of Commons of the United Kingdom about the sinking of PS Normandy and they would encumber the decks, and rather add to the danger than detract from it. It was not until after the sinking of the RMS Titanic on April 15,1912, the Titanic′s boats had a capacity of 1,178 people on a ship capable of carrying 3,330 people. This was resolved by the use of collapsible lifeboats, a number of which had been carried on the Titanic. The US Navy asked various groups and manufacturers to suggest solutions, the result was the first enclosed, self-righting lifeboat, manufactured in Delanco, New Jersey, USA, the first units were delivered in 1944. These radically new lifeboats were 24 feet in length and weighed 5,000 lbs and they had two enclosed cabins at each end which could hold a total of 25 persons.
The space in between was designed to help persons in the water be pulled aboard, and could be enclosed with a canvas top, the new type lifeboat could be driven either by a small motor or sail. Also, in 1943 the US developed a balsa wood liferaft that would not sink and these balsa liferafts were designed to hold five to ten men on a platform suspended on the inside or fifteen to twenty-five hanging lines placed on the outsides. They were inexpensive, and during the war thousands were stored in any space possible on US warships and these liferafts were intended only for use during a short term before lifeboats or another ship in the convoy or group could bring them aboard. Today, enclosed lifeboats are the preferred lifeboats fitted on modern merchant ships because of their protection against the elements. Generally each merchant ship has one fitted on the port side and one on the starboard side
A tsunami or tidal wave, known as a seismic sea wave, is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Unlike normal ocean waves which are generated by wind, or tides which are generated by the pull of the Moon and Sun. Tsunami waves do not resemble normal undersea currents or sea waves, Tsunamis generally consist of a series of waves with periods ranging from minutes to hours, arriving in a so-called internal wave train. Wave heights of tens of metres can be generated by large events, numerous terms are used in the English language to describe waves created in a body of water by the displacement of water, none of the terms in frequent use are entirely accurate. The term tsunami, meaning harbour wave in literal translation, comes from the Japanese 津波, while not entirely accurate, as tsunami are not restricted to harbours, tsunami is currently the term most widely accepted by geologists and oceanographers. Tsunami are sometimes referred to as tidal waves and this once-popular term derives from the most common appearance of tsunami, which is that of an extraordinarily high tidal bore.
Although the meanings of tidal include resembling or having the form or character of the tides, use of the tidal wave is discouraged by geologists. The term seismic sea wave is used to refer to the phenomenon, prior to the rise of the use of the term tsunami in English-speaking countries, scientists generally encouraged the use of the term seismic sea wave rather than tidal wave. The Sumatran region is not unused to tsunamis either, with earthquakes of varying magnitudes regularly occurring off the coast of the island, Tsunamis are an often underestimated hazard in the Mediterranean Sea and parts of Europe. The tsunami claimed more than 123,000 lives in Sicily, the Storegga Slide in the Norwegian sea and some examples of tsunamis affecting the British Isles refer to landslide and meteotsunamis predominantly and less to earthquake-induced waves. The cause, in my opinion, of this phenomenon must be sought in the earthquake, at the point where its shock has been the most violent the sea is driven back, and suddenly recoiling with redoubled force, causes the inundation.
Without an earthquake I do not see how such an accident could happen, the principal generation mechanism of a tsunami is the displacement of a substantial volume of water or perturbation of the sea. This displacement of water is attributed to either earthquakes, volcanic eruptions, glacier calvings or more rarely by meteorites. The waves formed in this way are sustained by gravity, tides do not play any part in the generation of tsunamis. Tsunami can be generated when the sea floor abruptly deforms and vertically displaces the overlying water and they grow in height when they reach shallower water, in a wave shoaling process described below. A tsunami can occur in any state and even at low tide can still inundate coastal areas. On April 1,1946, the 8.6 Mw Aleutian Islands earthquake occurred with a maximum Mercalli intensity of VI and it generated a tsunami which inundated Hilo on the island of Hawaii with a 14-metre high surge. Between 165 and 173 were killed, the area where the earthquake occurred is where the Pacific Ocean floor is subducting under Alaska
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. Earths volcanoes occur because its crust is broken into 17 major, therefore, on Earth, volcanoes are generally found where tectonic plates are diverging or converging. This type of volcanism falls under the umbrella of plate hypothesis volcanism, Volcanism away from plate boundaries has been explained as mantle plumes. These so-called hotspots, for example Hawaii, are postulated to arise from upwelling diapirs with magma from the boundary,3,000 km deep in the Earth. Volcanoes are usually not created where two plates slide past one another. Erupting volcanoes can pose hazards, not only in the immediate vicinity of the eruption. Historically, so-called volcanic winters have caused catastrophic famines, the word volcano is derived from the name of Vulcano, a volcanic island in the Aeolian Islands of Italy whose name in turn comes from Vulcan, the god of fire in Roman mythology.
The study of volcanoes is called volcanology, sometimes spelled vulcanology, at the mid-oceanic ridges, two tectonic plates diverge from one another as new oceanic crust is formed by the cooling and solidifying of hot molten rock. Most divergent plate boundaries are at the bottom of the oceans, most volcanic activity is submarine, black smokers are evidence of this kind of volcanic activity. Where the mid-oceanic ridge is above sea-level, volcanic islands are formed, for example, subduction zones are places where two plates, usually an oceanic plate and a continental plate, collide. In this case, the plate subducts, or submerges under the continental plate forming a deep ocean trench just offshore. In a process called flux melting, water released from the subducting plate lowers the temperature of the overlying mantle wedge. This magma tends to be very viscous due to its high content, so it often does not reach the surface. When it does reach the surface, a volcano is formed, typical examples of this kind of volcano are Mount Etna and the volcanoes in the Pacific Ring of Fire.
Because tectonic plates move across them, each volcano becomes dormant and is eventually re-formed as the plate advances over the postulated plume and this theory is currently under criticism, however. The most common perception of a volcano is of a mountain, spewing lava and poisonous gases from a crater at its summit, however. The features of volcanoes are more complicated and their structure. Some volcanoes have rugged peaks formed by lava domes rather than a summit crater while others have features such as massive plateaus