The Devonian is a geologic period and system of the Paleozoic, spanning 60 million years from the end of the Silurian, 419.2 million years ago, to the beginning of the Carboniferous, 358.9 Mya. It is named after Devon, where rocks from this period were first studied; the first significant adaptive radiation of life on dry land occurred during the Devonian. Free-sporing vascular plants began to spread across dry land, forming extensive forests which covered the continents. By the middle of the Devonian, several groups of plants had evolved leaves and true roots, by the end of the period the first seed-bearing plants appeared. Various terrestrial arthropods became well-established. Fish reached substantial diversity during this time, leading the Devonian to be dubbed the "Age of Fishes." The first ray-finned and lobe-finned bony fish appeared, while the placoderms began dominating every known aquatic environment. The ancestors of all four-limbed vertebrates began adapting to walking on land, as their strong pectoral and pelvic fins evolved into legs.
In the oceans, primitive sharks became more numerous than in the Late Ordovician. The first ammonites, species of molluscs, appeared. Trilobites, the mollusc-like brachiopods and the great coral reefs, were still common; the Late Devonian extinction which started about 375 million years ago affected marine life, killing off all placodermi, all trilobites, save for a few species of the order Proetida. The palaeogeography was dominated by the supercontinent of Gondwana to the south, the continent of Siberia to the north, the early formation of the small continent of Euramerica in between; the period is named after Devon, a county in southwestern England, where a controversial argument in the 1830s over the age and structure of the rocks found distributed throughout the county was resolved by the definition of the Devonian period in the geological timescale. The Great Devonian Controversy was a long period of vigorous argument and counter-argument between the main protagonists of Roderick Murchison with Adam Sedgwick against Henry De la Beche supported by George Bellas Greenough.
Murchison and Sedgwick named the period they proposed as the Devonian System. While the rock beds that define the start and end of the Devonian period are well identified, the exact dates are uncertain. According to the International Commission on Stratigraphy, the Devonian extends from the end of the Silurian 419.2 Mya, to the beginning of the Carboniferous 358.9 Mya. In nineteenth-century texts the Devonian has been called the "Old Red Age", after the red and brown terrestrial deposits known in the United Kingdom as the Old Red Sandstone in which early fossil discoveries were found. Another common term is "Age of the Fishes", referring to the evolution of several major groups of fish that took place during the period. Older literature on the Anglo-Welsh basin divides it into the Downtonian, Dittonian and Farlovian stages, the latter three of which are placed in the Devonian; the Devonian has erroneously been characterised as a "greenhouse age", due to sampling bias: most of the early Devonian-age discoveries came from the strata of western Europe and eastern North America, which at the time straddled the Equator as part of the supercontinent of Euramerica where fossil signatures of widespread reefs indicate tropical climates that were warm and moderately humid but in fact the climate in the Devonian differed during its epochs and between geographic regions.
For example, during the Early Devonian, arid conditions were prevalent through much of the world including Siberia, North America, China, but Africa and South America had a warm temperate climate. In the Late Devonian, by contrast, arid conditions were less prevalent across the world and temperate climates were more common; the Devonian Period is formally broken into Early and Late subdivisions. The rocks corresponding to those epochs are referred to as belonging to the Lower and Upper parts of the Devonian System. Early DevonianThe Early Devonian lasted from 419.2 ± 2.8 to 393.3 ± 2.5 and began with the Lochkovian stage, which lasted until the Pragian. It spanned from 410.8 ± 2.8 to 407.6 ± 2.5, was followed by the Emsian, which lasted until the Middle Devonian began, 393.3± 2.7 million years ago. During this time, the first ammonoids appeared. Ammonoids during this time period differed little from their nautiloid counterparts; these ammonoids belong to the order Agoniatitida, which in epochs evolved to new ammonoid orders, for example Goniatitida and Clymeniida.
This class of cephalopod molluscs would dominate the marine fauna until the beginning of the Mesozoic era. Middle DevonianThe Middle Devonian comprised two subdivisions: first the Eifelian, which gave way to the Givetian 387.7± 2.7 million years ago. During this time the jawless agnathan fishes began to decline in diversity in freshwater and marine environments due to drastic environmental changes and due to the increasing competition and diversity of jawed fishes; the shallow, oxygen-depleted waters of Devonian inland lakes, surrounded by primitive plants, provided the environment necessary for certain early fish to develop such essential characteristics as well developed lungs, the ability to crawl out of the water and onto the land for short periods of time. Late DevonianFinally, the Late Devonian started with the Frasnian, 382.7 ± 2.8 to 372.2 ± 2.5, during which the first forests took shape on land. The first tetrapods appeared in the fossil record in the ensuing Famennian subdivisi
Captain Matthew Flinders was an English navigator and cartographer who led the first circumnavigation of Australia and identified it as a continent. Flinders made three voyages to the southern ocean between 1791 and 1810. In the second voyage, George Bass and Flinders confirmed. In the third voyage, Flinders circumnavigated the mainland of what was to be called Australia, accompanied by Aboriginal man Bungaree. Heading back to England in 1803, Flinders' vessel needed urgent repairs at Isle de France. Although Britain and France were at war, Flinders thought the scientific nature of his work would ensure safe passage, but a suspicious governor kept him under arrest for more than six years. In captivity, he recorded details of his voyages for future publication, put forward his rationale for naming the new continent'Australia', as an umbrella term for New Holland and New South Wales – a suggestion taken up by Governor Macquarie. Flinders' health had suffered and although he reached home in 1810, he did not live to see the success of his praised book and atlas, A Voyage to Terra Australis.
The location of his grave was lost by the mid-19th century but archaeologists excavating a former burial ground near London's Euston railway station for the High Speed 2 project, announced in January 2019 that his remains had been identified. Matthew Flinders was born in Donington, England, the son of Matthew Flinders, a surgeon, his wife Susannah, née Ward, he was educated at Cowley's Charity School, from 1780 and at the Reverend John Shinglar's Grammar School at Horbling in Lincolnshire. In his own words, he was "induced to go to sea against the wishes of my friends from reading Robinson Crusoe", in 1790, at the age of fifteen, he joined the Royal Navy. Serving on HMS Alert, he transferred to HMS Scipio, in July 1790 was made midshipman on HMS Bellerophon under Captain Pasley. By Pasley's recommendation, he joined Captain Bligh's expedition on HMS Providence, transporting breadfruit from Tahiti to Jamaica; this was Bligh's second "Breadfruit Voyage" following on from the ill-fated voyage of the Bounty.
Flinders' first voyage to New South Wales, first trip to Port Jackson, was in 1795 as a midshipman aboard HMS Reliance, carrying the newly appointed governor of New South Wales Captain John Hunter. On this voyage he established himself as a fine navigator and cartographer, became friends with the ship's surgeon George Bass, three years his senior and had been born 11 miles from Donington. Not long after their arrival in Port Jackson and Flinders made two expeditions in two small open boats, named Tom Thumb and Tom Thumb II respectively: the first to Botany Bay and Georges River, the second, in the larger Tom Thumb II, south from Port Jackson to Lake Illawarra, during which expedition they had to seek shelter at Wattamolla. In 1798, Matthew Flinders, now a lieutenant, was given command of the sloop Norfolk with orders "to sail beyond Furneaux's Islands, should a strait be found, pass through it, return by the south end of Van Diemen's Land"; the passage between the Australian mainland and Tasmania enabled savings of several days on the journey from England, was named Bass Strait, after his close friend.
In honour of this discovery, the largest island in Bass Strait would be named Flinders Island. The town of Flinders near the mouth of Western Port commemorates Bass' discovery of that bay and port on 4 January 1798. Flinders never entered Western Port, passed Cape Schanck only on 3 May 1802. Flinders once more sailed Norfolk, this time north on 17 July 1799, he touched down at Pumicestone Passage and Coochiemudlo Island and rowed ashore at Clontarf. During this visit he named Redcliffe after the Red Cliffs. In March 1800, Flinders set sail for England. Flinders' work had come to the attention of many of the scientists of the day, in particular the influential Sir Joseph Banks, to whom Flinders dedicated his Observations on the Coasts of Van Diemen's Land, on Bass's Strait, etc.. Banks used his influence with Earl Spencer to convince the Admiralty of the importance of an expedition to chart the coastline of New Holland; as a result, in January 1801, Flinders was given command of HMS Investigator, a 334-ton sloop, promoted to commander the following month.
Investigator set sail for New Holland on 18 July 1801. Attached to the expedition were the botanist Robert Brown, botanical artist Ferdinand Bauer, landscape artist William Westall, gardener Peter Good, geological assistant John Allen, John Crosley as astronomer. Vallance et al. comment that compared to the Baudin expedition this was a'modest contingent of scientific gentlemen', which reflects'British parsimony' in scientific endeavour. On 17 April 1801, Flinders married his longtime friend Ann Chappelle and had hoped to bring her with him to Port Jackson; however the Admiralty had strict rules against wives accompanying captains. Flinders brought Ann on board ship and planned to ignore the rules, but the Admiralty learned of his plans and he was chastised for his bad judgement and told he must remove her from the ship; this is well documented in correspondence between Flinders and his chief benefactor, Sir Joseph Banks, in May 1801: I have but time to tell you that the news of your marriage, published in the Lincoln paper, has reached me.
The Lords of the Admiralty have heard that Mrs. Flinders is on board the Investigator, that you have some thought of carrying her to sea with you; this I was sorry to hear, if, the case I beg to give you my
Victoria is a state in south-eastern Australia. Victoria is Australia's smallest mainland state and its second-most populous state overall, thus making it the most densely populated state overall. Most of its population lives concentrated in the area surrounding Port Phillip Bay, which includes the metropolitan area of its state capital and largest city, Australia's second-largest city. Victoria is bordered by Bass Strait and Tasmania to the south,New South Wales to the north, the Tasman Sea to the east, South Australia to the west; the area, now known as Victoria is the home of many Aboriginal people groups, including the Boon wurrung, the Bratauolung, the Djadjawurrung, the Gunai/Kurnai, the Gunditjmara, the Taungurong, the Wathaurong, the Wurundjeri, the Yorta Yorta. There were more than 30 Aboriginal languages spoken in the area prior to the European settlement of Australia; the Kulin nation is an alliance of five Aboriginal nations which makes up much of the central part of the state. With Great Britain having claimed the half of the Australian continent, east of the 135th meridian east in 1788, Victoria formed part of the wider colony of New South Wales.
The first European settlement in the area occurred in 1803 at Sullivan Bay, much of what is now Victoria was included in 1836 in the Port Phillip District, an administrative division of New South Wales. Named in honour of Queen Victoria, who signed the division's separation from New South Wales, the colony was established in 1851 and achieved self government in 1855; the Victorian gold rush in the 1850s and 1860s increased both the population and wealth of the colony, by the time of the Federation of Australia in 1901, Melbourne had become the largest city and leading financial centre in Australasia. Melbourne served as federal capital of Australia until the construction of Canberra in 1927, with the Federal Parliament meeting in Melbourne's Parliament House and all principal offices of the federal government being based in Melbourne. Politically, Victoria has 37 seats in the Australian House of Representatives and 12 seats in the Australian Senate. At state level, the Parliament of Victoria consists of the Legislative Assembly and the Legislative Council.
The Labor Party led Daniel Andrews as premier has governed Victoria since 2014. The personal representative of the Queen of Australia in the state is the Governor of Victoria Linda Dessau. Victoria is divided into 79 municipal districts, including 33 cities, although a number of unincorporated areas still exist, which the state administers directly; the economy of Victoria is diversified, with service sectors including financial and property services, education, retail and manufacturing constitute the majority of employment. Victoria's total gross state product ranks second in Australia, although Victoria ranks fourth in terms of GSP per capita because of its limited mining activity. Culturally, Melbourne hosts a number of museums, art galleries, theatres, is described as the world's sporting capital; the Melbourne Cricket Ground, the largest stadium in Australia and the Southern Hemisphere, hosted the 1956 Summer Olympics and the 2006 Commonwealth Games. The ground is considered the "spiritual home" of Australian cricket and Australian rules football, hosts the grand final of the Australian Football League each year, drawing crowds of 100,000.
Nearby Melbourne Park has hosted the Australian Open, one of tennis' four Grand Slam events, annually since 1988. Victoria has eight public universities, with the oldest, the University of Melbourne, dating from 1853. Victoria, like Queensland, was named after Queen Victoria, on the British throne for 14 years when the colony was established in 1851. After the founding of the colony of New South Wales in 1788, Australia was divided into an eastern half named New South Wales and a western half named New Holland, under the administration of the colonial government in Sydney; the first British settlement in the area known as Victoria was established in October 1803 under Lieutenant-Governor David Collins at Sullivan Bay on Port Phillip. It consisted of 402 people, they had been sent from England in HMS Calcutta under the command of Captain Daniel Woodriff, principally out of fear that the French, exploring the area, might establish their own settlement and thereby challenge British rights to the continent.
In 1826, Colonel Stewart, Captain Samuel Wright, Lieutenant Burchell were sent in HMS Fly and the brigs Dragon and Amity, took a number of convicts and a small force composed of detachments of the 3rd and 93rd regiments. The expedition landed at Settlement Point, on the eastern side of Western Port Bay, the headquarters until the abandonment of Western Port at the insistence of Governor Darling about 12 months afterwards. Victoria's next settlement was on the south west coast of what is now Victoria. Edward Henty settled Portland Bay in 1834. Melbourne was founded in 1835 by John Batman, who set up a base in Indented Head, John Pascoe Fawkner. From settlement, the region around Melbourne was known as the Port Phillip District, a separately administered part of New South Wales. Shortly after, the site now known as Geelong was surveyed by Assistant Surveyor W. H. Smythe, three weeks after Melbourne, and in 1838, Geelong was declared a town, despite earlier European settlements dating back to 1826
Thorium is a weakly radioactive metallic chemical element with symbol Th and atomic number 90. Thorium is tarnishes black when it is exposed to air, forming thorium dioxide. Thorium is an electropositive actinide. All known thorium isotopes are unstable; the most stable isotope, 232Th, has a half-life of 14.05 billion years, or about the age of the universe. In the universe, thorium and uranium are the only three radioactive elements that still occur in large quantities as primordial elements, it is estimated to be over three times as abundant as uranium in the Earth's crust, is chiefly refined from monazite sands as a by-product of extracting rare-earth metals. Thorium was discovered in 1829 by the Norwegian amateur mineralogist Morten Thrane Esmark and identified by the Swedish chemist Jöns Jacob Berzelius, who named it after Thor, the Norse god of thunder, its first applications were developed in the late 19th century. Thorium's radioactivity was acknowledged during the first decades of the 20th century.
In the second half of the century, thorium was replaced in many uses due to concerns about its radioactivity. Thorium is still being used as an alloying element in TIG welding electrodes but is being replaced in the field with different compositions, it was material in high-end optics and scientific instrumentation, as the light source in gas mantles, but these uses have become marginal. It has been suggested as a replacement for uranium as nuclear fuel in nuclear reactors, several thorium reactors have been built. Thorium is a moderately soft, bright silvery radioactive actinide metal. In the periodic table, it lies to the right of actinium, to the left of protactinium, below cerium. Pure thorium is ductile and, as normal for metals, can be cold-rolled and drawn. At room temperature, thorium metal has a face-centred cubic crystal structure. Thorium metal has a bulk modulus of about the same as tin's. Aluminium's is 75.2 GPa. Thorium is about as hard as soft steel, so when heated it can be rolled into sheets and pulled into wire.
Thorium is harder than either of them. It becomes superconductive below 1.4 K. Thorium's melting point of 1750 °C is above both those of actinium and protactinium. At the start of period 7, from francium to thorium, the melting points of the elements increase, because the number of delocalised electrons each atom contributes increases from one in francium to four in thorium, leading to greater attraction between these electrons and the metal ions as their charge increases from one to four. After thorium, there is a new downward trend in melting points from thorium to plutonium, where the number of f electrons increases from about 0.4 to about 6: this trend is due to the increasing hybridisation of the 5f and 6d orbitals and the formation of directional bonds resulting in more complex crystal structures and weakened metallic bonding. Among the actinides up to californium, which can be studied in at least milligram quantities, thorium has the highest melting and boiling points and second-lowest density.
Thorium's boiling point of 4788 °C is the fifth-highest among all the elements with known boiling points. The properties of thorium vary depending on the degree of impurities in the sample; the major impurity is thorium dioxide. Experimental measurements of its density give values between 11.5 and 11.66 g/cm3: these are lower than the theoretically expected value of 11.7 g/cm3 calculated from thorium's lattice parameters due to microscopic voids forming in the metal when it is cast. These values lie between those of its neighbours actinium and protactinium, part of a trend across the early actinides. Thorium can form alloys with many other metals. Addition of small proportions of thorium improves the mechanical strength of magnesium, thorium-aluminum alloys have been considered as a way to store thorium in proposed future thorium nuclear reactors. Thorium forms eutectic mixtures with chromium and uranium, it is miscible in both solid and liquid states with its lighter congener cerium. All but two elements up to bismuth have an isotope, stable for all purposes, with the exceptions being technetium and promethium.
All elements from polonium onward are measurably radioactive. 232Th is one of the three nuclides beyond bismuth that have half-lives measured in billions of years. Four-fifths of the thorium present at Earth's formation has survived to the present. 232Th is the only isotope of thorium occurring in quantity in nature. Its stability is attributed to its closed nuclear shell with 142 neutrons. Thorium has a characteristic terrestrial isotopic composition, with atomic weight 232.0377. I
Camping is an outdoor activity involving overnight stays away from home in a shelter, such as a tent. Participants leave developed areas to spend time outdoors in more natural ones in pursuit of activities providing them enjoyment. To be regarded as "camping" a minimum of one night is spent outdoors, distinguishing it from day-tripping and other short-term recreational activities. Camping can be enjoyed through all four seasons. Luxury may be an element, as in early 20th century African safaris, but including accommodations in equipped fixed structures such as high-end sporting camps under the banner of "camping" blurs the line. Camping as a recreational activity became popular among elites in the early 20th century. With time, it grew more democratic, varied. Modern campers frequent publicly owned natural resources such as national and state parks, wilderness areas, commercial campgrounds. Camping is a key part of many youth organizations around the world, such as Scouting, which use it to teach both self-reliance and teamwork.
Camping describes a range of approaches to outdoor accommodation. Survivalist campers set off with as little as possible to get by, whereas recreational vehicle travelers arrive equipped with their own electricity and patio furniture. Camping may be combined with hiking, as in backpacking, is enjoyed in conjunction with other outdoor activities such as canoeing, climbing and hunting. Fastpacking involves both running and camping. There is no universally held definition of what is not camping. Just as with motels which serve both recreational and business guests, the same campground may serve recreational campers, migrant workers, homeless at the same time. Fundamentally, it reflects the nature of activities involved. A children's summer camp with dining hall meals and bunkhouse accommodations may have "camp" in its name but fails to reflect the spirit and form of "camping" as it is broadly understood. A homeless person's lifestyle may involve many common camping activities, such as sleeping out and preparing meals over a fire, but fails to reflect the elective nature and pursuit of spirit rejuvenation that are integral aspect of camping.
Cultures with itinerant lifestyles or lack of permanent dwellings cannot be said to be "camping", it is just their way of life. The history of recreational camping is traced back to Thomas Hiram Holding, a British travelling tailor, but it was first popularised in the UK on the river Thames. By the 1880s large numbers of visitors took part in the pastime, connected to the late Victorian craze for pleasure boating; the early camping equipment was heavy, so it was convenient to transport it by boat or to use craft that converted into tents. Although Thomas Hiram Holding is seen as the father of modern camping in the UK, he was responsible for popularising a different type of camping in the early twentieth century, he experienced the activity in the wild from his youth, when he had spent much time with his parents traveling across the American prairies. He embarked on a cycling and camping tour with some friends across Ireland, his book on his Ireland experience and Camp in Connemara led to the formation of the first camping group in 1901, the Association of Cycle Campers to become the Camping and Caravanning Club.
He wrote The Campers Handbook in 1908, so that he could share his enthusiasm for the great outdoors with the world. The first commercial camping ground in the world was Cunningham’s camp, near Douglas, Isle of Man, which opened in 1894. In 1906 the Association of Cycle Campers opened its first own camping site, in Weybridge. By that time the organization had several hundred members. In 1910 the Association was merged into the National Camping Club. Although WW1 was responsible for a certain hiatus in camping activity, the association received a new lease of life after the war when Sir Robert Baden-Powell became its president. In the US, camping may be traced to William Henry Harrison Murray 1869 publication of Camp-Life in the Adirondacks resulting in a flood of visitors to the Adirondacks that summer; the International Federation of Camping Clubs was founded in 1932 with national clubs from all over the world affiliating with it. By the 1960s camping had become an established family holiday standard and today camp sites are ubiquitous across Europe and North America.
Different types camping may be named after their form of transportation, such as with Canoe camping, car camping, RVing, backpacking, which can involve ultralight gear. Camping is labeled by lifestyle: Glamping combines camping with the luxury and amenities of a home or hotel, has its roots are in the early 1900s European and American safaris in Africa. Workamping allows campers to trade their labor variously for discounts on campsite fees, campground utilities, some degree of pay. Migrant camps are formed not as a temporary housing arrangement. Campgrounds for custom harvesters in the United States may include room to park combines and other large farm equipment. Another way of describing camping is by the manner of arrangement: reservation camping vs. drop camping. Campgrounds may require campers to check in with an employee or campground host prior to setting up camp, or they may allow "drop camping, where this is not required. Drop-in campsites may be free or a drop-box may be provided to accept payments on the honor system.
Although drop camping is specifically allowed by law, it may exist in a legal grey area, such as at California's Slab City. Social media oriented towa
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, gases to escape from a magma chamber below the surface. Earth's volcanoes occur because its crust is broken into 17 major, rigid tectonic plates that float on a hotter, softer layer in its mantle. Therefore, on Earth, volcanoes are found where tectonic plates are diverging or converging, most are found underwater. For example, a mid-oceanic ridge, such as the Mid-Atlantic Ridge, has volcanoes caused by divergent tectonic plates whereas the Pacific Ring of Fire has volcanoes caused by convergent tectonic plates. Volcanoes can form where there is stretching and thinning of the crust's plates, e.g. in the East African Rift and the Wells Gray-Clearwater volcanic field and Rio Grande Rift in North America. 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 core–mantle boundary, 3,000 km deep in the Earth.
Volcanoes are not created where two tectonic plates slide past one another. Erupting volcanoes can pose many hazards, not only in the immediate vicinity of the eruption. One such hazard is that volcanic ash can be a threat to aircraft, in particular those with jet engines where ash particles can be melted by the high operating temperature. Large eruptions can affect temperature as ash and droplets of sulfuric acid obscure the sun and cool the Earth's lower atmosphere. 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 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; because the crust is thin at these ridges due to the pull of the tectonic plates, the release of pressure leads to adiabatic expansion and the partial melting of the mantle, causing volcanism and creating new oceanic crust.
Most divergent plate boundaries are at the bottom of the oceans. Black smokers are evidence of this kind of volcanic activity. Where the mid-oceanic ridge is above sea-level, volcanic islands are formed. Subduction zones are places where two plates an oceanic plate and a continental plate, collide. In this case, the oceanic 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 melting temperature of the overlying mantle wedge, thus creating magma; this magma tends to be viscous because of its high silica content, so it does not attain the surface but cools and solidifies at depth. When it does reach the surface, however, a volcano is formed. Typical examples are the volcanoes in the Pacific Ring of Fire. Hotspots are volcanic areas believed to be formed by mantle plumes, which are hypothesized to be columns of hot material rising from the core-mantle boundary in a fixed space that causes large-volume melting.
Because tectonic plates move across them, each volcano becomes dormant and is re-formed as the plate advances over the postulated plume. The Hawaiian Islands are said to have been formed in such a manner; this theory, has been doubted. The most common perception of a volcano is of a conical mountain, spewing lava and poisonous gases from a crater at its summit; the features of volcanoes are much more complicated and their structure and behavior depends on a number of factors. Some volcanoes have rugged peaks formed by lava domes rather than a summit crater while others have landscape features such as massive plateaus. Vents that issue volcanic material and gases can develop anywhere on the landform and may give rise to smaller cones such as Puʻu ʻŌʻō on a flank of Hawaii's Kīlauea. Other types of volcano include cryovolcanoes on some moons of Jupiter and Neptune. Active mud volcanoes tend to involve temperatures much lower than those of igneous volcanoes except when the mud volcano is a vent of an igneous volcano.
Volcanic fissure vents are linear fractures through which lava emerges. Shield volcanoes, so named for their broad, shield-like profiles, are formed by the eruption of low-viscosity lava that can flow a great distance from a vent, they do not explode catastrophically. Since low-viscosity magma is low in silica, shield volcanoes are more common in oceanic than continental settings; the Hawaiian volcanic chain is a series of shield cones, they are common in Iceland, as well. Lava domes are built by slow eruptions of viscous lava, they are sometimes formed within the crater of a previous volcanic eruption, as in the case of Mount Saint Helen