151-723: Several types of volcanic eruptions —during which material is expelled from a volcanic vent or fissure —have been distinguished by volcanologists . These are often named after famous volcanoes where that type of behavior has been observed. Some volcanoes may exhibit only one characteristic type of eruption during a period of activity, while others may display an entire sequence of types all in one eruptive series. There are three main types of volcanic eruption: Within these broad eruptive types are several subtypes. The weakest are Hawaiian and submarine , then Strombolian , followed by Vulcanian and Surtseyan . The stronger eruptive types are Pelean eruptions , followed by Plinian eruptions ;
302-554: A ghost town ) and Fourpeaked Mountain in Alaska, which, before its September 2006 eruption, had not erupted since before 8000 BCE. Pompeii Pompeii ( / p ɒ m ˈ p eɪ ( i )/ pom- PAY( -ee) , Latin: [pɔmˈpei̯.iː] ) was a city in what is now the municipality of Pompei , near Naples , in the Campania region of Italy . Along with Herculaneum , Stabiae , and many surrounding villas ,
453-700: A magnitude of 4, but acoustic waves travel well in water and over long periods of time. A system in the North Pacific , maintained by the United States Navy and originally intended for the detection of submarines , has detected an event on average every 2 to 3 years. The most common underwater flow is pillow lava , a rounded lava flow named for its unusual shape. Less common are glassy , marginal sheet flows, indicative of larger-scale flows. Volcaniclastic sedimentary rocks are common in shallow-water environments. As plate movement starts to carry
604-886: A mid-ocean 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 also form where there is stretching and thinning of the crust's plates, such as in the East African Rift , the Wells Gray-Clearwater volcanic field , and the Rio Grande rift in North America. Volcanism away from plate boundaries has been postulated to arise from upwelling diapirs from
755-424: A 15th imperatorial acclamation among the emperor 's titles. These coins could not have been minted before the second week of September. Titus appointed two ex-consuls to organise a relief effort while donating large amounts of money from the imperial treasury to aid the victims of the volcano. He visited Pompeii once after the eruption and again the following year but no work was done on recovery. Soon after
906-497: A Peléan eruption are very similar to that of a Vulcanian eruption, except that in Peléan eruptions the volcano's structure is able to withstand more pressure, hence the eruption occurs as one large explosion rather than several smaller ones. Volcanoes known to have Peléan activity include: Plinian eruptions (or Vesuvian eruptions) are a type of volcanic eruption named for the historical eruption of Mount Vesuvius in 79 AD that buried
1057-420: A Plinian eruption, and reach up 2 to 45 km (1 to 28 mi) into the atmosphere . The densest part of the plume, directly above the volcano, is driven internally by gas expansion . As it reaches higher into the air the plume expands and becomes less dense, convection and thermal expansion of volcanic ash drive it even further up into the stratosphere . At the top of the plume, powerful winds may drive
1208-426: A base for the later wall) unusually enclosed a much greater area than the early town together with much agricultural land. That such an impressive wall was built at this time indicates that the settlement was already important and wealthy. The city began to flourish and maritime trade started with the construction of a small port near the mouth of the river. The earliest settlement was focused in regions VII and VIII of
1359-403: A coming eruption is the growth of a so-called Peléan or lava spine , a bulge in the volcano's summit preempting its total collapse. The material collapses upon itself, forming a fast-moving pyroclastic flow (known as a block -and- ash flow) that moves down the side of the mountain at tremendous speeds, often over 150 km (93 mi) per hour. These landslides make Peléan eruptions one of
1510-576: A first-hand account of the eruption of Mount Vesuvius from his position across the Bay of Naples at Misenum , but it was written approximately 27 or 28 years after the event. His uncle, Pliny the Elder , with whom he had a close relationship, died while attempting to rescue stranded victims. As admiral of the fleet, Pliny the Elder had ordered the ships of the Imperial Navy stationed at Misenum to cross
1661-437: A graduated spectrum, with much overlap between categories, and does not always fit neatly into only one of these three separate categories. The USGS defines a volcano as "erupting" whenever the ejection of magma from any point on the volcano is visible, including visible magma still contained within the walls of the summit crater. While there is no international consensus among volcanologists on how to define an active volcano,
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#17327660454941812-560: A house where modern archaeologists found a wall graffito saying "house dug". Over the following centuries, its name and location were forgotten, though it still appeared on the Tabula Peutingeriana of the 4th century. Further eruptions, particularly in 471–473 and 512, covered the remains more deeply. The area became known as the La Civita (the city) due to the features in the ground. The next known date that any part
1963-443: A paper was published suggesting a new definition for the word 'volcano' that includes processes such as cryovolcanism . It suggested that a volcano be defined as 'an opening on a planet or moon's surface from which magma , as defined for that body, and/or magmatic gas is erupted.' This article mainly covers volcanoes on Earth. See § Volcanoes on other celestial bodies and cryovolcano for more information. The word volcano
2114-401: A probable cause for higher levels of volcanism. The technology for studying seamount eruptions did not exist until advancements in hydrophone technology made it possible to "listen" to acoustic waves , known as T-waves, released by submarine earthquakes associated with submarine volcanic eruptions. The reason for this is that land-based seismometers cannot detect sea-based earthquakes below
2265-433: A single eruptive cycle. Volcanoes do not always erupt vertically from a single crater near their peak, either. Some volcanoes exhibit lateral and fissure eruptions . Notably, many Hawaiian eruptions start from rift zones . Scientists believed that pulses of magma mixed together in the magma chamber before climbing upward—a process estimated to take several thousands of years. Columbia University volcanologists found that
2416-607: A sort of autonomy. Nevertheless, Pompeii became a member of the Etruscan League of cities . Excavations in 1980–1981 have shown the presence of Etruscan inscriptions and a 6th-century BC necropolis . Under the Etruscans, a primitive forum or simple market square was built, as well as the Temple of Apollo , in both of which objects including fragments of bucchero were found by Maiuri . Several houses were built with
2567-408: A stable height of around 2,500 m (8,200 ft) for 18 minutes, briefly peaking at a height of 3,400 m (11,000 ft). Volcanoes known to have Hawaiian activity include: Strombolian eruptions are a type of volcanic eruption named after the volcano Stromboli , which has been erupting nearly continuously for centuries. Strombolian eruptions are driven by the bursting of gas bubbles within
2718-503: A type of volcanic eruption characterized by interactions between lava and ice , often under a glacier . The nature of glaciovolcanism dictates that it occurs at areas of high latitude and high altitude . It has been suggested that subglacial volcanoes that are not actively erupting often dump heat into the ice covering them, producing meltwater . This meltwater mix means that subglacial eruptions often generate dangerous jökulhlaups ( floods ) and lahars . Volcano A volcano
2869-489: A type of volcanic eruption named after the volcano Mount Pelée in Martinique , the site of a Peléan eruption in 1902 that is one of the worst natural disasters in history. In Peléan eruptions, a large amount of gas, dust, ash, and lava fragments are blown out the volcano's central crater, driven by the collapse of rhyolite , dacite , and andesite lava domes that often creates large eruptive columns . An early sign of
3020-445: A volcanic cone on Kilauea , erupted continuously for over 35 years. Another Hawaiian volcanic feature is the formation of active lava lakes , self-maintaining pools of raw lava with a thin crust of semi-cooled rock. Flows from Hawaiian eruptions are basaltic, and can be divided into two types by their structural characteristics. Pahoehoe lava is a relatively smooth lava flow that can be billowy or ropey. They can move as one sheet, by
3171-632: A volcano is largely determined by the composition of the lava it erupts. The viscosity (how fluid the lava is) and the amount of dissolved gas are the most important characteristics of magma, and both are largely determined by the amount of silica in the magma. Magma rich in silica is much more viscous than silica-poor magma, and silica-rich magma also tends to contain more dissolved gases. Lava can be broadly classified into four different compositions: Mafic lava flows show two varieties of surface texture: ʻAʻa (pronounced [ˈʔaʔa] ) and pāhoehoe ( [paːˈho.eˈho.e] ), both Hawaiian words. ʻAʻa
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#17327660454943322-555: A volcano is of a conical mountain, spewing lava and poisonous gases from a crater at its summit; however, this describes just one of the many types of volcano. The features of volcanoes are varied. The structure and behaviour of volcanoes depend on several 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 (including lava and ash ) and gases (mainly steam and magmatic gases) can develop anywhere on
3473-465: A volcano that has experienced one or more eruptions that produced over 1,000 cubic kilometres (240 cu mi) of volcanic deposits in a single explosive event. Such eruptions occur when a very large magma chamber full of gas-rich, silicic magma is emptied in a catastrophic caldera -forming eruption. Ash flow tuffs emplaced by such eruptions are the only volcanic product with volumes rivalling those of flood basalts . Supervolcano eruptions, while
3624-400: A wealth of examples of the largely lost Vulgar Latin spoken colloquially at the time, contrasting with the formal language of classical writers. Following its destruction, Pompeii remained largely undisturbed until its rediscovery in the late 16th century. Major excavations did not begin until the mid-18th century, which marked the emergence of modern archeology ; initial efforts to unearth
3775-526: Is a UNESCO World Heritage Site , owing to its status as "the only archaeological site in the world that provides a complete picture of an ancient Roman city." It is among the most popular tourist attractions in Italy, with approximately 2.5 million visitors annually. Pompeii in Latin is a second declension masculine nominative plural noun ( Pompeiī, -ōrum ). According to Theodor Kraus, "The root of
3926-739: Is a volcanic field of over 60 cinder cones. Based on satellite images, it has been suggested that cinder cones might occur on other terrestrial bodies in the Solar system too; on the surface of Mars and the Moon. Stratovolcanoes (composite volcanoes) are tall conical mountains composed of lava flows and tephra in alternate layers, the strata that gives rise to the name. They are also known as composite volcanoes because they are created from multiple structures during different kinds of eruptions. Classic examples include Mount Fuji in Japan, Mayon Volcano in
4077-445: 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. The process that forms volcanoes is called volcanism . On Earth, volcanoes are most often found where tectonic plates are diverging or converging , and because most of Earth's plate boundaries are underwater, most volcanoes are found underwater. For example,
4228-412: Is characterized by a rough, clinkery surface and is the typical texture of cooler basalt lava flows. Pāhoehoe is characterized by its smooth and often ropey or wrinkly surface and is generally formed from more fluid lava flows. Pāhoehoe flows are sometimes observed to transition to ʻaʻa flows as they move away from the vent, but never the reverse. More silicic lava flows take the form of block lava, where
4379-437: Is characterized by a series of short-lived explosions, lasting a few minutes to a few hours and typified by the ejection of volcanic bombs and blocks . These eruptions wear down the lava dome holding the magma down, and it disintegrates, leading to much more quiet and continuous eruptions. Thus an early sign of future Vulcanian activity is lava dome growth, and its collapse generates an outpouring of pyroclastic material down
4530-565: 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 . According to the theory of plate tectonics, Earth's lithosphere , its rigid outer shell, is broken into sixteen larger and several smaller plates. These are in slow motion, due to convection in
4681-584: Is expressed using the volcanic explosivity index (VEI), which ranges from 0 for Hawaiian-type eruptions to 8 for supervolcanic eruptions. As of December 2022 , the Smithsonian Institution 's Global Volcanism Program database of volcanic eruptions in the Holocene Epoch (the last 11,700 years) lists 9,901 confirmed eruptions from 859 volcanoes. The database also lists 1,113 uncertain eruptions and 168 discredited eruptions for
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4832-500: Is generally in the form of episodic explosive eruptions accompanied by the distinctive loud blasts. During eruptions, these blasts occur as often as every few minutes. The term "Strombolian" has been used indiscriminately to describe a wide variety of volcanic eruptions, varying from small volcanic blasts to large eruptive columns . In reality, true Strombolian eruptions are characterized by short-lived and explosive eruptions of lavas with intermediate viscosity , often ejected high into
4983-537: Is one of the several tuyas in the area of the Tuya River and Tuya Range in northern British Columbia. Tuya Butte was the first such landform analysed and so its name has entered the geological literature for this kind of volcanic formation. The Tuya Mountains Provincial Park was recently established to protect this unusual landscape, which lies north of Tuya Lake and south of the Jennings River near
5134-498: Is only moderately dispersed, and its abundance indicates a high degree of fragmentation , the result of high gas contents within the magma. In some cases these have been found to be the result of interaction with meteoric water , suggesting that Vulcanian eruptions are partially hydrovolcanic . Volcanoes that have exhibited Vulcanian activity include: Vulcanian eruptions are estimated to make up at least half of all known Holocene eruptions. Peléan eruptions (or nuée ardente ) are
5285-666: Is recorded in a fresco) and which led the Roman Senate to send the Praetorian Guard to restore order and to ban further events for ten years. The inhabitants of Pompeii had long been used to minor earthquakes (indeed, the writer Pliny the Younger wrote that earth tremors "were not particularly alarming because they are frequent in Campania"), but on 5 February 62 a severe earthquake did considerable damage around
5436-471: Is typically low in silica, shield volcanoes are more common in oceanic than continental settings. The Hawaiian volcanic chain is a series of shield cones, and they are common in Iceland , as well. Lava domes are built by slow eruptions of highly viscous lava. They are sometimes formed within the crater of a previous volcanic eruption, as in the case of Mount St. Helens , but can also form independently, as in
5587-418: The 1886 eruption of Mount Tarawera . Littoral cones are another hydrovolcanic feature, generated by the explosive deposition of basaltic tephra (although they are not truly volcanic vents). They form when lava accumulates within cracks in lava, superheats and explodes in a steam explosion , breaking the rock apart and depositing it on the volcano's flank. Consecutive explosions of this type eventually generate
5738-688: The Battle of Cumae . The first stable settlements on the site date to the 8th century BC when the Oscans , a population of central Italy, founded five villages in the area. With the arrival of the Greeks in Campania from around 740 BC, Pompeii entered the orbit of the Hellenic people. The most important building of this period is the Doric Temple, built away from the centre in what would later become
5889-617: The Cascade Volcanoes or the Japanese Archipelago , or the eastern islands of Indonesia . Hotspots are volcanic areas thought to be formed by mantle plumes , which are hypothesized to be columns of hot material rising from the core-mantle boundary. As with mid-ocean ridges, the rising mantle rock experiences decompression melting which generates large volumes of magma. Because tectonic plates move across mantle plumes, each volcano becomes inactive as it drifts off
6040-414: The Roman towns of Pompeii and Herculaneum and, specifically, for its chronicler Pliny the Younger . The process powering Plinian eruptions starts in the magma chamber , where dissolved volatile gases are stored in the magma. The gases vesiculate and accumulate as they rise through the magma conduit . These bubbles agglutinate and once they reach a certain size (about 75% of the total volume of
6191-662: The Temple of Jupiter , the Basilica, the Comitium, the Stabian Baths, and a new two-story portico. Pompeii was one of the towns of Campania that rebelled against Rome in the Social Wars and in 89 BC it was besieged by Sulla , who targeted the strategically vulnerable Porta Ercolano with his artillery as can still be seen by the impact craters of thousands of ballista shots in the walls. Many nearby buildings inside
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6342-804: The core–mantle boundary , 3,000 kilometres (1,900 mi) deep within Earth. This results in hotspot volcanism , of which the Hawaiian hotspot is an example. Volcanoes are usually not created where two tectonic plates slide past one another. Large eruptions can affect atmospheric temperature as ash and droplets of sulfuric acid obscure the Sun and cool Earth's troposphere . Historically, large volcanic eruptions have been followed by volcanic winters which have caused catastrophic famines. Other planets besides Earth have volcanoes. For example, volcanoes are very numerous on Venus. Mars has significant volcanoes. In 2009,
6493-410: The effusive eruption of very fluid basalt -type lavas with low gaseous content . The volume of ejected material from Hawaiian eruptions is less than half of that found in other eruptive types. Steady production of small amounts of lava builds up the large, broad form of a shield volcano . Eruptions are not centralized at the main summit as with other volcanic types, and often occur at vents around
6644-428: The eruption column . Base surges are caused by the gravitational collapse of a vaporous eruptive column, one that is denser overall than a regular volcanic column. The densest part of the cloud is nearest to the vent, resulting in a wedge shape. Associated with these laterally moving rings are dune -shaped depositions of rock left behind by the lateral movement. These are occasionally disrupted by bomb sags , rock that
6795-834: The landform and may give rise to smaller cones such as Puʻu ʻŌʻō on a flank of Kīlauea in Hawaii. Volcanic craters are not always at the top of a mountain or hill and may be filled with lakes such as with Lake Taupō in New Zealand. Some volcanoes can be low-relief landform features, with the potential to be hard to recognize as such and be obscured by geological processes. Other types of volcano include cryovolcanoes (or ice volcanoes), particularly on some moons of Jupiter , Saturn , and Neptune ; and mud volcanoes , which are structures often not associated with known magmatic activity. Active mud volcanoes tend to involve temperatures much lower than those of igneous volcanoes except when
6946-417: The magma . These gas bubbles within the magma accumulate and coalesce into large bubbles, called gas slugs . These grow large enough to rise through the lava column. Upon reaching the surface, the difference in air pressure causes the bubble to burst with a loud pop, throwing magma in the air in a way similar to a soap bubble . Because of the high gas pressures associated with the lavas, continued activity
7097-428: The magma chamber differentiates with upper portions rich in silicon dioxide , or if magma ascends rapidly. Plinian eruptions are similar to both Vulcanian and Strombolian eruptions, except that rather than creating discrete explosive events, Plinian eruptions form sustained eruptive columns. They are also similar to Hawaiian lava fountains in that both eruptive types produce sustained eruption columns maintained by
7248-415: The pyroclastic flows generated by material collapse, which move down the side of the mountain at extreme speeds of up to 700 km (435 mi) per hour and with the ability to extend the reach of the eruption hundreds of kilometers. The ejection of hot material from the volcano's summit melts snowbanks and ice deposits on the volcano, which mixes with tephra to form lahars , fast moving mudflows with
7399-528: The French occupied Naples in 1799 and ruled over Italy from 1806 to 1815. The land on which Pompeii lies was confiscated, and up to 700 workers were employed in the excavations. The excavated areas in the north and south were connected. Parts of the Via dell'Abbondanza were also exposed in the west–east direction, and for the first time, an impression of the size and appearance of the ancient town could be appreciated. In
7550-515: The Hawaiian volcano deity). During especially high winds these chunks may even take the form of long drawn-out strands, known as Pele's hair . Sometimes basalt aerates into reticulite , the lowest density rock type on earth. Although Hawaiian eruptions are named after the volcanoes of Hawaii, they are not necessarily restricted to them; the highest lava fountain recorded was during the 23 November 2013 eruption of Mount Etna in Italy, which reached
7701-570: The Philippines, and Mount Vesuvius and Stromboli in Italy. Ash produced by the explosive eruption of stratovolcanoes has historically posed the greatest volcanic hazard to civilizations. The lavas of stratovolcanoes are higher in silica, and therefore much more viscous, than lavas from shield volcanoes. High-silica lavas also tend to contain more dissolved gas. The combination is deadly, promoting explosive eruptions that produce great quantities of ash, as well as pyroclastic surges like
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#17327660454947852-605: The Pompeians were granted Roman citizenship and quickly assimilated into the Roman world. The main language in the city became Latin, and many of Pompeii's old aristocratic families Latinized their names as a sign of assimilation. The area around Pompeii became very prosperous due to the desirability of living on the Bay of Naples for wealthy Romans and due to the rich agricultural land. Many farms and villas were built nearby, outside
8003-459: The Romans. Pompeii covered a total of 64 to 67 hectares (160 to 170 acres) and was home to 11,000 to 11,500 people, based on household counts. Although best known for its Roman remains visible today, dating from AD 79, it was built upon a substantial city dating from much earlier times. Expansion of the city from an early nucleus (the old town ) accelerated after 450 BC under the Greeks following
8154-466: The Triangular Forum. At the same time the cult of Apollo was introduced. Greek and Phoenician sailors used the location as a safe port. In the early 6th century BC, the settlement merged into a single community centred on the important crossroad between Cumae , Nola , and Stabiae and was surrounded by a tufa city wall (the pappamonte wall). The first wall (which was also used as
8305-613: The USGS defines a volcano as active whenever subterranean indicators, such as earthquake swarms , ground inflation, or unusually high levels of carbon dioxide or sulfur dioxide are present. The USGS defines a dormant volcano as any volcano that is not showing any signs of unrest such as earthquake swarms, ground swelling, or excessive noxious gas emissions, but which shows signs that it could yet become active again. Many dormant volcanoes have not erupted for thousands of years, but have still shown signs that they may be likely to erupt again in
8456-467: The advancement of "toes", or as a snaking lava column. A'a lava flows are denser and more viscous than pahoehoe, and tend to move slower. Flows can measure 2 to 20 m (7 to 66 ft) thick. A'a flows are so thick that the outside layers cools into a rubble-like mass, insulating the still-hot interior and preventing it from cooling. A'a lava moves in a peculiar way—the front of the flow steepens due to pressure from behind until it breaks off, after which
8607-427: The air before hitting the ground, resulting in the accumulation of cindery scoria fragments; when the air is especially thick with clasts , they cannot cool off fast enough due to the surrounding heat, and hit the ground still hot, the accumulation of which forms spatter cones . If eruptive rates are high enough, they may even form splatter-fed lava flows. Hawaiian eruptions are often extremely long lived; Puʻu ʻŌʻō ,
8758-1083: The air. Columns can measure hundreds of meters in height. The lavas formed by Strombolian eruptions are a form of relatively viscous basaltic lava, and its end product is mostly scoria . The relative passivity of Strombolian eruptions, and its non-damaging nature to its source vent allow Strombolian eruptions to continue unabated for thousands of years, and also makes it one of the least dangerous eruptive types. Strombolian eruptions eject volcanic bombs and lapilli fragments that travel in parabolic paths before landing around their source vent. The steady accumulation of small fragments builds cinder cones composed completely of basaltic pyroclasts . This form of accumulation tends to result in well-ordered rings of tephra . Strombolian eruptions are similar to Hawaiian eruptions , but there are differences. Strombolian eruptions are noisier, produce no sustained eruptive columns , do not produce some volcanic products associated with Hawaiian volcanism (specifically Pele's tears and Pele's hair ), and produce fewer molten lava flows (although
8909-560: The architecture. Some buildings like the Central Baths were only started after the earthquake and were built to enhance the city with modern developments in their architecture, as had been done in Rome, in terms of wall-heating and window glass, and with well-lit spacious rooms. The new baths took over a whole insula by demolishing houses, which may have been made easier by the earthquake that had damaged these houses. This shows that
9060-465: The area. The period between about 450–375 BC witnessed large areas of the city being abandoned while important sanctuaries such as the Temple of Apollo show a sudden lack of votive material remains. The Samnites , people from the areas of Abruzzo and Molise , and allies of the Romans, conquered Greek Cumae between 423 and 420 BC. It is likely that all of the surrounding territory, including Pompeii,
9211-401: The bay to assist evacuation attempts. Volcanologists have recognised the importance of Pliny the Younger's account of the eruption by calling similar events " Plinian ". It had long been thought that the eruption was an August event based on one version of the letter, but another version gives a date of the eruption as late as 23 November. A later date is consistent with a charcoal inscription at
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#17327660454949362-609: The bay, and particularly to Pompeii. It is believed that the earthquake would have registered between 5 and 6 on the Richter magnitude scale . On that day in Pompeii, there were to be two sacrifices, as it was the anniversary of Augustus being named Pater Patriae ("Father of the Country") and also a feast day to honour the guardian spirits of the city. Chaos followed the earthquake; fires caused by oil lamps that had fallen during
9513-510: The boundary with the Yukon Territory . Mud volcanoes (mud domes) are formations created by geo-excreted liquids and gases, although several processes may cause such activity. The largest structures are 10 kilometres in diameter and reach 700 meters high. The material that is expelled in a volcanic eruption can be classified into three types: The concentrations of different volcanic gases can vary considerably from one volcano to
9664-605: The buildup of high gas pressure , eventually popping the cap holding the magma down and resulting in an explosive eruption. Unlike Strombolian eruptions, ejected lava fragments are not aerodynamic; this is due to the higher viscosity of Vulcanian magma and the greater incorporation of crystalline material broken off from the former cap. They are also more explosive than their Strombolian counterparts, with eruptive columns often reaching between 5 and 10 km (3 and 6 mi) high. Lastly, Vulcanian deposits are andesitic to dacitic rather than basaltic . Initial Vulcanian activity
9815-402: The case of Lassen Peak . Like stratovolcanoes, they can produce violent, explosive eruptions, but the lava generally does not flow far from the originating vent. Cryptodomes are formed when viscous lava is forced upward causing the surface to bulge. The 1980 eruption of Mount St. Helens was an example; lava beneath the surface of the mountain created an upward bulge, which later collapsed down
9966-522: The city and many have been excavated. These include the Villa of the Mysteries , Villa of Diomedes , several at Boscoreale , Boscotrecase , Oplontis , Terzigno , and Civita Guiliana. The city became an important passage for goods that arrived by sea and had to be sent toward Rome or Southern Italy along the nearby Appian Way . Many public buildings were constructed or refurbished and improved under
10117-520: The city was buried under 4 to 6 m (13 to 20 ft) of volcanic ash and pumice in the eruption of Mount Vesuvius in 79 AD . Largely preserved under the ash, Pompeii offers a unique snapshot of Roman life , frozen at the moment it was buried, as well as insight into ancient urban planning . It was a wealthy town of 10,000 to 20,000 residents at the time it was destroyed. It hosted many fine public buildings and luxurious private houses with lavish decorations, furnishings and artworks, which were
10268-537: The city was still flourishing rather than struggling to recover from the earthquake. In about 64, Nero and his wife Poppaea visited Pompeii and made gifts to the temple of Venus (the city's patron deity), probably when he performed in the theatre of Naples. By 79, Pompeii had a population of 20,000, which had prospered from the region's renowned agricultural fertility and favourable location, although more recent estimates are up to 11,500 based on household counts. The eruption lasted for two days. The first phase
10419-487: The city were haphazard or marred by looting, resulting in many items or sites being damaged or destroyed. By 1960, most of Pompeii had been uncovered but left in decay; further major excavations were banned or limited to targeted, prioritised areas. Since 2018, these efforts have led to new discoveries in some previously unexplored areas of the city, including a banquet hall adorned with rare well-preserved frescoes depicting various mythological scenes and figures. Pompeii
10570-528: The city's burial, survivors and possibly thieves came to salvage valuables, including the marble statues from the Forum and other precious materials from buildings. There is wide evidence of post-eruption disturbance , including holes made through walls. The city was not completely buried, and the tops of larger buildings would have been visible above the ash, making it obvious where to dig or salvage building material . The robbers left traces of their passage, as in
10721-454: The city. In the 2nd century BC, Pompeii enriched itself by taking part in Rome's conquest of the east, as shown by a statue of Apollo in the Forum erected by Lucius Mummius in gratitude for their support in the sack of Corinth and the eastern campaigns. These riches enabled Pompeii to bloom and expand to its ultimate limits. The Forum and many public and private buildings of high architectural quality were built, including The Large Theatre ,
10872-683: The city. Under Augustus , from about 30 BC, a major expansion in new public buildings, as in the rest of the empire, included the Eumachia Building , the Sanctuary of Augustus and the Macellum . From about 20 BC, Pompeii was fed with running water by a spur from the Serino Aqueduct , built by Marcus Vipsanius Agrippa . In AD 59, there was a serious riot and bloodshed in the amphitheatre between Pompeians and Nucerians (which
11023-432: The cone. Volcanoes known to have Surtseyan activity include: Submarine eruptions occur underwater. An estimated 75% of volcanic eruptive volume is generated by submarine eruptions near mid ocean ridges alone. Problems detecting deep sea volcanic eruptions meant their details were virtually unknown until advances in the 1990s made it possible to observe them. Submarine eruptions may produce seamounts , which may break
11174-420: The consistency of wet concrete that move at the speed of a river rapid . Major Plinian eruptive events include: Phreatomagmatic eruptions are eruptions that arise from interactions between water and magma . They are driven by thermal contraction of magma when it comes in contact with water (as distinguished from magmatic eruptions, which are driven by thermal expansion). This temperature difference between
11325-611: The currently visible walls with an outer wall of rectangular limestone blocks as a terrace wall supporting a large agger , or earth embankment, behind it. After the Samnite Wars from 290 BC, Pompeii was forced to accept the status of socii of Rome, maintaining, however, linguistic and administrative autonomy. From the outbreak of the Second Punic War (218–201 BC) in which Hannibal 's invasion threatened many cities, Pompeii remained faithful to Rome unlike many of
11476-451: The decomposed bodies, and so devised the technique of injecting plaster into them to recreate the forms of Vesuvius's victims. This technique is still in use today, with a clear resin now used instead of plaster because it is more durable and does not destroy the bones, allowing further analysis. Fiorelli also introduced scientific documentation. He divided the city into today's nine areas ( regiones ) and blocks ( insulae ) and numbered
11627-428: The display of antiquities reinforced Naples' political and cultural prestige. On 20 August 1763, an inscription [...] Rei Publicae Pompeianorum [...] was found and the city was identified as Pompeii. Karl Weber directed the first scientific excavations. He was followed in 1764 by military engineer Franscisco la Vega, who was succeeded by his brother, Pietro , in 1804. There was much progress in exploration when
11778-439: The earthquake of 62 and the eruption. It was thought until recently that some of the damage had still not been repaired at the time of the eruption, but this is doubtful as the evidence of missing forum statues and marble wall veneers are most likely due to robbers after the city's burial. The public buildings on the east side of the Forum were largely restored and were enhanced by beautiful marble veneers and other modifications to
11929-799: The enormous area they cover, and subsequent concealment under vegetation and glacial deposits, supervolcanoes can be difficult to identify in the geologic record without careful geologic mapping . Known examples include Yellowstone Caldera in Yellowstone National Park and Valles Caldera in New Mexico (both western United States); Lake Taupō in New Zealand; Lake Toba in Sumatra , Indonesia; and Ngorongoro Crater in Tanzania. Volcanoes that, though large, are not large enough to be called supervolcanoes, may also form calderas in
12080-454: The entrances of the individual houses ( domus ). Fiorelli also published the first periodical with excavation reports. Under his successors, the entire west section of the city was exposed. After those of Fiorelli, excavations continued in an increasingly more systematic and considered manner under several directors of archaeology though still with the main interest in making spectacular discoveries and uncovering more houses rather than answering
12231-650: The eruption and formation of the island of Surtsey off the coast of Iceland in 1963. Surtseyan eruptions are the "wet" equivalent of ground-based Strombolian eruptions , but because they take place in water they are much more explosive. As water is heated by lava, it flashes into steam and expands violently, fragmenting the magma it contacts into fine-grained ash . Surtseyan eruptions are typical of shallow-water volcanic oceanic islands , but they are not confined to seamounts. They can happen on land as well, where rising magma that comes into contact with an aquifer (water-bearing rock formation) at shallow levels under
12382-439: The eruption of Costa Rica's Irazú Volcano in 1963 was likely triggered by magma that took a nonstop route from the mantle over just a few months. It is important when studying the products of explosive eruptions to distinguish between...: George P. L. Walker , Quoted The volcanic explosivity index (commonly shortened to VEI) is a scale, from 0 to 8, for measuring the strength of eruptions but does not capture all of
12533-402: The eruption products and victims, merged with numerical simulations and experiments, indicates that at Pompeii and surrounding towns heat was the main cause of death of people, previously believed to have died by ash suffocation . The results of the study, published in 2010, show that exposure to at least 250 °C (480 °F) hot pyroclastic flows at a distance of 10 kilometres (6 miles) from
12684-462: The eruptive material does tend to form small rivulets). Volcanoes known to have Strombolian activity include: Vulcanian eruptions are a type of volcanic eruption named after the volcano Vulcano . It was named so following Giuseppe Mercalli 's observations of its 1888–1890 eruptions. In Vulcanian eruptions, intermediate viscous magma within the volcano make it difficult for vesiculate gases to escape. Similar to Strombolian eruptions, this leads to
12835-442: The explosive nature than thermal contraction. Fuel coolant reactions may fragment the volcanic material by propagating stress waves , widening cracks and increasing surface area that ultimately leads to rapid cooling and explosive contraction-driven eruptions. A Surtseyan (or hydrovolcanic) eruption is a type of volcanic eruption characterized by shallow-water interactions between water and lava, named after its most famous example,
12986-416: The finding secret. In 1689, Francesco Picchetti saw a wall inscription mentioning decurio Pompeiis ("town councillor of Pompeii"), but he associated it with a villa of Pompey . Francesco Bianchini pointed out the true meaning, and he was supported by Giuseppe Macrini, who in 1693 excavated some walls and wrote that Pompeii lay beneath La Civita. Herculaneum was rediscovered in 1738 by workers digging for
13137-431: The flow is covered with angular, vesicle-poor blocks. Rhyolitic flows typically consist largely of obsidian . Tephra is made when magma inside the volcano is blown apart by the rapid expansion of hot volcanic gases. Magma commonly explodes as the gas dissolved in it comes out of solution as the pressure decreases when it flows to the surface . These violent explosions produce particles of material that can then fly from
13288-627: The following years, the excavators struggled with a lack of money. Excavations progressed slowly, but with significant finds such as the houses of the Faun , of Menandro , of the Tragic Poet and the Surgeon . Giuseppe Fiorelli took charge of the excavations in 1863 and made greater progress. During early excavations of the site, occasional voids in the ash layer had been found that contained human remains. Fiorelli realised these were spaces left by
13439-610: The formation of a submarine volcano off the coast of Mayotte . Subglacial volcanoes develop underneath ice caps . They are made up of lava plateaus capping extensive pillow lavas and palagonite . These volcanoes are also called table mountains, tuyas , or (in Iceland) mobergs. Very good examples of this type of volcano can be seen in Iceland and in British Columbia . The origin of the term comes from Tuya Butte , which
13590-506: The foundations of a summer palace for the King of Naples, Charles of Bourbon . Due to the spectacular quality of the finds, the Spanish military engineer Roque Joaquín de Alcubierre made excavations to find further remains at the site of Pompeii in 1748, even if the city was not identified. Charles of Bourbon took great interest in the finds, even after leaving to become king of Spain because
13741-585: The future. In an article justifying the re-classification of Alaska's Mount Edgecumbe volcano from "dormant" to "active", volcanologists at the Alaska Volcano Observatory pointed out that the term "dormant" in reference to volcanoes has been deprecated over the past few decades and that "[t]he term "dormant volcano" is so little used and undefined in modern volcanology that the Encyclopedia of Volcanoes (2000) does not contain it in
13892-428: The general mass behind it moves forward. Pahoehoe lava can sometimes become A'a lava due to increasing viscosity or increasing rate of shear , but A'a lava never turns into pahoehoe flow. Hawaiian eruptions are responsible for several unique volcanological objects. Small volcanic particles are carried and formed by the wind, chilling quickly into teardrop-shaped glassy fragments known as Pele's tears (after Pele ,
14043-466: The glossaries or index", however the USGS still widely employs the term. Previously a volcano was often considered to be extinct if there were no written records of its activity. Such a generalization is inconsistent with observation and deeper study, as has occurred recently with the unexpected eruption of the Chaitén volcano in 2008. Modern volcanic activity monitoring techniques, and improvements in
14194-430: The growth of bubbles that move up at about the same speed as the magma surrounding them. Regions affected by Plinian eruptions are subjected to heavy pumice airfall affecting an area 0.5 to 50 km (0 to 12 cu mi) in size. The material in the ash plume eventually finds its way back to the ground, covering the landscape in a thick layer of many cubic kilometers of ash. The most dangerous eruptive feature are
14345-492: The interior of a continent and lead to rifting. Early stages of rifting are characterized by flood basalts and may progress to the point where a tectonic plate is completely split. A divergent plate boundary then develops between the two halves of the split plate. However, rifting often fails to completely split the continental lithosphere (such as in an aulacogen ), and failed rifts are characterized by volcanoes that erupt unusual alkali lava or carbonatites . Examples include
14496-422: The magma conduit) they explode. The narrow confines of the conduit force the gases and associated magma up, forming an eruptive column . Eruption velocity is controlled by the gas contents of the column, and low-strength surface rocks commonly crack under the pressure of the eruption, forming a flared outgoing structure that pushes the gases even faster. These massive eruptive columns are the distinctive feature of
14647-480: The main attractions for early excavators; subsequent excavations have found hundreds of private homes and businesses reflecting various architectural styles and social classes, as well as numerous public buildings. Organic remains, including wooden objects and human bodies, were interred in the ash; their eventual decay allowed archaeologists to create moulds of figures in their final moments of life. The numerous graffiti carved on outside walls and inside rooms provide
14798-425: The main questions about the city and its long term preservation. In the 1920s, Amedeo Maiuri excavated older layers beneath those of 79 AD for the first time to learn about the settlement history. Maiuri made the last excavations on a grand scale in the 1950s, and the area south of the Via dell'Abbondanza and the city wall was almost completely uncovered, but they were poorly documented scientifically. Preservation
14949-419: The mid-oceanic ridge is above sea level, volcanic islands are formed, such as Iceland . Subduction zones are places where two plates, usually an oceanic plate and a continental plate, collide. The oceanic plate subducts (dives beneath 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
15100-551: The modelling of the factors that produce eruptions, have helped the understanding of why volcanoes may remain dormant for a long time, and then become unexpectedly active again. The potential for eruptions, and their style, depend mainly upon the state of the magma storage system under the volcano, the eruption trigger mechanism and its timescale. For example, the Yellowstone volcano has a repose/recharge period of around 700,000 years, and Toba of around 380,000 years. Vesuvius
15251-423: The most dangerous in the world, capable of tearing through populated areas and causing serious loss of life. The 1902 eruption of Mount Pelée caused tremendous destruction, killing more than 30,000 people and completely destroying St. Pierre , the worst volcanic event in the 20th century . Peléan eruptions are characterized most prominently by the incandescent pyroclastic flows that they drive. The mechanics of
15402-415: The most dangerous type, are very rare; four are known from the last million years , and about 60 historical VEI 8 eruptions have been identified in the geologic record over millions of years. A supervolcano can produce devastation on a continental scale, and severely cool global temperatures for many years after the eruption due to the huge volumes of sulfur and ash released into the atmosphere. Because of
15553-438: The mud volcano is actually a vent of an igneous volcano. Volcanic fissure vents are flat, 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 generally do not explode catastrophically but are characterized by relatively gentle effusive eruptions . Since low-viscosity magma
15704-589: The new order; new buildings included the Amphitheatre of Pompeii in 70 BC, the Forum Baths, and the Odeon . In comparison, the Forum was embellished with the colonnade of Popidius before 80 BC. These buildings raised the status of Pompeii as a cultural centre in the region as it outshone its neighbours in the number of places for entertainment which significantly enhanced the social and economic development of
15855-452: The next. Water vapour is typically the most abundant volcanic gas, followed by carbon dioxide and sulfur dioxide . Other principal volcanic gases include hydrogen sulfide , hydrogen chloride , and hydrogen fluoride . A large number of minor and trace gases are also found in volcanic emissions, for example hydrogen , carbon monoxide , halocarbons , organic compounds, and volatile metal chlorides. The form and style of an eruption of
16006-876: The north side of the mountain. Cinder cones result from eruptions of mostly small pieces of scoria and pyroclastics (both resemble cinders, hence the name of this volcano type) that build up around the vent. These can be relatively short-lived eruptions that produce a cone-shaped hill perhaps 30 to 400 metres (100 to 1,300 ft) high. Most cinder cones erupt only once and some may be found in monogenetic volcanic fields that may include other features that form when magma comes into contact with water such as maar explosion craters and tuff rings . Cinder cones may form as flank vents on larger volcanoes, or occur on their own. Parícutin in Mexico and Sunset Crater in Arizona are examples of cinder cones. In New Mexico , Caja del Rio
16157-549: The ocean surface as new islands or floating pumice rafts . In May and June 2018, a multitude of seismic signals were detected by earthquake monitoring agencies all over the world. They took the form of unusual humming sounds, and some of the signals detected in November of that year had a duration of up to 20 minutes. An oceanographic research campaign in May 2019 showed that the previously mysterious humming noises were caused by
16308-420: The ocean surface, due to the rapid cooling effect and increased buoyancy in water (as compared to air), which often causes volcanic vents to form steep pillars on the ocean floor. Hydrothermal vents are common near these volcanoes, and some support peculiar ecosystems based on chemotrophs feeding on dissolved minerals. Over time, the formations created by submarine volcanoes may become so large that they break
16459-471: The ocean's surface. In the deep ocean basins, the tremendous weight of the water prevents the explosive release of steam and gases; however, submarine eruptions can be detected by hydrophones and by the discoloration of water because of volcanic gases . Pillow lava is a common eruptive product of submarine volcanoes and is characterized by thick sequences of discontinuous pillow-shaped masses which form underwater. Even large submarine eruptions may not disturb
16610-403: The one extreme there are effusive Hawaiian eruptions, which are characterized by lava fountains and fluid lava flows , which are typically not very dangerous. On the other extreme, Plinian eruptions are large, violent, and highly dangerous explosive events. Volcanoes are not bound to one eruptive style, and frequently display many different types, both passive and explosive, even in the span of
16761-468: The one that destroyed the city of Saint-Pierre in Martinique in 1902. They are also steeper than shield volcanoes, with slopes of 30–35° compared to slopes of generally 5–10°, and their loose tephra are material for dangerous lahars . Large pieces of tephra are called volcanic bombs . Big bombs can measure more than 1.2 metres (4 ft) across and weigh several tons. A supervolcano is defined as
16912-425: The overlying mantle wedge, thus creating magma . This magma tends to be extremely viscous because of its high silica content, so it often does not reach the surface but cools and solidifies at depth . When it does reach the surface, however, a volcano is formed. Thus subduction zones are bordered by chains of volcanoes called volcanic arcs . Typical examples are the volcanoes in the Pacific Ring of Fire , such as
17063-421: The plume away from the volcano . These highly explosive eruptions are usually associated with volatile-rich dacitic to rhyolitic lavas, and occur most typically at stratovolcanoes . Eruptions can last anywhere from hours to days, with longer eruptions being associated with more felsic volcanoes. Although they are usually associated with felsic magma, Plinian eruptions can occur at basaltic volcanoes, if
17214-619: The plume, and new volcanoes are created where the plate advances over the plume. The Hawaiian Islands are thought to have been formed in such a manner, as has the Snake River Plain , with the Yellowstone Caldera being part of the North American plate currently above the Yellowstone hotspot . However, the mantle plume hypothesis has been questioned. Sustained upwelling of hot mantle rock can develop under
17365-562: The properties that may be perceived to be important. It is used by the Smithsonian Institution 's Global Volcanism Program in assessing the impact of historic and prehistoric lava flows. It operates in a way similar to the Richter scale for earthquakes , in that each interval in value represents a tenfold increasing in magnitude (it is logarithmic ). The vast majority of volcanic eruptions are of VEIs between 0 and 2. Magmatic eruptions produce juvenile clasts during explosive decompression from gas release. They range in intensity from
17516-462: The quake added to the panic. The nearby cities of Herculaneum and Nuceria were also affected. Between 62 AD and the eruption in 79 AD, most rebuilding was done in the private sector and older, damaged frescoes were often covered with newer ones, for example. In the public sector, the opportunity was taken to improve buildings and the city plan, e.g. in the Forum. An important field of current research concerns structures that were restored between
17667-444: The relatively small lava fountains on Hawaii to catastrophic Ultra-Plinian eruption columns more than 30 km (19 mi) high, bigger than the eruption of Mount Vesuvius in 79 AD that buried Pompeii . Hawaiian eruptions are a type of volcanic eruption named after the Hawaiian volcanoes , such as Mauna Loa , with this eruptive type is hallmark. Hawaiian eruptions are the calmest types of volcanic events, characterized by
17818-503: The rising plate, lowering its melting point . Each process generates different rock; mid-ocean ridge volcanics are primarily basaltic , whereas subduction flows are mostly calc-alkaline , and more explosive and viscous . Spreading rates along mid-ocean ridges vary widely, from 2 cm (0.8 in) per year at the Mid-Atlantic Ridge , to up to 16 cm (6 in) along the East Pacific Rise . Higher spreading rates are
17969-435: The same time interval. Volcanoes vary greatly in their level of activity, with individual volcanic systems having an eruption recurrence ranging from several times a year to once in tens of thousands of years. Volcanoes are informally described as erupting , active , dormant , or extinct , but the definitions of these terms are not entirely uniform among volcanologists. The level of activity of most volcanoes falls upon
18120-492: The same way; they are often described as "caldera volcanoes". Submarine volcanoes are common features of the ocean floor. Volcanic activity during the Holocene Epoch has been documented at only 119 submarine volcanoes, but there may be more than one million geologically young submarine volcanoes on the ocean floor. In shallow water, active volcanoes disclose their presence by blasting steam and rocky debris high above
18271-420: The shops are typical of October – and conversely the summer fruit typical of August was already being sold in dried, or conserved form; nuts from chestnut trees were found at Oplontis , which would not have been mature before mid-September; wine fermenting jars had been sealed, which would have happened around the end of October; coins found in the purse of a woman buried in the ash include one with
18422-426: The site, discovered in 2018, which includes the date of 17 October and which must have been recently written. A collaborative study in 2022 determined a date of 24–25 October. An October/November eruption is clearly supported by many pieces of evidence: the fact that people buried in the ash appear to have been wearing heavier clothing than the light summer clothes typical of August; the fresh fruit and vegetables in
18573-460: The so-called Tuscan atrium , typical of this people. The city wall was strengthened in the early 5th century BC with two façades of relatively thin, vertically set slabs of Sarno limestone some four metres (13 ft) apart filled with earth (the orthostate wall). In 474 BC, the Greek city of Cumae , allied with Syracuse , defeated the Etruscans at the Battle of Cumae and gained control of
18724-622: The solidified erupted material that makes up the mantle of a volcano may be stripped away that its inner anatomy becomes apparent. Using the metaphor of biological anatomy , such a process is called "dissection". Cinder Hill , a feature of Mount Bird on Ross Island , Antarctica , is a prominent example of a dissected volcano. Volcanoes that were, on a geological timescale, recently active, such as for example Mount Kaimon in southern Kyūshū , Japan , tend to be undissected. Eruption styles are broadly divided into magmatic, phreatomagmatic, and phreatic eruptions. The intensity of explosive volcanism
18875-414: The south and partly to the west into the sea. Three layers of sediment from large landslides lie on top of the lava, perhaps triggered by extended rainfall. The city, once by the shoreline, is today circa 700 m (2,300 ft) inland. The mouth of the navigable Sarno River , adjacent to the city, was protected by lagoons and served early Greek and Phoenician sailors as a haven port, later developed by
19026-499: The southern cities. As a result, an additional internal wall was built of tufa and the internal agger and outer façade raised, resulting in a double parapet with a wider wall-walk. Despite the political uncertainty of these events and the progressive migration of wealthy men to quieter cities in the eastern Mediterranean, Pompeii continued to flourish due to the production and trade of wine and oil with places like Provence and Spain, as well as to intensive agriculture on farms around
19177-729: The strongest eruptions are called Ultra-Plinian . Subglacial and phreatic eruptions are defined by their eruptive mechanism, and vary in strength. An important measure of eruptive strength is the Volcanic Explosivity Index an order-of-magnitude scale, ranging from 0 to 8, that often correlates to eruptive types. Volcanic eruptions arise through three main mechanisms: In terms of activity, there are explosive eruptions and effusive eruptions . The former are characterized by gas-driven explosions that propel magma and tephra. The latter pour out lava without significant explosion. Volcanic eruptions vary widely in strength. On
19328-470: The summit and from fissure vents radiating out of the center. Hawaiian eruptions often begin as a line of vent eruptions along a fissure vent , a so-called "curtain of fire." These die down as the lava begins to concentrate at a few of the vents. Central-vent eruptions, meanwhile, often take the form of large lava fountains (both continuous and sporadic), which can reach heights of hundreds of meters or more. The particles from lava fountains usually cool in
19479-400: The surface and form volcanic islands. Submarine volcanism is driven by various processes. Volcanoes near plate boundaries and mid-ocean ridges are built by the decompression melting of mantle rock that rises on an upwelling portion of a convection cell to the crustal surface. Eruptions associated with subducting zones , meanwhile, are driven by subducting plates that add volatiles to
19630-406: The surrounding areas, and initially not seismically monitored before its unanticipated and catastrophic eruption of 1991. Two other examples of volcanoes that were once thought to be extinct, before springing back into eruptive activity were the long-dormant Soufrière Hills volcano on the island of Montserrat , thought to be extinct until activity resumed in 1995 (turning its capital Plymouth into
19781-434: The theory of plate tectonics. For example, some volcanoes are polygenetic with more than one period of activity during their history; other volcanoes that become extinct after erupting exactly once are monogenetic (meaning "one life") and such volcanoes are often grouped together in a geographical region. At the mid-ocean ridges , two tectonic plates diverge from one another as hot mantle rock creeps upwards beneath
19932-444: The thinned oceanic crust . The decrease of pressure in the rising mantle rock leads to adiabatic expansion and the partial melting of the rock, causing volcanism and creating new oceanic crust. Most divergent plate boundaries are at the bottom of the oceans, and so most volcanic activity on Earth is submarine, forming new seafloor . Black smokers (also known as deep sea vents) are evidence of this kind of volcanic activity. Where
20083-460: The third Samnite war and in the war against Pyrrhus . In the late 4th century BC, the city began expanding from its nucleus into the open-walled area. The street plan of the new areas was more regular and more conformal to Hippodamus 's street plan. The city walls were reinforced in Sarno stone in the early 3rd century BC (the limestone enceinte , or the "first Samnite wall"). It formed the basis for
20234-476: The town (the old town) as identified from stratigraphy below the Samnite and Roman buildings, as well as from the different and irregular street plan. By 524 BC the Etruscans had settled in the area, including Pompeii, finding in the river Sarno a communication route between the sea and the interior. Like the Greeks, the Etruscans did not conquer the city militarily, but simply controlled it, and Pompeii enjoyed
20385-422: The two causes violent water-lava interactions that make up the eruption. The products of phreatomagmatic eruptions are believed to be more regular in shape and finer grained than the products of magmatic eruptions because of the differences in eruptive mechanisms. There is debate about the exact nature of phreatomagmatic eruptions, and some scientists believe that fuel-coolant reactions may be more critical to
20536-431: The underlying ductile mantle , and most volcanic activity on Earth takes place along plate boundaries, where plates are converging (and lithosphere is being destroyed) or are diverging (and new lithosphere is being created). During the development of geological theory, certain concepts that allowed the grouping of volcanoes in time, place, structure and composition have developed that ultimately have had to be explained in
20687-400: The vent was sufficient to cause instant death, even if people were sheltered within buildings. The people and buildings of Pompeii were covered in up to twelve different layers of tephra , in total, up to 6 metres (19.7 ft) deep. Archaeology in 2023 showed that some buildings collapsed due to one or more earthquakes during the eruption, killing the occupants. Pliny the Younger provided
20838-427: The volcano can cause them. The products of Surtseyan eruptions are generally oxidized palagonite basalts (though andesitic eruptions do occur, albeit rarely), and like Strombolian eruptions Surtseyan eruptions are generally continuous or otherwise rhythmic. A defining feature of a Surtseyan eruption is the formation of a pyroclastic surge (or base surge ), a ground hugging radial cloud that develops along with
20989-466: The volcano's slope. Deposits near the source vent consist of large volcanic blocks and bombs , with so-called " bread-crust bombs " being especially common. These deeply cracked volcanic chunks form when the exterior of ejected lava cools quickly into a glassy or fine-grained shell, but the inside continues to cool and vesiculate . The center of the fragment expands, cracking the exterior. The bulk of Vulcanian deposits are fine grained ash . The ash
21140-442: The volcano, consisting of high speed, dense, and scorching ash clouds, knocking down wholly or partly all structures in their path, incinerating or suffocating the remaining population and altering the landscape, including the coastline. By the evening of the second day, the eruption was over, leaving only haze in the atmosphere through which the sun shone weakly. A multidisciplinary volcanological and bio-anthropological study of
21291-492: The volcano. Solid particles smaller than 2 mm in diameter ( sand-sized or smaller) are called volcanic ash. Tephra and other volcaniclastics (shattered volcanic material) make up more of the volume of many volcanoes than do lava flows. Volcaniclastics may have contributed as much as a third of all sedimentation in the geologic record. The production of large volumes of tephra is characteristic of explosive volcanism. Through natural processes, mainly erosion , so much of
21442-460: The volcanoes away from their eruptive source, eruption rates start to die down, and water erosion grinds the volcano down. The final stages of eruption cap the seamount in alkalic flows. There are about 100,000 deepwater volcanoes in the world, although most are beyond the active stage of their life. Some exemplary seamounts are Kamaʻehuakanaloa (formerly Loihi), Bowie Seamount , Davidson Seamount , and Axial Seamount . Subglacial eruptions are
21593-424: The volcanoes of the East African Rift . A volcano needs a reservoir of molten magma (e.g. a magma chamber), a conduit to allow magma to rise through the crust, and a vent to allow the magma to escape above the surface as lava. The erupted volcanic material (lava and tephra) that is deposited around the vent is known as a volcanic edifice , typically a volcanic cone or mountain. The most common perception of
21744-528: The walls were also destroyed. Although the battle-hardened troops of the Social League, headed by Lucius Cluentius , helped in resisting the Romans, Pompeii was forced to surrender after the conquest of Nola. The result was that Pompeii became a Roman colony named Colonia Cornelia Veneria Pompeianorum. Many of Sulla's veterans were given land and property in and around the city, while many who opposed Rome were dispossessed of their property. Despite this,
21895-479: The word Pompeii would appear to be the Oscan word for the number five, pompe , which suggests that either the community consisted of five hamlets or perhaps it was settled by a family group ( gens Pompeia )." Pompeii was built approximately 40 m (130 ft) above sea level on a coastal lava plateau created by earlier eruptions of Mount Vesuvius (8 km or 5 mi distant). The plateau fell steeply to
22046-623: Was already conquered around 424 BC. The new rulers gradually imposed their architecture and enlarged the town. From 343 to 341 BC in the Samnite Wars , the first Roman army entered the Campanian plain bringing with it the customs and traditions of Rome, and in the Roman Latin War from 340 BC, the Samnites were faithful to Rome. Although governed by the Samnites, Pompeii entered the Roman orbit, to which it remained faithful even during
22197-421: Was described by Roman writers as having been covered with gardens and vineyards before its unexpected eruption of 79 CE , which destroyed the towns of Herculaneum and Pompeii . Accordingly, it can sometimes be difficult to distinguish between an extinct volcano and a dormant (inactive) one. Long volcano dormancy is known to decrease awareness. Pinatubo was an inconspicuous volcano, unknown to most people in
22348-604: Was flung out by the explosive eruption and followed a ballistic path to the ground. Accumulations of wet, spherical ash known as accretionary lapilli are another common surge indicator. Over time Surtseyan eruptions tend to form maars , broad low- relief volcanic craters dug into the ground, and tuff rings , circular structures built of rapidly quenched lava. These structures are associated with single vent eruptions. If eruptions arise along fracture zones , rift zones may be dug out. Such eruptions tend to be more violent than those which form tuff rings or maars, an example being
22499-512: Was haphazard, and his reconstructions were difficult to distinguish from the original ruins, which is a great handicap for studying genuine antique remains. Questionable reconstruction was also done after the severe earthquake of 1980, which caused great destruction. Since then, work has been confined to the excavated areas except for targeted soundings and excavations. Further excavations on a large scale are not planned, and today archaeologists are more engaged in reconstructing, documenting and slowing
22650-427: Was of pumice rain ( lapilli ) lasting about 18 hours, allowing most inhabitants to escape. Only approximately 1,150 bodies have so far been found on site, which seems to confirm this theory, and most escapees probably managed to salvage some of their most valuable belongings; many skeletons were found with jewellery, coins, and silverware. At some time in the night or early the next day, pyroclastic flows began near
22801-400: Was unearthed was in 1592, when architect Domenico Fontana , while digging an underground aqueduct to the mills of Torre Annunziata , ran into ancient walls covered with paintings and inscriptions. His aqueduct passed through and underneath a large part of the city and would have had to pass through many buildings and foundations, as they still can be seen in many places today. However, he kept
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