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79-561: It was formed by a volcanic eruption around 4.5 million years ago. Roque Nublo is in the 263,074 km (101,573 sq mi) Nublo Rural Park  [ es ] , in the Tejeda municipality, near the geographical centre of Gran Canaria. Near to the Roque Nublo are other rock formations, such as the Roque del Fraile and La Rana . This Canary Islands location article is

158-701: 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

237-409: A stub . You can help Misplaced Pages by expanding it . Volcanic eruption 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

316-498: 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

395-420: 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

474-406: 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

553-401: 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 ;

632-422: A practical method of assigning an absolute measure of magnitude. First, to span the wide range of possible values, Richter adopted Gutenberg's suggestion of a logarithmic scale, where each step represents a tenfold increase of magnitude, similar to the magnitude scale used by astronomers for star brightness . Second, he wanted a magnitude of zero to be around the limit of human perceptibility. Third, he specified

711-401: 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

790-469: A seismogram recorded by a Wood-Anderson torsion seismometer. Finally, Richter calculated a table of distance corrections, in that for distances less than 200 kilometers the attenuation is strongly affected by the structure and properties of the regional geology. When Richter presented the resulting scale in 1935, he called it (at the suggestion of Harry Wood) simply a "magnitude" scale. "Richter magnitude" appears to have originated when Perry Byerly told

869-408: A seismograph anywhere in the world, so long as its sensors are not located in the earthquake's shadow . The following describes the typical effects of earthquakes of various magnitudes near the epicenter. The values are typical and may not be exact in a future event because intensity and ground effects depend not only on the magnitude but also on (1) the distance to the epicenter, (2) the depth of

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948-531: A shallow earthquake in a populated area with soil of certain types can be far more intense in impact than a much more energetic deep earthquake in an isolated area. Several scales have been historically described as the "Richter scale", , especially the local magnitude M L   and the surface wave M s   scale. In addition, the body wave magnitude , mb , and the moment magnitude , M w  , abbreviated MMS, have been widely used for decades. A couple of new techniques to measure magnitude are in

1027-405: A similar large-scale rupture occurred elsewhere). Such an earthquake would cause ground motions for up to an hour, with tsunamis hitting shores while the ground is still shaking, and if this kind of earthquake occurred, it would probably be a 1-in-10,000-year event. Prior to the development of the magnitude scale, the only measure of an earthquake's strength or "size" was a subjective assessment of

1106-435: 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

1185-410: 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

1264-577: 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 . Richter magnitude scale The Richter scale ( / ˈ r ɪ k t ər / ), also called

1343-490: 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

1422-446: 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

1501-419: Is about 7 and about 8.5 for M s  . New techniques to avoid the saturation problem and to measure magnitudes rapidly for very large earthquakes are being developed. One of these is based on the long-period P-wave; The other is based on a recently discovered channel wave. The energy release of an earthquake, which closely correlates to its destructive power, scales with the 3 ⁄ 2 power of

1580-437: 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

1659-502: 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

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1738-500: 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

1817-420: 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

1896-629: The California Institute of Technology and the Carnegie Institute , a network of seismographs stretching across Southern California . He also recruited the young and unknown Charles Richter to measure the seismograms and locate the earthquakes generating the seismic waves. In 1931, Kiyoo Wadati showed how he had measured, for several strong earthquakes in Japan, the amplitude of the shaking observed at various distances from

1975-606: The Richter magnitude scale , Richter's magnitude scale , and the Gutenberg–Richter scale , is a measure of the strength of earthquakes , developed by Charles Richter in collaboration with Beno Gutenberg , and presented in Richter's landmark 1935 paper, where he called it the "magnitude scale". This was later revised and renamed the local magnitude scale , denoted as ML or M L  . Because of various shortcomings of

2054-521: 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

2133-410: 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

2212-428: 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

2291-419: 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

2370-430: 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

2449-417: 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

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2528-568: 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

2607-460: The M L   value. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude. In terms of energy, each whole number increase corresponds to an increase of about 31.6 times the amount of energy released, and each increase of 0.2 corresponds to approximately a doubling of the energy released. Events with magnitudes greater than 4.5 are strong enough to be recorded by

2686-626: The Richter scale, numerical values are approximately the same. Although values measured for earthquakes now are M w  , they are frequently reported by the press as Richter values, even for earthquakes of magnitude over 8, when the Richter scale becomes meaningless. The Richter and MMS scales measure the energy released by an earthquake; another scale, the Mercalli intensity scale , classifies earthquakes by their effects , from detectable by instruments but not noticeable, to catastrophic. The energy and effects are not necessarily strongly correlated;

2765-494: The Wood–Anderson seismograph as the standard instrument for producing seismograms. Magnitude was then defined as "the logarithm of the maximum trace amplitude, expressed in microns ", measured at a distance of 100 km (62 mi). The scale was calibrated by defining a magnitude 0 shock as one that produces (at a distance of 100 km (62 mi)) a maximum amplitude of 1 micron (1 μm, or 0.001 millimeters) on

2844-468: 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

2923-427: 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 ,

3002-1087: 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

3081-407: The amplitude of a precisely defined wave. All scales, except M w  , saturate for large earthquakes, meaning they are based on the amplitudes of waves that have a wavelength shorter than the rupture length of the earthquakes. These short waves (high-frequency waves) are too short a yardstick to measure the extent of the event. The resulting effective upper limit of measurement for M L  

3160-413: The amplitudes of different types of elastic waves must be measured. M L   is the scale used for the majority of earthquakes reported (tens of thousands) by local and regional seismological observatories. For large earthquakes worldwide, the moment magnitude scale (MMS) is most common, although M s   is also reported frequently. The seismic moment , M 0   , is proportional to

3239-402: The area of the rupture times the average slip that took place in the earthquake, thus it measures the physical size of the event. M w   is derived from it empirically as a quantity without units, just a number designed to conform to the M s   scale. A spectral analysis is required to obtain M 0  . In contrast, the other magnitudes are derived from a simple measurement of

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3318-606: 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

3397-434: 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

3476-422: 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

3555-449: The development stage by seismologists. All magnitude scales have been designed to give numerically similar results. This goal has been achieved well for M L  , M s  , and M w  . The mb  scale gives somewhat different values than the other scales. The reason for so many different ways to measure the same thing is that at different distances, for different hypocentral depths, and for different earthquake sizes,

3634-426: The earthquake's focus beneath the epicenter, (3) the location of the epicenter, and (4) geological conditions . ( Based on U.S. Geological Survey documents. ) The intensity and death toll depend on several factors (earthquake depth, epicenter location, and population density, to name a few) and can vary widely. Millions of minor earthquakes occur every year worldwide, equating to hundreds every hour every day. On

3713-897: The energy released. The elastic energy radiated is best derived from an integration of the radiated spectrum, but an estimate can be based on mb  because most energy is carried by the high-frequency waves. These formulae for Richter magnitude   M L   {\displaystyle \ M_{\mathsf {L}}\ } are alternatives to using Richter correlation tables based on Richter standard seismic event (   M L = 0   , {\displaystyle {\big (}\ M_{\mathsf {L}}=0\ ,}   A = 0.001   m m   , {\displaystyle \ A=0.001\ {\mathsf {mm}}\ ,}   D = 100   k m   )   . {\displaystyle \ D=100\ {\mathsf {km}}\ {\big )}~.} In

3792-489: The epicenter. He then plotted the logarithm of the amplitude against the distance and found a series of curves that showed a rough correlation with the estimated magnitudes of the earthquakes. Richter resolved some difficulties with this method and then, using data collected by his colleague Beno Gutenberg , he produced similar curves, confirming that they could be used to compare the relative magnitudes of different earthquakes. Additional developments were required to produce

3871-650: 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

3950-441: 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

4029-466: 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

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4108-444: 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,

4187-429: 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 ,

4266-434: 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

4345-454: The intensity of shaking observed near the epicenter of the earthquake, categorized by various seismic intensity scales such as the Rossi–Forel scale . ("Size" is used in the sense of the quantity of energy released, not the size of the area affected by shaking, though higher-energy earthquakes do tend to affect a wider area, depending on the local geology.) In 1883, John Milne surmised that

4424-423: 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

4503-476: The middle of the scale). Due to the variance in earthquakes, it is essential to understand the Richter scale uses common logarithms simply to make the measurements manageable (i.e., a magnitude 3 quake factors 10³ while a magnitude 5 quake factors 10 and has seismometer readings 100 times larger). The Richter magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs. Adjustments are included to compensate for

4582-424: 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

4661-403: 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

4740-408: The original M L   scale, most seismological authorities now use other similar scales such as the moment magnitude scale (M w  ) to report earthquake magnitudes, but much of the news media still erroneously refers to these as "Richter" magnitudes. All magnitude scales retain the logarithmic character of the original and are scaled to have roughly comparable numeric values (typically in

4819-523: The other hand, earthquakes of magnitude ≥8.0 occur about once a year, on average. The largest recorded earthquake was the Great Chilean earthquake of May 22, 1960, which had a magnitude of 9.5 on the moment magnitude scale . Seismologist Susan Hough has suggested that a magnitude 10 quake may represent a very approximate upper limit for what the Earth's tectonic zones are capable of, which would be

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4898-425: The particular circumstances refer to it being defined for Southern California and "implicitly incorporates the attenuative properties of Southern California crust and mantle." The particular instrument used would become saturated by strong earthquakes and unable to record high values. The scale was replaced in the 1970s by the moment magnitude scale (MMS, symbol M w  ); for earthquakes adequately measured by

4977-422: 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

5056-442: The press that the scale was Richter's and "should be referred to as such." In 1956, Gutenberg and Richter, while still referring to "magnitude scale", labelled it "local magnitude", with the symbol M L  , to distinguish it from two other scales they had developed, the surface-wave magnitude (M S ) and body wave magnitude (M B ) scales. The Richter scale was defined in 1935 for particular circumstances and instruments;

5135-639: 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

5214-500: 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

5293-708: The result of the largest known continuous belt of faults rupturing together (along the Pacific coast of the Americas). A research at the Tohoku University in Japan found that a magnitude 10 earthquake was theoretically possible if a combined 3,000 kilometres (1,900 mi) of faults from the Japan Trench to the Kuril–Kamchatka Trench ruptured together and moved by 60 metres (200 ft) (or if

5372-572: 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

5451-519: The shaking amplitude (see Moment magnitude scale for an explanation). Thus, a difference in magnitude of 1.0 is equivalent to a factor of 31.6 ( = ( 10 1.0 ) ( 3 / 2 ) {\displaystyle =({10^{1.0}})^{(3/2)}} ) in the energy released; a difference in magnitude of 2.0 is equivalent to a factor of 1000 ( = ( 10 2.0 ) ( 3 / 2 ) {\displaystyle =({10^{2.0}})^{(3/2)}} ) in

5530-560: The shaking of large earthquakes might generate waves detectable around the globe, and in 1899 E. Von Rehbur Paschvitz observed in Germany seismic waves attributable to an earthquake in Tokyo . In the 1920s, Harry O. Wood and John A. Anderson developed the Wood–Anderson seismograph , one of the first practical instruments for recording seismic waves. Wood then built, under the auspices of

5609-732: 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

5688-471: 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

5767-400: 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

5846-424: 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

5925-537: The variation in the distance between the various seismographs and the epicenter of the earthquake. The original formula is: where A is the maximum excursion of the Wood-Anderson seismograph , the empirical function A 0 depends only on the epicentral distance of the station, δ {\displaystyle \delta } . In practice, readings from all observing stations are averaged after adjustment with station-specific corrections to obtain

6004-429: 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

6083-467: 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

6162-463: 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

6241-605: 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

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