Axial Seamount (also Coaxial Seamount or Axial Volcano) is a seamount and submarine volcano located on the Juan de Fuca Ridge, approximately 480 km (298 mi) west of Cannon Beach, Oregon. Standing 1,100 m (3,609 ft) high, Axial Seamount is the youngest volcano and current eruptive center of the Cobb–Eickelberg Seamount chain. Located at the center of both a geological hotspot and a mid-ocean ridge, the seamount is geologically complex, and its origins are still poorly understood. Axial Seamount is set on a long, low-lying plateau, with two large rift zones trending 50 km (31 mi) to the northeast and southwest of its center. The volcano features an unusual rectangular caldera, and its flanks are pockmarked by fissures, vents, sheet flows, and pit craters up to 100 m (328 ft) deep; its geology is further complicated by its intersection with several smaller seamounts surrounding it.
Axial Seamount was first detected in the 1970s by satellite altimetry, and mapped and explored by Pisces IV, DSV Alvin, and others through the 1980s. A large package of sensors was dropped on the seamount through 1992, and the New Millennium Observatory was established on its flanks in 1996. Axial Seamount received significant scientific attention following the seismic detection of a submarine eruption at the volcano in January 1998, the first time a submarine eruption had been detected and followed in situ. Subsequent cruises and analysis showed that the volcano had generated lava flows up to 13 m (43 ft) thick, and the total eruptive volume was found to be 18,000–76,000 km3 (4,300–18,200 cu mi). Axial Seamount erupted again in April 2011, producing a mile-wide lava flow. There was another eruption in 2015.
|Summit depth||1,410 m (4,626 ft)|
|Height||1,100 m (3,609 ft)|
|Location||Juan de Fuca Ridge|
|Type||Seamount (Submarine volcano), Hotspot volcano|
|Volcanic arc/chain||Cobb–Eickelberg Seamount chain|
|Last activity||April 2015|
|Last eruption||April 2015|
|Discovered by||NOAAS Surveyor|
Axial Seamount is the youngest volcano and current eruptive center of the Cobb–Eickelberg Seamount chain, a chain of seamounts that terminates south of Alaska. Axial lies where the chain intersects with the Juan de Fuca Ridge, approximately 480 km (298 mi) west of Oregon. It is a product of the Cobb hotspot, but now sits on an ocean spreading center between the Juan de Fuca Plate and the North American Plate, offset by the Blanco Fracture Zone to the south and a ridge-built triple junction to the north.
This position is not yet entirely understood. It is believed that the chain, formed over millions of years by the now-inactive Cobb hotspot, is older than the mid-ocean ridge it bisects. Between 200,000 and 700,000 years ago, the hotspot was encroached by the tectonic spreading center, displacing it by as much as 20 km (12 mi) and building up the 500 km (311 mi) long Juan de Fuca Ridge. At least 7 spreading centers have been recognized, and plate measurements near Axial show that the ridge is separating at a rate of 6 cm (2 in) per year,[n 2] producing a complex system of oceanic basins and ridges. However some scientists have questioned this theory, pointing out that the high density of the chain's overlapping seamounts is incompatible with such an origin, as a hotspot would form a well organized, widely spaced chain. Although the exact nature of Axial Seamount remains unknown, its complex origins makes it one of the most geologically interesting features in the North Pacific.
Axial Seamount is the most active volcanic site in the North Pacific. Study of magnetic delineations along the seamount have modeled the ridge's history up to 30 million years ago, and shown that growth has progressed mostly in the north, with some southward progression dating back 3.5 million years. The base of Axial Seamount is a long, low-lying plateau, and the eastern part of the seamount is defined by a series of linear scarps. Axial Seamount has two major volcanic rifts extending approximately 50 km (31 mi) north and south of its main summit, as well as several much smaller, ill-defined ones aligned in a roughly similar pattern. Basins around the volcano increase its irregularity, making it unusually complex (most seamounts of roughly the same size are circular or flattened in shape.)
Axial Seamount's summit is marked by an unusual rectangular caldera, 3 km × 8 km (2 mi × 5 mi) in area, ~3° in slope, and breached on the southeast side. The area is offset by the two rift zones and defined on three sides by boundary faults up to 150 m (492 ft) deep. The caldera is roughly 50 m (164 ft) deeper at the north side then it is in the south. Flows within the caldera consist mostly of sheet flows pocketed by lava ponds and pit craters. Less common are pillow lavas; their arrangement along the caldera walls suggests that they were an important component in the volcano's early growth. There are several dome-like structures within the caldera with heights of 100–300 m (328–984 ft). There are several small craters within the region, the largest of which, nicknamed the D.D. Cone, is 2 km (1 mi) in diameter and 100 m (328 ft) in relief. However, most of the features do not range over 30 to 40 m (98 to 131 ft) deep and 1 km (1 mi) across.
The northern rift zone of Axial Seamount is a 5 km (3 mi) long ridge running 10 to 20 degrees northeast of the main caldera. The rift is pocketed by multiple fissures, 100–200 m (328–656 ft) in length, as far as 7 km (4 mi) from Axial Volcano's center, and reaching up to 400 m (1,312 ft) long and 20 m (66 ft) deep. The area contains high amounts of volcanic glass; a major eruption is still visible in the form of an elongated glassy lava flow extending off the caldera wall, east of the main rift line. Dives in 1983 found extensive low-temperature venting at the northern half of the fissure. The shorter, newer southern rift zone consists of a topographically plunging rift, surrounding by subtle, discontinuous faults. Camera tows along the southern flank reveal that the area is built of delineated sheet flows, small lava ponds, and lava channels.
The youngest of the flows on Axial Seamount are aligned along the two rift zones, followed by flows inside the summit caldera; the oldest appear to originate from directly around the caldera, where most of the basalt is completely covered in accumulated sediment. This suggests a bilateral growth pattern, a trend also found in Hawaiʻian volcanics and other well-known seamounts, for instance Jasper Seamount.
Axial Seamount's growth has intersected the growth of many of the smaller seamounts around it. The largest of these is Brown Bear Seamount, to which it is connected by a narrow ridge running roughly perpendicular to its western caldera wall. However, little evidence of interactions between the two seamounts has been found. On the other hand, Axial Seamount's southern rift zone bisects Vance Seamount by as much as 30 km (19 mi), creating a zone of intense fissuring at the northern edge of the smaller volcano.[n 3] Interactions with Cobb Seamount to the north are more complex, forming an unusual "bent spreading center." In addition there are four smaller structures directly east, north, and south of Axial.
The first volcanoes along the Juan de Fuca ridge, including Axial Seamount, were detected in the 1970s by satellite altimetry. Axial Seamount's proximity to the western coast and shallow depth make it one of the most easily accessible seamounts in the world, and its unique geological setting and active state also makes it one of the most interesting, rivaling Davidson Seamount to the south in scientific interest.
The first bathymetry of the seamount was compiled by the NOAAS Surveyor in 1981, as part of SeaBeam trials in the North Pacific. The survey was specifically meant to find and link seafloor hydrothermal activity to geomorphic features. Four areas of increased temperature concentration, indicative of hydrothermal activity were found, and the then-unnamed Axial Seamount was among them. Submersible dives with Pisces IV and DSV Alvin in 1983 and 1984 discovered the first active black smoker vents in the north Pacific. Soon after Axial Seamount was named for its central position on the intersection of the Cobb–Eickelberg Seamount chain and Juan de Fuca Ridge. That same year, the National Oceanic and Atmospheric Administration (NOAA) founded its VENTS program, providing impetus for studying the volcano more closely.
Between 1987 and 1992, a variety of pressure sensors, tilt sensors, temperature probes, and seismometers were dropped on the volcano in what came to be known as the Volcanic Systems Moninters (VSN). Further bathymetries by the NOAAS Discoverer in 1991 and RV Sonne in 1996 detailed the seamount further, making it one of the best known features in the North Pacific. Also in 1996, the New Millennium Observatory (NeMO) was established on Axial Seamount, to study volcanic perturbations and the effect they have on hydrothermal communities.
The 1998 eruption of Axial Seamount was preceded by several large earthquake swarms, common indicators of volcanic activity. The swarms correlated to magma movements in the volcano; bottom pressure recorders deployed on the volcano between 1987 and 1992 recorded five instances of deflation in the summit surface (caused by lava movement), ranging from 3 to 10 cm (1 to 4 in). In 1991, the National Oceanic and Atmospheric Administration (NOAA) was granted access to the United States Navy's SOSUS system, a chain of submerged hydrophones in the North Pacific originally used by the Navy to detect Russian submarines during the Cold War. Since 1993, the NOAA has maintained a real-time monitoring system that alerts the organization whenever an event occurs. The hydrophones are able to detect even very small earthquakes (~ magnitude 1.8) by listening for the acoustic waves generated by T-waves. These waves can propagate over large distances with minimal loss in power, making them an ideal way to record otherwise unnoticeable submarine earthquakes; over the course of the eruption, only 3 earthquakes were strong enough to register on land-based systems. However, they cannot interpret earthquake depth or what caused them.
Between 1991 and 1996 Axial Seamount experienced a single earthquake swarm of over 50 events. Between May and November 1997 this activity increased markedly, with SOSUS recording 5 such swarms, culminating with a massive 11-day, 8247-quake event around the time of the eruption, in January 1998. The seismicity began at the summit, but within 6 hours had begun to migrate south as well; by 29 November 1997 the swarm had moved south by 50 km (31 mi). This coincided with lava release along the summit and southern flank. The seamount remained absolutely quiet thereafter, suggesting the completion of an eruptive cycle at the volcano. In all, 9055 earthquakes were detected, and 1669 were strong enough to be located. Earthquake activity was concentrated around the summit and southern rift zones, with the majority of events centered inside the summit caldera; temperature probes and pressure recorders in the caldera recorded an average 0.6 °C (33.1 °F) increase and 3.3 m (11 ft) height deflation, respectively, during the event. This close monitoring gives the 1998 eruption the distinction of being the only submarine eruption ever observed in situ.
The first post-eruption expedition was organized and conducted by RV Wecoma on 12 February 1998, which conducted conductivity, temperature, depth, and optical casts to unusual results. In May, a dedicated bathymetric survey of the seamount showed topographical changes along the volcano's southern flank, which estimated the thickest flows to 13 m (43 ft). In July DSV Alvin made several dives on the seamount's summit caldera, followed in August through September by an extensive observation and collection program using ROV ROPOS, confirming the bathymetric estimates. A sheet flow more than 3 km (2 mi) long and 500 to 800 m (1,640 to 2,625 ft) wide was produced from Axial Seamount's upper southern flank, on the site of what was formerly an active geothermal field. The southern flows were in an area marked by a difference between older sediments and newer, glassier rock, and the maximum ridge generated by the eruption, at the crest of the southern flow, was 13 m (40 ft) high. The total eruptive volume was roughly 0.018–0.076 km3 (0.004–0.018 cu mi).
The development, eruption, and close monitoring of Axial Seamount provided a fertile model on submarine volcanic eruptions to scientists; several scientific papers on the topic were published soon after.
Seismic activity at Axial Seamount virtually disappeared after the 1998 eruption, and monitoring of the volcano was done principally with bottom pressure recorders deployed on the volcano's flanks, supplemented since 2000 by annual measurements using pressure sensors mounted on Remotely operated underwater vehicles (ROVs) and applied to local benchmarks. The sensors have shown that Axial Seamount is slowly reflating; just after the eruption the seamount was swelling at 20 cm (8 in) per month, a number that decreased to 15 cm (6 in) by 2006. In eight years Axial Seamount recovered approximately 50% of its 3.2 m (10.5 ft) of pre-eruption swelling, and in 2006, William Chadwick of the Oregon State University and his associates calculated that the next eruption would occur in approximately 2014:
Axial Seamount behaves in a more predictable way than many other volcanoes; likely due to its robust magma supply coupled with its thin crust, and its location on a mid-ocean ridge spreading center. It is now the only volcano on the seafloor whose surface deformation has been continuously monitored throughout an entire eruption cycle.— Scott L. Nooner, Columbia University
In July 2011, a dive using ROV Jason discovered new lava flows on the volcanoes that had not been present a year ago. The expeditionary crew recovered two bottom-pressure recorders and two hydrophones (a third was found buried in lava) off the volcano, which together showed that the eruption had occurred during April, starting on 6 April 2011. Although the instruments recorded hundreds of seismic events, only a handful had been noticed by SOSUS and land-based seismometers, as many components of the system had been offline at the time. The volcano subsided by more than 2 m (7 ft) and produced a 2 km (1 mi) wide lava flow during the event, which was as much as three times larger than the 1998 eruption.
In 1983, a Canadian–American collaborative expedition, named the Canadian American Seamount Expedition (CASM), visited the northwestern edge of Axial Seamount's summit caldera to investigate a persistent temperature anomaly in the region. In a series of eight dives conducted by Pisces IV, the scientists discovered a vibrant hydrothermal vent community on the leading edge of a 300 m (984 ft) fissure within the caldera. Vent temperatures were measured around 35 °C (95 °F), approximately 30 °C (86 °F) hotter than the surrounding environment. Camera tows and submersible dives through the 1980s and 1990s revealed Axial Seamount's active state, including the only known black smoker in the northwest Pacific. Three venting centers have been recognized: the original site, named CHASM; a southwestern caldera field discovered in the late 1980s, named ASHES; and a site located on its southeastern rift zone, named CASTLE. All are primarily sulfur/sulfide emitting.
The temperature and composition of Axial Seamount's hydrothermal vents changes over time, but always maintains a roughly common identity, as do the vents' individual microbial communities. Vents generally have a lower pH than the surrounding fluid, and are acidic and alkaline as a result. The temperature of the magma feeding the system is uncertain, and may vary between 300 and 550 °C (572 and 1,022 °F). Curiously, vent fluid are heavily enriched in helium, containing five times the amount of the element as similar vents in the Galápagos, and 580 times that of regular seawater.
Tube worms of the Pogonophora family thicket the largest vents on Axial Seamounts, forming colonies up to 6 m2 (65 sq ft) thick in places; smaller, less nutritious vents feed bacterial mats, smaller tube worms, and limpets. The three most common microbial groups are bacterial epsilonproteobacteria, archaeon thermophilics of the Methanococcus family, and archaeons of the Euryarchaeota family. The most common flora at Axial Seamount's hydrothermal vents is the worm Ridgeia piscesae, which is found at hydrothermal sites of all descriptions on the Juan de Fuca ridge, and is the base of Axial Seamount's hydrothermal ecosystem.[n 6] Other species on the seamount include the tube worm P. palmiformis, the sea snail Lepetodrilus fucensis, the bristle worm Amphisamytha galapagensis, and the sea spider Sericosura verenae.
Axial may refer to:
one of the anatomical directions describing relationships in an animal body
Axial Seamount and submarine volcano off Oregon, USA
Axial, Colorado, a ghost town
In geometry:a geometric term of location
an axis of rotationthe Axial age in China, India, etc.
a type of modal frame, in music
axial-flow, a type of fanBrown Bear Seamount
Not to be confused with Bear Seamount.Brown Bear Seamount is a seamount (underwater volcano) approximately 300 mi (483 km) west of the coast of Oregon. It is connected to the larger Axial Seamount by a small ridge. Brown Bear Seamount was created by the Cobb hotspot, and is located on the near west of the Juan de Fuca Ridge. It has not been affected by ocean spreading as much as its neighbor, and is therefore not quite as geologically complex. Brown Bear is the second youngest volcano in the chain, after Axial. No eruptions are known.Cobb hotspot
The Cobb hotspot is a marine volcanic hotspot at (46˚ N, 130˚ W), which is 460 km (290 mi) west of Oregon and Washington, North America, in the Pacific Ocean. Over geologic time, the Earth's surface has migrated with respect to the hotspot through plate tectonics, creating the Cobb-Eicklberg seamount chain. The hotspot is currently collocated with the Juan de Fuca Ridge.Cobb–Eickelberg Seamount chain
The Cobb-Eickelberg seamount chain is a range of undersea mountains formed by volcanic activity of the Cobb hotspot located in the Pacific Ocean. The seamount chain extends to the southeast on the Pacific Plate, beginning at the Aleutian Trench and terminating at Axial Seamount, located on the Juan de Fuca Ridge.The seamount chain is spread over a vast length of approximately 1800 km. The location of the Cobb hotspot that gives rise to these seamounts is 46° N -130° W. The Pacific plate is moving to the northwest over the hotspot, causing the seamounts in the chain to decrease in age to the southeast. Axial is the youngest seamount and is located approximately 480 km west of Cannon Beach, Oregon. The most studied seamounts that make up this chain are Axial, Brown Bear, Cobb, and Patton seamounts. There are many other seamounts in this chain which have not been explored.Geology of the Pacific Ocean
The Pacific Ocean evolved in the Mesozoic from the Panthalassic Ocean, which had formed when Rodinia rifted apart around 750 Ma. The first ocean floor which is part of the current Pacific Plate began 160 Ma to the west of the central Pacific and subsequently developed into the largest oceanic plate on Earth.The tectonic plates continue to move today. The slowest spreading ridge is the Gakkel Ridge on the Arctic Ocean floor, which spreads at less than 2.5 cm/year (1 in/year), while the fastest, the East Pacific Rise near Easter Island, has a spreading rate of over 15 cm/year (6 in/year).Hotspot (geology)
In geology, the places known as hotspots or hot spots are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the surrounding mantle. Their position on the Earth's surface is independent of tectonic plate boundaries. There are two hypotheses that attempt to explain their origins. One suggests that hotspots are due to mantle plumes that rise as thermal diapirs from the core–mantle boundary. The other hypothesis is that lithospheric extension permits the passive rising of melt from shallow depths. This hypothesis considers the term "hotspot" to be a misnomer, asserting that the mantle source beneath them is, in fact, not anomalously hot at all. Well-known examples include the Hawaii, Iceland and Yellowstone hotspots.Juan de Fuca Ridge
The Juan de Fuca Ridge is a mid-ocean spreading center and divergent plate boundary located off the coast of the Pacific Northwest region of North America. The ridge separates the Pacific Plate to the west and the Juan de Fuca Plate to the east. It runs generally northward, with a length of approximately 500 kilometers (300 miles). The ridge is a section of what remains from the larger Pacific-Farallon Ridge which used to be the primary spreading center of this region, driving the Farallon Plate underneath the North American Plate through the process of plate tectonics. Today, the Juan de Fuca Ridge pushes the Juan de Fuca Plate underneath the North American plate, forming the Cascadia Subduction Zone.List of submarine volcanoes
A list of active and extinct submarine volcanoes and seamounts located under the world's oceans. There are estimated to be 40,000 to 55,000 seamounts in the global oceans. Almost all are not well-mapped and many may not have been identified at all. Most are unnamed and unexplored. This list is therefore confined to seamounts that are notable enough to have been named and/or explored.List of volcanoes in the Pacific Ocean
A list of active and extinct volcanoes in the Pacific Ocean.NOAAS Surveyor (S 132)
NOAA Ship Surveyor (S 132) was an oceanographic survey ship in commission in the National Oceanic and Atmospheric Administration (NOAA) from 1970 until 1995. Prior to her NOAA career, she was in commission in the United States Coast and Geodetic Survey from 1960 to 1970 as USC&GS Surveyor (OSS 32). She was the second and last Coast and Geodetic Survey ship named Surveyor and has been the only NOAA ship thus far to bear the name.Ocean Networks Canada
Ocean Networks Canada is a University of Victoria initiative that operates the NEPTUNE and VENUS cabled ocean observatories in the northeast Pacific Ocean and the Salish Sea. Additionally, Ocean Networks Canada operates smaller community-based observatories offshore from Cambridge Bay, Nunavut., Campbell River, Kitamaat Village and Digby Island. These observatories collect data on physical, chemical, biological, and geological aspects of the ocean over long time periods. As with other ocean observatories such as ESONET, Ocean Observatories Initiative, MACHO and DONET, scientific instruments connected to Ocean Networks Canada are operated remotely and provide continuous streams of freely available data to researchers and the public. Over 200 gigabytes of data are collected every day.The VENUS Observatory is situated at three main sites in the Salish Sea, including Saanich Inlet (depth 100 m), the eastern and central Strait of Georgia (depths 170–300 m), and the Fraser River delta.
The NEPTUNE observatory is situated off the west coast of Vancouver Island in Barkley Sound, along the Cascadia subduction zone, on the Cascadia Basin abyssal plain, and on the Endeavour segment of the Juan de Fuca Ridge.Altogether, the system includes 3 observatories, 5 shore stations, 850+ km of seafloor backbone cables, 11 instrumented sites, 32 instrument platforms, 6 mobile instrument platforms, 400+ instruments and over 2000 scientific sensors deployed.Scientific topics of study that are enabled by data from these observatories include Arctic oceanography, deep-sea biodiversity, marine ecosystem function, marine forensics, gas hydrates, hydrothermal vents, marine mammals, sediment and benthic dynamics and tsunami studies.Ocean Observatories Initiative
The Ocean Observatories Initiative (OOI) is a National Science Foundation (NSF) Division of Ocean Sciences program that focuses the science, technology, education and outreach of an emerging network of science driven ocean observing systems. It is a networked infrastructure of science-driven sensor systems to measure the physical, chemical, geological and biological variables in the ocean and seafloor as well as the overlying atmosphere, providing an integrated system collecting data on coastal, regional and global scales.
OOI is funded by the National Science Foundation (NSF).
OOI's goal is to deliver data and data products for a 25-year-plus time period within a scalable architecture that can meet emerging technical advances in ocean science. These data are freely accessible online through the OOI cyberinfrastructure.Outline of oceanography
The following outline is provided as an overview of and introduction to Oceanography.Regional Scale Nodes
The National Science Foundation's (NSF) Ocean Observatories Initiative (OOI) Regional Scale Nodes (RSN) component is an electro-optically cabled underwater observatory that directly connects to the global Internet. It is the largest cable-linked seabed observatory in the world, and also the first of its kind in the United States.
Located on the southern part of the Juan de Fuca plate, off the coast of Washington and Oregon, it is the first ocean observatory to span a tectonic plate.
RSN utilizes several high-power, high-bandwidth sub-sea terminals called primary nodes which are linked together by fiber-optic cable and provide support to oceanographic sensors at key locations.
Upon completion of the network in 2014, RSN will cover a distance of over 900 kilometers at depths of up to 3000 meters. Implementation of the OOI Regional Scale Nodes is led by the University of Washington's (UW) School of Oceanography, the UW Applied Physics Laboratory, and L-3 MariPro.
Live RSN data from >100 seafloor and water column instruments will be made available live on the Internet. This will allow both scientists and the general public to study long-term changes in ocean systems over the next 25 years.
Construction of RSN will be completed in 2014. Efforts are substantially aided by the crews of ROPOS (Remotely Operated Platform for Observation Sciences. The 83-day VISIONS ’14 expedition aboard the 274-foot global-class R/V Thomas G. Thompson is responsible for the observatory's final implementation.Seamount
A seamount is a mountain rising from the ocean floor that does not reach to the water's surface (sea level), and thus is not an island, islet or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise abruptly and are usually found rising from the seafloor to 1,000–4,000 m (3,300–13,100 ft) in height. They are defined by oceanographers as independent features that rise to at least 1,000 m (3,281 ft) above the seafloor, characteristically of conical form. The peaks are often found hundreds to thousands of meters below the surface, and are therefore considered to be within the deep sea. During their evolution over geologic time, the largest seamounts may reach the sea surface where wave action erodes the summit to form a flat surface. After they have subsided and sunk below the sea surface such flat-top seamounts are called "guyots" or "tablemounts".There are more than 14,500 seamounts, of which 9,951 seamounts and 283 guyots, covering a total of 8,796,150 km2 (3,396,210 sq mi) have been mapped but only a few have been studied in detail by scientists. Seamounts and guyots are most abundant in the North Pacific Ocean, and follow a distinctive evolutionary pattern of eruption, build-up, subsidence and erosion. In recent years, several active seamounts have been observed, for example Loihi in the Hawaiian Islands.
Because of their abundance, seamounts are one of the most common marine ecosystems in the world. Interactions between seamounts and underwater currents, as well as their elevated position in the water, attract plankton, corals, fish, and marine mammals alike. Their aggregational effect has been noted by the commercial fishing industry, and many seamounts support extensive fisheries. There are ongoing concerns on the negative impact of fishing on seamount ecosystems, and well-documented cases of stock decline, for example with the orange roughy (Hoplostethus atlanticus). 95% of ecological damage is done by bottom trawling, which scrapes whole ecosystems off seamounts.
Because of their large numbers, many seamounts remain to be properly studied, and even mapped. Bathymetry and satellite altimetry are two technologies working to close the gap. There have been instances where naval vessels have collided with uncharted seamounts; for example, Muirfield Seamount is named after the ship that struck it in 1973. However, the greatest danger from seamounts are flank collapses; as they get older, extrusions seeping in the seamounts put pressure on their sides, causing landslides that have the potential to generate massive tsunamis.Types of volcanic eruptions
Several types of volcanic eruptions—during which lava, tephra (ash, lapilli, volcanic bombs and volcanic blocks), and assorted gases are 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 different types of eruptions. The most well-observed are magmatic eruptions, which involve the decompression of gas within magma that propels it forward. Phreatomagmatic eruptions are another type of volcanic eruption, driven by the compression of gas within magma, the direct opposite of the process powering magmatic activity. The third eruptive type is the phreatic eruption, which is driven by the superheating of steam via contact with magma; these eruptive types often exhibit no magmatic release, instead causing the granulation of existing rock.
Within these wide-defining 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; 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 Volcanic Explosivity Index (VEI), an order of magnitude scale ranging from 0 to 8 that often correlates to eruptive types.