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Axial Seamount (also Coaxial Seamount or Axial Volcano) is a seamount, submarine volcano, and underwater shield volcano in the Pacific Ocean, located on the Juan de Fuca Ridge, approximately west of Cannon Beach, Oregon. Standing 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 trending to the northeast and southwest of its center. The volcano features an unusual rectangular caldera, and its flanks are pockmarked by , vents, sheet flows, and up to 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 up to thick, and the total eruptive volume was found to be . Axial Seamount erupted again in April 2011, producing a wide lava flow. There was another eruption in 2015 and another is expected in 2025.
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 and building up the 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 per year,In south to north order, they are the Cleft, Vance, Coaxial, Cobb, Endeavor, and West Valley segments. producing a complex system of and . 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's summit is marked by an unusual rectangular caldera, 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 deep. The caldera is roughly deeper at the north side than it is in the south. Flows within the caldera consist mostly of sheet flows pocketed by lava ponds and . Less common are ; their arrangement along the caldera walls suggests that they were an important component in the volcano's early growth. There are several lava dome-like structures within the caldera with heights of . There are several small volcanic crater within the region, the largest of which, nicknamed the D.D. Cone, is in diameter and in relief. However, most of the features do not range over deep and across.
The northern rift zone of Axial Seamount is a long ridge running 10 to 20 degrees northeast of the main caldera. The rift is pocketed by multiple , in length, as far as from Axial Volcano's center, and reaching up to long and 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 .
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 Hawaii hotspot 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 Seamounts by as much as , creating a zone of intense fissuring at the northern edge of the smaller volcano.The Vance Seamounts is a group of individually nameless volcanoes, so its northernmost member is sometimes called Vance Seamount for convenience. 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 bathymetry of the seamount was compiled by the 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 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.
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 . 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 (1.08 °F) increase and 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 on 12 February 1998, which conducted conductivity, temperature, depth, and optical casts to unusual results. In May, a dedicated bathymetry of the seamount showed topographical changes along the volcano's southern flank, which estimated the thickest flows to . 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 long and 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 high. The total eruptive volume was roughly .
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.
In July 2011, a dive using ROV Jason discovered new lava flows on the volcanoes that had not been present a year before. 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 and produced a wide lava flow during the event, which was as much as three times larger than the 1998 eruption.
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 respectively. The temperature of the magma feeding the system is uncertain, and may vary between . Curiously, vent fluid are heavily enriched in helium, containing five times more than the amount of the element in similar vents in the Galápagos, and 580 times that of regular seawater.
of the Pogonophora family thicket the largest vents on Axial Seamounts, forming colonies up to thick in places; smaller, less nutritious vents feed , smaller tube worms, and . The three most common microbial groups are bacterial Campylobacterota, Methanococcaceae, and Euryarchaeota family. The most common fauna 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.The morphology of the worm changes with local morphology, so much so that it was originally considered two distinct species. 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.
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