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.

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Cobb-Eickelberg seamount chain extending all the way to Aleutian Trench

Formation

Seamounts are created at hot spots. These are isolated areas within tectonic plates where plumes of magma rise through the crust and erupt at the surface. This creates a chain of submarine volcanoes and seamounts.

The Cobb hot spot is located at the Juan de Fuca Ridge in the Pacific Ocean. The Pacific Plate is moving north-westward direction at a speed of ~5.5 cm per year. Periodic volcanic events have led to magma eruption onto the seafloor, forming seamounts. The last known volcanic activity was at Axial Seamount, which is currently directly overlying the hot spot. The total magmatic flux from the Cobb Hot Spot is about 0.3 cubic m/yr.[1]

Although the Cobb hotspot is currently located beneath the Juan de Fuca ridge, this has not always been the case. It went under the Juan de Fuca Ridge when the Pacific plate started moving northwest and eventually the boundary came right on top of the hot spot.

Currently the Axial seamount is the only active seamount. The most recent eruption took place in April–May 2015.[2]

Seamounts

1. Axial Seamount (46° 03′ 36″ N, 130° 00′ 0″ W)

Axial seamount earthquake distribution
The most recent seamount

Axial Seamount is the youngest seamount in the Cobb Eickelberg Seamount chain. Since this is the most active of all the Cobb-Eickelberg Seamounts, it is studied the most: to help understand the dynamics of seamounts, volcanic activity, earthquakes, biodiversity, geology and chemistry. The Axial Volcano is about 700 meters higher than the Juan de Fuca Ridge and about 1000 meters higher than the front of the flanking basins on either side. The Axial Volcano plateaus on top, and has a relatively smooth relief, with a rectangular shape of size 3 km x 8 km.[3] 42% of the lava surrounding the volcano ranges from ropy, whorly, or lineated pāhoehoe to jumbled chaotic form. The remaining area is mostly pillow basalt.[4] Colonial protozoans, bacterial mats, pogonophorans, metazoans, polychaetes, bivalves, tubeworms, copepods and many other organisms are found in the region where there are hydrothermal vents present in the caldera. This helps with the study of varying biodiversity at great depths.

Axial seamount is the only active seamount in the chain, because it is on top of the Cobb hot spot. All the other seamounts in this chain are inactive because their source of magma, the Cobb hot spot, has moved out from underneath them.

2. Brown Bear Volcano (46° 02′ 24″ N, 130° 27′ 36″ W)

With an age of 0.5-1.5 million years, Brown Bear Seamount is the second youngest seamount in the Cobb-Eickelberg chain. It is northwest of Axial Seamount and connected by a small ridge. Due to its separation from the Juan de Fuca ridge, spreading has very little effect on Brown Bear, so it is not as geologically complex and is not studied in detail. It has a volcanic cone of width 5 km and rises approximately 1000 meters from the ocean floor. The Brown Bear Seamount summit is at a depth of 1400 meters.[5] Geographically, the Brown Bear Seamount is separated into two areas, Northwestern and Southeastern, with distinct morphological features.[6] This is thought to be caused by the influence of the mid ocean ridge extensional stress regime. The morphology of the western portion also suggests that it was formed before the hot spot interacted with the Juan de Fuca ridge.[6] The northwestern section of Brown Bear is dominated by a large (5 km diameter) rounded volcanic cone structure.[6] The southern portion extends south of 46.1 degrees North Latitude, and consists of relatively small (1–2 km diameter) volcanic cones.[6]

3. Cobb Seamount (46° 44′ 0″ N, 130° 47′ 0″ W)

The Cobb Seamount rises from a 2750 m basin to within 37 meters of the ocean surface.[7] It is located just 100 km to the northwest of the hot spot.[8] This seamount is at least 3.3 million years old.[8] The Cobb seamount has been extensively studied for its geological features. Cobb seamount was once an island that was 914 meters above sea level [9] which due to erosion became a seamount.[9] Samples collected from this location were used in studies that determined the age and geological composition of the rocks. The Cobb Seamount is all basalt and contains phenocrysts of plagioclase and clinopyroxene; the intergranular/interstitial matrix was found to have iron and titanium oxides.[8] Video and photographs collected in 2012 from Cobb Seamount have shown a wide variety of biodiversity at the location. 17 benthic taxa were observed through pictures collected from the ROV dives. Most common species included sea cucumbers, squat lobsters, thornyheads, and corals.[10]

4. Patton Seamount (54° 34′ 48″ N, 150° 26′ 24″ W)

Patton Seamount
3-D bathymetric map of Patton Seamount, a Gulf of Alaska seamount, with two smaller seamounts in the foreground. Deep areas are blue, and shallow areas are red.

Patton Seamount is about 33 million years old.[11] Although there is not much information about its geology, the biology at the Patton Seamount is very well studied. The seamount's summit is 183 meters below the ocean surface, and its height from the seafloor is 3048 meters.

In July 1999, DSV Alvin was used to explore the biodiversity at the Patton Seamount.[12] The shallow water community mostly consisted of rockfish, flatfish, sea stars and attached suspension feeders. The community at mid-depths consisted of attached suspension feeding organisms like corals, sponges, crinoids, sea anemones and sea cucumbers. The common fish species were the sablefish and the giant grenadier. The deep water community consisted of fewer attached suspension feeders and more highly mobile species like Pacific grenadier, popeye grenadier, Pacific flatnose and large mobile crabs.[12]

Volcanic activity in the past and eruptions

Currently, the only active seamount is Axial Seamount, located directly overtop the hotspot at Juan de Fuca Ridge. The most recent eruption was in April–May 2015, with a prior eruption in 2011.[2] Another eruption was detected seismically in January 1998. Lava erupted from a 9 km long fissure, and the caldera subsided by 3 meters during the eruption.[13] In 1983, hydrothermal venting was discovered.[13]

Foraminiferan fossil studies have suggested that Cobb Seamount was a pre-late Eocene volcano.[9] Thus, it was likely volcanically active approximately 40 million years ago, and remained volcanically active until about 3.3 million years ago when the Cobb seamount was formed.

Ar40-Ar39 dating of deep basalt from the Patton seamount shows it to be 33 million years old, which coincides with the time when the seamount was above the Cobb hot spot.[11] However, there are samples collected from shallower depths of basalt which are younger, suggesting that even after the hotspot volcanism ceases, non-hotspot volcanism can sometimes take place.[11]

Other seamounts of Cobb Eickelberg seamount chain[8]

  • Thompson
  • Son of Brown Bear
  • Corn
  • Pipe
  • Warwick
  • Eickelberg
  • Forster
  • Miller
  • Murray

References

  1. ^ West, Michael; Menke, William; Maya, Tolstoy (February 2003). "Focused magma supply at the intersection of the Cobb hotspot and Juan de Fuca ridge" (PDF). Cite journal requires |journal= (help)
  2. ^ a b Wilcock, William S. D.; Tolstoy, Maya; Waldhauser, Felix; Garcia, Charles; Tan, Yen Joe; Bohnenstiehl, DelWayne R.; Caplan-Auerbach, Jacqueline; Dziak, Robert P.; Arnulf, Adrien F. (2016-12-16). "Seismic constraints on caldera dynamics from the 2015 Axial Seamount eruption". Science. 354 (6318): 1395–1399. doi:10.1126/science.aah5563. ISSN 0036-8075. PMID 27980204.
  3. ^ "Growth, Demise, and Recent Eruption History of the Eastern Cobb-Eickelberg Seamounts at the Intersection with the Juan De Fuca Ridge". ResearchGate. Retrieved 2017-05-31.
  4. ^ "High-Resolution Studies of the Summit of Axial Volcano". ResearchGate. Retrieved 2017-05-31.
  5. ^ Bobbitt, Andra M.; Merle, Susan G.; Steinker, Paula J.; Dziak, Robert P. (November 8, 2000). "Full-coverage multibeam bathymetry of Cobb and Brown Bear Seamounts, Northeast Pacific Ocean". American Geophysical Union : Washington, DC, United States. 81. CiteSeerX 10.1.1.603.7320. doi:10.1002/(ISSN)2324-9250.
  6. ^ a b c d Duarte, Joao C; Schellart, Wouter P, eds. (2016). "11.Growth, Demise, and Recent Eruption History of the Eastern Cobb-Eickelberg Seamounts at the Intersection with the Juan De Fuca Ridge". Plate Boundaries and Natural Hazards. Hoboken, New Jersey: Wiley - AGU. ISBN 978-1-119-05397-2.
  7. ^ "Diving to Cobb Seamount: A Report on Diving Operations on Pr... : Journal of Occupational and Environmental Medicine". LWW. Retrieved 2017-05-15.
  8. ^ a b c d Desonie, Dana L.; Duncan, Robert A. (10 August 1990). "The Cobb-Eickelberg Seamount Chain: Hotspot volcanism with mid-ocean ridge basalt affinity". Journal of Geophysical Research: Solid Earth. 95: 12697. doi:10.1029/jb095ib08p12697.
  9. ^ a b c Budinger, Thomas F.; Enbysk, Betty J. (1960). "Cobb Seamount, a Deep-sea Feature Off the Washington Coast: Topography, Geology, Biology, and Hydrography". Dept. of Oceanography, University of Washington, Seattle. hdl:1773/15993.
  10. ^ Preez, Cherisse Du; Curtis, Janelle M. R.; Clarke, M. Elizabeth (2016-10-28). "The Structure and Distribution of Benthic Communities on a Shallow Seamount (Cobb Seamount, Northeast Pacific Ocean)". PLOS ONE. 11 (10): e0165513. doi:10.1371/journal.pone.0165513. ISSN 1932-6203. PMC 5085030. PMID 27792782.
  11. ^ a b c Keller, Randall A.; Fisk, Martin R.; Duncan, Robert A.; White, William M. (1997-06-01). "16 m.y. of hotspot and nonhotspot volcanism on the Patton-Murray seamount platform, Gulf of Alaska". Geology. 25 (6): 511–514. doi:10.1130/0091-7613(1997)025<0511:MYOHAN>2.3.CO;2. ISSN 0091-7613.
  12. ^ a b Hoff, Gerald R.; Stevens, Bradley (Summer 2005). "Faunal Assemblage Structure on the Patton Seamount (Gulf of Alaska, USA)" (PDF). The Alaska Fisheries Research Bulletin. 11.
  13. ^ a b Seach, John. "Volcano Live".
Axial Seamount

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.

Brown 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 Seamount

Cobb Seamount is a seamount (underwater volcano) and guyot located 500 km (310 mi) west of Grays Harbor, Washington, United States. Cobb Seamount is one of the seamounts in the Cobb–Eickelberg Seamount chain, a chain of underwater volcanoes created by the Cobb hotspot that terminates near the coast of Alaska. It lies just west of the Cascadia subduction zone, and was discovered in August 1950 by the U.S. Fish and Wildlife Service fisheries research vessel R/V John N. Cobb (FWS 1601). By 1967, over 927 km (576 mi) of soundings and dozens of samples from the seamount had been collected.

Cobb Seamount is geologically interesting for its terraced, pinnacle structure, and its biological community. Like many other seamounts, Cobb Seamount acts as a biological center of diversity, and supports a dense oceanic ecosystem. Relatively convenient access and an interesting biological setting have made the seamount an object of several scientific cruises and dives.

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.

Outline of oceanography

The following outline is provided as an overview of and introduction to Oceanography.

Patton Seamount

Patton Seamount is a prominent seamount (underwater volcano) in the Cobb–Eickelberg Seamount chain in the Gulf of Alaska. Located 166 nmi (307 km; 191 mi) east of Kodiak Island and reaching to within 160 m (520 ft) of the ocean surface, Patton is one of the largest seamounts in the Cobb–Eickelberg Seamount chain. It was originally created near the coast of Oregon by the Cobb hotspot 33 million years ago, and was moved to its present location by tectonic plate movement. Patton is one of the most well-understood seamounts, as a major expedition using DSV Alvin in 1999 and another in 2002 helped define the scope of the seamount's biological community. Like other large seamounts, Patton acts as an ecological hub for sea life. Dives have revealed that the volcano is heavily encrusted in sea life of various forms, including sea stars, corals, king crabs, demersal rockfish, and other species.

Shield volcano

A shield volcano is a type of volcano usually composed almost entirely of fluid lava flows. It is named for its low profile, resembling a warrior's shield lying on the ground. This is caused by the highly fluid (low viscosity) lava erupted, which travels farther than lava erupted from a stratovolcano, and results in the steady accumulation of broad sheets of lava, building up the shield volcano's distinctive form.

Volcanoes of the Galápagos Islands

The Galápagos Islands are an isolated set of volcanoes, consisting of shield volcanoes and lava plateaus, located 1,200 km (746 mi) west of Ecuador. They are driven by the Galápagos hotspot, and are between 4.2 million and 700,000 years of age. The largest island, Isabela, consists of six coalesced shield volcanoes, each delineated by a large summit caldera. Española, the oldest island, and Fernandina, the youngest, are also shield volcanoes, as are most of the other islands in the chain. The Galápagos Islands are perched on a large lava plateau known as the Galápagos Platform, which creates a shallow-water depth of 360 to 900 m (1,181 to 2,953 ft) at the base of the islands, which stretch over a 174 mi (280 km)-long diameter. Since Charles Darwin's famous visit to the islands in 1835, over 60 recorded eruptions have occurred in the islands, from six different shield volcanoes. Of the 21 emergent volcanoes, 13 are considered active.The Galápagos are geologically young for such a big chain, and the pattern of their rift zones follows one of two trends, one north-northwest, and one east-west. The composition of the lavas of the Galápagos shields are strikingly similar to those of the Hawaiian volcanoes. Curiously, they do not form the same volcanic "line" associated with most hotspots. They are not alone in this regard; the Cobb–Eickelberg Seamount chain in the North Pacific is another example of such a delineated chain. In addition, no clear pattern of age is seen between the volcanoes, suggesting a complicated, irregular pattern of creation. How exactly the islands were formed remains a geological mystery, although several theories have been posited.

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