Geography of North Arch volcanic field

Map of the North Arch volcanic field, with Oahu and sampling sites marked
270km 168miles South Arch volcanic field North Arch volcanic field
Coordinates	24°N158°W / 24°N 158°W /24; -158[1]
Area
• Total	25,000 km2 (9,700 sq mi)
• Land	0%
• Water	100%
Highest point	3,900 m (12,800 ft) below sealevel[2]
Lowest point	4,700 m (15,400 ft) below sealevel[3]
Terrain	Basalt flow with other volcanic features such ascones

North Arch volcanic field^[a] is anunderwatervolcanic field north ofOahu,Hawaii. It covers an area of about 25,000 square kilometres (9,700 sq mi) and consists of large expanses ofalkali basalt,basanite andnephelinite that form extensivelava flows andvolcanic cones. Some lava flows are longer than 100 kilometres (62 mi).

This volcanic field appears to be somehow related to theHawaii hotspot, although the exact mechanisms are debated. Similar volcanic units are also found on the adjacent islands, such as theHonolulu Volcanics onOahu. The volcanic field was formed througheffusive andexplosive eruptions between 1.5 and 0.5 million years ago, although eruptions before and after these dates also took place.

The North Arch volcanic field lies 200–400 kilometres (120–250 mi)^[5] north ofOahu at 3,900–4,380 metres (12,800–14,370 ft) depth;^[2] within theExclusive Economic Zone of theHawaiian Islands. The existence of lava flows in this geological region was already postulated in 1964, but it wasGLORIA sidescan sonar eventually identified an anomalously reflective seafloor that constitutes the North Arch volcanic field;^[6] the discovery was first published in 1968^[7] and rock samples taken in 1988.^[6] The volcanic field lies on theHawaiian Arch^[b][2] and on its southern slope.^[10]

The volcanic field covers an area of about 25,000 square kilometres (9,700 sq mi)^[2][c] and consists of about 100 observedvolcanic cones with associatedlava flows. The volcanic cones range from shield-like to steep^[11] tolava dome-like and long ridges^[12] andlava flows range fromlava lakes forming flat-topped cones^[10] to over 100 kilometres (62 mi) long lava flows despite the flat terrain;^[11] small hills in the lava flows may bevents.^[8] The average thickness of the lava flows probably amounts to either less than 10 metres (33 ft)^[13] or about 40–50 metres (130–160 ft).^[14]Fissure vents also occur in the field; one of these has been the source of a 3,600 square kilometres (1,400 sq mi) lava flow.^[15] Owing to the sizes of the lava flows, the field has also been called aflood basalt in analogy to its usually much larger continent-based counterparts.^[4] The volcanoes are located amongCretaceousseamounts and sediment-covered seafloor, which formskipukas between the lava flows.^[16] In turn, lava flows are covered with over 1 metre (3 ft 3 in) thick sediments.^[8] A noticeable swell where clusters of volcanic cones occur may reflect the presence ofsills.^[10] The total volume of the volcanic field is about 1,250–1,000 cubic kilometres (300–240 cu mi),^[16] much less than a typical Hawaiianshield stage volcano,^[17] and rocks occur in the form of e.g. glassshards,pillow lavas,scoria andvolcaniclastic material.^[11] Aseismic velocity anomaly at depth appears to be associated with the North Arch volcanic field.^[18][19]

The bulk of the volcanism of aHawaiian volcano is made up by theshield stage during whichtholeiitic lavas are erupted.^[20] This stage is after between 250,000 – 2,500,000 years followed by a "post-erosional" or "rejuvenated" stage during whichlavas ofalkali basalt,melilitite andnephelinite composition; such volcanism has been observed onKoolau,Kauai andNiihau^[2] and covers large parts of the Hawaiian Islands even though it makes up less than one percent of their volume.^[1] The North Arch volcanic rocks resemble the composition of theserejuvenated stage volcanics of Hawaii such as theHonolulu Volcanics,^[21] and another unit of these volcanics forms theSouth Arch volcanic field^[20] 200 kilometres (120 mi) south ofKilauea,^[22] and have been deemed "peripheral lavas" of theHawaii hotspot.^[23] In fact, it has been proposed that the rejuvenated volcanics of Niihau, Kauai, Oahu andMolokai might actually be part of the North Arch volcanic field.^[24] Other volcanic units of a probably similar nature have been found betweenOahu andKauai.^[25]

While theshield stage volcanics of Hawaii are linked to themantle plume, the origin of volcanism in the North Arch volcanic field is debated:^[1]

• Low-degree melting ofgarnetlherzolite.^[1]
• The flexure of the crust under the weight of the Hawaiian volcanoes forms the Hawaiian Arch, where the crust breaks up and melts can concentrate; this would provide both pathways and sources formagma to ascend to the surface.^[26]
• Magma ascent along fissures associated with theMolokai fracture zone.^[13][27]
• Changes inlithosphere thickness associated with the Molokai Fracture Zone.^[13][27]
• Development and spreading of a refractory mantle root underneath theHawaii hotspot.^[28]
• Secondaryconvection away from the mainmantle plume.^[29]

The volcanism of the North Arch volcanic field might nevertheless have a common cause as therejuvenated stage volcanism of Hawaiian Islands.^[13] Volcanic units interpreted similarly to the North Arch volcanic field are also known fromKerguelen.^[30]

Samples taken from the North Arch volcanic field are fine grained or glassy^[31] with varying quantities of vesicles^[32] and consist ofalkali basalt,basanite^[31] andnephelinite,^[21] with variablechlorine,magnesium,potassium andsulfur contents.^[33]Olivine is the most importantphenocryst, althoughclinopyroxene^[34] andspinel are also found.^[35] This diverse composition stems from variable melting from a common source rock with subsequent fractionation ofolivine^[36] andgarnet;^[37] the source rock appears to be ultimately a product of bothmantle andmantle plume components.^[38]Hyaloclastite is also found;^[8]clay andpalagonite found in the field may have formed from volcanic ash^[39] through e.g.hydrothermal alteration^[32] andmanganese oxide crusts cover many flows.^[8]

Volcanism on the North Arch volcanic field occurred during thePlio-Pleistocene^[16] between 1.5 million years ago^[d] and less than 500,000 years ago, at the same time as theHonolulu Volcanics^[1] although even younger flows have been found.^[15] The timing of volcanic activity has been inferred from the thicknesses ofpalagonite and sediment layers as well as the stratigraphy^[8] and overlaps with rejuvenated volcanism onOahu,Kauai andNiihau.^[9] In contrast, theSouth Arch volcanic field was emplaced in the last 10,000 years.^[41]

Apparently the central part of the field was emplaced first, with the surrounding parts forming later; in addition, lava flows which make up the surrounding parts appear to preferentially originate at the foot ofCretaceous ridges and seamounts;^[42] perhaps these ridges and seamounts can act as pathways for magma to reach the surface.^[43] Total magma production is about 0.001 cubic kilometres per year (0.00024 cu mi/a).^[44]

The North Arch volcanic field has been the site of voluminouslava effusion^[10] and the formation of shallowmagma chambers which gave rise topit craters when they collapsed.^[15] Despite the great depth of the North Arch volcanic field, traces ofexplosive eruptions has been found there^[16] such as steep and low volcanic cones and deposits fromfire fountaining;^[45] the formation of bubbles from volatile substances in themagma^[45] and processes involvingsupercritical fluids and magma-water interaction are probably involved in the explosive volcanism there.^[46]

• Pliocene volcanoes
• Pleistocene volcanoes

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