Английская Википедия:Hollister Ridge
Шаблон:Short description Шаблон:Infobox seamount
Hollister Ridge is a group of seamounts in the Pacific Ocean. They lie west from the Pacific-Antarctic Ridge and form three ridges that form a line; one of the ridges rises to a depth of Шаблон:Convert and in the past formed an island. The seamounts are composed out of basaltic and other rocks and their ages range from about 2.5 million years ago to latest Pleistocene; an acoustic swarm recorded in the southern Pacific Ocean in 1991-1992 is considered to be the manifestation of a historical eruption of the Hollister Ridge.
The origin of the Hollister Ridge is unclear, with various proposed mechanisms involving the neighbouring Pacific-Antarctic Ridge and crustal weaknesses, but most involve the Louisville hotspot in some way.
History
The ridge was discovered either by gravimetry from satellitesШаблон:Sfn or by the research ship EltaninШаблон:Sfn in 1965 and first named "Hollister Ridge" in a 1995 publication.Шаблон:Sfn Rock samples were taken at the ridge in 1996.Шаблон:Sfn
Geography and geomorphology
The Hollister Ridge is an aseismic ridge in the Pacific Ocean, west of the Pacific-Antarctic Ridge.Шаблон:Sfn It consists of three separate ridges which are lined up in east-southeast to north-northwest direction, starting from the axis of the Pacific-Antarctic Ridge and ending in the direction of the Louisville seamount chain. The eastern ridge is Шаблон:Convert long and rises to a depth of Шаблон:Convert below sea level, the central ridge is Шаблон:Convert long and rises to a depth of Шаблон:Convert below sea level, the western ridge is Шаблон:Convert long and rises to a depth of Шаблон:Convert below sea level.Шаблон:Sfn The central ridge formed an island in the past.Шаблон:Sfn
Geology
The ridge rises from a seafloor whose age decreases from 7-8 to 0-1 million years ago southeastward.Шаблон:Sfn Three fracture zones, the Heezen, Tharp and Hollister fracture zones, extend northwestward across the seafloor northeast of the Hollister Ridge;Шаблон:Sfn at least the first two are considered to be part of the Eltanin fracture zone.Шаблон:Sfn A scarp lies south of the Hollister Ridge,Шаблон:Sfn and even farther south lies the Udintsev fracture zone.Шаблон:Sfn The Pacific-Antarctic Ridge close to the Hollister Ridge is the site of an isolated geoid anomaly which has been interpreted as a product of magmatic upwelling.Шаблон:Sfn
Rocks sampled from the Hollister Ridge have yielded basalts,Шаблон:Sfn alkali basalts, hawaiites, picrites and tholeiitesШаблон:Sfn as well as granites, which are most likely dropstones transported to the ridge by icebergs. The basalts range from aphyric to porphyric and contain phenocrysts of olivine and plagioclase.Шаблон:Sfn
Several mechanisms have been proposed to explain its origin:Шаблон:Sfn
- The ridge may be the present-day location of the Louisville hotspot.Шаблон:SfnШаблон:Sfn Petrological differences between the volcanoes formed by this hotspot and the Hollister Ridge make this hypothesis problematic,Шаблон:SfnШаблон:Sfn as are misfits between the reconstructed path of the Louisville hotspot and the position of the Hollister Ridge.Шаблон:Sfn Even later plate reconstructions have endorsed this model of origin.[1]
- A "mini-hotspot", which however is not consistent with the geometry of the ridge (which is at an angle to the motion of the Pacific Plate).Шаблон:Sfn Such a mini-hotspot may be a branch of the Louisville hotspot.Шаблон:Sfn
- Asthenosphere may be flowing from the Louisville hotspot to the Pacific-Antarctic Ridge.Шаблон:Sfn Seamounts and aseismic ridges have been observed in other regions of the world where such flow is expected to occur.Шаблон:Sfn
- Lineaments in the crust allowed the ascent of magma from the mantle.Шаблон:Sfn Such lineaments may be produced by tectonic stresses related to crustal spreading; this theory is supported by the geometry of the Hollister Ridge and the ages of its components. There may be some influence by the Louisville hotspot.Шаблон:Sfn Pliocene changes in the plate motion patterns of the region may have generated the lineaments.[2]
- One variation of the "lineament" theory posits that the ridge at first was built by magma ascending through crustal weaknesses; later material from the Louisville hotspot flowed south towards the Hollister Ridge and increasingly interacted with the lineament, thus influencing the composition of the ridge rocks.Шаблон:Sfn A change in lithospheric thickness across the Eltanin fracture zone would divert the mantle flow from the Louisville hotspot southward.Шаблон:Sfn
Eruptive history
Argon-argon dating has yielded ages ranging from a mean age of 2.531 ± 0.036 million years ago for the western ridgeШаблон:Sfn over 0.487 ± 0.03 million years ago and 0.343 ± 0.008 million years ago for the eastern ridge to 91,000 ± 12,000 and 0 years ago for the central ridge. This implies that volcanism is still activeШаблон:Sfn at the central ridge, which is also the shallowest sector of the Hollister Ridge.Шаблон:Sfn
There is evidence of historical eruptions at the Hollister Ridge.Шаблон:Sfn Between 10 March 1991 and 12 June 1992 a strong acoustic swarm was recorded in the southern Pacific Ocean from several stations in French PolynesiaШаблон:Sfn and its source identified with a segment of the Hollister Ridge.Шаблон:Sfn Anthropogenic and biological origins were considered unlikely sources for the swarm,Шаблон:Sfn and it is thus interpreted to be a volcanic swarm.[3] The acoustic swarm may have resulted from the interaction between seawater and a subaqueous lava lake;Шаблон:Sfn the acoustic patterns are not consistent with a simple explosive eruption.[3]
References
Sources
- Шаблон:Cite journal
- Шаблон:Cite journal
- Шаблон:Cite journal
- Шаблон:Cite journal
- Шаблон:Cite journal
- Шаблон:Cite journal
