Hollister Ridge is a group ofseamounts in thePacific Ocean. They lie west from thePacific-Antarctic Ridge and form threeridges that form a line; one of the ridges rises to a depth of 100 metres (330 ft) and in the past formed an island. The seamounts are composed out ofbasaltic and other rocks and their ages range from about 2.5 million years ago to latestPleistocene; an acoustic swarm recorded in the southernPacific 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 andcrustal weaknesses, but most involve theLouisville hotspot in some way.
The ridge was discovered either bygravimetry fromsatellites[2] or by the research shipEltanin[3] in 1965 and first named "Hollister Ridge" in a 1995 publication.[4]Rock samples were taken at the ridge in 1996.[2]
The Hollister Ridge is anaseismic ridge in thePacific Ocean, west of thePacific-Antarctic Ridge.[2] 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 theLouisville seamount chain. The eastern ridge is 70 kilometres (43 mi) long and rises to a depth of 1,400 metres (4,600 ft) below sea level, the central ridge is 207 kilometres (129 mi) long and rises to a depth of 100 metres (330 ft) below sea level, the western ridge is 50 kilometres (31 mi) long and rises to a depth of 1,500 metres (4,900 ft) below sea level.[5] The central ridge formed an island in the past.[6]
The ridge rises from a seafloor whose age decreases from 7-8 to 0-1 million years ago southeastward.[5] Threefracture zones, the Heezen, Tharp and Hollister fracture zones, extend northwestward across the seafloor northeast of the Hollister Ridge;[6] at least the first two are considered to be part of theEltanin fracture zone.[7] Ascarp lies south of the Hollister Ridge,[8] and even farther south lies the Udintsev fracture zone.[9] The Pacific-Antarctic Ridge close to the Hollister Ridge is the site of an isolatedgeoid anomaly which has been interpreted as a product ofmagmaticupwelling.[10]
Several mechanisms have been proposed to explain its origin:[2]
The ridge may be the present-day location of theLouisville hotspot.[12][2] Petrological differences between the volcanoes formed by this hotspot and the Hollister Ridge make this hypothesis problematic,[13][12] as are misfits between the reconstructed path of the Louisville hotspot and the position of the Hollister Ridge.[14] Even later plate reconstructions have endorsed this model of origin.[15]
A "mini-hotspot", which however is not consistent with the geometry of the ridge (which is at an angle to the motion of thePacific Plate).[8] Such a mini-hotspot may be a branch of the Louisville hotspot.[16]
Asthenosphere may be flowing from the Louisville hotspot to the Pacific-Antarctic Ridge.[2] Seamounts and aseismic ridges have been observed in other regions of the world where such flow is expected to occur.[9]
Lineaments in thecrust allowed the ascent ofmagma from themantle.[17] 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.[18]Pliocene changes in the plate motion patterns of the region may have generated the lineaments.[19]
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.[20] A change inlithospheric thickness across the Eltanin fracture zone would divert the mantle flow from the Louisville hotspot southward.[21]
Argon-argon dating has yielded ages ranging from a mean age of 2.531 ± 0.036 million years ago for the western ridge[22] 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 active[23] at the central ridge, which is also the shallowest sector of the Hollister Ridge.[22]
There is evidence of historical eruptions at the Hollister Ridge.[14] Between 10 March 1991 and 12 June 1992 a strongacoustic swarm was recorded in the southernPacific Ocean from several stations inFrench Polynesia[24] and its source identified with a segment of the Hollister Ridge.[25]Anthropogenic and biological origins were considered unlikely sources for the swarm,[26] and it is thus interpreted to be a volcanic swarm.[1] The acoustic swarm may have resulted from the interaction between seawater and a subaqueouslava lake;[27] the acoustic patterns are not consistent with a simpleexplosive eruption.[1]
Castillo, Paterno R.; Natland, James H.; Niu, Yaoling; Lonsdale, Peter F. (January 1998). "Sr, Nd and Pb isotopic variation along the Pacific–Antarctic risecrest, 53–57°S: Implications for the composition and dynamics of the South Pacific upper mantle".Earth and Planetary Science Letters.154 (1–4):109–125.Bibcode:1998E&PSL.154..109C.CiteSeerX10.1.1.331.8707.doi:10.1016/S0012-821X(97)00172-6.ISSN0012-821X.
Géli, Louis; Aslanian, Daniel; Olivet, Jean-Louis; Vlastelic, Ivan; Dosso, Laure; Guillou, Hervé; Bougault, Henri (December 1998). "Location of Louisville hotspot and origin of Hollister Ridge: geophysical constraints".Earth and Planetary Science Letters.164 (1–2):31–40.Bibcode:1998E&PSL.164...31G.doi:10.1016/S0012-821X(98)00217-9.ISSN0012-821X.
Vlastelic, I.; Dosso, L.; Guillou, H.; Bougault, H.; Geli, L.; Etoubleau, J.; Joron, J.L. (August 1998). "Geochemistry of the Hollister Ridge: relation with the Louisville hotspot and the Pacific–Antarctic Ridge".Earth and Planetary Science Letters.160 (3–4):777–793.Bibcode:1998E&PSL.160..777V.doi:10.1016/S0012-821X(98)00127-7.ISSN0012-821X.
