Hollister Ridge

The ridge was discovered either by gravimetry from satellites[2] or by the research ship Eltanin[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 an aseismic ridge in the Pacific Ocean, west of the Pacific-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 the Louisville 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] Three fracture 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 the Eltanin fracture zone.[7] A scarp 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 isolated geoid anomaly which has been interpreted as a product of magmatic upwelling.[10]

Rocks sampled from the Hollister Ridge have yielded basalts,[6] alkali basalts, hawaiites, picrites and tholeiites[11] 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.[6]

Several mechanisms have been proposed to explain its origin:[2]

• The ridge may be the present-day location of the Louisville 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 the Pacific 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 the crust allowed the ascent of magma from the mantle.[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 in lithospheric 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 strong acoustic swarm was recorded in the southern Pacific Ocean from several stations in French 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 subaqueous lava lake;[27] the acoustic patterns are not consistent with a simple explosive eruption.[1]