Hollister Ridge

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]