| Hollister Ridge | |
| Coordinates: | -53.998°N -139.845°W |
| Age: | Pliocene-Pleistocene |
| Last Activity: | 1991-1992 |
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 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.
The ridge was discovered either by gravimetry from satellites or by the research ship Eltanin in 1965 and first named "Hollister Ridge" in a 1995 publication. Rock samples were taken at the ridge in 1996.
The Hollister Ridge is an aseismic ridge in the Pacific Ocean, west of the Pacific-Antarctic Ridge. 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 long and rises to a depth of below sea level, the central ridge is long and rises to a depth of below sea level, the western ridge is long and rises to a depth of below sea level. The central ridge formed an island in the past.
The ridge rises from a seafloor whose age decreases from 7-8 to 0-1 million years ago southeastward. Three fracture zones, the Heezen, Tharp and Hollister fracture zones, extend northwestward across the seafloor northeast of the Hollister Ridge; at least the first two are considered to be part of the Eltanin fracture zone. A scarp lies south of the Hollister Ridge, and even farther south lies the Udintsev fracture zone. 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.
Rocks sampled from the Hollister Ridge have yielded basalts, alkali basalts, hawaiites, picrites and tholeiites 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.
Several mechanisms have been proposed to explain its origin:
Argon-argon dating has yielded ages ranging from a mean age of 2.531 ± 0.036 million years ago for the western ridge 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 at the central ridge, which is also the shallowest sector of the Hollister Ridge.
There is evidence of historical eruptions at the Hollister Ridge. 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 and its source identified with a segment of the Hollister Ridge. Anthropogenic and biological origins were considered unlikely sources for the swarm, and it is thus interpreted to be a volcanic swarm. The acoustic swarm may have resulted from the interaction between seawater and a subaqueous lava lake; the acoustic patterns are not consistent with a simple explosive eruption.