The Louisville Ridge, often now referred to as the Louisville Seamount Chain, is an underwater chain of over 70 seamounts located in the Southwest portion of the Pacific Ocean. One of the longest seamount chains on Earth, it stretches some 4,300 km (2,700 mi) from the Pacific-Antarctic Ridge northwest to the Tonga-Kermadec Trench, where it subducts under the Indo-Australian Plate as part of the Pacific Plate. The chain's formation is best explained by movement of the Pacific Plate over the Louisville hotspot, although others have suggested that it formed by leakage of magma from the shallow mantle up through the Eltanin fracture zone, which it follows closely for some of its course. Depth-sounding data first revealed existence consistent with a seamount chain in 1972, although some of the seamounts had been described as a ridge in 1964 linked to the Eltanin fracture zone system, hence the name.
Geology
The oldest volcanic rocks of the chain come from Osbourn Seamount at 78.8 ± 1.3 Ma; ages become younger in a nonlinear fashion towards the southeast, with a youngest age of 1.1 Ma. Composition studies of the erupted dominantly alkali basalt are consistent with a single Louisville mantle source distinct from other hotspots, and the composition has remained homogeneous over at least the last 70 million years. In the past 25 million years, magma upwelling rates may have decreased. There is almost certainly a deep plume origin to the hotspot. The Louisville hotspot chain passes through the western and eastern branches of the Wishbone scarp, and while the seamounts show no compositional change as they cross the scarps, the East Wishbone scarp crossing point is associated with a distinct decrease in the volume of the younger seamount eruptives from that point east into the Pacific Plate.
Tectonics Volcanic hotspot chains are used to suggest the net movements of tectonic plates, and so in the case of the large Pacific Plate, validation of models of its movement and indeed the hotspot hypothesis itself relies on data from several hot spot chains. As well as the Louisville hotspot, there is data over tens of millions of years available from the movements of the Hawaii hotspot and the Arago hotspot. While the model of Pacific Plate movement, including bends in the hotspot track, can be made to fit very well, there has been long debate on the timings of such bends as mismatches of a few million years appeared to exist.
Subduction The area of subduction of the Louisville chain into the Tonga Trench is associated with a relative seismic gap beneath the Tonga forearc. This implies that the subduction of the volcanoes compared to normal sediment has a significant impact in terms of normal relief of stress, but it is unclear if the subducted volcanoes relieve it as suggested by some or increase the potential for sudden release. Further, a postulated historic change in the trend of the subducted Louisville chain compared to present is backed up by compositional analysis of more recent arc volcanism as the volcanics from the Louisville chain are recycled. A bathymetric high c. 2 km (1.2 mi) northwest of the Osbourn Seamount has been interpreted as the currently subducting portion of the Louisville chain, but this continuation is not aligned with the extant chain.
Ecology Some of the seamounts are known coral reef stoney habitats, with typical species including the coral Solenosmilia variabilis, brisingid starfishes (Order Brisingida), and sea-lilies and feather stars (Class Crinoidea). They can be a fishery resource for species such as the orange roughy (Hoplostethus atlanticus) that can be fished by bottom trawling.
Seamounts The Louisville Ridge includes the following:
See also Louisville hotspot Hollister Ridge Hotspot (geology) Zealandia
References
Sources
External links Expedition 330 - Louisville Seamount Trail, Integrated Ocean Drilling Program, 13 December 2010 to 11 February 2011 The Louisville Ridge – Tonga Trench collision: Implications for subduction zone dynamics, RV Sonne Research Expedition SO215 Cruise Report, 25 April 2011 to 11 June 2011