Louisville Ridge

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. As one of the longest seamount chains on Earth it stretches some 4300 km 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 chains formation is best explained by movement of the Pacific Plate over the Louisville hotspot although others had suggested 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 assigned 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 and ages become younger in a non linear fashion towards the south east 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 hot spot 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 timing of such bends as mismatchs 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 say increase potential for sudden release. Further a postulated historic change in 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 north-west of the Osbourn Seamount has been interpreted as the currently subducting portion of the Louisville chain, but this continuation is not aligned with the existent 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: