482 SEAMOUNTS a central feeder vent The steep slopes and flat summits of the Galapagos volcanoes, which essentially have the same form as flat-topped seamounts, are attributed to lateral extension by ring-dike intrusion Circumferential fractures observed in the summit areas of some near-ridge seamounts support the ring-dike model Ring dikes are thought to form when magma stalls within the crust, creating a small laccolith Ring dikes shoot upwards from the edge of the laccolith as it inflates A variation on this model proposes dipping cone sheets instead of more vertical ring dikes as the intrusive body In the caldera-fill model proposed by Clague and others, large calderas form early in seamount growth and are filled by later eruptions A repeated sequence of caldera collapse and re-filling allows the seamount to grow upwards and outwards while maintaining a flat top This model is based on new high-resolution mapping that suggests that lava shields (erupted from central vents) formed on the summits of flat-topped volcanoes in the north-eastern Pacific In the neutral-buoyancy model proposed by Barone and Ryan, low-density basaltic crust counterbalances the buoyancy of the magma so that eruptions will rise only to a limited height Seamounts will grow only to the height that balances the magma pressure in their magma chambers and will then grow laterally instead of upwards, forming a flat top Other studies, however, have questioned why this model would lead to circular volcano outlines and what continues to drive eruptions once the critical height is reached Broader Effects of Seamounts seamount chains and aseismic ridges coincides with a reduced dip angle in the subducting plate, for example in the case of the Juan Fernandez chain and the flat slab in northern Chile Hydrothermal Circulation Active seamounts may host high-temperature focused hydrothermal flow The Loihi seamount in the Hawaiian chain hosts high-temperature hydrothermal vents within its summit caldera, which are very similar to vents found at mid-ocean ridges (Figure 2) Most seamounts have neither the magma supply volume nor the longevity of Loihi, but sulphide hydrothermal deposits have been found on the summits of several other seamounts during submersible dives These hydrothermal deposits indicate that seamount and mid-ocean ridge hydrothermal systems are broadly similar and contribute the same ions to seawater Possibly more significant is the role that inactive seamounts play as basement outcrops that provide easy paths for the escape of fluid and heat Oceanographic Circulation The topography created by seamounts has a large effect on the local water masses, primarily generating upwelling and enhancing currents Anticyclonic circulation over the tops of seamounts creates a socalled ‘cold dome’ as a result of enhanced upwelling along the sides of the edifice Also, long aseismic ridges and seamount chains are barriers to deep- and midwater circulation Aseismic ridges standing 2–4 km above the seafloor and stretching for hundreds of kilometres across the Pacific deflect north–south currents at depth Subduction Asperities Seamounts being subducted erode the fore arc and may also cause erosion at the base of the overriding plate Huge furrows, mass wasting, and structural disruption of the fore arc are the effects attributed to seamount subduction (Figure 5) There are many areas that exhibit the morphological signs of seamount subduction, but proving the existence of buried seamounts depends on interpreting seismic and magnetic data However, seamounts being subducted are sometimes directly visible, for example Daiichi-Kashima Guyot in the Japan Trench and the Bannock (Seamount) Structure in the Mediterranean Seamount subduction may also be responsible for seismic asperities on the subduction thrust-fault interface Seamount tunnelling, the process by which a seamount going down on the subducting plate scrapes material off the overriding plate, has been inferred to have an important role in crustal development and fluid flow in the margin The subduction of large Critical Habitat Seamounts play a key role in ocean environments by providing habitats for fishes and suspension feeders The enhanced upwelling combined with the potential for a reef environment in very shallow seamounts make large seamounts sites of high primary productivity Seamounts have been known as productive fishing grounds for centuries, but their role in oceanic biodiversity has been appreciated only in the last 50 years Seamounts host a relatively large percentage (estimated at 15–35%) of endemic species and may be important sites of speciation for deep-sea fauna Seamounts in the south-west Pacific show highly localized species distributions and fairly limited recruitment among seamounts, even between closely spaced seamounts The formation of atolls by coral growth on subsiding seamounts creates an important habitat niche for filter feeders and reef fishes Finally, many humans find ocean islands and