SEAMOUNTS 481 Figure The growth stages of seamounts from (A) early deep water stage, through (B) shoaling stage, showing the development of rift zones, and (C) ocean island stage, including the development of a subaerial shield volcano and mass wasting, to (D) an atoll in the guyot stage within the edifice itself aids the development of rift zones on large seamounts by enabling lateral dikes and flank eruptions in focused zones of weakness Within about km of sea-level, reduced hydrostatic pressure allows exsolution of gas, and explosive eruptions become frequent Lava flows of scoriaceous pillows and lapilli breccia become important lava types Emergent stage At sea-level, eruptions become surtseyan in type, producing fine-grained base surge and air-fall tephra deposits Besides the famous and welldescribed eruption of Surtsey off Iceland (1963– 1967), the only recorded observations during the emergent stage are from Graham Island (1831), Metis Shoal (1968, 1979), Myujin-sho (1952–1953), and Shin-Iwo-Jima (1986) Phreatomagmatic eruptions form tuff rings and cones consisting of easily eroded deposits of loose lapilli and ash If eruptions stop at this stage, wave action will quickly erode the seamount The small islands of Myujin-sho disappeared within months of formation Ocean island stage Once the volcanic vent rises above sea-level, phreatomagmatic eruptions become rare and effusive eruptions resume (Figure 4C) Pahoehoe and aa are the important lava types, and in most cases a lava shield is created from numerous low-relief lava flows Giant landslides may cover the flanks of the island with debris from slumping or debris avalanches Some of the largest landslides on Earth (involving volumes of 5000 km3) have occurred around the Hawaiian Islands (Figure 2), and some studies speculate that up to 50% of the original volume of the older Hawaiian Islands has been lost owing to mass wasting Guyot stage Thermal subsidence and erosion combine to drown all islands after volcanic activity ceases Wave erosion planes off a flat summit plateau, but the sides of the volcano remain steep, creating the distinctive guyot cross-section Atolls (including the Marshall, Tuamotu, Kiribati, and Maldive Islands) are a type of guyot found in tropical oceans, where coral forms a fringing reef around an ocean island If the upward growth of the coral reef keeps pace with the subsidence of the seamount, a ring of coral islands form, encircling a lagoon, and a thick layer of carbonate sediment develops, which caps the old seamount basement rocks (Figure 4D) Why Seamounts Have Flat Tops The characteristic steep-sided flat-topped shape of seamounts is a morphology rarely seen in subaerial volcanoes The flat tops of guyots can be explained by wave erosion, but many seamounts with flat tops have never been near the wave base Several models have been advanced to explain why so many seamounts have a summit plateau or flat-topped shape It is still unclear whether one model can explain all the occurrences of flat-topped volcanoes, or whether each of various models is appropriate under certain conditions In the ring-dike model proposed by Batiza and Vanko, eruptions occur from a series of circumferential dikes encircling the summit area rather than from