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They comprise mainly basaltic rocks that are geochemically distinct from basalts sampled at extensional (e.g. The flanks of intra-plate oceanic islands and seamounts in the Atlantic and Pacific oceans. Indeed, the distribution raises important scientific questions about Earth’s ‘magmatic pulse’ and the origin of intra-plate volcanism, as well as societal questions about the role that seafloor volcanoes play in navigation, fisheries and geohazards.įigure 3. 3), the spatial extent of magmatic activity on Earth changes even more dramatically. Here I show that when ‘seamounts’ are added into the mix, most of which are also volcanic in origin (Fig. (Data sources: Nunn, Ocean Islands 1994 Goldberg, Atoll Res. c) Seamounts (filled red circles) with height above seafloor that is the same or greater than the height of Ben Nevis above sea-level, the highest mountain peak in the UK (1,344 m). Note: distribution of guyots is incomplete and probably more extensive than shown. Guyots (unfilled circles) were once islands. b) Ocean islands (filled blue circles), atolls (×). a) Holocene to Recent volcanoes (filled red triangles), according to the Smithsonian Global Volcanism Program ( ), compared to boundaries of major plates (blue, subduction zones orange, mid-ocean ridges black, transform/strike-slip faults). Global distribution of volcanoes, ocean islands and seamounts. Yet, the large majority of guyots and atolls are located in the interior of plates, far from plate boundaries (Fig. the circum-Pacific) and extensional plate boundaries, where the plates are moving apart (e.g. The Smithsonian Global Volcano Program (SGVP), for example, lists 1,535 volcanoes that have been active since the Holocene, the large majority of which are associated with compressional plate boundaries, where one plate is underthrust by another (e.g. While we now attribute the subsidence of guyots and atolls to sinking of the oceanic plate as it ages and cools, their spatial distribution raises questions about the origin of volcanic activity on Earth. 47 are active volcanoes and approximately 439 are atolls (Goldberg, Atoll Research Bulletin 2016), which Charles Darwin hypothesized in 1842 comprise coral reefs that had grown upwards on the summit of volcanoes as they subsided below sea level. Science 1946), and considered them as volcanic oceanic islands that had been wave trimmed prior to subsidence below sea level.Īfter more than three centuries of discovery on sailing ships, we know there are 1,770 ocean islands (all but one was discovered by 1840). He named them guyots, in honour of the Swiss born geographer and Princeton Professor, Arnold H. The Princeton academic, Harry Hess, who had been given command of the troop-carrying ship USS Cape Johnston, for example, used a PDR to chart 160 flat-topped bathymetric features in the Pacific Ocean, rising up to 4.5 km above the seafloor.
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Arguably the most important was the Precision Depth Recorder (PDR), which used a hull-mounted acoustic transducer/receiver to continuously measure two-way reflection time and hence depth. The development of new technologies during World War II dramatically altered this view. Abyssal plains (‘deeps’) correspond to depths >3,000 fathoms (5,486 m). Constructed from ~3,200 soundings made with pre-stressed hemp rope and lead weights on British and other survey ships. Sir John Murray and his ‘bathymetrical chart’ of the Atlantic, western Indian and eastern Pacific oceans published in 1912. The profiles showed, however, that apart from the prominence of a few widely scattered islands such as the Azores, the seafloor of the oceans was smooth and featureless-a view that persisted for about the next four decades.įigure 1. Compiled from lead-line surveys during expeditions such as Challenger and Michael Sars, the coloured contour map revealed for the first time the nature of Earth’s surface beneath the oceans and the outline of the continental margins, the Mid-Atlantic Ridge and the intervening abyssal plains (Fig. It has been more than one hundred years since the publication of Sir John Murray’s ‘bathymetrical chart’ of the Atlantic Ocean.
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Watts, A., Science, seamounts and society. In reviewing the distribution and formation of seamounts, Tony Watts highlights the societal implications of these abundant oceanic features and the urgent need for more seafloor mapping