Origin of Some Flat-Topped Volcanoes and Guyots

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Abstract
Many ancient ring complexes give evidence of having fed surface flows from rows of circumferential vents around a broad circle many kilometers in diameter. The volcanoes of the Galápagos Islands provide contemporary illustrations of this process. Whenever flows from these vents are viscous or small in volume, the circumferential feeder zone will grow vertically at the expense of the outer flanks. The area within this zone, however, being limited in extent and receiving flows from all sides, will soon fill and grow vertically along with the circumferential zone. The result should be a flat-topped volcano. The Galápagos volcanoes have gently sloping outer flanks which steepen to 35° approaching the circumferential vent zone, but the zone itself is nearly horizontal as the full top appears to have been before the development of central calderas. Filling of calderas in the waning stages of volcano growth may then return the flat-topped morphology. The shapes of several Galápagos volcanoes are strikingly similar to those of many flat-topped seamounts on the ocean floor, and there seem to be no obvious reasons why the processes that built the Galápagos volcanoes could not operate below sea level as well as above. Wave base truncation and subsidence are clearly responsible for many flat-topped seamounts, or guyots, but others may simply exhibit primary shapes controlled by circumferential vents during growth of the volcano.

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