HUNDREDS OF SMALL VOLCANOS ON THE MEDIAN VALLEY FLOOR OF THE MID-ATLANTIC RIDGE AT 24-30-DEGREES-N

WE have identified 481 seamounts from ∼6,000 km2 of Sea Beam swaths of the median valley floor of the Mid-Atlantic Ridge (MAR) between 24 and 30°N. We find that seamount abundances are an order of magnitude larger (on average, 80 per 1,000 km2 with relief above 50 m) than estimated in previous studies along mid-ocean ridges in the Pacific1 and South Atlantic2, suggesting that at this section of the MAR, seamount volcanism plays an important part in the formation of new oceanic lithosphere. The neovolcanic ridge, often observed on the inner valley floor3, appears to be constructed totally of individual coalesced seamounts, so that our counts represent a minimum estimate. The frequency distribution of summit heights of the MAR seamounts is consistent with the exponential model describing the heights of off-axis seamounts in the eastern Pacific4-6, but with different parameters. Such a distribution of heights may be a fundamental aspect of seamount volcanism, related to lithospheric properties and/or magmatic plumbing. Seamount-dominated axial volcanism must create a crustal structure very different from classic fissure-fed models. © 1990 Nature Publishing Group.