The marine geomorphology of American Samoa : shapes and distributions of deep sea volcanics

Abstract

The geologic processes at work in American Samoa have long been a point of scientific debate. Of its numerous volcanic formations, few breach sea level, leaving an enormous proportion of their mass unavailable to traditional observation. This study aims to describe the deep sea geomorphology of American Samoa through compilation, quantitative analysis, and qualitative interpretation of multibeam bathymetry datasets in an effort to contribute a new perspective on volcanic origins. Compilation of multibeam bathymetry datasets collected by various primary sources over the last two and half decades is accomplished using the multibeam processing software package MB-System by Caress and Chayes (1996). The high-resolution product is then employed to measure shape parameters of small seamounts (height < 1,000 m). Methods of quantitative analysis established by Jordan et al. (1983) and Smith (1988) are then used to assess the geomorphologic implications of shape parameter relationships. These relationships suggest that morphologies of small seamounts in American Samoa are typical of Pacific seamounts, though infrequent departures show forms indicative of mid-ocean ridge type magmas. A distribution analysis of small seamounts follows, calculating areal density with the exponential distribution model conceived by Jordan et al. (1983) and modified by Smith and Jordan (1988). Distribution analysis yields a predicted density of 2.8 seamounts per 1,000 km2 and a characteristic height of 139 m, both within expectations for seamounts in the Pacific. Finally, a qualitative interpretation of the entire study area is undertaken that includes mapping of major volcanic features, morphologic descriptions of large seamounts, and considerations of age-progression based on arrangements of volcanic lineaments.