Sea MARC 1 side-scan sonar imaging near the East Pacific Rise



Six seamounts and many small constructional volcanic cones were recently investigated during a combined Sea MARC 1 and Seabeam survey of the East Pacific Rise (EPR) axis between 9.5°N and 13°N as part of an overall program funded by the National Science Foundation designed to characterize the along-strike morphology and structure of the axis of the EPR [Rise Axis Tectonic Team, 1983]. The larger seamounts studied fall into two principal categories: those that exhibit purely constructional volcanic terrain and those where the summits and flanks show significant structural and erosional features such as summit calderas (cover photo) and collapse craters and edifice flanks which have been extensively modified owing to mass-wasting processes (Figure 1). The seamounts studied include three volcanos just west of the EPR axial graben at 9°52′, which form a linear group that trends N60°W. The flanks and summits of these seamounts were insonified on overlapping traverses by using Sea MARC 1 side-scan sonar and Seabeam, multi-beam echosounding. Sea MARC 1 is a midrange side-scan system developed by International Submarine Technology of Redmond, Washington, and Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York. It can insonify swaths of seafloor as great as 5 km (full width) and depicts seafloor morphology and structure as small as a few meters with great accuracy. The resulting bathymetric base a n d sonar images have clearly identified an evolutionary pattern in the progressive structural and morphological development of these seamounts as one moves away from the EPR axis, with the details of the sonar records having important implications for constructional volcanism on the flank of the EPR.