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The only near-ridge seamount chain off the Gorda Ridge is the President Jackson Seamounts, located on the Pacific Plate. The most easterly volcano is located about 53 kilometers west of the ridge axis. A SeaBeam bathymetric survey imaged most of the chain (R. Embley, written communication, 1985), but poor navigation and incomplete coverage of the volcanoes limit the usefulness of the survey. A sidescan survey with the GLORIA system (EEZ-Scan 84 Scientific Staff, 1986; Masson et al., 1988) further established the rough sizes and locations of the volcanoes. A sampling cruise by the U. S. Geological Survey (Clague and Holmes, 1987) recovered pillow fragments and hyaloclastite from four volcanoes. These samples consist entirely of N-MORB that is generally more primitive than that erupted along the nearby Gorda Ridge (Davis and Clague, 2000). MBARI recently did two dives on the cluster of cones southeast of the main chain of central volcanoes. The following descriptions are summarized from Clague et al. (2000), who analyzed the Simrad data presented here in detail.
The chain consists of eight volcanoes, of which four are isolated volcanoes, less than 10 kilometers in diameter and nearly circular in plan view. The other four form two morphologically complex, coalesced flat-topped structures. In addition, a transit line to our survey crossed an elevated hummocky area of volcanic cones and flows about 13 kilometers southeast of the southeastern central volcano. The volcanoes form a linear chain and are all relatively small, with an average volume of 47±14 km3 and a range of 24 to 68 km3. The summits of all but the second from the northwestern end are flat, and most are modified by nested or crosscutting calderas and smaller pit craters (29 in total), some of which embay the flanks of the volcanoes. In many cases the younger calderas have removed all but small remnants of the earlier crater walls. The third volcano from the southeast has a trapdoor caldera whose floor slopes down towards the southeast. The flanks and tops of many of the central volcanoes have numerous small cones. Large hummocky aprons, probably of debris, extend away from several volcanoes. The flanks of several volcanoes are also fluted or embayed by erosional gullies or landslide chutes.
|Index Map||Axial Volcano||Gorda Ridge||Cleft Segment||Taney Seamounts|
|Guide, Gumdrop, Pioneer Seamounts||Mendocino Fracture Zone||President Jackson Seamount||Davidson Seamount||Vance Seamounts|