Geomorphology of two seamounts offshore Ascension Island, South Atlantic Ocean

Ascension Island is an intra-plate volcanic island in the South Atlantic Ocean, located 80 km west of the Mid-Atlantic Ridge (MAR) and 50 km south of the Ascension Fracture zone. The onshore lithologies show a magma chamber evolution from basic to felsic and physical textures suggest deposits erupted within the past several hundred years. The subaerial portion of the island accounts for less than 1% of the volcano´s total volume and the 60 km basal diameter lies in 3200 m water depth. It lies on a 5- to 6-million-year-old portion of the South American Plate, between magnetic anomalies 3 and 4. The volcano emplacement may have caused significant crustal loading. Ascension Island´s offshore geology has not been studied in much detail, in part because of its remoteness. During a July 2002 engineering survey, high-resolution data were acquired for portions of the island´s eastern slope, which includes two seamounts. These seamounts have significantly different geomorphology. A prominent northwest-trending ridge deepens from the Ascension volcano to the slopes of what was regarded as the northern seamount for the duration of the survey. The seamount rises to within 800 m of the ocean surface and deepens to 3800 m. Its basal diameter is approximately 16 km and it is nearly conical in relief. Geomorphic features suggest it is the result of several flank eruptions. Detailed imagery delineates areas of exposed rock, rock debris, channelized deposits, and sediment ponds. Channel systems, probably created by slope failures, emanate near the central portion of the seamount, forming a radial pattern. Steep slopes outline numerous semi-circular terraces, giving the seamount a stepped appearance. These terraces appear to truncate against the northwest trending ridge, suggesting that seamount eruptions postdate a majority of the Ascension eruptions. The ridge is likely an extension of a regional northwest trending structure, a possible conduit from the Ascension mantle chamber t- > - > o the seamount. This shoal does not appear to be a seamount by definition, but rather an eruptive center on the flanks of the Ascension volcano. Along the southern slope north-south trending ridges appear at the base of the Ascension volcano. These structures probably formed as a result of MAR extensional tectonics or by dike intrusion that accommodated extension and crustal loading. Graben-like depressions between these structures suggest that they are fault controlled. High-resolution acoustic imagery suggests a relationship between the north-south trending structures and a prominent northeast-southwest elongate seamount. The base of the seamount is in 3200 m water depth, it rises to 1540 m, and its dimensions are 9 km by 19 km. Geomorphology suggests that this seamount is a Pele/spl acute/ean dome. It is relatively flat-topped, covered locally by sediment, and has a central craggy ridge trending the length of the dome. A debris apron flanks its steep, rocky slopes and there are indications that pyroclastic flows may have emanated from the northern side. The dome probably erupted from one of the north-south trending structures.