Geomorphology of Two Seamounts Offshore Ascension Island, South Atlantic Ocean

摘要

Ascension Island is an intra-plate volcanic island in the South Atlantic Ocean, located 80 kilometers west of the Mid- Atlantic Ridge (MAR) and 50 kilometers 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 kilometer basal diameter lies in 3200 meters 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 meters of the ocean surface and deepens to 3800 meters. Its basal diameter is approximately 16 kilometers 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.