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Instituto de Investigação
em Vulcanologia e Avaliação de Riscos
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Referência Bibliográfica

WUNDERMAN, R., BARRIGA, F., NISHIMURA, C., PACHECO, J., VOGT, P., GASPAR, J.L., QUEIROZ, G., SANTOS, R. (2003) - Faults, Post-1720 explosion craters and the remains of a lava lake at Castro Bank Seamount (Azores). Fall 2003 AGU Meeting, 8-12 December 2003 (Comunicação Oral).​


During 25-28 July the US Navy submarine NR-1 dove on the seamount D. João de Castro Bank, compiling reconnaissance sonar and visual data. Castro Bank sits along strike and between the eastern Azorian islands of Terceira and S.Miguel, occupying a seismically active region ~60 km from each of these islands and apparently controlled by the same underlying tectonics as other islands found along the Azores northern margin. Castro Bank’s last recorded eruptions built a ~1 km diameter ephemeral island in the 1720s. The bathymetry of the uppermost 40 m or so of the Bank is rather well known via single beam sonar, scuba diving and AUV surveys (IH, DOP/UA and ISR/IST, unpublished work). Our dives compiled data in concentric rings along contours, collecting side- and forward-looking sonar along an overall track length of ~20 km, with the deepest ring approaching ~200m depth. To document key features we came near the sea floor and took videos with typical visibility of ~10-15 m.
This is the first progress report on our work, which found the edifice morphologically complex and irregular. We noted that the seamount was often covered by aerially extensive yellow-brown hyaloclastic tuffs that were presumably products of the 1720s eruption, but also cut by faults and fissures (with offsets of ten’s of meters) exposing abundant areas of older edifice. The faults typically lacked sediment cover, and in one case very fresh, sediment-free fault trended along the base of a steep cliff. This suggested the faults were much younger than the 1720 eruption, an observation in accord with intense seismicity recorded in this area. The faults provided exposures of older rocks, which included abundant breccias and lesser clearly identified pillows or thick lava flows. The NW quadrant contains two small, shallow, elliptical craters. These lie side-by-side and crosscut inferred 1720s-age tuffs. One crater held a lava lake, the body of which apparently withdrew or subsided slightly, stranding the lake’s chilled upper surface (~20 cm thick). Unsupported from below, that fragile surface has largely broken and dropped on the order of a meter or two. Much of it lies strewn across the crater floor in a pattern of broken, interlocking plates. The lava lake’s upper surface appeared surprisingly clean-devoid of sediments or colonizing organisms. This, taken together with the craters cutting across the inferred 1720s tuffs, suggest that castro Bank underwent post-1720s, and were perhaps deposited in the past few decades (?). In conclusion, we suggest that Castro Bank underwent post 1720s faulting and volcanism. The complexity and difficulty of mapping the seamount suggest multiple studies and techniques will be required to portray its morphology and history.