The small, 1.4 x 1.7 km Boisa, or Aris Island consists of a large basaltic-andesitic cone with a summit crater open to the north whose rim extends to the island's east coast. The crater of the basaltic older cone is filled by two steep-sided andesitic lava domes, the highest of which forms the 240-m high point of the island. A third lava dome, separated from the eastern cone by a low saddle, forms a peninsula on the western side of the island. No historical eruptions or present-day thermal activity have been observed, although the last eruption may have been quite recent judging by the youthful morphology of the island (Gust and Johnson, 1981).