From Wood and Kienle (1990) 
: "Davidof, Khvostof, and nearby small islands Pyramid and Lopy rise 100 m above a submarine platform as the remnants of a collapsed caldera. This 'Aleutian Krakatoa' is thought to have formed during the late Tertiary, but the volcano is essentially unstudied. The islands are covered by vegetation; however lava flows can be recognized on aerial photographs. Lavas and pyroclastic layers form the islands, and rocks on the northern part of Davidof and Lopy Island are intensely hydrothermally altered."