The main part of Robinson Crusoe (Mas a Tierra) Island, in the central part of the Juan Fernandez Island group, is composed of four overlapping shield volcanoes with calderas breached to the NE.  The SW part of the basaltic island consists of smaller altered cones.  Potassium-Argon dates for the island range from 5.8 to 3.8 million years ago (Steussy et al., 1984) and the island is more dissected than Alexander Selkirk (Mas a Fuera) Island to the west.  An eruption at El Yunque was reported in 1743, but is considered uncertain (Catalog of Active Volcanoes of the World).  Several reports exist of a submarine eruption about 1.5 km north of the island in 1835 that produced an explosion and flames that lit up the island.