Bare slopes and landslide scarps indicate high rates of soil erosion. A layer of reddish silt is observed along the coast of Bakkao. The dominant land cover is grass, which covers about 81 hectares of the island's total land area. Groundwater is available both in the volcanic substrate as well as in the coralline sand flatlands. Water in the volcanic rocks is available in the weathered portion.
Water stored in the fractures of the basalt apparently recharges water along the coastal area.
With an estimated land area of 76 hectares, Boan is the second largest island in the Turtle Islands Group. It has a low relief with topography varying from flat to low hills at the northeastern end. The substrate of the island is sandstone. Continued extrusion of mud by diapiric activity built up the hill, though residents report that the outflow of mud has ceased for a number of years now. The flat part of the island is made up of an accumulation of coralline sand, coralline rubbles and some sandstone boulders.