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. The coastline of Boan is dominated by a rocky and shingles beach made up of coralline rubbles. A small patch of mangrove is located at the northeastern side. The sandy part of the coastline is now occupied by settlements.
Shoreline retreat is evident in the toppled coconut trees at the eastern part.
Scarp also rims this part of the island. To protect houses from shore erosion, fences of round logs have been set up in some parts of the sandy beach. Several berms made up of corraline rubble have been mapped at the northeastern part. The land cover is dominantly agriculture in the wooded areas in the northern part of the island. The flat part is made up of an accumulation of coralline sand, coralline rubbles and some sandstone boulders.