Henderson Island is a raised coral atoll. With Pitcairn, Ducie and Oeno islands, forms the Pitcairn Island Group. The nearest major landmass is more than away. This coral limestone island sits atop a conical (presumed volcanic) mound, rising from a depth of roughly . Its surface is mostly reef-rubble and dissected limestone. Except for the north end, the island is encircled by steep limestone cliffs up to high.