Like the other islands in the archipelago, Malaita is near the Andesite line and thus forms part of the Pacific Ring of Fire. Earthquakes are common on the island, but there is little evidence of current volcanic activity. The main structural feature of Malaita is the central ridge which runs along the length of the island, with flanking ridges and a few outlying hills. There is a central hilly country, between Auki and the Kwai Harbor, which separates the central ridge into northern and southern halves, the latter being somewhat longer. The northern ridge reaches a height of about 3,200 feet (975 m), while the southern goes up to 4,275 feet (1,303 m). Geologically, Malaita has a basaltic intrusive core, covered in most places by strata of sedimentary rock, especially limestone and chert, and littered with fossils. The limestone provides numerous sinkholes and caverns.