The two largest islands, Espiritu Santo (or Santo) and Malakula, account for nearly one-half of the total land area. They are volcanic, with sharp mountain peaks, plateaus, and lowlands. The larger islands of the remaining half also are volcanic but are overlaid with limestone formations; the smaller ones are coral and limestone. Volcanic activity is common with an ever-present danger of a major eruption, the last of which occurred in 1945. Rainfall averages about 2,360 millimeters (94 in.) per year but can be as high as 4,000 millimeters (160 in.) in the northern islands.