Land

All the large islands and most of the smaller ones are wholly or partly mountainous. Except on Borneo and New Guinea, the ranges were formed largely by volcanic action, mainly along a line running through Sumatra, Java, and the Lesser Sundas and looping back through the Moluccas and Celebes. The mountains are highest and most continuous along Sumatra's west coast, where they are known as the Barisan Mountains. The nonvolcanic Maoke Mountains of western New Guinea have Indonesia's highest peaks, some rising more than 16,500 feet (5,030 m) above sea level.

Dotting the islands are scores of volcanoes, many of which are active. On Sumatra, Java, and Lombok, volcanic cones rise to more than 12,000 feet (3,660 m). Only slightly lower are those on Celebes, the country's most mountainous island. Volcanic eruptions and earthquakes are frequent and sometimes severe. Undersea earthquakes often cause highly destructive tsunamis, or seismic sea waves. The explosion of Krakatau in 1833 was one of the most violent volcanic eruptions in recorded history.

Lowland plains and gently sloping cultivable mountainsides flank some of the country's coasts. Eastern Sumatra and southern Borneo have extensive lowlands, but they are often marshy, swampy, and jungle-covered.

Water

Large rivers occur only on Sumatra, Borneo, and New Guinea, where they often run through swamps and jungles. Major rivers include the Musi, Hari, Kampar, and Rokan on Sumatra; the Kapuas, Barito, Mahakam, and Kayan on Borneo; and the Digul and Mamberamo on New Guinea. Toba, in the mountains of northern Sumatra, is the country's largest lake.

Climate

Indonesia has a tropical climate with relatively high temperatures prevailing except in the mountains and coastal areas. Temperatures at sea level average about 80° F. (27° C.) annually and vary little from month to month.

November to March is the wet season for most of Indonesia—the time of the north monsoon. The rest of the year, especially June through September, which is the time of the south monsoon, generally brings less rain. Except in the extreme southeastern islands, yearly rainfall is heavy. Some mountain areas are drenched by 150 inches (3,810 mm) or more; lowland areas normally receive 70 to 120 inches (1,780 to 3,050 mm). High humidity makes the weather oppressive much of the time.