Here the sea depth is similar to that of the Sunda Shelf. Back to Top Located between these two shelves is the island group of Nusatenggara, Maluku and Sulawesi, where the sea depth reaches 15,000 feet. Coastal plains have been developed around the islands of Sumatra, Java, Kalimantan and Irian Jaya. The land area is generally covered by thick tropical rain forests, where fertile soils are continuously replenished by volcanic eruptions like those on the island of Java. The country is predominantly mountainous with some 400 volcanoes, of which 100 are active.
Many rivers flow throughout the country.
On Java rivers are important for irrigation purposes, i.e., the Bengawan Solo, Citarum and Brantas rivers. The displacement of the latter follows the north-south movement of the sun and its relative position from the earth, in particular from the continents of Asia and Australia, at certain periods of the year. Thus, the west and east monsoons, or the rainy and dry seasons, are a prevalent feature of the tropical climate. The climate changes every six months. The air contains vapor which precipitates and produces rain in the country.