Geography of Indonesia - Geology

Geology

Sumatra, Java, Madura, and Kalimantan lie on the Sunda Shelf and geographers have conventionally grouped them, (along with Sulawesi), as the Greater Sunda Islands. At Indonesia's eastern extremity is western New Guinea, which lies on the Sahul Shelf. Sea depths in the Sunda and Sahul shelves average 200 metres (656 ft) or less. Between these two shelves lie Sulawesi, Nusa Tenggara (also known as the Lesser Sunda Islands), and the Maluku Islands (or the Moluccas), which form a second island group where the surrounding seas in some places reach 4,500 metres (14,764 ft) in depth. The term "Outer Islands" is used inconsistently by various writers but it is usually taken to mean those islands other than Java and Madura.

Borneo is the third largest island in the world and the original vegetation was mostly Borneo lowland rain forests although much of this has been cleared with wildlife retreating to the Borneo montane rain forests inland.

Nusa Tenggara consists of two strings of islands stretching eastward from Bali toward Papua. The inner arc of Nusa Tenggara is a continuation of the chain of mountains and volcanoes extending from Sumatra through Java, Bali, and Flores, and trailing off in the volcanic Banda Islands, which along with the Kai Islands and the Tanimbar Islands and other small islands in the Banda Sea are typical examples of the Wallacea mixture of Asian and Australasian plant and animal life. The outer arc of Nusa Tenggara is a geological extension of the chain of islands west of Sumatra that includes Nias, Mentawai, and Enggano. This chain resurfaces in Nusa Tenggara in the ruggedly mountainous islands of Sumba and Timor.

The Maluku Islands (or Moluccas) are geologically among the most complex of the Indonesian islands. They are located in the northeast sector of the archipelago, bounded by the Philippines to the north, Papua to the east, and Nusa Tenggara to the south. The largest of these islands include Halmahera, Seram and Buru, all of which rise steeply out of very deep seas and have unique Wallacea vegetation. This abrupt relief pattern from sea to high mountains means that there are very few level coastal plains. The islands of North Maluku are the original Spice Islands, a distinct rainforest ecoregion.

Geomorphologists believe that the island of New Guinea, of which Papua is a part, may once have been part of the Australian continent. The breakup and tectonic action created towering, snowcapped mountain peaks lining the island's central east-west spine and hot, humid alluvial plains along the coasts. The New Guinea Highlands range some 650 kilometres (404 mi) east to west along the island, forming a mountainous spine between the north and south coasts. A number of islands off the coast of New Guinea have their own distinctive habitats, including the limestone islands of Biak, in the entrance to the large Cenderawasih Bay at the northwest end of the island.