Indonesia is the largest country in Southeast Asia, with a maximum dimension from east to west of about 3,200 miles (5,100 km) and an extent from north to south of 1,100 miles (1,800 km). It shares a border with Malaysia in the northern part of Borneo and with Papua New Guinea in the centre of New Guinea. Indonesia is composed of some 17,500 islands, of which more than 7,000 are uninhabited. Almost three-fourths of Indonesia’s area is embraced by Sumatra, Kalimantan, and western New Guinea; Celebes, Java, and the Moluccas account for most of the country’s remaining area.
The major Indonesian islands are characterized by densely forested volcanic mountains in the interior that slope downward to coastal plains covered by thick alluvial swamps that, in turn, dissolve into shallow seas and coral reefs. Beneath this surface the unique and complex physical structure of Indonesia encompasses the junction of three major sections of the Earth’s crust and involves a complicated series of shelves, volcanic mountain chains, and deep-sea trenches. The island of Borneo and the island arc that includes Sumatra, Java, Bali, and the Lesser Sunda chain sit on the Sunda Shelf, a southward extension of the continental mass of Asia. The shelf is bounded on the south and west by deep-sea trenches, such as the Java Trench (about 24,440 feet [7,450 metres] deep at its lowest point), which form the true continental boundary. New Guinea and its adjacent islands, possibly including the island of Halmahera, sit on the Sahul Shelf, which is a northwestern extension of the Australian continental mass; the shelf is bounded to the northeast by a series of oceanic troughs and to the northwest by troughs, a chain of coral reefs, and a series of submarine ridges. The third major unit of the Earth’s crust in Indonesia is an extension of the belt of mountains that forms Japan and the Philippines; the mountains run southward between Borneo and New Guinea and include a series of volcanoes and deep-sea trenches on and around Celebes and the Moluccas.
The relation between these three landmasses is not clearly understood. The present land-sea formations are somewhat misleading because the seas that lie on the Sunda and Sahul shelves are shallow and of geologically recent origin; they rest on the continental mass rather than on a true ocean floor. The Sunda Shelf in the vicinity of the Java Sea has relatively low relief, contains several coral reefs, and is not volcanic. The mountain system that stretches along the South China and Celebes seas of this shelf and that marks the outer edge of the continental mass of Asia, however, is an area of strong relief and is one of the most active volcanic zones in the world.
The outer (southern) side of the chain of islands from Sumatra through Java and the Lesser Sundas forms the leading edge of the Southeast Asian landmass. It is characterized by active volcanoes, bounded to the south and west by a series of deep-sea trenches. On the inner (northern) side of the islands the volcanic mountains grade into swamps, lowlands, and the shallow Java Sea. This sheltered sea was formed at the close of the Pleistocene Epoch (about 12,000 years ago), and there is evidence of former land bridges, which facilitated the migration of plants and animals from the Asian continent.
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