Sumatra (sŏŏmä´trə), island (1990 pop. 36,471,731), c.183,000 sq mi (473,970 sq km), Indonesia, in the Indian Ocean along the equator, S and W of the Malay Peninsula (from which it is separated by the Strait of Malacca) and NW of Java (across the narrow Sunda Strait). The westernmost and second largest island of Indonesia, Sumatra is c.1,110 mi (1,790 km) long and c.270 mi (435 km) wide and is fringed with smaller islands off its western and eastern coasts. The Bukit Barisan, a volcanic mountain range with more than 30 active volcanoes, traverses its length, reaching 12,467 ft (3,800 m) at Mt. Kerinci. Rising in the Barisan range are several large rivers, including the Hari, Indragiri, and Musi; some rivers are being developed for hydroelectric power. In the north is the great salt lake Toba. Because of the hot, moist climate and heavy rainfall, the vegetation is luxuriant, and much of the eastern half of the island is swampland. The interior is covered largely by impenetrable rain forests. Among the native animals are elephants, clouded leopards, tapirs, tigers, Malayan bears, and snakes.
Sumatra has great natural wealth; about 70% of the country's income is produced there. The island has some of Indonesia's richest oil fields, its finest coalfields, and deposits of gold and silver. Its offshore islands are known for their tin and bauxite. Most of the country's rubber and coffee is grown in Sumatra; pepper, tea, sugarcane, and oil palms are also grown on plantations. The Deli region around Medan is famous for its tobacco. Rice, corn, and root crops are raised for local consumption. Timber cut includes camphor and ebony.
Sumatra comprises eight provinces of Indonesia. It is a sparsely settled island, with principal centers at Medan and Palembang; also important are Jambi, Padang, and Bandar Laumpung. There are state universities in Jambi, Medan, Padang, Pakanbaru, and Palembang. The four largest ethnic groups are the Acehnese, Batak, Minangkabau, and coastal Malays. In the interior highlands are found the Gayo-Alas and the Rejang-Lebong groups. Islam is the predominant religion, though there are many Christians among the Batak and the Gayo-Alas. Chinese, Arabs, and Indians live on the coasts, and some 15 different languages are spoken on the island.
Sumatra had early contact with Indian civilization, and by the 7th cent. AD the powerful Hindu-Sumatran kingdom of Sri Vijaya (with its capital in or near Palembang) flourished under the house of Sailendra. The kingdom extended its control over a large part of Indonesia and also over the Malay Peninsula. By the 14th cent., Sumatran supremacy had waned, and the island fell under the Javanese kingdom of Majapahit. The Arabs, who may have arrived as early as the 10th cent., established the sultanate of Achin (now Aceh), which reached its height in the 17th cent. and controlled most of the island.
The first European to visit Sumatra was Marco Polo, who was there briefly c.1292. Following the Portuguese, who came in 1509, the Dutch arrived in 1596 and gradually gained control of all the native states including Achin. The British had brief control over parts of the island in the late 18th and early 19th cent. The Achinese (Acehnese) launched a rebellion in 1873 and were not subdued by the Dutch until 1904. In World War II, Japanese troops landed (Feb., 1942) in Sumatra and occupied it throughout the war.
After Indonesian independence was granted (1949), all of Sumatra was included in the new republic. Since then there has been much indigenous agitation and repeated demands for local autonomy. The Acehnese have waged occasional guerrilla warfare against the government, and in 1958 a full-scale rebellion was launched by dissident army officers. It spread to other islands before being quelled by the government. Sentiment for autonomy or independence remains strong among the Acehnese. Guerrilla attacks and demonstrations in Aceh increased in 1999 and 2000 after the end of Indonesian authority in East Timor. Indonesian legislation in 2001 granted Aceh limited local autonomy, including the right to implement Islamic law, but sentiment in favor of independence remained strong and fighting escalated. A peace pact with the rebels (Dec., 2002) only paused the conflict for a few months. In Dec., 2004, an earthquake and resulting tsunami devastated the coastal Aceh and North Sumatra. Most of Indonesia's 167,000 deaths from the event occurred on Sumatra. In Aug., 2005, a new peace accord with signed with Aceh's rebels; it led to rebel disarmament and, in 2006, the beginning of the establishment of local self-government. Aceh and North Sumatra suffered disastrous flooding from heavy rains in Dec., 2006; more than 400,000 were displaced. Earthquakes in 2007 and 2009 caused significant destruction and deaths in the area around Padang, and a 2010 temblor's tsunami devastated coastal areas in North and South Pagai in West Sumatra's Mentawai islands.
See F. M. Schnitger, Forgotten Kingdoms in Sumatra (1989).