Sumatra, (Indonesian: Sumatera), the westernmost of the Greater Sunda Islands of Indonesia. It lies between the Indian Ocean and the South China and Java seas, separated from the Malay Peninsula by the Strait of Malacca and from Java by Sunda Strait. The sixth largest island in the world, Sumatra has an area of 163,557 square miles (423,611 km2) and is about 1,050 miles (1,690 km) long and up to 250 miles (400 km) wide.
The volcanic Barisan Mountains extend the length of western Sumatra. Numerous peaks, some of them active volcanoes, exceed 9,000 feet (2,700 m) above sea level. Mount Kerinci, at 12,467 feet (3,800 m), is the highest. The eastern part of the island consists of a lowlying coastal plain, much of it covered by swamps. Major rivers, including the Kampar, Indragiri, Hari, and Musi, flow eastward from the mountains to the Strait of Malacca. Lake Toba, in the north, is the largest of several mountain lakes. Lying on the Equator, Sumatra has a humid tropical climate, marked by year-round high temperatures, except in the mountains. Rainfall is extremely heavy, especially on the west coast, where it exceeds 170 inches (4,300 mm) a year. Dense tropical forests cover much of the island.
Most Sumatrans live by farming, growing foods, such as rice, corn, and root crops, and a few cash crops, notably rubber and spices, on small plots. Estate agriculture has declined since Indonesian independence but nevertheless remains important, yielding rubber, tobacco, coffee, tea, and palm kernels for export. Sumatra has rich oil fields, and the extraction and refining of petroleum is the island's leading industry. There is also some coal mining and some manufacturing of light consumer goods. Except for air and water travel, transportation is poorly developed. Good roads and railways are limited to the developed areas around Medan, Palembang, and Padang.
Sumatra's inhabitants are Malays, except for a small number of Chinese. Coastal Malays make up the largest ethnic group, followed by Minangkabaus, Bataks, and Acehnese. Of the many languages and dialects spoken, Bahasa Indonesia, the Malay-based national language, is the most widely understood. Islam is the predominant religion. Sumatra has a population of about 43,309,707. Major cities include Medan (1,988,200), Palembang (1,415,500), and Padang (757,900).
A Buddhist kingdom called Sri Vijaya arose on Sumatra by the seventh century A.D. and dominated western Indonesia and Malaya for 600 years. During the 13th century the kingdom gradually came under the control of a Hindu empire centered on Java. About the same time Islam was brought to the island by Arab and Indian traders.
European influence began with the arrival of the Portuguese in the 1500's; the Dutch and English followed a century later. Although the Dutch established coastal settlements in the 1600's, little effort was made to colonize Sumatra until the late 19th century. The Atjehnese in the far north resisted Dutch control until 1908. Japan occupied the island from early 1942 until the end of World War II. Following the occupation, Sumatra was declared independent of the Netherlands by nationalist leaders and became part of the Republic of Indonesia in 1950.