A Vast Island of Dramatic Contrasts
Sumatra is the fourth largest island in the world after Greenland, New Guinea and Borneo, with a land area of some 473,481 sq km-roughly the size of California or Spain. It is an island of enormous regional diversity, that for administrative purposes has been divided into eight large provinces. To some extent, these divisions have been made along ethnic, and not geographic lines. The provinces of Aceh and Lampung at Sumatra's northern and southern tips, for example, are home to the Acehnese and Lampung peoples, while North and West Sumatra are the Batak and Minangkabau homelands, respectively. Other provincial boundaries, however, have been determined more by the island's lofty central mountain range and the large river systems flowing eastward from it. Sumatra's largest province is South Sumatra, which encompasses a major slice of territory on the east coast and the large islands of Bangka and Belitung.
-migrating continents
Contrary to earlier beliefs, recent geological research has demonstrated that Sumatra once formed part of the great southern continent called Gondwanaland, and not part of Asia. About 160 million years ago it broke away and began to migrate northwards-eventually lodging, with Malaya, Thailand and Burma, againts what is now Indochina.
About 40 million years later, India started its own northward journey, crashing slowly but powerfully into Tibet some 80 million years later. The dense oceanic plate on top of which India rested then nudged eastward againts the relatively light and rigid Sunda Plate. This caused massive buckling, faulting and shearing-resulting in the formation of a series of small islands to the west of Sumatra, as well as of Sumatra's central Barisan Range and the valleys in between.
Today, part of the Indian Ocean plate continues to slide below the west of Sumatra, causing frequent but minor earthquakes, eruptions and landslides. The strees and shearing has been relived by a long rift valley stretching from Semangko Bay at the island's southern tip, north through Lake Kerinci, Bukittinggi's Ngarai Sianok Canyon.
All along its length, the rift has been filled in by some 65 volcanic peaks, several of which are still active. The fault countinues southward into the straits between Sumatra and Java, where it has created the most infamous of Indonesia's many volcanoes-the mighty Krakatau.
-parallel slices
The physical geography of Sumatra is best considered as a series of parallel longitudinal slices running northwest to southeast along the main axis of the island. In the extreme west is a sparsely inhabited chain of island-from Simeulue in the north to Enggano in the south-that is separated from Sumatra by of 2,000 meters.
The western fringe of Sumatra itself is a narrow but complex coastal strip, alternating between fertile alluvial soils where rice is grown, and inhospitable swamps that are gradually being drained for agriculture. Rugged foothills rise steeply from this coastal fringe to the mighty Barisan Mountains-several of which tower above 3,000meters. The highest non-volcanic peaks (about 3,400 meters) lie within The Gunung Leuser National Parks in the northern part of the island. Sumatra's highest volcano is Mt. Kerinci-at 3,800 meters, the highest Indonesia mountain outside of Irian Jaya.
Also found along this central axis is dramatic Lake Tujuh-a spectacular crater lake, 5 sq km in area, perched at an altitude of 2,000 meters atop Mt. Tujuh. It is best seen from the southern slopes of adjacent Mt. Kerinci, but can also found be reached directly from the town of Sungaipenuh. Impressive mountains are also found in the limestone areas around Bukittinggi and Payakumbuh in West Sumatra, and in the the Lhoknga area of Aceh. These are not particularly high, but the unusual karst formations with steep sides, often studded with caves, are very interesting.
East of the Barisan Range lie irregular foothills and broad, dry lowlands which have seen rapid development in this century as plantations and as urban and agricultural centers. These lowlands are now also the focus of industrial development based on food processing and fossil fuels.
As you approach the coast, particularly in the central and southern regions, the dry lands turns to various types of swamps-the largest extent of swamplands in indonesia outside of Irian Jaya. These swamps are crisscrossed by large, meandering rivers. Where the peat is not too thick, the land has been cleared for rice cultivation; where it is thicker, pineapples and coconuts grow. Where the peat is really thick-and it can reach an incredible 22 meters in parts of Riau-agricultural crops cannot be grown. Logging continues, however, and land drainage in other areas threatens to eliminate many of these magnificent raised swamps.
Further east, off the central and southern coasts, are the low-lying eastern island at the southern and of the Malacca Strait. Here, extensive areas are mined for bauxite and tin, but Batam Island is also being developed now as a leisure, business and industrial alternative to Singapore-with everything from industrial estates to luxury beach resorts.
Far out in the South China Sea about 500 km from the mainland are the Natuna Islands. These have little agricultural potential but are important strategically, as well as possessing important fishing grounds and rich reserves of oil and natural gas.
-Riches of the earth
Sumatra is extremely rich in fossil fuels-oil, gas and coal. Oil is found mainly in the east, from Riau to South Sumatra, both on land and offshore. New technologies had to be developed to tap this wealth, especially to facilitate drilling in the thick peat deposits of coastal Riau. Roads there are built on top of floating plastic carpets, and derricks perch atop tall piles reaching down to the bedrock below. Almost all of the oil from this central basin is pumped to the port of Dumai on the east coast. In southeast Aceh is the enormous Arun gas field , which ranks as one of the world's most productive. In South Sumatra, at Muaraenim, is a very large opent-cast coal mine. The coal is taken from here by rail south to Teluk Betung and thence to java.
Small amounts of gold are mined, though the important metals are aluminum (bauxite) and tin. Bauxite is mined on Bintan, one of the Riau Island, then shipped to Japan for processing into alumina, then back to Sumatra again, where it is smelted into aluminum at the Inalum plant on the coast of North Sumatra.
-Sumatra's 'big bang'
Huge Lake Toba in North Sumatra is Southeast Asia's largest lake, covering 1,146 sq km, and so far defying efforts to measure its depth-though it is known to be deeper than 450 m. It occupies the caldera of a massive volcano which exploded some 100,000 years ago in the most powerful volcanic eruption ever known. Early man was probably living in Sumatra at the time, and the scale of devastation caused by the explosion is hard for us to imagine-equivalent to hundreds of hydrogen bombs detonating simultaneously.
Shortly before the eruption, a giant boil of a mountain grew above the increasing quantities of molten material below. When the solid ground finally gave way, a searingly hot plume of ash was thrown into the sky-its remains can be traced today over an area of 20-30,000 sq km. Around the volcano the ash settled in a layer more than 600 m thick. This eruption set the world record for the quantity of material ejected-some 1,500-2,000 cubic km, compared with just 0,6 cubic km for the much-publicised eruptionof Mt. St. Helens in Washington State, USA. Because of the massive quantity of ejecta, the empty cone soon collapsed in on itself to form the world's largest caldera. This became an ancient lake, which was itself destroyed about 30,000 years ago when a second, much smaller eruption occurred. The island of Samosir in the center of the lake, and the lake's eastern shore-the main tourist centers today-are all that remain of the shattered cone.
The first Europeans to see Lake Toba were a pair of British missionaries, Messrs Burton and Ward, who traveled here in 1824. They were told that the lake level rose and fell like the sea. Although much of the area was already deforested and under rice cultivation, one area on the eastern shore was still covered with jungle. At the time, the lake's outlet was unknown, but eventually it became clear that the Asahan River flowed out through a "spirit-filled" forest at what is now Porsea on the southeastern shore.
One reason for the forest's mystique may have been the impressive 100-meter Sigura-gura waterfall, which races through a narrow gorge-now rendered somewhat less dramatic by the presence of a hydroelectric dam. A short distance downstream is the 150-meter Sampuran Harimau waterfall which cascades into a large natural amphitheater near Tangga. Tumbling into this spectacular gorge is Sumatra's highest waterfall, at a point where the small Ponot River drops 280 meters from the mountains above and joins up with the Asahan River below. Although the falls are now used for hydroelectric power to supply the massive Inalum plant, the natural beauty of the area has not been lost. On a clear day these falls can be seen from the windows of jets plying the Medan-Jakarta route.
The climate of Sumatra has changed dramatically over the past million years. During the ice ages, glaciers covered the higher peaks in the north, in what is now Gunung Leuser National park. The largest glacier on Mt. Leuser covered some 100 sq km.
Sea levels have also been as much as 50 meters higher and 180 meters lower than they are at present. As a result, the low lying eastern swamps and plains have been inundated and exposed several times. At times of low sea levels, land bridges connected Sumatra with Peninsular Malaysia, Borneo and Java, and the estuaries of large rivers such as the Musi, the Kampar and the Indragiri were situated some 700 km farther east from the present-day coastline, near what are now the Natuna Islands.
-Fierce tropical storms
The western parts of Sumatra are by far the wettest; few areas here receive less than 4,000 mm of rain per annum, quite evenly distributed throughout the year. The wettest spots are the hills behind Padang, which receive 5,000 mm, and the northern Bengkulu mountains, which are drenched with more than 6,000 mm (about 10 times the rainfall of London!). The driest places are on the east coast and in the central rift valley, but even most of these receive more than 2,000 mm.
Although the difference between seasons is not very distinct, dry periods often occur in the early part of the year and in June and July. During these periods, wild fires occur, often started by farmers clearing patches of land. The problem is growing worse because the changing world climate is prolonging the dry seasons. Logging has also opened up the moist forests, allowing forested areas to dry out-as have drainage schemes on the east coast, which dry out the peat deposits, causing them to burn out of control. It is not unusual for air traffic approaching Singapore to be re-routed because of the smoke from fires in southern Sumatra.
While there is considerable local variability, winds in Sumatra generally blow from the north between December and March, and from the south between May and September. Many of the seasonal winds are particularly fierce, and have been given names by the local residents. A stormy wind that occurs in the region of Lake Tawar, in Aceh, is given the name Angin Depok-after the depik fish that is easily caught in the lake around this time. There are also two warm, dry winds-the Angin Bohorok which flows eastwards from the mountains behind Bohorok (in North Sumatra) and the the Angin Padang Lawas which blows east of Padang Sidempuan (in West Sumatra)-which often damage crops and make agriculture in some regions almost impossible because the soils become so dry.
