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Huge island full of contrasts

Sumatra is, after Greenland, New-Guinee and Borneo, the biggest island in the world with a surface of 473,481 - ten times as big as Holland. The island knows big regional differences and it's cut in eight pieces for governmental reasons. Until a certain level, this partition is fixed more over ethnical barriers than by geographical borders. So are Sumatra's opposites, the northern Aceh and the southern Lampung, habited by the Acehnese and the Lampung, while the north is the living area of the Batak and western Sumatra of the Minangkabau.

Other provincial borders however are marked by the location of a mountain range and the location of big rivers, which flow from the mountains towards the east. The biggest province is Sumatra Selatan (South Sumatra), which consists of a significant part of the eastern coast, and the islands of Banka and Belitung.

Moving continents

Recent geological survey prooved that Sumatra used to be a part of Gondwanaland, instead of Asia. About 160 milion year ago it managed to get awaw from the super-continent, and steadily moved towards the north. It finally got connected with the current Indo-China, together with the Malay peninsula, Thailand, Vietnam and Burma.

About 40 milion year later, India also started to move to the north, until it, 80 milion years later, finally collided into Tibet with tremendous force. The compact ocean plate where India was built on, also collided a little with the smaller, but stronger Sunda plate. This caused enormous movements, cracks and faults. This caused the existance of a row of smaller islands west of Sumatra, as well as the Barisan mountain range with it's valleys on the island itself.

Still the Indian plate is moving under the Sumatran plate, which still causes many, but light earthquakes, landslides and volcanic eruptions. Some of the earthquakes can be very devastating, as was prooved in 2000, when a big quake jolted the southern city of Bengkulu, leaving many hundreds dead.

The pressure and cracks are decreased by a long valley on a fault, stretching from Teluk Semanko (Semanko Bay) at the southern tip of the island, to the far north over Danau Kerinci (Lake Kerinci), Bukittinggi and the Alas Valley towards Banda Aceh. The most known part of the fault is Ngarai Sianok (Hole of the Buffalo), near Bukittinggi.

This fault-line valley system is stuffed with 65 volcanoes over it's entire length, some of them still active. The fault stretched south into the sealane between Sumatra and Java, where it created Indonesias most famous, and most notorious volcano; the mighty Krakatau.

Parallel segments

The fysical geography of Sumatra is best explained as a number of parallel segments, which run from the Northwest towards the Southeast of the island. In the far west is a low-populated string of islands - from Simeulue in the north to Enggano in the south -, separated from Sumatra by a trench in the ocean floor with a depth of about 2000 metres.

The western border of Sumatra itself consists of a fertile line of silted up earth where rice is being produced, varied by mountainious swamps which are gradually dried for agricultural targets. Rough hills raise themselfves at the mighty Bukit Barisan, and can reach an altitude of over 3000 metres. The highest, non-volcanic, peak (about 3400 metres) can be found in Gunung Leuser National park in the north of the island. Gunung Kerinci is the highest volcano (3800 metres), except the Puncak Jaya in Papua.

Also located along this central axis, is Gunung Tujuh, with Lake Tujuh on an altitude of 2000 metres, a spectaculair craterlake with a surface of 5 km2. The southen slopes of the bordering Kerinci offer a splendid view over the lake. Impressife mountains can also be found in the limestone area around Bukittinggi and Payakumbuh in Western Sumatra, and in the Lhoknga area in Aceh.

East of the Barisan mountain range are irregular hills and big, dry plateaus, which have developed into plantations, agricultural centres and cities in a fast pace over the last century. Nowadays these lowlands are the centre of industrial development, based on foodprocessing and fossile fuel.

More towards the coast the landscape changes, especially in the central and Southern part. The dry grass is replaced by various types of swamps, crossed by big, slow moving meandering rivers. This is the most extended swamp area of Indonesia, outside Papua. Where the peat is not too thick, it has been removed in favour of rice producing; on thicker layers pineapples and coconut trees are planted. Where the layers are most thick, in Riau sometimes 22 metres, there is no possibility of agricultural activities.
In front of the coast of Central- and Southern Sumatra, in the southern part of the Selat Malaka (Malaca Strait), are the low, eastern islands. Here bauxite and tin are mined in big numbers. The developmend of Batam is still stepping up. This island still has to become the center of trade, industry and recreation, and eventually the alternative of Singapore.

At about 550 kilometres from the coast are the Natuna islands, somewhere in the China Sea. These islands are less useable for agriculture, but they are from big strategic importance, since they offer access to important fishing watera and they also posess oil and natural gas reserves.

Natural resources

Sumatra is extraordinary rich of fossile fuel: natural oil and gas and coal. The oil is mainly found in the east, between Riau and South Sumatra, on the main island, as well as outside the shore. New techniques have to developed to mine for these natural resources, especially to make possible drilling in the thick layers of peat in Riau.

Almost all oil from this central place is transported to the seaport of Dumai, at the eastern coast. The enormous gas field near Arun in Southeastern Aceh, belongs to the most productive in the world. In South Sumatra, near Muara Enim, is an enormous coalmine, at the surface. The coal is transported to Teluk Betung by train, and from there is it shipped to Java for further processing.

Gold is still mined in small numbersm, but bauxite and tin are the most important minerals. Bauxite is mined on Bintan, in the archipelago of Riau, most of it is transported to Japan where it is made into aluminiumoxydes. This is shipped to Sumatra again, where aluminium is made out of it.

Sumatra's "Big Bang"

Danau Toba, the biggest lake in Southeast Asia, covers 1146 Until now the exact depth is not known, but it's certainly deeper than 450 metres. It's filling the caldera of an immense volcano, which exploded about 100.000 years ago, in the most powerfull eruption we have ever known. In that time it was most likely people already inhabited the island and the damage done by thay eruption is almost impossible to imagine.

Shortly before the eruption, the superheated mass pushed an entire new mountain to the surface. When the pressure eventually seemed too big, a pyroclastic ashcloud was spit into the sky. Remains of this cloud can still be found in an area ob 20 to 30.000 Around the volcano a layer of ash of 600 metres thick was formed. This eruption had the biggest ever spewed out mass of earth: 1500 to 2000, against on 0.6 of Mount Saint Helens, in the State Washington in the US.

Because of the tremendous exhaust, the cone collapsed and the world's biggest caldera was formed. A lake was formed soon after, and just 30.000 years ago, most of the lake was destroyed again by another, smaller eruption. The island Samosir and the eastern shore of the lake are the remains of that eruption.

In 1853 the Dutch language specialist H.N. van der Tuuk saw Lake Toba as the first European. However it was already deforested for a big part, and planted witrh rice, the area around the eastern banks were still covered in original rainforests. In that time they could only guess about the drainage of the huge lake, but eventually it was prooved that the river Asahan flowed out of the lake, through the 'Forest of the Spirits', to mouth in the China Sea near Porsea, at the southeastern shore.

Maybe it was the impressive, hundred-metre-high Siguragura-fall which streamed through the narrow clove, which created the mystical air in the forest. But with the construction of a power station the reality won from mysticallity. Just a little downstream the fall falls stairwise in a big natural amphitheathre near Tangga. In this spectaculair clove, Sumatra's biggest fall can be found, the place where the small river Ponot falls over 280 metres from the mountains to unite itself with the river Asahan.

However the fallsare now used for making power for the factories of aluminium, the natural beauty of the area didn't suffer at all. With clear weather these falls can be seen from the plane when you fly from Medan to Jakarta, or the other way.

The climate of Sumatra changed drastically over the last milion years. In the Ice ages the high peaks in the north, the nowadays National park Gunung Leuser, were covered with glaciers. The biggest glacies on Gunung Leuser covered over 100

The sealevel has changed over the time, sometimes 50 metres lower, sometimes 180 metres higher. The low easters swamps were sometimes above, and sometimes below the sea. Whenever the sealevel dropped, the Malay peninsula and Sumatra were connected, as well as Borneo and Java. The mouths of the big rivers like the Musi, Kampar and Indragiri, were approximately 700 km more east, on the current place of the Natuna-islands.

Heavy tropical storms

By far the most wet areas of Sumatra are the western parts. Most places get more than 4,000 millimeters in one year, gradually spread over the entire year. The most wet ares are the hills behind Padang (5,000 millimeter) and the northern hills of Bengkulu (over 6,000 millimeter), about seven times as much as in Holland. The most dry place is the central fault valley, but even they get over 2,000 milimetres on most spots.

However there are no clear differences between the wet and the dry season, the dry periods are mostly in the beginning of the year and around June and July. In this time, big bushfires are common, they are started by farmers who want to prepare their fields for new harvests. The problems of drought are getting bigger because the dry season is extended because the worlds climate is changing.

Forest clearing activities broke up the humid forests, causing the forested areas to dry out. Above that, the irrigation systems in the east also cause the soil to get poor, so the fires can't be controlled anymore. Air traffic in Singapore sometimes has to be detoured because of these fires.

However there are big differences locally, winds normally blow from the north between December and March, and from the south between May and September. Many of these seasonal winds are powerful and are named by the locals. A stormy wind in the area of Danau Tawar (Tawar Lake) in Aceh is named Angin Depek, to the fish that is easy to catch in the lake.

There are also two warm, dry winds; the Angin Bohorok which comes from the mountains behind Bohorok and goes towards the east, and the Angin Padang Lawas, which blows from Padang Sidempuan towards the east. These winds make agriculture difficult in some areas, if not bearly possible, because they dry out the soil dramatically

Last revised on September 02, 2011
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