It has been suggested that western Sulawesi collided with eastern Borneo in the late Pliocene (3 Ma ago) thereby closing the Makassar Straits which opened again only during the Quaternary (Katili 1978), although there is no great weight of data to support this. Thick sediments in the Straits from before the Miocene indicate that Borneo and Sulawesi have been separated for at least 25 Ma. During periods of low sea-level (p. 16) it is quite likely that islands would have existed particularly in the area west of Majene and around the Doangdoangan shoals (Audley-Charles 1981). In the latter area a drop in sea-level of 100 m would have exposed an expanse of almost continuous land between southeast Borneo and southwest Sulawesi. One interesting observation, however, is that along the northern (and deeper) section of the Makassar Straits the 1,000 m submarine contour of eastern Borneo more or less exactly matches that of western Sulawesi (Katili 1978) so it is possible that the Straits was once narrower.

      As can be deduced from the active lateral movement along the Gorontalo, Palu-Koro, Matano and Sorong faults (fig. 1.3), the island of Sulawesi is at present undergoing a process of fragmentation. The end result could be a cluster of islands separated by narrow straits resembling the complex pattern of the Philippine Archipelago in which the original double island-arc structure is no longer recognizable (Katili 1978).

      Volcanoes

      The most devastating eruption in Sulawesi in recent times was that of Colo volcano on Una-una Island in Tomini Bay. As far as is known this had erupted only once before, in 1898, when a large eruption spewed out 2.2 km³ of ash which was deposited over 303,000 km², reaching nearly as far as the border between Sarawak and East Kalimantan 800 km away (Umbgrove 1930). For some days after 14 July 1983 a large number of earthquakes—up to 100 per day—shook Una-una and on 18 July a large 'phreatic' eruption occurred; that is, water caught below hot volcanic material was turned into high pressure steam and exploded, generating a column of steam and debris some 500 m high. The first magmatic eruption occurred in the morning of 23 July, and this sent a plume of ash and other material 1,500 m into the air. The violent climax of the activity came that afternoon when most of the island blew apart and most of the vegetation was destroyed by a 'nuée ardente' (Katili and Sudrajat 1984),⁵ a glowing mass of turbulent, superheated gases and incandescent solid particles (Francis 1978). Ash from the 15,000 m-high cloud reached Pulau Laut, an island 900 km away off southeast Kalimantan, and covered 90% of the remains of Una-una. All the island's inhabitants were evacuated in time, a wonderful achievement, but all houses, crops, animals, coral and inshore fish were destroyed except along a narrow strip on the east of the island (Katili and Sudrajat 1984).

      Colo volcano is just one of a number of volcanoes on or near Sulawesi which have greater and lesser effects on the surrounding human population. By far the most destructive of human life has been Mt. Awu on Sangihe Island (fig. 1.4; table 1.2). Apart from the obvious damage to the surrounding land, ash clouds can have serious effects on aircraft. For example, a Qantas jet had to be grounded for major repairs in 1985 after it had flown into an ash cloud from Soputan volcano.

      Figure 1.4. Location of active volcanoes. Numbers refer to table 1.2.

      Sulawesi has 11 active volcanoes, compared with 17 on Java, 10 on Sumatra and 6 on Halmahera, and numerous old cones, the most beautiful of which is probably Manado Tua in Minahasa. The volcanoes are associated with the subduction zones north of Toli-Toli (in the case of Colo on Una-una), and east of Minahasa and Sangihe (in the case of the remainder). These regions are often shaken by earthquakes (McCaffrey and Sutardjo 1982). The epicenters or positions on the earth's surface where these originated are frequently cited, but less attention is paid to the depths at which the earthquakes originate. For example, the floor of the Sulawesi Sea is moving southwards but instead of buckling and piling up it is forced down at an angle of about 60° under the northern peninsula. The enormous forces and friction involved generate both earthquakes and heat, and the zone where these occur is referred to as the Benioff Zone. The heat can be so intense that the rocks of the descending plate melt, and the molten material or magma forces its way upwards. In most cases this magma never reaches the surface, hundreds of kilometers above, but cools down in pockets in the earth's crust. The magma which does reach the surface, however, is ejected in volcanic eruptions as lava or pyroclastic deposits such as ash and larger rock particles (fig. 1.5).

      A - volcano with eruptions in historical time

       B - volcano in solfatara and fumarole stage

       C - solfatara and fumarole field

      After van Bemmelen 1970; Anon. 1979

      Figure 1.5. Vertical section through North and Central Sulawesi showing the Benioff Zone below Una-una.

      Based on Katili and Sudrajat 1984

      Minerals

      Petroleum. Petroleum deposits are the remains of microscopic plants and animals which were buried in mud and sand of shallow prehistoric seas, underwent slow decomposition by bacteria and left a residue of hydrocarbon compounds which, under conditions of high temperature and pressure, were converted into oil and gas. Petroleum in Indonesia is usually found in thick Tertiary deposits (fig. 1.6) but there are leakages of natural gas in Tanjung Api Nature Reserve on the northern coast of the northeast peninsula of Sulawesi, and of oil at various inland and coastal locations in the northeast arm. The largest is in a mangrove area in Kolo Bay on the southern shore where the people have used the viscous oil for corking boats. The first oil well on Sulawesi was drilled in 1902 over one such deposit near the mouth of the Lariang River in northwest South Sulawesi. A gas field to the northeast of Lake Tempe has recently been found by British Petroleum and it is possible that an ammonia or urea plant might one day be built nearby. Development has been postponed, however, due to the low international price of liquified natural gas (Anon. 1986). The first commercially-viable source of oil in the Sulawesi region was found in 1985 by Union Texas 25 km south of Kolo Bay on the southern coast of the northeast Peninsula, and more test wells are planned.

      Asphalt. Asphalt is a black, sticky mixture of bitumen or tarry hydrocarbons and mineral matter. About 70,000 ha of limestone in the southeast of Butung is impregnated with asphalt to the extent of 10%-40% by weight (fig. 1.7). These hydrocarbon mixtures have migrated upwards along faults above deep deposits into recent, relatively porous rocks and the lighter fractions have evaporated, leaving the viscous asphalt behind. The asphalt deposits with concentrations of 20% - 30% were first exploited in the 1920s, primarily for tarring roads, and remain the only source of natural asphalt in Southeast Asia (van Bemmelen 1970; Anon. 1985a). About 500,000 tons of asphaltic rock are processed each year (Anon. 1984a).

      Coal. Coal, the fossilized remains of plants, has never been mined in Sulawesi and occurs only around Pangkajene, Enrekang, Makale and the Karama River, all in the southwest peninsula. None of these young Tertiary deposits is economically viable (van Bemmelen 1970).

      Limestone. Just north of Maros at the edge of the karst hills of elevated Miocene coral reefs (p. 468), are the P.T. Semen Tonasa limestone quarry and cement factories from which about 400,000 tons of cement is produced each year, which supplies the entire needs of eastern Indonesia (Anon. 1984a).

      Figure 1.6. Distribution of Tertiary deposits (dark shade) and the location of past and present petroleum exploration activities (dashed lines).

      After Anon. 1984a

      Copper. Copper ores are found primarily in the northern arm and near the nickel areas of Soroako. None has yet been mined but there were plans

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