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 to do so just east of Gorontalo6 (Lowder and Dow 1978), and possibly in the Latimojong Mountains of the southwest peninsula. An analysis of leaves from herbs collected on Salayar Island, south of the southwest peninsula, revealed high concentrations of copper (80-600 mg/g compared with normal concentrations of <50 mg/g), indicating that rich deposits might also be present there7 (Brooks et al. 1978).
Gold. Gold is generally associated with copper deposits, and the exploration for copper in North Sulawesi has revealed locations of potential large, commercial gold mines. Four mines that were worked early this century have been exhausted for commercial exploitation. New exploration licenses, however, have recently been issued.
Figure 1.7. Distribution of asphalt-impregnated limestone on Butung Island.
After Anon. 1985a
Nickel. Nickel ores are derived from the weathering of ultrabasic rocks and are also found in the molasse deposits derived from these rocks. The ore deposits around Soroako and Lake Matano are mined by P.T. Inco and those around Pomala'a, south of Kolaka, are mined by P.T. Aneka Tambang.
Minor Products. Deposits of sulphur are known only from Soputan and Mahawu volcanoes in Minahasa and these have been exploited since the 1920s, although production is currently not very active. Kaolin or white clay is produced on a small scale in North Sulawesi for the ceramics industry. Salt is produced in coastal salt pans by small-scale operators in South Sulawesi. Quartz sands and silica were mined in South Sulawesi up to 1977 since when it has not been economically viable to do so.
SOILS
The description of soil types is hampered because there is no system of soil classification that has gained universal acceptance in terms of definitions or names. From experience and observation it has been said that "scientists who are otherwise reasonable and unemotional are liable to behave quite differently when discussing this topic" (Mulcahy and Humphries 1967). In many cases, hybrid systems have evolved using names from various systems with the result that those with little knowledge can become extremely confused. For the purposes of this section the FAO system is used because it has wide recognition and because the names are easily pronounceable.
The major soil types on Sulawesi and their approximate distributions are shown below (table 1.3; fig. 1.8).
* Horizons are layers of soil roughly parallel to the surface which have fairly distinctive characteristics.
After Young 1976
Figure 1.8. Approximate distribution of major soil types on Sulawesi.
After
