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Figure 1.1. Changing locations of components of Southeast Asia since the first rifting from east Gondwanaland, showing schematic ocean spreading ridges, subduction trenches (triangles) and continental margins (dotted lines). Present coastlines shown for reference only. Horizontal lines: 0°, 30° and 60° South.
After Audley-Charles 1987
The most dramatic event in Indonesian geological history occurred in the Miocene when the northward-drifting Australian Plate caused the bending to the west of the eastern part of the Banda Arc. This westward movement, coupled with the westward thrust along the east-west Sorong fault system from western Irian Jaya, modified the two major landmasses that would form the peculiar shape of Sulawesi we recognize today. It has been proposed that this collision occurred 19-13 Ma ago (Sasajima et al. 1980; Audley-Charles 1987). The Banggai-Sula Islands formed the continental platform section of the east Sulawesi fragment. The Talaud Islands and the small islands of Mayu and Tifore between North Sulawesi and Halmahera are probably also part of the collision suture that formed between Sundaland and Gondwanaland (Audley-Charles 1987).
Thus eastern Sulawesi was like a spearhead that hit western Sulawesi and caused the southwest peninsula to rotate anticlockwise by about 35°, thereby opening the Gulf of Bone (Haile 1978) and causing the northern peninsula to pivot around its northern end, rotating clockwise through nearly 90°. This would have caused subduction3 along the North Sulawesi Trench in Gorontalo Bay (Otofuji et al. 1981) and obduction4 of the ultrabasic rocks of east and Southeast Sulawesi over the erosion debris or molasse deposits of younger rocks.
The physical history of eastern Indonesia has made it one of the most geologically complex regions in the world (Audley-Charles 1981). It is this complexity and the strange shape of Sulawesi, described variously as an orchid, a demented spider and a wobbly 'K', which have long attracted the interest of geologists and others (Davis 1976; Otofuji et al. 1981).
Sulawesi comprises three distinct geological 'provinces' brought together by movements of the earth's crust as described above. These are West and East Sulawesi (divided by the north-northwest fault between Palu and the Gulf of Bone-the Palu-Koro fault), and the Banggai-Sula province comprising the Tokala region behind Luwuk on the northeast peninsula, the Banggai Islands, Butung Island and the Sula Islands (actually part of the political province of Maluku) (fig. 1.2).
West Sulawesi is underlain in the south by a basement of schists (metamorphic rocks of continental origin that split easily along their mineral plates) and ultrabasic rocks (derived from the mantle), and in the north by schists and gneiss (banded, coarse, metamorphic rocks that do not split easily). These are overlain by marine sediments including limestone (primarily calcium carbonate from animal shells), sandstones (consolidated sand), cherts (a compact flint-like variety of silica) and shales (thin layers of consolidated mud, clay and silt). Volcanism began in the Eocene but became widespread in the Miocene, and the volcanic arc that ran from the south to the north deformed the existing sedimentary rocks. The ash and dust from the volcanoes mixed with eroded sedimentary rock derived from the uplifting of the eastern part of the Sulawesi collision zone, to form the Celebes molasse, a generally poorly-consolidated conglomerate rock of gravels, sands, silts and muds formed in terrestrial or shallow-water environments. Molten igneous rocks such as granite and diorite have forced their way by intrusion into Miocene and older rocks at a number of locations (Sukamto 1975a, b; Otofuji et al. 1981). East Sulawesi consists mainly of basic and ultrabasic igneous rocks associated with schists in the west and with Mesozoic limestones in the east and the south.
Figure 1.2. Geological map.
After Katili 1978
The Banggai-Sula province comprises a basement of Palaeozoic metamorphic rocks intruded by granites. Triassic and Permian effusive rocks were deposited locally on the basement. These rocks are overlain by Mesozoic shale, sandstone, conglomerate and marl (consolidated mud and calcium carbonate) deposited in both continental and marine shelf environments. Butung Island is included in this province by virtue of distinctive Jurassic shales which it shares with Banggai-Sula but which do not occur in East Sulawesi (Sukamto 1975b).
Figure 1.3. Tectonic features of Sulawesi.
After Katili 1978
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.
