Indonesian Journal of Geology, Vol. 7 No. 1 March 2012: 39-54 Morphostructure Control Towards the Development of Mahawu Volcanic Complex, North Sulawesi Kontrol Morfostruktur Terhadap Perkembangan Kompleks Gunung Api Mahawu, Sulawesi Utara S. PoedjoPrajitno Centre for Geological Survey, Geological Agency, Jln. Diponegoro No. 57 Bandung, Indonesia 40122 AbstrAct The studied area, situated in northeastern part of North Sulawesi Arm, is dominantly occupied by the Mahawu, Linau, Tompusu, and Kasurutan volcanic rocks. Using remote sensing data, such as landsat image, black and white panchromatic aerial photograph, and IFSAR image, morphology-origin unit and morphology lineament can be interpreted. Four morphology-origin units, those are Mahawu Volcano Complex, Intra-montane Plain structure, Linau Volcano Complex, and Lacustrine Plain are recognized. Furthermore, morphological lineament pattern was statistically processed to find out the general stress direction in the area to determine the probability of the structural morphology occurrence in the Mahawu Volcano Complex. The result shows that generally the development pattern of volcanic cones are irregular, except the Mahawu Volcano Complex showing a linear pattern. This lineament pattern is interpreted as a NW - SE fault pattern controlling the rise of magma. At least, two tectonic and two eruption periods occurred regularly at different time from the Quaternary age till the present. Keywords: morphostructure, Mahawu Volcano cone, fault, lineament, magma, North Sulawesi Arm Sari Daerah penelitian, yang terletak di bagian timur laut Lengan Utara Sulawesi, didominasi oleh batuan vulkanik hasil kegiatan Gunung Api Mahawu, Linau, Tompusu, dan Kasurutan. Dengan menggunakan data inderaan jauh, yakni citra landsat, foto udara pankromatis hitam-putih, dan citra IFSAR, satuan bentukan- asal dan kelurusan morfologi ditafsirkan. Empat satuan bentukan-asal, yakni kompleks Gunung Api Mahawu, struktur dataran antar-gunung, Kompleks Gunung Api Linau, dan dataran danau telah terdeteksi. Selanjutnya pola kelurusan morfologi diolah secara statistik untuk penentuan arah umum tegasan dan kemungkinan kehadiran morfologi strukturnya. Hasilnya menunjukkan bahwa arah pertumbuhan kerucut gunung api tidak berpola, kecuali arah pertumbuhan kompleks kerucut Gunung Api Mahawu yang memperlihatkan garis lurus. Pola kelurusan kerucut gunung api ini ditafsirkan sebagai pola sesar berarah barat laut - tenggara yang mengontrol keluarnya magma ke permukaan. Dua periode tektonik dan dua periode erupsi yang teratur dan tidak pernah bersamaan telah terjadi di daerah kajian pada kala Kuarter hingga sekarang. Kata kunci: morfostruktur, kerucut Gunung Api Mahawu, sesar, kelurusan, magma, lengan utara Sulawesi IntroductIon with 80 m width. Its surface temperature reaches ± 65.50 C, and the estimated depth is ± 10 m. A weak Mount Mahawu is a stratovolcano with an eleva- explosive eruption expelling white smoke occurred tion of ± 1324 m asl., located at the east of Lokon on April 1987 (SEAN, 1987). Beforehand, in 1974 Empung active Volcano. One of the volcanic cones outpouring of mud with 0.5 m height took place. was cut off and developing to become a crater lake Furthermore, in 1977, 1978, 1987, and 1990 there Manuscript received: December 7, 2011, final acceptance: March 09, 2012 Coresponding Author: 08122482072/[email protected] 39 40 Indonesian Journal of Geology, Vol. 7 No. 1 March 2012: 39-54 were activities of fumarole, mudspot, and hot water was statistically processed to find out the general outpouring along the crater lake periphery having stress direction in the area. Supported by the result green water (Sumpena and Djuhara, 1992). of the aerial photograph interpretation and by finding The Mahawu Complex is located at the coordi- out the general stress direction, the probability of the nates of 1240 49’ - 1240 54’ E and 10 19’ 30” - 10 21’ occurrence of the structural morphology in Mahawu 18” N. Administratively, it is included into the Mina- Volcano Complex was determined. hasa Regency, North Sulawesi Province (Figure 1). Based on the geologic map of Manado Quadran- gle scale 1:250.000 (Effendi and Bawono, 1977) no regIonAl geology geological structure exists in the Mahawu Volcanic Complex. That is why the morphological structure The Mahawu Volcano Complex is situated in the of Mahawu Volcano is need to be discussed here to eastern part of the north arm of Sulawesi. The region find out its relation with the probability of the oc- is occupied by volcanic rocks composed of andesite currence of Mahawu Fault. bordered with basalt as the result of Lokon volcanic activities. Both volcanoes are categorized as young stratovolcanoes of Quaternary age (Qv). The material Methodology produced is lava, bomb, lapilli, and ash. The oldest deposit around the studied area is Tertiary volcanics The research method was carried out by using re- (Tmv) (Figure 2). The studied area is included into mote sensing data, comprising landsat, panchromatic a volcanic arc due to the multiple subduction of the black-white aerial photograph, and IFSAR images. North Sulawesi subduction belt in the north, and of Morphological lineament was interpreted on landsat East Sangihe subduction belt located in the east and images which then the lineament interpretation result south parts of the north arm (Hall, 2001). The sub- 121° E 122° E 124° E 126° E 0 25 50 75 km MANADO Sulawesi Sea . Mahawu Volcano North Sulawesi Studied Area 120º Index Map GORONTALO Studied Area S u la we s I Sea MANADO 0° 0° GOR ONTALO Tomini Bay PALU Sula Islands S u la we si Island Bu ru Sea KENDARI Central Sulawesi MAKAS AR Ba n d a Sea 120º 121° E 122° E 124° E 126° E Figure 1. Locality map of the studied area. Morphostructure Control Towards the Development of Mahawu Volcanic Complex, North Sulawesi 41 (S. Poedjoprajitno) o 1 24 4 5 ’ E 1 25o 0 0 ’ E o o 1 30 ’ N 130’ N MANADO Qa l Qs La kit S a wa n ga n Ma u mbi Sulawesi Sea Teling QTv Ka wa ng ko a n Malalayang Qv S a re n gs o n g Mt. TETE MP ANGAN Qv Koka QTv Ke n d e s Mt. TATAWIR AN Ru me ng ko ko Mt. LOKON Mt. MAHAWU To n d a n o QTv Tmv Mahawu Qv Ma ra wa s Volcano Qs QTv Mt. KALUTA Lake Linau Mt. TOMBUS U Lake Ton da n o o Ka s u ra ta n o 115’ N 1 15’ E o o 1 2 4 45 ’ E 1 2 5 0 0’ N Legend: Qs = Fluvial and lacustrine deposits Mount Qv = Young volcanic rock Hot Spring Qtv = Tondano Tuff Studied Area 0 5 km Tmv = Tertiary volcanic rocks Figure 2. Geological map of the Mahawu Volcano Complex and its vicinity, Minahasa Regency, North Sulawesi (part of the Geologi Map of Manado Quadrangle, scale 1: 250.000, Effendi and Bawono, 1977). duction caused the magmatic activities to form the regIonAl geoMorphology Mahawu, Lokon, Soputan, Klabat, Tongkoko, Tom- pusu, Lengkoan, and Batuangus Volcanic Complexes According to Suharsono et al. (2007) nearly 50 % (Figure 3). Based on the similar occurrences of the of the Manado area originated from a volcanic mor- volcanic distribution patterns in the North Sulawesi phology (Figure 4). The others were derived from arm and Sangihe subduction, it is inferred that the denudated, marine, fluvial, and structural volcanic Sangihe subduction belt more affected the formation morphology. The morphological origin is generally of volcanic cones than the North Sulawesi subduction. of young active volcano (Quaternary) of which its 42 Indonesian Journal of Geology, Vol. 7 No. 1 March 2012: 39-54 Mindano 6º N 0 130 260 km Legend: Thrust fault of active fore-arc Talaud Subduction 4º N Islands Quaternary Volcano Sangihe Sulawesi Sea Maluku mid-oceanic ridge Studied Area 2º N Mayu Te rn a te North Arm Tidore 0º Una -una Goronta lo Maluku Basin Sea To m in i Togia n Isl. Bay Bacan Sula Islands East Arm Ba ngga i Ta la b u Ma ngkole 2º S SULAWESI Island Sula Besi 120º E 122º E 124º E 126º E Figure 3. Geography and the main feature of the North Sulawesi tectonics (modified from Hall, 2001). spread followed the regional tectonic pattern, in Geomorphology of the Mahawu Volcano NE-SW direction beginning from Mount Lokon, Complex and the surrounding Volcanoes Mahawu, Klabat, Duabersaudara, Batunuang, and On the basis of the interpretation result of the Batuangus parasite. black-white aerial photograph plotted on a topo- The old volcanic tracks that can be observed graphical map of the Mahawu Volcano and the sur- among others are the old caldera walls of Tertiary rounding volcanoes, scale 1:50.000 (Figure 5) four Tondano Volcano shown by the series of Kawatak, morphological origins are recognized in the area. Kaweng, Kamingtan, and Kaluta Hills. Suharsono et al. (2007) classified the hill series as dissected Unit of Morphological Origin of the Mahawu longitudinal volcanic ridge (VD3), while Verstap- Volcano Complex pen (2000) called it as Tondano Escarpment. It is Aerial photograph interpretation indicates that presumed that a paleoparasitary developed in the the unit consists of six volcanic cone landforms north flank of old Tondano Caldera. At present, the developing close to each other, grouped, and they parasite is shown by a hill landform with dissected occur almost in one line. The eruption product of peak and wide plain in the middle part. Suharsono et each Mahawu volcanic cone is difficult to be differ- al. (2007) classified the hill landform with dissected entiated. However, on the landsat image combined peak as a medium dissected crater wall (VD7), with DEM, twelve volcanic cones can be identified while the wide plain in the middle was classified (Figure 6). All eruption material of this Mahawu as a medium dissected crater plain landform (VD8) Volcano Complex is presumed to cover most of the (Figure 4). residual body of Paleo-Mahawu volcanic cone.