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Coll. and Res. (2004) 17: 83-92 83 The Topographic Landscapes of Volcanic Necks in the Coastal Range and Lutao, Eastern Taiwan Wen-Shing Juang1* and Ju-Chin Chen2 1 Department of Geology, National Museum of Natural Science, Taichung, Taiwan 404, R.O.C. 2 Institute Oceanography, National Taiwan University, Taipei, Taiwan 106, R.O.C. (Received October 29, 2004; Accepted November 15, 2004) Abstract. Hot magma was transmitted through a conduit system or fractures to the ground surface in a volcanic eruption. These volcanic conduits are mostly formed through the consolidation of the magma. Generally speaking, there are quite resistant to weathering processes, and can be preserved through long periods of time. In the Coastal Range and the offshore islet Lutao of Taiwan well-developed volcanic necks, typified by the occurrence of columnar joints, are found in Tafengfeng, Tuweihsi (Chungan) in the Coastal Range and Lantzuhu, Yotzuhu, Haishenping and Kungkuantsun in Lutao. The columnar joints found in Lutao andesites or in the basalts of the Coastal Range are either horizontal or clino to pleated-skirt formed manifesting geological structure specific to volcanic conduits present in the volcanic necks. The shapes of the columnar jointing in the volcanic necks depend on the diameter of the passage conduit, the thickness of the emitted lava, the composition of the magma, and the topographic features of the strata underlying the site of eruption. The occurrence of volcanic necks in the coastal Range and in Lutao is highly diversified and these volcanic necks are worthy of permanent preservation simply for tourism and educational purposes. We report 3 typical examples of volcanic conduits occurred in the Coastal Range and Lutao. (1) Haishenping, Lutao: a typical volcanic crater with clino columnar jointing. (2) Tuweihsi, Coastal Range: a volcanic dike with horizontal to clino-columnar jointing in basaltic the cliff wall of lava breccias. The pillow lavas and hyaloclastite flows indicate possible submarine eruption. Both of (1) and (2) may be formed in the trunk of the volcanic neck, Tuweihsi's position maybe lower than that of Haisenping's. (3) Tafonfon, the Coastal Range: the exposed volcanic neck in basaltic rock at Tafonfon is small, but the pleated-skirt configuration is perfect with a well exposed volcanic conduit in the upper portion looking like a corolla or crown. We suggest that it may be used as the Logo for the Coastal Range Volcanic Scenic Area. This outcrop illustrates well the mechanism to form columnar jointing in a volcanic conduit. Key words: Columnar jointing, Eastern Coastal Range, Lutao, Taiwan, volcanic neck. INTRODUCTION Taiwan is located at the eastern margin of the Eurasian plate. Formation of this island was a consequence of the collision between the Luzon arc and the China continent (Chai, 1972), most of the Taiwan orogen is still bering pushed up by the collision (Teng, 1996). Since the Cenozoic Era, intraplate volcanism related to continental rifting has prevailed in the *Corresponding author. E-mail: [email protected] passive continental margin of China. As a portion of this region, Taiwan and the neighboring islets of the Penghu Islands (Penghu) were inevitably affected by such tectonics. Therefore, products of Miocene volcanism such as alkali basalts and tholeiites in western Taiwan and Penghu seem to follow prevailing physiographic trends. In eastern Taiwan, the convergence of the Manila Trench is marked by eastward subduction of the South China Sea plate underneath the Luzon volcanic arc system on the Philippine Sea plate along the Eastern Longitudinal Valley. In eastern Taiwan, andesitic volcanism occurred mainly during 84 Coll. and Res. (2004) 17: 83-92 Miocene and Pliocene to Pleistocene. Additionally, Taiwan is also situated in the western end of the Ryukyu arc-trench system. The subduction of the Philippine Sea Plate beneath the Eurasia plate is marked by the Volcano Groups in northern Taiwan. Thus, based on the tectonic framework, three volcanic provinces can be identified: the western province, of a continental-rifting type; the eastern province, of an island-arc type; and the northern province, of a continental-margin type (Fig. 1). In the northern volcanic province, the andesitic rocks are predominated; accordingly the columnar joints are scant. In the western province, especially in the Penghu Islands, the basaltic lava flows are prolific in columnar joints. In the same western volcanic province along the western Foothills of Taiwan, outcrops of columnar joints are very rare. One locality with perfect volcanic neck at Sanshai, Taipei County was reported by Tsao (1989). Since 1988, the landform found at this site has disappeared, owing to the urban development (Juang, 1999). In eastern volcanic province of Taiwan, several volcanic necks with columnar jointing are exhibited; a typical example was found at Hsianpiyan (Elephant Truck Rock) in Lanyu (Juang and Chen, 2002). It is important to spread the geological information (Lo et al., 1993; Wang Fig. 1. Map showing the principal tectonic features in the Ryukyu-Taiwan and Luzon areas and 3 igneous provinces in Taiwan: A. western, B. eastern and C. northern districts. Black areas show the distribution of basalts; RRF, Red River Fault; EVBF, East Vietnam Boundary Fault. et al., 1992; Hsu, 1956) to the public, so that we can reduce the risk of runining of the beautiful landforms from farmer excavation, fishery plantation or rock materials mining. SUMMARY OF VOLCANIC GEOLOGY OF THE COASTAL RANGE The north-south trending Coastal Range of eastern Taiwan has a length of 140 km and an average width of 10 km. Its maximum elevation is 1,500 m above sea level. This Pacific-facing range is separated by a longitudinal rift valley from the Central Range to the west. It has been suggested that the convergent boundary between the Eurasian plate and the Philippine Sea plate runs on the island of Taiwan along the Coastal Range (Biq, 1976). The incorporation of the Coastal Range onto Taiwan means that a part of the advancing edge of the Philippine Sea plate has been welded to the continental margin of Eurasia. The rock formations of the Coastal Range (Fig. 2) can be divided into the following three units (Yen, 1968) from oldest to youngest: Lower (L) unit (late Mesozoic or early Cenozoic), Middle (M) unit (early Miocene or older) and Upper (U) (late Miocene to Pleistocene). The L unit consists of phyllitic siltstone beds and masses of peridotite-serpentinite and gabbro of mélange materials. The M unit crops out widely in the whole Coastal Range, consisting exclusively of andesitic agglomerates. Hornblende-pyroxene andesites and pyroxene andesites are the principal rock types for most boulders of the andesitic agglomerates; hornblende-pyroxene andesites agglomerates constitute most of the unit (Phase A) (Yen, 1968) and pyroxene andesites agglomerates form the uppermost part (Phase B). The U unit consists mainly of alternating sandstone and shale with conglomerate beds, basaltic-andesitic lava and agglomerate and limestone lenses. Based on the lithic facies, the U unit can be subdivided into two beds: the Us beds representing shale with or without sandstone or alternating beds of sandstone and shale and the Uc beds representing conglomerate beds (Yen, 1969). Pyroclastic beds (pyroxene andesitic and basaltic) appears sporadically at the base of the Us beds (Phase C) or at the horizons near the boundary between the Us and Uc beds (Phase D). There are several younger intrusive hornblende-pyroxene andesitic masses (Phase E) in the central part of Juang and Chen : Volcanic Necks of Eastern Taiwan 85 andesites (agglomerates), early Miocene or older. B. Local effusion of pyroxene andesites (agglomerates), early Miocene or older. C. Local effusion of pyroxene andesites and basalts (agglomerates), early Miocene or late Miocene and / or Pliocene. D. Local effusion of pyroxene andesites and basalts (agglomerates and lavas), Pliocene. E. Local intrusion of hornblende andesites (massive), Pliocene, F. Local intrusion of basalts (dikes), Pleistocene to Recent. The locations of the outcrops of volcanic neck or basaltic dike are shown in Fig. 2. The host rock of the conglomerate or volcanic breccias beds may belong to M unit of A phase (Yen, 1968). According to Hsu (1956), it belongs to Miocene Tulunshan Formation. Based on the explanatory text of the geologic map of Chengkung and Tungho (Lo et al., 1993), the Tuluanshan Formation consists of volcanic breccias, lava flow, lapillus (lapilli), conglomerates, agglomerates and tuffs of Miocene age. The Formation can be further divided into three members, namely the Shihmen volcanic Breccia, the Shitiping Tuff and the Pehchi Member, in ascending order. The vesicular lava of the Shimen Volcanic Breccia is exposed in the Tuweichi. The vesicles are filled with zeolite (Fig. 3), which may represent the product of submarine eruptions. Lutao is an islet off eastern Taiwan located at about 34 kilometers southeast of Taitung. This islet is underlain mainly by andesitic agglomerate with some scattered exposures of andesite lavas. Fig. 2. Geologic map of the Coastal Range shown the location of study areas (modified from Yen, 1968). U, upper unit; M, middle unit; L, lower unit; Is, lower Miocene limestone. A, B, C, D, E, and F, the volcanic rocks of the A, B, C, D, E, and F phases, respectively. the Coastal Range. Some of them captured basicultramafic plutonic blocks and were locally cut by few basaltic dikes (Phase F). The main rock types and geologic ages of the six phases are summarized as follows: A. regional effusion of hornblende and pyroxene-hornblende Fig. 3. The vesicle-rich lava of the Shihmen Volcanic Breccia exposed in the Tuweichi. The vesicles are filled with zeolitic minerals. Lava flows, which may represent the products of submarine eruptions, are generally concentrated in the lower part of the Shihmen Volcanic Breccia. 86 Coll. and Res. (2004) 17: 83-92 Fig. 5. A topographic feature of the Haishenping, Lutao, and an explosive crater is suggested. Fig. 4. Geological map of Lutao and the location of volcanic necks or dikes. Geologic map modified from Ichimura, (1936). Lateritic terrace gravel covers the andesite in the central part of the islet. Raised and recent coral reefs fringe almost the whole. Hornblende-bearing andesite is the dominant rock type both in the flows and as clasts in the agglomerates. The andesite and agglomerate in Lutao have been classified into Niouzihsan, Youzihhu Agglomerate, and Queiwan Agglomerate as three members of the Lutao Formation (Fig. 4). Expulsion of volcanic ash, lapilli, breccia and other forms of ejecta may build a ring around a volcanic vent and produce a crater. In Lutao, several volcanic conduits were found and they form the volcanic necks or dikes (Fig. 4). Volcanic Necks in the Coastal Range and Lutao Three typical examples of volcanic necks found in the Coastal Range and in Lutao are described as follows: Haishenping is located at the eastern coast of Lutao, which is a marine terrace rich in tidal flat biocoenose, especially winsome sea cucumber. The explosive crater in the agglomerate formed a circular rim on the cliff walls. Based on the topographic point of view the landform was named Pug or sleeping Beauty (Fig. 5). Columnar joints, topographic features specific to volcanic rocks are defined as parallel, prismatic columns commonly found in basaltic flows and sometimes in other volcanic rocks. Following the extrusion of lava into the mantling formations, the temperature of the magma began to fall (Bates and Jackson, 1984). The lava cools rapidly from the outside toward the center, causing shrinkage cracks to form, commonly, in a hexagonal pattern (Huber and Rinechart, 1968; Peck and Minakim, 1968). The polygonal shrink joints are similar to the shrinkage cracks developed in sedimentary rocks that had been exposed to curing under the sun. Sedimentary mud-cracks owe their occurrence to shrinkage due to desiccation; whereas the columnar joints in the volcanic rocks occurred as a result of contraction of the rock mass induced by temperature drop (Peck and Minakim, 1968) as shown in Figs. 6 and 7. In a homogeneous mass of molten lava, similar degree of shrinkage may be ubiquitous within many spots of the mass; as a result, hexagonal joints were formed during cooling of the lava. Once such a shrinkage crack forms, the crack-forming process rapidly spreads over the entire surface of the lava mass, while cooling proceeds from surface of the mass towards the inner parts of the mass of the otherwise molten lava. A series of columnar joints was thus formed. Besides the ideal hexagonal jointing, trigonal, tetragonal, pentagonal, and octagonal columns can also be formed (Ryan and Shammis, 1978; Spry, 1962). Based on K-Ar geochronological data, the age of biotite-hornblende andesite of Haiseng ping (2.9 0.2Ma) is comparable to the upper portion of Lutao lava flow (2.4 0.1Ma, Juang and Chen, 1990). We suggest that the remnant volcanic neck may be the conduit of the upper lava flow; through Juang and Chen : Volcanic Necks of Eastern Taiwan 87 Fig. 6. The columns form due to stress as the lava cools. The lava contracts as it cools, forming cracks. Fig. 8. The basaltic dike in Tuweishsi, with horizontal to clino columnar joint. Fig. 7. Columnar joints viewed in cross section showing many Y-intersection. it the magma was transferred to the surface. Tuweihsi is located between Chengkung to Changping in the Coastal Range with an aboriginal village Tuwei. At the hill edge along the irrigation ditch or flume of farming, down to the valley of Tuweihsi (Tuwei stream), a type locality of the Shihmen Volcanic Breccia of Tuluanchan Formation (Lo et al., 1993) is found. The riverbeds are generally composed of volcanic breccias or lava flow. The vesicle-rich breccias may represent the product of submarine eruptions. Clino-columnar joints (Fig. 8) in basaltic dike were found on the cliff walls. The occurrence and petrology of the dikes are very similar to those found at Wukungtung (the five-hole cave), Lanyu (Juang and Chen, 2002). Both the dikes are composed of olivine tholeiites ejected into agglomeratic country rocks. Along the Coastal Range highway, from Changyuan to Takankou, two spines ran along the coastal area. Based on geological map of the Coastal Range, Eastern Taiwan (Hsu, 1954), these outcrops are considered as blocks with andesitic composition in the Tuluanshan Formation of Miocene age. A "spine" is defind as a pointed mass or monolith of solidified lava that occurs over the throat of a volcano. It may be formed by low, forced extrusion of viscous lava, or it may represent magma in the pipe that was exposed by differential weathering. The magnificent outcrops at Tafonfon (Fig. 9) must be related to an explosive crater. Expulsion of volcanic ash, lappilli, bomb and ejecta build a ring around the volcanic neck and produce a crater similar to that 88 Coll. and Res. (2004) 17: 83-92 Fig. 9. The pleated-skirt formed columnar joint in Tafonfon, Taitung. Over half of the site was dug out for building material. found at Haisenping in Lutao (Fig. 5). Based on the occurrence of the columnar joints found at Lanyu (Juang and Chen, 2002), Lutao and the Coastal Range, the mechanism that formed the columnar jointing in the volcanic conduit is suggested in Fig. 10. Columnar joints formed under tensional stress that produced cracks caused by shrinkage due to cooling. The tensile stress is subjected to temperature control, and is directly related to isothermal contours (Hill, 1972; Jaeger, 1961). Although there is no practical theory to explain why the maximum tensile stress should run parallel to isotherm contours, nevertheless as the curvatures of the isothermal contours are negligible, the hypothesis will hold true and reasonable. Thus, as a lava flow or intrusion dyke shrinks due to cooling, the directions of shrinkage contraction will run parallel to the instantaneous isotherm surfaces, and hence the columnar structures will stand up vertical to the isotherm surfaces, its development will gradually proceed from the outer lateral sides of the volcanic body towards the center. It thus stands to reason that the attitudes of such joints will indicate the attitudes of the intrusive rock or dyke (Fig. 10). The layout of columnar joints of a volcanic neck depends on the elevation of the location of the lava conduit (Fig. 10). When the igneous rock formed deep within the magmatic conduit as basic dykes, the contours of isotherm of cooling will run parallel to the magmatic conduit or the basic dyke. The columnar joints formed will be normal to the magmatic conduit or the basic dyke. (Area A in Fig. 10-A). The scenic spots at Wukungtung, Haisen ping and Tuweihsi provide good examples (Fig. 10-A). If the lava was ejected out of the volcanic neck onto the ground surface and formed lava flow, the lava flow would consolidate at the place where the neck made a turn. The isothermal contours of cooling will change in accordance with change in depths. The columnar joints thus formed would show corresponding gradual changes. (Area B in Fig. 10-B). Joints at Hsianpiyen and Laojenyen, Lanyu are probably good examples for this type of joint development (Fig. 10). Furthermore, if the lava flow became solidified above the volcanic neck, vertical, towering columnar joints would be formed (Fig. 10-C). This is illustrated by the towering columnar jointing at Tafonfon of the Coastal Range (Area C; Figure 10-C). There are several places in the United States where volcanic neck columnar jointing is exhibited, a typical example is, Devil's Postpile in California, Devil's Tower in Wyoming, Sheep Eaters Cliffs in Yellowstone National Park, and Palisades sill in New Jersey (Spry, 1962; Harris and Kiver 1985; Huber and Rinehart, 1965; Robinson, 1956). The Penghu islands, situated at the southern part of the Taiwan Strait are widely covered by several thick layers of basaltic lava. Several grand and beautiful columnar jointing landscape sites are found in Penghu Islands. The columnar jointing patterns in Penghu are summarized in Fig. 11. The extension in horizontal plane developed in the Penghu lava flow may be the major difference between the Eastern Volcanic Province and the Western Province (Juang and Chen, 1999). The high-T basaltic lava with low silica content may have higher flow velocity and may spread over large areas to form a mesa with vertical columnar jointing. Vertical columnar jointing or fanwise type joint was not found in the Coastal Range volcanic rocks up to now. CONCLUSION Volcanic necks landscapes with scenic importance are found in the Coastal Range, Lutao and Lanyu. They are worthy of permanent preservation and we strongly recommend legislation to protect these topographic features in the planned National Geopark in eastern Taiwan. Furthermore the combination of volcanic topography, pit hole of Shitiping lapilli's tuff, overturned turbidite sedimentary structure at Hsaioyelieu, Longitudinal Valley active fault of Tapo and the aborigines' culture in eastern Taiwan will offer a good chance for local and foreign Juang and Chen : Volcanic Necks of Eastern Taiwan 89 C. The pleated-skirt formed columnar joint in Tafengfeng. fracture B. Allocation of columnar joints changed gradually. isotherm A. The horizontal to clino columnar joint occurred in rim of volcanic neck at Tafengfeng. A. The horizontal to clino columnar joint occurred in basaltic dike in Tuweihsi. Fig. 10. Possible mechanism for the formation of columnar jointing in volcanic conduit. 90 Coll. and Res. (2004) 17: 83-92 Chimeiyu fracture isotherm B. Yuanbeiyu B. Siyu A. Toujinyu Fig. 11. Possible mechanism for the formation of columnar jointing in Penghu basalt. Juang and Chen : Volcanic Necks of Eastern Taiwan visitors to become familiar with the unique geological landforms in Taiwan. We also recommend application of these topographic features to the United Nations Educational, Scientific and Cultural Organization for future listing. ACKNOWLEDGMENTS We would like to thank Mr. G.J. Chiang of Taitung Elementary School for assistance during field survey, to Mr. J.Y. Juang for technical assistance. The authors are grateful to Drs. Haotsu Chu, S.S. Chang, and Christopher C. K. Fong for reviewing the manuscript, their comments and suggestions are very helpful for the improvement of this paper. This research was supported by the National Science Council under contract NSC941201-29-05-14-03-00-02. REFERENCES Bates, R.L. and J. A. Jackson. 1984. Dictionary of Geological term. Doubleday, New York. Biq, C.C. 1976. A tale of two orogens. Bull. Geol. Surv. Taiwan 25: 149-166. Chai, B.H.T. 1972. Structure and tectonic evolution of Taiwan. American Journal Science 272: 389-422. Harris, D.V. and E. P. Kiver 1985. The Geologic story of the national parks and monuments, 464pp., John Wiley & Sons, Inc. Hill, E.S. 1972.Elements of structural geology. John Wiley and Sons Inc., New York. Huber, N.K. and C.D. Rinehart. 1965. "The Devil's Postpile National Monument." Calif. Div. Mines and Geology, Mineral Information Service: 18, no. 6: p. 109-118. Huber, N.K. and C.D. Rinehart. 1967. Cenozoic volcanic rocks of the Devils Postpile Quadrangle, Eastern Sierra Nevada, California. U.S. Geol. Surv., Prof. Paper 554-D: D1-D19. Hsu. T.L. 1956. Geology of the Coastal Range, eastern Taiwan. Bull. Geo. Surv. Taiwan 8: 1541. Jaeger, J.C. 1961. The cooling of irregularly shaped igneous bodies. Am. Jour. Sci. 259: 721-734. Ichimura, T. 1936 Geology of Kasyoto (Lutao), Taiwan. Bull. Volcan. Soc. Japan 3(1):1-27. (in 91 Japanese) Juang, W.S. and J.C. Chin. 1990. Geochronology and chemical variations of volcanic rocks along the arc-continent collision zone in eastern Taiwan. Bull. Natl. Mus. Nat. Sci. 2: 89-118. Juang, W.S. 1993. Diversity and origin of Quaternary basaltic magma series in northern Taiwan. Bull. Natl. Mus. Nat. Sci. 4: 125-166. Juang, W.S. 1999. Volcanic activity and igneous rock of Taiwan. Pub. National Museum Natural Science. Taichung, 323pp. (2nd ed., in Chinese) Juang W.S. and J.C. Chen 1999. The nature and origin of Penghu basalts: a review. Bulletin Central Geological Survey 12: 147-200. Juang W.S. and J.C. Chen. 2002. The Topographic landscapes of volcanic necks on Lanyu, Taitung, Taiwan. Coll. and Res. 15: 87-95. Lo. H.J., W.S. Chen, and S.R. Song. 1993. Cheng Kung and Tunho explanatory text of the Geological Maps of Taiwan Sheet 60, 63pp. Pub. Central Geological Survey. Peck, D.L. and T. Minakim. 1968. Formation of columnar joints in the upper part of the Kilauea Lava Lake. Geol. Soc. Amer. Bull. 79: 1151-1169. Robinson, C.S. 1956. "Geology of Devils Tower, Wyoming." U.S. Geol. Survey Bull. p. 1021-I. Ryan, R.A. and C.G. Sammis. 1978. Cyclic fracture Mechanism in cooling basalt. Geol. Soc. Amer. Bull. 89: 1295-1308. Spry, R.A. 1962. The origin of columnar jointing, particularly in basalt flows. Geol. Soc. of Australia J. 8: 191-216. Streckeisen, A. 1976. To each plutonic rock its proper name. Earth Sci. Rev. 12: 1-33. Tsao, S.C. 1989. Volcanic columnar jointing of Henghsi (Tuchen, Taipei). Ti-Chih 9: 127-130. Teng, L. S. 1996. Extensional collapse of the northern Taiwan mountain belt. Geology 24: 949-952. Wang, Y., C.N. Yang, and W.S. Chen 1992. YuLi, Explantory text of the geological Map of Taiwan Sheet 48, 81pp. pub. Central Geological Survey. Yen, T.P. 1968. Volcanic geology of the coastal range, eastern Taiwan. Proc. Geol. Soc. China 11: 74-88. 92 Coll. and Res. (2004) 17: 83-92 1 1 2 2