Download The Topographic Landscapes of Volcanic Necks in the Coastal

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