Volcanism and Volcanic Rocks
The earliest documented volcanic activity in the western foothills is recorded by late Oligocene tuffaceous
shale and sandstone in the Tsukeng Formation in central Taiwan. Limited exposures have precluded detailed
geologic study of this phase of volcanism. Volcanism continued intermittently in the depositional basin of
the western foothills during the Miocene but essentially ceased in Pliocene time. Basaltic tuffs or tuff
breccia and some lava flows are found in almost every Miocene unit in northern Taiwan and north-central
Taiwan. The Miocene volcanism in southern Taiwan is represented by small tuffaceous lenses sparsely
scattered in the younger Miocene sediments only.
The Plio-Pleistocene volcanic activities are most important in the geologic evolution of Taiwan. Extensive
volcanic eruptions were active at the northern tip of Taiwan and on offshore islands. Two important groups
of andesitic and dacitic volcanoes, the Tatun volcano Group on the west and the Chilung Volcano Group on the
east, were built up by successive eruptions near the northern coast. The five offshore islands in
northeastern Taiwan are all volcanic islands of the same phase of andesite volcanism. Great volumes of
basaltic lava rose up quietly in fissure eruptions through the Neogene sediments on the Penghu Islands in
the Taiwan Strait. The basalt flows form an extensive sheet of plateau basalt, covering 63 island and islets
of the Penghu Group. The age of the basalt is in question now. Pleistocene or Miocene?
MIOCENE VOLCANISM
Contemporaneous volcanism took place sporadically during the deposition of the Miocene sediments in the
western basin, but the volcanic rocks are distributed in different areas at different geologic times. The
volcanic eruptions are largely of basaltic composition and pyroclastic deposits are more abundant than lava
flows. Basaltic pyroclastic piles were built up from various eruptive centers in the different Miocene
formations. They transgressed different beds of the Miocene strata in different places, forming a number of
discrete tuff bodies of varying thicknesses. These are the products of contemporaneous erosion and
redeposition of the tuff bodies. They occur now as lenticular masses or irregular beds interbedded in the
rocks.
In northern or north-central Taiwan, nearly all the Miocene units contain contemporaneous volcanic effusions
locally, each varying widely in size and in areal distribution. The distribution and extent of volcanism,
however, are quite restricted. The Miocene volcanism in western Taiwan can be divided into three volcanic
stages (see Fig. 6).
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| Figure 6. Miocene volcanism in the northern part of the western foothills |
VOLCANIC DEPOSITS IN THE YEHLIU GROUP (Kungkuan volcanic stage)
This is the most extensive Miocene volcanism in western Taiwan and extends from the northern coast southwest
to the Tahanchi drainage on the boundary between Hsinchu-hsien and Taoyuan-hsien. Scattered
volcanic bodies are distributed in the Mushan Formation and the Taliao Formation of the early sedimentary
cycle in the Miocene. This phase of volcanism is called the Kungkuan volcanic stage (Yen, 1950 and 1958; Ho,
1969b) discussed earlier in the section on the Taliao Formation. The tuffaceous bodies in this stage assume
various geometric shapes, as elongate lenticular bodies, diverging or coalescing thin layers or irregular
pods. The tuffaceous bodies are usually gradational to the surrounding strata and wedge out laterally. The
thickest tuffaceous body is 200 meters thick at Nanshihchio in Taipei-hsien, and the thinnest layers are
less than one meter thick. Lateral extension of the volcanic layers ranges from several meters to several
kilometers.
VOLCANIC DEPOSITS IN THE JUIFANG GROUP (Chienshih volcanic stage)
This phase of volcanism occurred in the middle sedimentary cycle of the Miocene rocks. The volcanic
activity was apparently minor and only limited volcanogenic deposits are found in the Shihti and Nankang
Formations. The volcanic bodies and layers are small or thin and mostly concealed underground. They have
been revealed largely in the underground mine workings in the coal fields of Taoyuan- hsien,
Hsinchu-hsien and Miaoli-hsien and have been described in the section on the Shihti
Formation. The volcanogenic rocks in this stage are distributed sparsely in a belt close to the Central
Range on the east.
The exposed volcanic bodies in the Nankang Formation are very limited and are distributed mainly in the area
of Neiwan and Maohoshan near Chienshih in Hsinchu-hsien (Ho and Hsu, 1951). These volcanic bodies are
stratigraphically associated with limestone. Small tuffaceous outcrops have also been found in the Nankang
Formation in the Nanchuang coal field (Ho and others, 1954). In the Chinkuashih gold district on the
northern coast, a lenticular bed of glassy tuff and other tuffaceous sediment is exposed in the Nankang
Formation (Ho, 1964). Basalt flows and basaltic tuff layers have also been found in units equivalent to the
Nankang Formation in holes drilled by the Chinese Petroleum Corporation in the Hsinchu area and offshore
regions. Based on the geographic occurrence, the volcanic activities in the Juifang Group are named the
Chienshih volcanic stage in this text.
VOLCANIC DEPOSITS IN THE SANHSIA GROUP (Chiopanshan volcanic stage).
Volcanism was active again in the late Miocene sedimentary cycle when the coal- bearing Nanchuang Formation
was being deposited. This latest Miocene volcanic phase is named the Chiopanshan stage by Yen
(1950 and 1958); A number of tuffaceous beds of various extension and length are intercalated in the rocks
of the Nanchuang Formation exposed in Taoyuan-hsien and Hsinchu-hsien (Tsan, 1961).
The distribution of tuffaceous bodies or layers extends intermittently in a northeast-trending belt about 60
kilometers long. The northwest boundary of this belt is the Hsintien Fault. The volcanic rocks in the
Nanchuang Formation have been discussed in the section on the Nanchuang Formation. Volcanic rocks are very
scarce in the Kueichulin Formation, although basalt flows and tuff have been reported in some holes drilled
in the area of Hsinchu and Taoyuan. Small volcanic bodies are found in strata equivalent to the Kueichulin
Formation in southern Taiwan.
PLEISTOCENE VOLCANISM
Large-scale andesitic eruptions took place in Pleistocene time in northern Taiwan and in some northeastern
offshore islands, depositing both lava and pyroclastic rocks. Two important Pleistocene andesitic volcano
groups were formed, the Tatun Volcano Group and the Chilung Volcano Group, located on the west and the east
side of the Chilung harbor respectively. No volcano is known to be active in Taiwan in the historical
record, but geothermal evidence suggests that these volcanoes may not be entirely extinct (Chen and Wu 1971).
Recent age dating indicates that incipient volcanism in these two volcano groups may have taken place in
late Pliocene time and gradually increased in tempo toward the Pleistocene Epoch (Chuang and Bellon, 1984).
In the plate tectonic setting, the andesites of northern Taiwan are formed by partial melting of the
subducting wet basaltic oceanic crust and associated oceanic sediments. These volcanic rocks represent the
western extension of the volcanic arc. In the Ryukyu Arc System which will be discussed later. In addition
to the Pleistocene andesites in northern Taiwan, Pleistocene flood basalt is widely distributed on the
Penghu Island Group in the Taiwan Strait according to early reports. Recent age dating (unpublished)
indicates, however, that the age of Penghu basalt may be Miocene.
TATUN VOLCANO GROUP
The Tatun Volcano Group comprises a series of andesitic volcanoes in the northernmost part of Taiwan, the
nearest being 15 kilometers north of Taipei. About 20 volcanoes and volcanic cones are included in this
volcano group, among which Chihsingshan (1,119.6 meters) is the highest and youngest volcano in the group.
Hot springs, fumaroles, and solfataras are found at many places in this volcano group, mostly on the
southeast side of the Chinshan longitudinal fault, and exploration for geothermal resources has been carried
on for many years (Chen and Wu, 1971). Sulphur and pyrite are being produced by solfataric activity in
several places of this volcano group. Craters are preserved at the summit of some of the cones. A few have
been eroded to craterless volcanoes or were erupted as high-viscosity volcanic domes. This group of
volcanoes are mainly strato-volcanoes, built by successive eruptions of andesitic flows, volcanic ash, and
coarse pyroclastic ejecta. The Tatun andesites commonly contain augite, hornblende hypersthene or
combinations of these.
The Kuanyinshan volcano is separated from the main volcanic chain by the Tanshuiho river. This volcano,
rising to 611.5 meters above sea level, built atop Pleistocene clastic sediments and upper Miocene
coal-bearing strata. Kuanyinshan and Hunglushan in the main Tatun group are the only two volcanic cones in
which minor high-alumina basalt has been discovered (C. H. Chen, 1975) in addition to the main andesite
flows. The basalt is believed to be a little older than the andesite.
In the Tatun Volcano Group, effusion of andesite lavas commonly alternated with explosive eruption of
pyroclastic debris. At least 15 separate lava flows and three tuff breccia formations have been mapped by
Chen and Wu (1971). Based on recent K-Ar dating (Chuang and Bellon, 1984), the andesites are divided into
three age categories. The earliest phase is 2.5 m.y. (Pliocene), including biotite- bearing hornblende
andesite and hypersthene andesite. The second phase is 0.75 m.y. (Pleistocene), and consists largely of
augite andesite. The third phase is 0.5 m.y. (Pleistocene), and includes olivine-augite-hornblende andesite,
hypersthene-hornblende andesite and glassy augite andesite.
CHILUNG VOLCANO GROUP
Numerous quartz andesite or dacite flows are found on the northeastern coast of Taiwan, east of Chilung
harbour. These flows and associated pyroclastic rocks constitute the Chilung Volcano Group. Ore-forming
fluids accompanying the dacite intrusions have deposited valuable gold and copper ores in the Miocene host
rocks or at the intrusive margins, forming the three principal deposits at Chinkuashih, Chiufen, and
Wutankeng. The host rocks are mainly calcareous sandstone and subordinately carbonaceous shale. Porous
dacite is also the favorable loci for metallization. Six dacite bodies are exposed in this area, and a few
more are concealed underground. These dacite bodies are of two main types. The Tsaoshan and Chimuling bodies
are extrusive, with well preserved explosion craters and containing pyroclastic ejecta. No significant
hydrothermal effects are observed around these dacite bodies. The other four bodies, Chilungshan, Penshan,
Hsinshan, and Wutankeng, are intrusions in the form of chonoliths or irregular bodies that cut through the
Miocene strata. The Chilungshan dacite is the most conspicuous, forming a prominent conical hill about 2.47
kilometers across and 588.5 meters above sea level on the northern coast. The other intrusive dacite bodies
have suffered intense hydrothermal alterations and vary considerably in texture and composition. The age of
volcanism in this group is ascribed to Pleistocene, although radiometric age has been limited to the
Chilungshan volcano only which has yielded a K-Ar age of 0.2 to 1.7 m.y., (Chuang and Bellon, 1984).
VOLCANIC ISLANDS OFF THE NORTHERN COAST
Five islands off the north- eastern coast of Taiwan are chiefly volcanic islands (Lin, 1967). Four of them,
Mienhuahsu, Huapinghsu, Pengchiahsu, and Kueishantao, are built up of andesitic lava flows. The fifth,
Chilungtao, which is closest to the Chilung harbour, is made up of dacite and is believed to be genetically
related to the Chilung Volcano Group. Andesitic agglomerate and tuff breccia are found in Kueishantao and
Pengchiahsu, being much more extensive in the former island. Solfataras and fumaroles are still active on
the Kueishantao, indicating hidden volcanic activities in the underground. The age of volcanism in these
islands has been assumed to be Pleistocene. Only the basaltic andesite on the Pengchiahsu island has been
dated by K-Ar analysis (Chuang and Bellon, 1984), yielding an age spread of 1.5 to 3.6 m.y.
(Plio-Pleistocene).
BASALT ON PENGHU ISLANDS
The Penghu Island Group in the Taiwan Strait exposes extensive plateau basalts of Pleistocene age given in
earlier reports. The age of the basalt has recently been dated to be Miocene by K-Ar method, but the report
has not yet been published. These basalts cover the entire island group except the westernmost island,
Huahsu. They form flat-topped tablelands reaching a maximum elevation of 50 meters above sea level. Horizons
of sand, clay, and carbonized wood fragments are interbedded with the basalt flows. The sediments contain
mollusks and foraminifers indicating a reported Pleistocene age (Lin, et al., 1957) and a shallow marine
environment. Recent paleontologic studies, however, have suggested that these fossils may be older, possibly
Pliocene or even latest Miocene (unpublished report). Further faunal studies are thus needed. The Penghu
basalt and its associated sediments are believed to form a thin blanket directly overlying the basement,
which is exposed on Huahsu. This basement is quartz porphyry with quartz veins and xenoliths of phyllite and
orthoquartzite, and has yielded a radiometric date of 55 m.y. (Jahn and Chen, 1976). This porphyritic rock
is considered to be related to the granite batholith exposed in the Fukien province of mainland China.
Eastward to the main islands of the Penghu Group, a thin wedge of Neogene sediments generally intervenes
between the basalt flows and the basement. One hole recently drilled on Penghu recorded 320 meters of basalt
overlying about 200 meters of Neogene sediments which in turn overlie Mesozoic (?) basement composed of
indurated siliceous shale and quartzite (Chou, 1969).
The Penghu basalt erupted quietly, as pyroclastic deposits are comparatively rare. The eruption produces
nine successive flows (Lin and others, 1957). This mafic rock contains plagioclase laths, dark augite, and
some olivine. Olivine basalt is the main basalt type, and the rock commonly weathers to a rusty brown mass
with well-developed spheroidal weathering. The upper surfaces of basalt flows have been decomposed into red
soil under tropical sun and poor surface drainage. The basalt is either dense and compact or amygdaloidal.
Amygdules are generally filled with aragonite that is called "wenshih" in local terminology. These
amygdaloidal basalts provide an ornamental stone for which Penghu is noted.
TSAOLINGSHAN BASALT
About 4 kilometers south of the town of Tachi in Taoyuan-hsien, a conical volcanic hill rises to 347 meters
above sea level. It is made up of vesicular alkaline basalt with a small amount of pyroclastic deposits
exposed on the southern slope. The basalt flow contains phenocrysts of olivine, augite, plagioclase, and
minor accessory minerals with a partly glassy groundmass (Yen, 1949). Eruption of the basalt is inferred to
have taken place in Pleistocene time along the Hsintien Fault, a major tectonic feature in northern Taiwan.
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