Volcanic Activities
Taiwan is situated on the Pacific Ring of Fire, a region that encircles the Pacific Ocean where plate tectonics are intensively active. However, compared to the nearby countries like Japan, Philippines and Indonesia where volcanic eruptions occur frequently, there have been no clear records of volcanic eruptions in Taiwan to date. Currently, the only areas in Taiwan where there are signs of volcanic activities are the Datun Volcano Group in northern Taiwan, the Guishan Island off the coast of Yilan, and the undersea volcanoes in the waters off north eastern Taiwan. Scientists have hypothesized that the current volcanic activities in Taiwan are related to the northward subduction of the Philippine Sea Plate, underneath the Eurasian Plate in the waters off the coast of northeastern Taiwan.
Recent studies on geomorphology and geology indict that the eruption activities of the Datun Volcano Group began approximately 1 million years ago, and continued until 200,000 years ago. Sequence stratigraphic analysis of pyroclastic rocks indicate that the Datun Volcano Group previously had seven phases of eruption sequence strata, with phase 2 to phase 4 covering the largest scale and range of eruptions, and phase 3 producing the most amount of volcanoes. The volcanic activities of Datun Volcano Group are related to the location of the fault structures. Lava is erupted through crevices along or created by faults; these crevices also provided channels for hot springs to flow through.
It is generally considered that the volcanic activities of Datun Volcano Group are related to the extensional collapse of environments set up by the late-stage continental collision in northern Taiwan. Sequence stratigraphic analysis of lahar sediments and studies on rock dating have determined that the peak of volcanic activities to be approximately eight hundred to two hundred thousand years ago. Results from helium isotope studies have also indicated that materials in the lower mantle have infiltrated upward to the Datun Volcano Group and concentrated earthquake swarms are also detected around Mt. Qixing and Dayoukeng, accompanied by occasional volcanic earthquakes. Dating of volcanic ash suggests that the last eruption of the Datun Volcano Group was approximately 6,000 years ago, which would classify the Datun Volcano Group as dormant volcanoes. However, as there is still magma activity within the crust of the volcanic region and possible future eruptions have not been completely dismissed.
Volcanic geological terrain
The appearance of volcanoes may be classified based on their type of eruption and different compositions: shield volcanoes, cone volcanoes and composite volcanoes (stratovolcanoes). Shield volcanoes are so named for their low-profile and flat appearances, with a large base and gentle slopes resembling a shield that laid flat on the ground. The lava flow of shield volcanoes are mainly composed of fluidic basaltic lava; there are no volcanoes of this kind in the Yangmingshan National Park as the volcanoes in the Park are composed mainly of andesite. Cone volcanoes have a steeper conical appearance; volcanoes that are formed from layers of pyroclastic cinders are called cinder cones. Volcanoes formed by the more viscous, less fluidic felsic or intermediate lava appear in an inverted bell-shape and lack distinctive craters. Mt. Shamao and Mt. Dajianhou are conical volcanoes in Yangmingshan National Park shaped like an inverted bowl. The shape of the composite volcano is conical, the upper slopes have steeper gradient, while the lower slopes are less steep. Composite volcano is formed from alternating layers of lava flows and pyroclastic rocks and is also known as a “stratovolcano." It is the most common type of volcano in the world, especially those that are found on island arcs. The volcanoes in the Yangmingshan National Park are mostly composite volcanoes, such as Mt. Qixing, Mt. Xiaoguanyin, Mt. Datun, Mt. Zhuzi and Mt. Huangzui.
As the underground magma rises to the surface to form a volcano, the peak of the volcano often forms a sunken basin that is called a volcanic crater. Volcanic crater can be divided into three kinds based on their mechanisms of formation: explosive crater, caldera and cauldron. An explosive crater is formed when the top of the lava channel explodes on the volcano, forming a sunken crater; the caldera is formed when the magma chamber become empty during the late stages of volcanic eruption. Without a supply of new magma, the chamber caves in on itself due the weight of the top rock layers, forming a sunken depression. A cauldron is related to a volcanic structure and is formed before a volcano erupts: as magma rises, the top of the magma chamber collapses to form fractures, and lava flows from vents within these fractures to the surface. Most of the major volcanoes in the Yangmingshan National Park feature explosive craters, however they have become less distinctive due to years of weathering and erosion.
Crater lakes are formed when rain water collects in the sunken craters; an example being the Xiangtian Pond in the Park. However, due to fractures in the ground, the pond would only manifest itself during periods of torrential rain falls. Creeks in the vicinity of volcanoes are often blocked by lava flow, and the resulting lake is called a barriered lake. Zhuzihu between Mt. Qixing and Mt. Datun was formed in this way. However its water has since drained away, the lake has dried up and is now used to grow vegetables and flowers.
Due to the influence of volcanic terrain, the creeks in the Park radiate outward in all directions. The creeks in the Park are short, steep and swift-flowing, creating many valleys and waterfalls. Some of the well-known waterfalls in the Park are Datun Waterfall, Juansi Waterfall, Fenglin Waterfall and Shengren Waterfall. The main creeks are Beihuang Creek that originates from the northern foothills of Mt. Qixing; the Nanhuang Creek that originates from the southern foothills of Mt. Qixing; the south-flowing Shuangxi Creek and the east-flowing Masu Creek. Water levels in these creeks vary considerably with seasonal rainfall.
After the volcanic activities ceased, residual heat still remained deep underground. As surface water seeps deep in the ground, heated and stored in rock layers with good permeability. Some of this water moves to the surface through fissures in the faults, forming hot springs. If the temperature of the hot water exceeds the boiling point, it becomes water vapor and rises to the surface, forming fumaroles. These activities are known as post-volcanic activities. Most of the hot spring and fumarole locations of the Datun Volcano Group are situated in a belt-like distribution that runs in a northeast to southwest direction. This may be due to the Xinzhuang Fault between Beitou and Jinshan, which is 18km long and 3km wide. The fumaroles are highly active in Dahuangzui, Xiaoyoukeng, Macao, Dayoukeng, Sihuangziping and Gengziping areas, and the most well-known hot springs of Yangmingshan are also located within these areas.
The hot springs of Yangmingshan National Park can be divided into three types according to their chemical composition: acid sulfate (white sulfur) hot springs, such as those at Dahuangzui, Zhuzihu, Sihunagziping and Gengziping; acid sulfate chloride (green sulfur), such as those at Macao and Dayoukeng; and neutral iron-bearing hot springs, such as those at Dingpi Bridge and Lengshuikeng, which have lower temperatures and mostly consist of indirect heated groundwater. One of life’s greatest pleasures is to soak in a steaming, silky-smooth hot spring set amid beautiful natural scenery. No wonder so many people make frequent trips up the mountain to visit these places regularly. The lure of a hot spring is too great to miss!
The main rock strata in the Park is composed of igneous and sedimentary rocks. Igneous rocks are primarily composed of andesite formed during the volcanic activities of the Pleistocene era. Its modes of occurrence include lava flow, agglomerate and tuff, with some basaltic magma and lahar sediments (volcanic mudflow). Andesite is a rock of intermediate composition, and usually contains minerals like hornblende, pyroxene, plagioclase and olivine. Andesite is further classified based on its mineral assemblage, such as the hornblende-pyroxene andesite, Two-pyroxene andesite and olivine-pyroxene andesite. Basalt is a mafic igneous rock, containing minerals like pyroxene, plagioclase and low quantities of olivine.
Sedimentary rock was formed from sediments in a shallow sea environment on the edge of Eurasia continent, during the late Oligocene to Pliocene Era. The main sedimentary strata include the Wuchishan formation, Mushan formation, Talio formation, Shihti formation, Nanchung formation and Kueichoulin formation. Igneous rocks are deposited on top of sedimentary rocks in an unconventional fashion (discontinuous time intervals).