Besides the thirteen hot springs and fumaroles areas where great clouds of geothermal steam rush out of fumaroles of all sizes encrusted with yellow sulfur crystals other typical geological features of the Datun volcano roup are crater lakes and geological faults. Volcanoes can be classified as cones and tholoids.
Volcanic cones are formed by the piling up of lava flows and pyroclastic rocks around the volcanic crater. Mt. Qixing is a good example of this type. At an elevation of 1120 meters above sea level, it is the highest peak in Yangmingshan National Park. This is one of the younger volcanoes in the park, and it has the most intact conical shape.
When large volcanoes are formed, molten lava may flow out of the hillside and form another smaller volcano nearby, known as a parasitic volcano. Mt. Miantian, for example, is a parasitic volcano of Mt. Datun, and Mt. Shamao is a parasitic volcano of Mt. Qixing. Lava with high silica content is highly viscous and will form a low round hill like an inverted bell, known as a tholoid volcano. Volcanic activity in the Yangmingshan area tailed off after the formation of Mt. Shamao some 300,000 years ago, leaving the various signs of post-volcanic activity we see today.
Volcanic craters are characterized by steep-sided depressions like those on Mt Xiangtian, >Mt. Huangzui andMt. Xiaoguanyin. In contrast to craters, steam vents are formed when high temperature volcanic gases spurt out through the earth's surface. The early topography around Lengshuikeng shows intact explosion vents. The active fumarolic steam at Dayoukeng,Xiaoyoukeng, Macao and Gengziping is strongly acidic and corrosive. It attacks and disintegrates the adjacent rock, causing the rock to collapse or slide, particularly when earthquakes or rainstorms occur.
Crater lake such as Xiangtian Pond is formed by rainwater collecting in a crater. Rivers in the vicinity of volcanoes can be blocked by lava flow, and the resulting lake is called a lava-dammed lake. Zhuzihu ("hu" means lake) between Mt. Qixing and Mt. Datun was formed in this way, although subsequent erosion breached the south rim of this lake and allowed the water to drain away. Today the lake has dried up and the area is used to grow vegetables and flowers.
The streams in the park area radiate out in all directions. Being short, steep and swift-flowing, these streams have many gullies and waterfalls. The park's best known waterfalls are Datun Waterfall, Fenglin Waterfall, Juansi Waterfall and Shengren Waterfall. The main streams are Beihuang Stream and Nanhuang Stream, which rise in the northern and southern foothills of Mt. Qixing respectively, together with the south- flowing Shuang Stream and the east-flowing Masu Stream. Water levels in these streams vary considerably with seasonal rainfall.
Hot springs require three conditions for their formation: an underground supply of hot water, hydrostatic pressure to force it upwards, and deep fissures in the rock through which it can penetrate to the surface. In Yangmingshan National Park, the distribution of hot springs and fumaroles is controlled by a sandstone formation that underlies the 13 geothermal areas. Forces inside the earth's crust cause the rock to rupture.
If rocks on both sides of the fracture move relative to each other, a fault is formed. When surface water seeps down and is heated by an underground heat source, most of it is retained inside porous rock but a small proportion rises to the surface along the fissure or fault and emerges as a hot spring. If the water temperature is above boiling point, the water vaporizes underground and emerges as fumarolic steam. All of Yangmingshan's geothermal activity is confined to a narrow strip between Beitou and Jinshan about 18 kilometers long and 3 kilometers wide, in which solfataras are also distributed, The most active steam vents are those at Dahuangzui, Xiaoyoukeng, Macao, Dayoukeng, Sihuangziping and Gengziping.
Yangmingshan's popular hot springs can be divided into three types according to their chemical composition:
- Acid sulfate (white sulfur) hot springs, such as those at Dahuangzui, Zhuzihu, Sihuangziping and Gengziping.
- Acid sulfate chloride (green sulfur) hot springs, such as the one at Xinbeitou.
- Neutral iron-bearing hot springs, such as those at Dingpi Bridge and Lengshuikeng; these are cooler and mostly consist of indirectly heated groundwater.
Enjoying a good soak in a steaming, silky-smooth hot spring set amid beautiful natural scenery is one of life's greatest pleasures. No wonder so many people make the trip up to Yangmingshan on a regular basis. They wouldn't miss their daily hot spring dip for the world!