The Tatun (Datun) Volcanic Group (TVG) is a number of andesitic lava domes in the northern portion of the island of Taiwan. There are at least 20 volcanoes, the southernmost (and largest) of which are located only 15 km N of Taipei City. This puts over 5 million within 5 km of the field and nearly 8 million within 30 km.
The region is well developed, with a modern industrial infrastructure including two nuclear power plants built at the foot of the volcanoes. There is a national park centered around the volcano group visited daily by thousands, so any activity could potentially have deadly consequences.
Taipei City is the capital of the Republic of China. It is located on an ancient lakebed bounded by a pair of river valleys. It is the political, economic, educational and cultural center of Taiwan and one of the major economic hubs in East Asia. Tourism is a major part of the economy, with over 6 million overseas visitors in 2013.
The climate is a humid subtropical climate with long, hot and humid summers with occasional heavy rains and typhoons. Winters are short, generally warm and foggy. Typhoon season is June – Oct.
The group was mostly active 2.8 – 0.2 Ma. There is geothermal and fumarole activity among these volcanoes. There appears to be a substantial magma chamber beneath the field.
The group was thought to be mostly dormant with the last eruption thought to be 200,000 years ago. However, new monitoring equipment installed a decade ago and further investigation into the group revised eruption dates as recently as 5,000 years ago. This caused quite the stir in local media, both due to the possibility of future eruptions and its close proximity to Taipei.
Data from this improved monitoring led to Taiwanese geologists taking a closer look at the field. They found active fumaroles, at least one small phreatic eruption, a possible offshore eruption north of the island, and worst of all a pair of what appear to be substantial magma chambers beneath the field itself and a related island just off the north shore of Taiwan. Erik Klemetti mentioned this in two posts, one in 2011, the other in 2017.
Taiwanese geologists set up the Taiwan Volcano Observatory (TVO) at Tatun in 2011. The observatory included various real-time monitoring systems to carry out long-term monitoring of the Tatun Volcanic Group and submarine volcanoes off the coast to Guishan Island. TVO monitors seismic activity, fumaroles, hot springs, inflation and ground temperatures. They announced results in 2019, concluding that both the TVG and Guishan Island are active volcanoes. They define an active volcano as one that has erupted in the last 10,000 years.
The TVG covers around 400 km2 on the northern end of Taiwan. The basement under the TVG is moderately folded and cut by multiple faults, with several major thrust faults striking to the NE. There are two principal volcanic ridges – an E-W ridge intersected at its western end by a SW-NE ridge. Each ridge is around 15 km long with an average elevation of 800 – 1,000 m. The E-W ridge has gentle slopes. The SW-NE ridge had a sharp ridge, steep alpine slopes, and extensive water erosion.
The field has produced primarily andesites. There appears to be a change in water content of the magma and potentially a change in the plumbing system as volcanic activity shifted from the SW-NE ridge to the E-W ridge. There is only a small cone (Mt Hunglu) located in the western portion of the E-W ridge.
There appear to be two magma chambers north of Taipei City. The one 20 km under the TVG is 15 x 16 km across and 4 – 10 km thick. The one under Guishan (Turtle) Island is 1.5 times as large. The magma body under the TVG appears to be a single lens of magma or a stack of sills. It could be as large as 350 km3 depending on how much of it is molten. The percentage of molten, eruptible magma is estimated to be far below the 60% required for an eruption.
There were two major pulses of volcanism in the region, 2.8 – 2.5 Ma and 0.8 – 0.2 Ma. Cising (Qixing), Dajianhou and Huangzuei appear to be the youngest volcanoes in the group, 0.3 – 0.2 Ma. Basaltic Hunglu may be as young as 0.11 Ma. Deeply eroded volcanoes in the SW-NE ridge are estimated at hundreds of thousands of years old. Similar age estimates of young volcanoes on the E-W ridge may be too old based on the excellent preservation of their primary volcanic landforms.
Dating the youngest eruptions has only recently been done. Several layers of volcanic ash were found in drill cores from the nearby Taipei Basin. These were dated less than 20,000 years separated in time by 3,000 years. This means that previous estimates of eruptions from these volcanoes of several hundred thousand of years may be significantly off, with the actual activity being much more recent.
Activity in the TVG was generally more extrusive than explosive, creating well-preserved steep domes and cones. Many of these have steep sides, indicating extrusion of very stiff, almost completely degassed and solidified magmas. Most of these are half spherical, indicating they were erupted in single episodes of continuous magma extrusion – monogenetic dome eruptions. The domes are surrounded by short, steep sided aprons of coarse gravel deposits. There are small volume deposits of hot avalanches. Few block and ash flows were found. Explosive episodes like Merapi-type dome collapses were small volume and apparently rare.
Several domes have crater-like depressions on their summits. No pyroclastic deposits were found on the crater rims. The craters were likely formed by subsidence after the extrusive eruption. The Cising volcano is different from the other volcanoes, as it appears to be formed by multiple extrusive episodes separated by periods of repose. It also had explosive activity. It is a polygenetic effusive lava dome structurally transitioning to a stratovolcano. It has lava domes and thick lava flows.
There are 9 well-preserved lava flows with clearly defined outer boundaries. Thicknesses vary from 80 – 150 m. Most of these are a single long branch. Two have multiple branches. These flows have structures typical for andesitic lava flows including coulees, pressure ridges, coulees, large vesicular lava blocks, massive joints. Effusive eruptions for the TVG produced relatively low discharge rates and lasted up to several years. Maximum flow length was 5.6 km and maximum volume around 0.6 km3.
There are few pyroclastic deposits preserved. While erosion may have removed much of what was erupted, it appears that these were only a few percent of magmas erupted in the TVG. Most of these are tephra falls. Block and ash and surge deposits are rare. No ignimbrites are found. A significant part of the explosive deposits were remobilized by lahars shortly after deposition. A single tephra ring is found on the flanks of the Cising volcano. It is primarily made of pumice and dates around 0.5 Ma.
Mt Hunglu was a basalt shield buried by a tuff cone. Its most recent eruption appears to be a wet phreatomagmatic eruption, which may mean that the old Hunglu crater contained a crater lake or swamp. While the underlying lavas of this cone date 0.11 Ma, the tuff cone may be much younger.
The slopes of the youngest volcanic edifices are smooth and little eroded. There are several horseshoe shaped amphitheaters 0.5 – 1 km across identical to those caused by large scale gravitational flank collapses. The scars are not filled by younger volcanics, indicating that there was no volcanic activity following these collapses, meaning the collapses took place some period of time after volcanic activity stopped. Some of these collapses took place shortly after dome or lava flow emplacement while they were still hot.
Many of these scars are located where the volcanic edifices are intersected by tectonic faults. Avalanches of Datun, Siaoguanyin and Cising were confined to a creek valley where they cover one another. The largest of these collapses came from Datun has a volume of 0.1 km3.
The Siaoguanyin debris avalanche was hot during its emplacement, as the debris has radial cooling joints, the occasional bread crust surfaces. The deposits from this avalanche are similar to Merapi-type pyroclastic flows from a newly extruded lava dome. The material is crystalized, degassed and still hot. This avalanche was fairly mobile and flowed as far as 5 km from the volcano. It ran 80 m up the flanks of neighboring Mr Huangsi
The youngest collapse was from Mr Cising. A lava flow collapsed and underwent weak disintegration. The 0.05 km3 deposit is poorly sorted and has is mostly large blocks. As it flowed, the leading edge of the avalanche hit the flanks of Mt Datun, made a left turn and traveled for another kilometer.
There are small volume collapses at Mr Cigu and Dajianhou that appear to be more similar to large rockfalls than debris avalanches.
Collapses from these volcanoes took place after eruptive activity stopped. They were not directly associated with volcanic activity. There does not appear to be a significant amount of hydrothermally altered rocks associated with the collapses, though hydrothermal fields are common in the avalanche scars (amphitheater). Collapsed parts of the volcanoes appear to be detached by tectonic motions with collapses at least in part caused by seismic activity.
Lahars form a significant amount of volcanic debris in the TVG. Individual lahars are massive, poorly sorted, with thicknesses 2 – 10 m. There are two types of lahars present – poorly sorted debris flows and better sorted flood flows. There are little clays present in the lahars, indicating that most took place shortly after emplacement of the material mobilized by the lahars.
There are no records of the Tatun Volcanic Group (TVG) erupting in recorded human history on Taiwan. This is similar to historic records of the Ontake volcano in Japan prior to 1979. The group was thought to be inactive to extinct until those conclusions were changed based on geochemical analysis, monitoring of volcanic tremor, and recent dating of volcanic ash. Newly installed monitoring also discovered long-period seismic signals, what is called “heartbeat-like seismicity”, and identified a magma chamber beneath the group.
While activity of the entire field has not been dated, recent activity of the western portion has. This area included Datun, Siangtian, Bailaka, Siaoguanyin, Hunglu and Cising.
The earliest large event was the flank collapse of Mr Datun which completely filled the neighboring valley to the S and E. The collapse was followed by a period with no significant deposit events. A deep canyon eroded into the avalanche deposit. This was followed some 23,000 years ago by a dome extrusion on the southern slope of Mt Siaoguanyin. Shortly after extrusion, part of the hot dome collapsed creating a high-speed debris avalanche. Multiple lahars followed.
There were several Plinian eruptions from Mr Cising 23,000 – 16,000 years ago. The largest VEI 4 formed the Siaoyoukeng tephra ring. Multiple relatively small lahars redeposited the erupted pumice. Several small to moderate vulcanian eruptions took place 16,000 – 13,000 years ago depositing thin layers of ashfall.
One of the last magmatic eruptions at Mt Cising took place around 13,000 years ago. It started with a small to moderate vulcanian eruption which was followed by extrusion of a lava flow. Eruptive products including a Merapi-style block and ash pyroclastic flow were mobilized by lahars, filling a nearby lake. Around 6,000 years ago, a large fissure opened the summit of Mt Cising. This was likely caused by a strong earthquake. The event caused a small phreatic eruption and a gravitational collapse of the western slope of Cising. This low mobility debris flow was 0.05 km3 in size. Summit lava flows are thought to be extruded not long before the fissure opened. This means the youngest magmatic eruption at Cising took place around 6,000
Past eruptive behavior of a volcanic field is predictive of how it is expected to erupt in the future. With this in mind, the vast majority of volcanic activity in the TVG are monogenetic lava domes formed by extrusion of andesitic, degassed, highly crystalline, viscous lavas. Volume of these domes is 0.05 – 0.3 km3. Eruptions forming them took several years. Future eruptions are expected to be dome-forming eruptions taking several years to complete. Likewise, lava flows in the TVG are exceptionally long and thick, also taking 4-5 years to erupt. Future lava flows like this may present a real threat to neighboring nuclear power stations. Explosive eruptions have been mostly mild, with relatively small volumes of ejected pyroclastics. The most vigorous eruptions were at Cising with a VEI 4 Plinian eruption. Generally, the TVG produced weakly explosive eruptions, much less explosive than typical andesitic arc volcanoes. There are at least 5 flank collapse scars on TVG volcanoes. Large scale collapses may occur at any time. So, might large lahars.
The first scientifically recorded eruption took place March 16, 2014. It was not initially reported or observed. Analysis of seismic and acoustic data afterwards identified a source near the Hsiaoyukeng fumarole. Acoustic data accelerated 60x normal, similar to steam released from a kettle. A 2017 paper by Lin suggests that there might be a dynamic connection with this event and a M6.7 quake in Chile. A second event took place Jan 5, 2015. This one was immediately identified and coincident with a M5.0 in Japan. If these connections are real, the TVG is one of the few places on earth where volcanic activity appears to be tied to seismic activity.
Erik Klemetti suggests that the sea discoloration viewed in the preceding video was more likely activity from underwater steam or gas vents off the coast of Kueishantao Islands rather than an actual eruption in 2015.
Tectonics of Taiwan are governed by the collision of the Philippine Sea Plate with the Eurasian Plate. Taiwan itself is on the continental shelf of the Eurasian Plate and has been built by accreting terranes and mountains pushed up by the collision
The collision is complex, with the Eurasian Plate subducting under the Philippine Sea Plate to the south and the Philippine Sea Plate subducting under the Eurasian Plate to the north. The body of Taiwan is where the subduction regime changes, not unlike what we see in New Zealand. Unlike New Zealand, this change in subduction has not led to extensive volcanism, though there are two major volcanic arcs involved. Like New Zealand, Taiwan is very seismically active.
Although there are only two plates involved in this collision, the lines of collision are complex. To the south, we have the Manilla Trench to the west of Luzon stretching northward to Taiwan. In this case, the Eurasian Plate is subducting under the Philippine Sea Plate.
The Luzon Volcanic Arc is a volcanic forearc created by this portion of the collision stretching from the middle of Luzon Island in the Philippines north to where it is being subducted in central Taiwan. Collision of the Luzon volcanic arc beneath the Eurasian Plate started around 400 km east of the current location of Taiwan. It has migrated westward over the last 9 Ma. The collision proceeds today at around 5.6 cm/yr.
Subduction of the Eurasian Plate stops somewhere under Taiwan where it changes both in type and direction. North of Taiwan, the Philippine Sea Plate is subducting under the Eurasian Plate under they Ryukyu Trench. To the NW of this line is the Ryukyu Volcanic Arc. North of that it the spreading center that is the Okinawa Trough. The pivot point for the change in subduction is under the northern end of Taiwan, which is also where the TVG is located.
The Ryukyu volcanic arc descends around 45-degree angle under northern Taiwan. The TVG is located slightly off the lateral edge of the subducting plate. Magmas of the TVG appear to be subduction magmas, though they may be coming from the gradual westward unzipping of the Okinawa Trough, the back-arc basin of the Ryukyu volcanic arc. On the other hand, the TVG may be associated with the Taiwan orogen.
Given the existence of at least two substantial magma chambers under northern Taiwan, observation of possible offshore subsea eruptions, active fumaroles, and at least one small hydrothermal eruption over the last five years, this system should be considered active, dangerous, and something to watch closely, which the Taiwanese appear to be doing. While explosive activity appears to be somewhat minimized in the TVG, its proximity to nearly 8 million residents makes it a potentially dangerous neighbor.