Mount Hakone (Hakoneyama) is a complex stratovolcano / caldera system perhaps 80 km SW of Tokyo. There are a pair of overlapping calderas, the largest 10 x 11 km formed as the result of two massive eruptions 180,000 and 60,000 – 49,000 years ago. Since the last caldera-forming event, there at least half a dozen domes formed along a SW – NE lineament through the center of the caldera. Dome growth progressed generally southward over time. The largest (and most recent) of these is Kami-yama (Kamiyama) and is the high point of the system at 1,438 m.
The calderas are breached to the east by the Haya-kawa (Hayakawa) canyon. Lake Ashi (Ashinoko) partly fills the southern caldera and is bounded by the SW caldera wall and the dome system. It was created some 3,000 years ago by a debris avalanche from Kamiyama around the time of the last magmatic eruption of the system. There have been several other more recent phreatic eruptions, mostly from the vicinity of Kamiyama.
Hakone has an active and vigorous hydrothermal system that powers fumaroles, hot springs, and hot mud pits. The most popular of these is located in the collapse crater on Kamiyama.
Mount Hakone is located around 30 km ESE from Mount Fuji. It is located some 80 km SW from Tokyo. As such, it is an incredibly popular tourist site located for the nearly 40 million located in the Tokyo metropolitan area. Yearly tourist traffic was 21 million in 2018, 600,000 of them foreigners.
As with any heavily trafficked tourist area, Mount Hakone is well-connected to major roads, rail lines, and other infrastructure. Hakone is a small community in the caldera on the shores of Lake Ashi with a population of over 13,000. It boasts hot springs, Shinto Shrines, tour boats and other tourist attractions.
The Hakone Shrine is a Shinto shrine on the shores of Lake Ashi in the town of Hakone. It was relocated from the peak of Komagatake to the shores of Lake Ashi in 1667. It has burned at least once down during samurai wars. The shrine is listed as a nationally significant shrine.
Owakudani is the area around the crater created by the last eruption of Mount Hakone some 2,900 years ago. The area is an active volcanic zone with sulfurous fumes, hot springs and hot rivers. Mount Fuji is visible on clear days. There is a walking trail and a ropeway into the volcanic zone. Local vendors sell eggs cooked in the hot water with shells blackened by the sulfur. The black eggs are said to extend life by seven years and are not unexpectedly quite popular with visitors.
There is a hiking trail from the ropeway station to the top of Mount Kamiyama, on to Mount Komagatake, and then via second ropeway into the caldera to Lake Ashi (Ashinoko). It is about a two-hour hike one way. Conditions can be wet, slippery and quite windy. There is a second trail that splits after Kamiyama to the lakeshore, around the lakeshore and on to the main Hakone Ropeway. This trip is 4.5 hours. Transportation across the lake by commercial boat is available. In this part of the world, the term ‘ropeway’ describes transportation via gondola suspended by steel cables on pylons, similar to what we see worldwide going on up and down mountains.
The main tourist site was closed for six months in 2019 after an increase in volcanic alert level. It was reopened in November. The presence of so many people inside the calderas and active volcanic area on a regular basis makes precise observations and monitoring of activity imperative.
The andesitic to basaltic to dacitic Hakone volcano has been active for around 400,000 years ago. Early activity built an andesitic stratovolcano similar to Mt Fuji, topping out around 2,700 m. The original Hakone also had a pair of parasitic volcanoes – the andesitic Kintokisan on the NW flanks and the dacitic Makuyama on the SE flanks. The first caldera forming eruption sequence took place around 180,000 years ago. This caldera was enlarged by erosion during the quiescent period following caldera formation.
The original caldera was formed by the collapse of a volcano perhaps 2,700 m high, similar in shape and size to neighboring Mount Fuji. It took place during two separate eruptive periods. These events were followed by periods of relative quiet and erosion. The central portion of the caldera has been refilled by thick piles of lavas and post-caldera cones and domes. Drilling for geothermal wells found that subsidence from the base of the present caldera wall is at least 1,200 m and as much as 1,800 m in the center of the caldera. This is massive subsidence, with as much as 600 m 2 – 3 km from the base of the current caldera. Subsidence took place along ring faults with eventual tilting of individual blocks toward the center of the caldera creating table-shaped mountains.
Following the original caldera forming eruptions, activity resumed around 130,000 years ago and built a 300 m thick dacitic shield volcano that filled the original caldera. The second caldera was formed with at least three Plinian eruptions producing airfall and ignimbrite flows. These eruptions took place 60,000 – 49,000 years ago. Remains of flow deposits are found 50 km east near Yokohama City and 25 km south near Izunagaoka. This series of eruptions left the newest caldera measuring 12 x 8 km.
Kamiyama started growing around 46,000 years ago with at least three major pumice eruptions. Mode of eruption changed after an eruption 39,000 years ago. The original cone suffered a flank collapse at the time of a pair of eruptions 38,000 – 37,000 years ago. Eruption mode after the flank collapse was repeated block and ash flows, effusion of (likely dacitic) lavas, and growth of lava domes. Eruptive rates from Kamiyama decrease gradually over time.
Cone building activity resumed some 30,000 years ago and built the Kozukayama, Daigatake domes. It rebuilt the current Kamiyama stratovolcano in the caldera. The remaining 1325 Peak, Komagatake, Kami-futago and Shimofutago domes formed on the S and E parts of the Kamiyama stratovolcano. Ejecta from Kami-futogo has been dated 4,840 years ago, making it perhaps the most recent dome (spine).
An eruption sequence around 2,900 years ago from Kamiyama included multiple phreatic eruptions, a moderate flank collapse from Kamiyama that dammed the outflow canyon for water out of the calderas. This in turn created Lake Ashi. This sequence ended with the last magmatic eruption from Hakone (Kamiyama) and creation of a new dome. The Owakudani thermal field is the most vigorous remaining fumarole field on the volcano.
Seven natural hot springs are known on Hakone. Wells have been drilled in the volcano and new hot spas developed. Today, there are 15 hot springs with over 220 wells in Hakone. One of these wells provided a path for hydrothermal fluids to blow out of the system and was involved in the 2015 phreatic eruption.
Mount Hakone has been a prolific volcanic system over the last 400,000 years, producing 85 – 125 km3 bulk volume of mainly tephras, pumices and pyroclastic flows. While there is some effusive activity, the majority of all eruptive products came from explosive eruptions.
The VOGRIPA database lists 16 mostly VEI 4 eruptions over the last 100,000 years. Average bulk volume of these eruptions is in the vicinity of 0.1 km3. There was a sequence of 4 larger eruptions between 88,500 – 64,000 years ago with a pair around VEI 5 and VEI 6. The largest was a VEI 6.1 around 67,500 years ago that produced at least 20 km3 bulk volume and the Tokyo Pumice. These eruptions created the second and most recent caldera.
The 10,000-year period 109,000 – 99,000 years ago was quite active with 8 major eruptions between VEI4 – VEI 6. The largest of these took place 103,500 years ago, a VEI 6 that produced at least 10 km3 of the Hakone Daruma 4 deposit.
In this database, the size of earlier eruptions creeps up as the date of the eruption gets older. My personal speculation is that the larger earlier eruptions are easier to determine as their deposits tend to overwhelm the smaller ones, especially from a system that is as vigorous as this one was.
The period 150,000 – 110,000 years ago had another 14 VEI 4 – VEI 6 eruptions. The largest of these took place around 126,000 years ago, producing the Hakone Daruma 1 pumice with at least 1 km3 bulk volume erupted.
The system quieted a bit 190,000 – 160,000 years ago after the formation of the original caldera with only 8 eruptions in the VEI 4 – VEI 6 range. The 10,000-year period between 190,000 – 178,000 was particularly active with four VEI 6+ eruptions that produced 8 – 12 km3 bulk volume and the Tama A and B pumices.
There were 7 VEI 5 – 5.8 eruptions between 310,000 – 210,000 mostly producing around 1 km3 per eruption and the Tama B and C series of pumice layers. The largest of these produced 6 km3.
The most recent magmatic eruption some 2,900 years ago took place at Kamiyama. Initial activity was phreatic shortly followed by a flank collapse of the NW side of Kamiyama. This collapse dammed a breach in the caldera called the Hayakawa Valley creating Lake Ashi. The collapse was shortly followed by the last magmatic eruption that produced a pyroclastic flow and a dome in the collapse crater. The crater is still active as the Owakudani fumarole field, a popular tourist destination. It is also the largest fumarole field on the volcano.
The Owakudani Tephra Group is a recently discovered set of phreatic eruption deposits on the northern slope of Kamiyama and the Owakudani fumarole area of Hakone. They were produced some time after the most recent magmatic eruption of Hakone some 2,900 years ago. There are five units. The oldest units Hk-Ow 1 and 2 are tephra fall deposits and secondary debris flow deposits. Hk-Ow 2 is also associated with surge deposits. The most recent three are tephra fall deposits. There is no juvenile material in any of these deposits with the possible exception of Hk-Ow 2 which contained trace amounts of volcanic glass fragments. While the eruption is considered to be phreatic, the surrounding edifice was deformed indicating possible magma intrusion to shallow depths. The most recent three deposits were all erupted in the latter half of the 12th and 13th centuries. Hk-Ow1 was erupted shortly after the most recent magmatic eruption 2,900 years ago. Hk-Ow 2 was erupted around 900 years later, some 2,000 years ago. The eruption ages of the Owakudani (Hk-Ow) tephra group tends to correspond with seismic events nearby and suggest that activity at Hakone may be related to tectonic events in the region.
Hakone has been long known for earthquake swarms. Among the earliest recorded were 1786 AD, with over 100 felt quakes in two days triggering rockfalls that damaged local buildings. An earthquake swarm in 1917 had nearly 250 felt quakes in a single day. Two months later, a hot mud spring opened up. Felt swarms in 1934 and 1952 triggered earthquakes during and after the swarms stopped. A 1966 swarm is connected to a sudden rise in hot spring temperatures around 10 months later.
There were repeated earthquake swarms on Hakone in 2001. Additional swarm events were observed in 2006, 2008 – 2009, 2013 and 2015. The 2015 swarm let to a phreatic eruption. It was the largest ever observed on the volcano. Inflation was measured during swarm events, with cracks opening NE from Owakudani during 2001 and 2013. Earthquake swarms occur at Hakone once every several years.
The most recent unrest at Hakone took place in 2015 with the first recorded phreatic eruption at Owakudani. This was accompanied by an earthquake swarm. Inflation took place weeks before the earthquake swarm and was attributed to either activity in the magma chamber or hydrothermal activity. What was described as abnormal fumarole activity was observed at a hot spring supply facility (steam well) about a week following the swarm. In response the Japanese Meteorology Agency (JMA) raised the alert level a few days later. There was a local uplift around the steam well. Activity waxed and waned until volcanic tremors on June 29, a crack opening, and ashfall, all indicating a smallish phreatic eruption centered around two wells that continued until July 1. Seismic activity decreased following the eruption and the alert level was returned to normal by Sept 11 after which the tourists also returned. Fumarole activity continued to be elevated following the eruption.
The complex tectonics of central Japan are driven by the subduction of the Philippine Sea Plate beneath the Eurasian and Okhotsk Plates. The subduction is taking place along the Suruga and Sagami troughs. The Izu-Bonin arc previously discussed in our Bayonaise Rocks post has been colliding with central Japan for the last 15 Ma. Multiple volcanoes in the vicinity of Hakone, Fuji, Ashitaka and a large number of monogenic volcanoes on the Izu peninsula just to the south of Hakone are fueled by magma making its way to the surface in this region. Just to keep things simple, the Pacific Plate is subduction from east to west under the colliding Philippine Sea Plate.
KarenZ touched on the regional tectonics in a post in the old VC in 2014. There are multiple papers on the crust and mantle beneath the region, though most are tied to investigations of Mount Fuji. A 2015 paper by Kinoshita, et al is one of several examples as is a 2004 paper by Aizawa, et al.
Crust thickness under the region is 20 – 30 km. There is a second velocity boundary 40 – 50 km. That thickness is interpreted as the bottom of subducting Philippine Sea Plate. There is a gap at the 50 km depth underneath Fuji some 30 km NW of Hakone, which is suspected to be a location that mantle fluids rise to the surface. This gap does not exist under Hakone. The Philippine Sea Plate subducts to a depth of 140 km to the NW of the collision zone. There are not continuous high velocity anomalies beneath the region. The north-dipping interface defining the top of the Philippine Sea Plate is located 25 – 35 km below the surface. This is defined by earthquake analysis. The upper boundary of the Philippine Sea Plate is 20 km deep, some 50 km N of Hakone.
One of the questions out of this analysis is to describe the sheer size of Hakone, Fuji and neighboring volcanoes. They are much larger than would be expected out of arc volcanic systems.
Hakone volcano is a large, active volcanic system. While it does have a extended period of repose, it continues to have a vigorous hydrothermal system that drives the occasional phreatic eruption. Given the tens of millions of people who are on the mountain on a daily basis and its close proximity to nearly 40 million in greater Tokyo, it should be considered a possible danger for years to come.