The Towada Caldera in northern Honshu is a double caldera that was formed by as many as six major explosive eruptions 53,000 – 13,000 years ago. The main caldera is filled with Lake Towada and measures around 11 km across. Following the caldera-forming eruptions, the basaltic Goshikiiwa stratovolcano grew in the SSE portion of the caldera. This was followed by explosive eruptions that formed the Nakanoumi caldera, a 2-km diameter caldera inside the older and larger caldera. Two of the rims of this caldera stretch into the lake. Final dome building eruption built the Ogurayama lava dome on the NE rim of the Nakanoumi caldera. The most recent eruption took place from Ogurayama 915 years ago.
The entire area is part of the Towada Hachimantai National Park that gets over 2.5 million visitors a year. Surrounding towns are Towada City, with 66,000 people, Hirakawa City, with 34,000 people, Shingo Village with 3,000, and Kosaka Town, 6,600 people. The national park spans Aomori and Akita Prefectures.
The lake is over 60 km2 with a maximum depth of 327 m. It is locally famous for fall leaf colors and have an active boat excursion service. Both boat service and local bus service are suspended during the winter. There is local hiking, a revered local sculpture, the Otome no Zo statue, boats, canoes and kayaks available for individual boating, restaurants and souvenir shops along the shoreline. The Oirase Stream is 30 minutes away with a cluster of waterfalls. Best access is via public bus. Total travel time from Tokyo is just over 5 hours bus time. Busses run mid-April – Nov. There is also daily air travel into the local Aomori Airport.
Only 12,000 live within 10 km of the lake. 103,000 within 30 km, and 2.2 million within 100 km.
Climate is cold humid with cool summers, cool winters and heavy snowfall. September is the wettest month. Local economy is largely based on agriculture and tourism.
The lake sits 400 m above sea level. Water renewal from the lake takes 8.5 years. The catchment area is nearly 130 km2. It is fed by up to 70 rivers and streams. Half of them are intermittent. The only outlet is the Oirase River that drains into the Pacific Ocean. This river flows through a scenic gorge with numerous rapids and waterfalls.
While activity in this part of Honshu has been taking place for 2 Ma, recent activity at Towada is generally traced back around 200,000 years. The basement rocks for Towada are Miocene volcanic rocks and sediments including rhyolitic tuffs, andesitic and dacitic lavas. Most have undergone intense hydrothermal alteration which has turned the region into a mining area.
At least five stratovolcanoes formed in the area: Zakura Misaki. Minami Hakkoda, Towada-yama, Towariyama and Heraidake. The Zakura Misaki volcano is generally made of andesitic lava flows. It is partially exposed on the NW corner of the main caldera. Minami Hakkoda is a cluster of cones constructed mostly of lava flows on the N of the caldera. The Ohanabeyama lava flow on the middle part of the N caldera wall is one of its lava flows. The latter three volcanoes are nearly all conical and located E of the caldera. They are thickly covered by younger pyroclastic flows and rarely expose andesitic lava flows they were built from.
There are three units of subaqueous tuffs located under thick pyroclastic deposits. These are the Aoni Tuff, 1.6 – 1.3 Ma, the Tose Tuff, and the Nagakogi Tuff (which may be related to the Aoni Tuff. There are about 100 m of exposed sediment, fallout beds, and rhyolitic pumice beds.
There is an exposed section of a scoria cone 8.6 km NNE of the inner caldera. This cone, the Oirase scoria cone, may have been erupted under shallow water. It is covered by a water-laid rhyolitic fine tuff bed. The lake was buried by voluminous ignimbrites (Ishigedo Tuff) erupted from the 10 x 7 km Tashirotai caldera some 20 km N. The lake had extended over the SE half of the current Towada caldera before the eruption of the Ishigedo Tuff.
Volcanic structures younger than the Ishigedo Tuff are called the Towada volcano. Eruptive episodes are a number of eruptions separated by a dormant period. More than 20 eruptive episodes are identified from hundreds of pyroclastic sections on the slopes of the volcano. Activity of Towada started on the border line between the Ishigedo Tuff and basement mountains. There are at least two stratovolcanoes (Aobuna and Hakka) exposed on the caldera walls. Aobuna is on the NE corner of the caldera and is a pile of pyroclastic deposits up to 200 m thick. There is a 110,000-year-old ash deposit from the Toya volcano found on top of these deposits. Activity at Aobuna is thought to have started around 200,000 years ago. Hakka is exposed on the opposite side of the caldera wall. It is generally andesitic lava flows and a small amount of scoria flow deposits. Only a relatively small area of the current caldera was occupied by a few relatively small pre-caldera volcanoes, not unlike Fisher Caldera in Alaska.
The main caldera is thought to have grown incrementally in response to as many as six moderate sized eruptive episodes 53,000 – 13,000 years ago. The volume lost in formation of the caldera is estimated at 30 km3 and total eruptive volume estimated at 48 km3 DRE. The caldera was fully formed following the eruption 13,000 years ago.
Post-caldera activity started around 2 km SSE of the center of the caldera, building the small Goshikiiwa stratovolcano. The central crater was enlarged with successive Plinian eruptions and finally connected to the lake water. These eruptions created the inner caldera, the Nakanoimi crater, a 3 km diameter inner caldera. It is surrounded by steep cliffs up to 200 m high and constructed by basaltic andesite lava flows with some pyroclastic materials. Radial dikes are exposed. The Goshikiiwa volcano grew in a short period of time.
There are two relatively newer lava domes in the caldera. The Gomonishi lava dome is to the N, with a top near the level of the lake surface. Its eruptions were not pyroclastics so it is as yet undated. The Ogurayama dome is on the NE slopes of the Goshikiiwa volcano. It is not covered by any pyroclastics meaning it was the last thing built in the caldera. There is columnar joining on its flanks which may indicate that part of it may be a thick lava flow or coulee. The Ogurayama dome formed some 7,600 years ago. The Mikadoishi lava dome is estimated at 12,000 – 2,800 years old based on variations in post-caldera magma compositions.
There are no known current fumarolic activity or hot springs associated with Towada. However, there are multiple hot springs, mud pits and active fumaroles in the surrounding region which has multiple active volcanoes. These are popular tourist attractions to the surrounding Towada – Hachimantai National Park.
The VOGRIPA database lists Towada with 24 major eruptions between 111,000 – 915 years ago. These eruptions range from VEI 4.4 – 6.7. The two VEI 6.7 eruptions took place 370 years apart around 14,000 years ago. They ejected 50 km3 of bulk volume apiece according to VOGRIPA. One of these had an estimated column height of 37 km. Magma eruption rates increased through the caldera stage of Towada. VOGRIPA dates tend to be listed as generally older than the 1985 Pyroclastic Geology of Towada Volcano by Hayakawa.
There are older pyroclastic deposits that are poorly known due to limited exposures and the sheer volume of newer deposits.
The first of the caldera-forming eruptions is thought to have taken place 55,000 – 53,000 years ago. This produced the massive Okuse ignimbrite, measuring some 24 m thick 18 km NE of the main vent. 20 m of these deposits are the Okuse ignimbrite. Variation in the layers indicates vertical zoning of the magma chamber. It was a VEI 6.1 producing some 12 km3 of material.
Two smaller eruptions took place some 45,000 and 35,000 years ago. Each produced deposits measuring around a meter thick 23 – 34 km NE of the main vent. Data on these eruptions is relatively scarce. Both of these were VEI 5.5 producing around a km3 of material.
The Ofudo ignimbrite was produced 25,000 years ago. 9 m of mostly pyroclastics are measured 9 km NE of the main vent. It wass a VEI 6.7 that produced 46 km3 of material.
An eruption 17,000 years ago produced the Maira ash, at least 6 m of ashfall and ignimbrites measured 25 km E of the vent. The combined 100 cm isopach extends well into the Pacific to the E. It was VEI 5.8 that produced around a km3 of material.
The Hachinohe ash was erupted some 13,000 years ago. It is the largest volume of tephra erupted from the volcano at 16 km3. VOGRIPA lists it at 50 km3. Deposits alternate pumice lapillis and fine ash. It is primarily rhyodacitic and was produced in a phreatomagmatic eruption as the magma interacted with lake water in the larger Towada caldera. Rate of eruption varied over time, which also varied interaction of magma with lake water. Some portions of the eruption were highly phreatomagmatic. Some were mostly magmatic. Ash tended to clump in the wetter portions of the eruption resulting in accretionary lapilli up to 20 mm in diameter in some places. Prevailing winds were generally W to E, so the majority of the material was deposited to the E of the volcano. There are at least six clearly identifiable ash beds. The 100 cm isopach for this eruption stretches well into the Pacific Ocean to the E of the eruption. These pumice beds are deposited on pyroclastic flows from previous eruptions.
Initial eruption was phreatoplinian with the interaction of water and magma. Water access to the initial column was not significant, though enough to allow a relatively constant phreatoplinian column to establish itself. This was the HP1 ash, a closely packed accretionary lapilli. Discharge rate of magma increased over time which decreased the mass ratio of water to magma, leading to relatively magmatic eruptions. This happened at least three times over the course of the eruption creating the end of the HP1 bed, the HP2 and HP4 beds. The vent radius increased over the course of the eruption.
A stable phreatoplinian column was sustained for a few hours creating the HP5 ash. Toward the end of the eruption, available lake water was apparently used up, leading to a mostly magmatic plume. This one varied significantly in height. The caldera subsided and the eruptive column collapsed creating pyroclastic flows from the newly formed ring vents. Ignimbrite sheets cover around 40 km2 around the caldera. Total duration of the eruption was a day or two.
There were at least six explosive eruptions between the end of the Hachinohe eruption 13,000 years ago and the Towada – Chuseri eruption 6,293 years ago. They produced varying volumes of ashfall. The only significant pyroclastic production took place 8,500 years ago, producing the Nambu Pumice, measured around 2.5 m thick 11 km E of the source from the Goshikiiwa vent. There is a series of fallout deposits 13,000 – 11,000 years ago described as the Ninokura scoria. Pyroclastic Geology of Towada describes these as poorly defined, but the VOGRIPA database lists these eruptions as a VEI 6.7 producing another 50 km3 of bulk volume. I have not yet found anything to resolve this discrepancy.
The Towada – Chuseri eruption took place 6,293 – 6,029 years ago, producing a significant ashfall. There are three eruptive units from this eruption: an opening Plinian fall (Chuseri), middle phreatomagmatic Kanagasawa pumice fall, and final Utarube ash, a dilute pyroclastic flow. All of these units are generally to the E of the volcano. Underlying soils are dated 6,313 – 6,180 years ago. The initial Plinian Chuseri pumice fall is more homogeneous than the later units. The Utarube unit was generally deposited within the caldera, with at least a few deposits outside it. Reconstruction of the eruptive sequence has the
The initial Chuseri eruptive column significantly varied in intensity at least once until settling into its largest output for a time. The column shut down and after a short dormant period the second Kanagasawa column started out as explosive to a steady plume. Towards the end of this phase, it was intermittently explosive before shutting down once again. After a very short dormant phase, the eruption transitioned to a phreatomagmatic eruption.
The second most recent eruption took place some 3,000 years ago from the Nakanoumi crater in the Goshikiiwa volcano. It produced mainly ashfall, with multiple layers measuring some 1.3 m thick 5.6 km E of the main vent. This ashfall is widely dispersed and suggests a relatively extended eruption of many days to weeks.
The most recent eruption took place some 1,250 years ago, 915 AD. It produced a thin pyroclastic deposit around the source. The deposits were sampled some 10 km S of its source from the Ogurayama dome. The top deposit was 2 m of white pyroclastics with scattered pumice lapilli. It is underlain with three layers of pumice and ashfall another 21 cm thick. The ashfall layers were produced by Plinian eruptions.
The most recent pyroclastic deposit distributed in the Towada area dates some 1,000 years ago. Its source is suspected to be the Baegdusan Volcano on the border of China and North Korea some 1,100 km W.
Tectonics of northern Honshu is dominated by subduction of the Pacific Plate under the Okhotsk Plate. The subduction trench is the Japan Trench offshore to the east. Immediately west of Honshu, a bit into the Japan Sea is a line of subduction of the Amur Plate beneath the Okhotsk Plate. The Pacific Plate is moving 9 cm/year to the W. The Amur Plate is moving 1 cm/year to the E. Combined movement has produced a typical volcanic island arc, alternately known as the Northeastern Japan Arc or the Northeastern Honshu Arc.
The present arc system is dominated by E – W compression. In NE Honshu, the compression episode seems to have begun some 3 – 2 Ma, beginning at the E margin of the Sea of Japan where a fold belt of mountains developed. The backbone range of NE Honshu has been rising since 2 Ma. The present mode of tectonics in the Japanese arc system started 4 – 3 Ma and was fully developed by 2 Ma.
Crust thickness in northern Honshu is in the vicinity of 35 km. Subducting Pacific Plate slab has a variety of low velocity zones imaged beneath Honshu to depths of 150 – 200 km under the volcanic arc. Results appear to indicate that water and fluids brought into the mantle depths are released by dehydration and subsequently transported to the surface by upwelling flow from the mantle wedge. There is a low velocity zone about 10 thick on the top of the subducting Pacific Plate that disappears at a depth of 70 – 90 km. Imaging suggests multiple rising magma diapirs along the spine of Honshu.
Towada is an active explosive system currently in repose. While it has been more than 1,000 years since its last eruption, this time of little activity is not uncommon in its history. Rising mantle fluids generated by dehydrating Pacific Plate volatiles are currently rising from location to location along the volcanic front in Honshu. As long as the tectonic subduction continues, the magma supply will be uninterrupted. While current lack of fumaroles or an active hydrothermal system is good news, that is more than balanced by continued rise of mantle fluids beneath the region.