Question: In what year did the eruption occur that laid down the lava upon which a famous forest visible from the Tenjō-Yama Park Mt. Kachi Kachi Ropeway? 
Context: The Huckleberry Ridge Tuff is a tuff formation created by the Huckleberry Ridge eruption that formed the Island Park Caldera that lies partially in Yellowstone Park, Wyoming and stretches westward into Idaho into a region known as Island Park.  This eruption of 2,500 km³ of material is thought to be the largest known eruption in the Yellowstone Hotspot's history.  This eruption, 2.1 million years ago, is the third most recent large caldera forming eruption from the Yellowstone hotspot.  It was followed by the Mesa Falls Tuff and the Lava Creek Tuff eruptions. The Mt. Kinka Ropeway (金華山ロープウェー , Kinkazan Rōpuwē ) is Japanese aerial lift line in Gifu, Gifu.  This is the only line Gifu Kankō Ropeway (岐阜観光索道 , Gifu Kankō Sakudō ) operates.  The company belongs to Meitetsu Group.  The line, opened in 1955, climbs Mount Kinka, linking Gifu Park and Gifu Castle. Mount Nyiragongo is an active stratovolcano with an elevation of 3470 m in the Virunga Mountains associated with the Albertine Rift.  It is located inside Virunga National Park, in the Democratic Republic of the Congo, about 20 km north of the town of Goma and Lake Kivu and just west of the border with Rwanda.  The main crater is about two kilometres wide and usually contains a lava lake.  The crater presently has two distinct cooled lava benches within the crater walls – one at about 3175 m and a lower one at about 2975 m .  Nyiragongo's lava lake has at times been the most voluminous known lava lake in recent history.  The depth of the lava lake varies considerably.  A maximum elevation of the lava lake was recorded at about 3250 m prior to the January 1977 eruption – a lake depth of about 600 m .  A recent very low elevation of the lava lake was recorded at about 2700 m .  Nyiragongo and nearby Nyamuragira are together responsible for 40% of Africa's historical volcanic eruptions. Mount Sinabung (Indonesian: "Gunung Sinabung", also "Dolok Sinabung", "Deleng Sinabung", "Dolok Sinaboen", "Dolok Sinaboeng" and "Sinabuna") is a Pleistocene-to-Holocene stratovolcano of andesite and dacite in the Karo plateau of Karo Regency, North Sumatra, Indonesia, 25 miles from the Lake Toba supervolcano.  Many old lava flows are on its flanks and the last known eruption, before recent times, occurred in the year 1600.  Solfataric activities (cracks where steam, gas, and lava are emitted) were last observed at the summit in 1912; recent documented events include an eruption in the early hours of 29 August 2010 and eruptions in September and November 2013, January, February and October 2014.  A pyroclastic flow in May 2016 killed 7 people.  Between 2013 and 2014 the alert for a major event was increased with no significant activity.  On 2 June 2015 the alert was again increased, and as of 26 June 2015 at least 10,000 people have been evacuated, fearing a major eruption.  The long eruption of Mount Sinabung is similar to Mount Unzen in Japan, which erupted for five years after lying dormant for 200 years. The Tsugaike Ropeway (栂池ロープウェイ , Tsugaike Rōpuwei ) is Japanese aerial lift line in Otari, Nagano, operated by Tsugaike Gondola Lift (栂池ゴンドラリフト , Tsugaike Gondora Rifuto ) .  The Tōkyū Group company also operates Tsugaike Gondola (Gondola Lift Eve), a gondola lift line connected to Tsugaike Ropeway.  The two lines are collectively called Tsugaike Panorama Way (栂池パノラマウェイ , Tsugaike Panorama Wei ) .  Although it departs from Tsugaike Kōgen Ski Resort ( ) , Tsugaike Ropeway "closes" in winter.  It transports tourists to Tsugaike Natural Park, mainly in summer.  Tsugaike Gondola Lift, on the other hand, openes whole year, and mainly transports skiers. Aokigahara (青木ヶ原 ) , also known as the Suicide Forest or Sea of Trees (樹海 , Jukai ) , is a forest on the northwestern flank of Japan's Mount Fuji thriving on 30 square kilometers (12 square miles) of hardened lava laid down by the last major eruption of Mount Fuji in 864 CE.  The western edge of Aokigahara, where there are several caves filled with ice, is a popular destination for tourists and school trips.  Parts of Aokigahara are very dense, and the porous lava absorbs sound, helping to provide visitors with a sense of solitude. The Hōei Eruption of Mount Fuji (宝永大噴火 , Hōei dai funka ) started on 16 December 1707 (23rd day of the 11th month of the year Hōei 4) and ended about 1 January 1708 (9th day of the 12th month of the year Hōei 4) during the Edo period.  Although it brought no lava flow, the Hōei eruption released some 800 e6m3 of volcanic ash, which spread over vast areas around the volcano, even reaching Edo almost 100 km away.  Cinders and ash fell like rain in Izu, Kai, Sagami, and Musashi provinces.  In Edo, the volcanic ash was several centimetres thick.  The eruption is rated a 5 on the Volcanic Explosivity Index. Browns Island or Motukorea is a small New Zealand island, in the Hauraki Gulf north of Musick Point, one of the best preserved volcanoes in the Auckland volcanic field.  The age of eruption is currently unknown.  Due to centuries of cultivation, little native bush remains except on the north-eastern cliffs, leaving the volcanic landforms easily visible.  It exhibits the landforms from three styles of eruption.  The island consists of one main scoria cone with a deep crater, a small remnant arc of the tuff ring forming the cliffs in the northeast, and the upper portions of lava flows.  The area was dry land when the eruptions occurred, but much of the lava is now submerged beneath the sea. The Tenjō-Yama Park Mt. Kachi Kachi Ropeway (天上山公園カチカチ山ロープウェイ , Tenjōyama Kōen Kachikachiyama Rōpuwei ) , officially the Lake Kawaguchi Mt. Tenjō Ropeway (河口湖天上山ロープウェイ , Kawaguchiko Tenjōyama Rōpuwei ) , is a Japanese aerial lift line in Fujikawaguchiko, Yamanashi, operated by Fuji Kyuko.  Opened in 1959, the line climbs Mount Tenjō from the shores of Lake Kawaguchi, one of the Fuji Five Lakes.  The line name comes from "Kachi-kachi Yama" (lit.  "Mt. Kachikachi"), a folktale which took place on the mountain.  The observatory has a view of the lake, as well as Aokigahara forest, and Mount Fuji. A Hawaiian eruption is a type of volcanic eruption where lava flows from the vent in a relatively gentle, low level eruption; it is so named because it is characteristic of Hawaiian volcanoes.  Typically they are effusive eruptions, with basaltic magmas of low viscosity, low content of gases, and high temperature at the vent.  Very little amounts of volcanic ash are produced.  This type of eruption occurs most often at hotspot volcanoes such as Kīlauea on Hawaii's big island and in Iceland, though it can occur near subduction zones (e.g. Medicine Lake Volcano in California, United States) and rift zones.  Another example of Hawaiian eruptions occurred on the island of Surtsey in Iceland from 1964 to 1967, when molten lava flowed from the crater to the sea.
Answer: 864 CE