Quaternary Volcanoes > Active Volcanoes > Unzen

Older Unzen volcano / Younger Unzen volcano

Older Unzen volcano
   Older activities of the volcano began about 500,000 years before present. Drill core and samples of lavas from peripheral locations gave ages between 400,000 and 500,000 years before present. After the rocks of those ages were erupted, volcanoes such as Sarubayama, Takadake ( those two are outside of the map area ), Takaiwayama, Kinugasayama, Azumadake, and Kusenbudake, along with the basal parts of Tarukidaichi Plateau and Mayuyama, were formed in 300,000 to 100,000 something years before present. Early volcanoes are mainly consisting of lavas and lava domes, and volcanic morphology is not obvious due to fault movements related to activities of Unzen graben and erosion. In this map older ejecta are shown grouped together except for some pyroclastic flow deposits and debris avalanche deposits showing coherent distributions.

   Iwarego pyroclastic flow deposit distributed along Hijikuro river, northward flowing from Myokendake, is up to 20 m thick and made up of finely-vesiculated white dacite blocks and fine-grained ash of the same material. Different from rocks of other volcanoes in Unzen, it is characteristically lacking in biotite and quartz. Its age is given as about 200,000 years before present by K-Ar method. Yasumiba pyroclastic flow deposit distributed in gentle southern slopes of Takaiwayama consists of a little grayish pink pumice and fine-grained ash of the same composition. The pumice show elongated vesicles with small diameters containing a little plagioclase and biotite. Unzen-Onsen debris avalanche deposit is widespread in fumarole zone around Unzen Hot Spring and seems to have originated from an area showing avalanche scar topography in the east. The deposit is altered to white colored materials by fumarolic actions but broken up lava blocks are still recognized in them. Nitamachi debris avalanche deposit forms higher terrace surface than fan deposits along Mizunashigawa river near Nita south of Mayuyama. The deposit consists of broken up lavas and pyroclastic rocks and shows hummocky topography. The source is not clear. Not shown on the map but existence of debris avalanche deposit is confirmed in a part of a plateau of Tateno and to its east, north of Mayuyama.

   Pyroclastic rocks forming gentle slopes from older volcanic bodies to the coast had been called Tatsuishi Formation at one time. These are re-interpreted as originating from debris avalanche and small scale pyroclastic flow developed in pace with the growth of older volcanic bodies, and shown in this map as fan deposit-1.

Younger Unzen volcano
   The activities of younger volcanoes started about 100,000 years ago on the eastern part of central peninsula and built volcanoes Nodake, Myokendake, and Fugendake successively. In particular, Fugendake erupted twice in historical times and the latest activity started in 1990 and continued today.

   Mayuyama volcano erupted about 4,000 years ago independently from Fugendake in east of Shimabara Peninsula. The volcanic bodies are largely made up of lava domes and thick lava flows of andesite to dacite. On the base of the volcano spread pyroclastic flows, debris avalanches, and mudflow deposits forming gentle slopes. Classification of rock units and stratigraphic relationships are shown in Fig. 1 The younger ejecta are, different from older ones, not much disrupted by displacements along Unzen graben. Although clear fault scarps are exposed at north of Myokendake and south of Nodake, the amount of displacement is comparatively small.

1 Nodake volcano
   Nodake volcano is a volcanic body south of Fugendake. Its northern half is lost by collapse and Myokendake volcano is created inside of collapsed area. Nodake volcano is made up of Fukkoshi lava forming mountain base, Yugawachi pyroclastic flow deposit spreading in the south, Tawaraishi debris avalanche deposit and Nodake lava forming mountain top. Fukkoshi lava is distributed in Fukkoshi to the west of Myokendake and gentle slopes of Iwatokoyama and environs. The rock is white to blue-grey biotite hornblende dacite lava. Groundmass is glassy and often well vesiculated. Yugawachi pyroclastic flow deposit occupies northern parts of Arie, south of Nodake. It is compact blue-grey to white hornblende andesite to dacite consisting of partly vesiculated essential blocks and fine-grained materials of the same lithology. Tawaraishi debris avalanche deposit is originated from the collapse of mountain and spreading south of Tawaraishi Lookout. It contains white dacite blocks closely similar to Fukkoshi lava and brown weathered ashy soils, and covered by regional tephra, Aira Tn ash ( about 24,000 years before present ), originated from southern Kyushu. The topography of provenance area is obscured probably buried under Nodake lava. Nodake lava is andesite forming the lava domes of Nodake. Near the top it contains olivine phenocrysts. Its K-Ar ages give 60,000 to 80,000 years before present. When did the Nodake volcano become active is not clear, because the ages of Fukkoshi lava at the base and Yugawachi pyroclastic flow deposit are not known yet. However, as the volcanic topography is clearly preserved, the age is probably later than 100,000 years before present.

2 Myokendake volcano
   Myokendake is a volcano about 4 km in diameter made up of andesite lava with a collapse topography of 1.5 km diameter opening to SE. It consists of main body, and Maidake-Minami pyroclastic flow deposit and Ipponmatsu pyroclastic deposit distributed to the north of the main body, in addition to Tarukidaichi debris avalanche deposit from the collapse.
The rocks forming the main body of Myokendake volcano spread from peaks of Myokendake, Kunimidake, Emarudake, to valleys of Oshigadani and Tansansuidani in the east. On the cross section of collapsed wall thick lava flow is exposed. Therefore, original Myokendake seems not to be a typical stratovolcano but a collection of thick lavas and lava domes. The ages of the main body formation were determined to be 20,000 to 30,000 years before present. Maidake-Minami pyroclastic deposit is distributed to the south of Maidake forming flat surface. The rocks are compact hornblende andesite blocks and sandy ash of the same lithology. Ipponmatsu pyroclastic deposit distributed to the north foot of the volcano contains two essential materials, compact andesite and vesiculated dacite. It overlies Aso-4T pyroclastic flow deposit ( Tosu orange pumice flow deposit ) and in turn overlain by Aira Tn ash. In upstream areas of Yuegawa river, a dacitic pyroclastic flow deposit is distributed under Kureishibaru pyroclastic deposit ( Fugendake volcano ). The dacite here is lithologically different from that of Ipponmatsu pyroclastic flow, but because of intense activity of volcano, detailed field study could not be performed. It is shown, therefore, included in Ipponmatsu pyroclastic flow. Debris avalanche deposits formed by collapse of Myokendake are not clearly defined. Tarukidaichi Plateau debris avalanche deposits covering the surface of Tarukidaichi Plateau west of Mayuyama may correspond to them.

3 Fugendake volcano
   Fugendake volcano consists of several lava flows and lava domes which have erupted from not only inside of Myokendake collapse wall but also outside of the wall. Several pyroclastic flow deposits of similar lithology to those of 1991-1995 are found around Fugendake. Gentle slope of Botanyama, SE of Fugendake, is made up of Furue pyroclastic flow deposit which contains hard fist- to head-sized hornblende dacite blocks of essential nature and sandy ash of identical composition. On the other hand, Kureishibaru pyroclastic flow deposit is distributed from north slope of Fugendake to a slope of Kureishibaru covered by fan deposit, and the plateau NE of Maidake and Tateno which is slightly higher than surrounding area. The thickness of the deposits is thin ( 0.5 - 3 m ) with abundant essential blocks but little fine-grained materials. From carbonized wood found included in it, ages from 14,000 to 19,000 years before present have been reported using 14C method. Taruki-Higashi dacite lava is very rich in phenocrysts. Its K-Ar age is about 20,000 years before present. Inaoyama lava dome is also dacite very rich in phenocrysts and fresh lava crinkle textures have been recognized at the top. It does not contain pyroxene as is Taruki-Higashi lava. Senbongi lava is andesite lava erupted from outside of the collapse wall. Its age was reported to be 13,000 years before present by fission-track method. Shimabara debris avalanche deposit forming hummocky ground at north of Mayuyama, City of Shimabara, overlies Kikai-Akahoya ash fall ( about 6,300 years before present ) and seems to have been ejected from Fugendake but its exact source is not known.
   Three lava domes erupted within the collapse ( Kazaana lava, Fugendake-Sancho lava, and Shimanomine lava ) are all hornblende dacite similar in chemical compositions to each other, but the order of eruptions and ages are not well known. Only a fission track age for Shimanomine lava is known to be 6,000±2,000 years before present. On the other hand, Mizunashigawa pyroclastic flow deposit distributed in Akamatsudani valley and upstream area of Mizunashigawa river gave about 4,000 years before present by 14C method. As Shimanomine lava occurs in the upstream side of Mizunashigawa pyroclastic flow deposit, and as the ages of both are close to each other within the experimental error, Mizunashigawa pyroclastic flow deposit may have erupted at the same time of Shimanomine lava. Other pyroclastic flow deposits which may have erupted at the same time of two lava dome formation have not been identified. Heisei lava which is currently continuing eruption is the fourth lava dome erupted within the Myokendake collapse ( see later section ).

5. 4 Mayuyama volcano
   Mayuyama volcano situated in the eastern edge of Unzen volcano group consists of two dacite domes, Shichimenzan and Tenguyama. Those are the largest domes in Unzen volcano group. The lava gave ages of about 5,000 years ( fission track method ) and about 3,000 years ( thermoluminescence method ) suggesting that the age is to be about 4,000 years before present. In the north of Shichimenzan overlying Kikai-Akahoya ash fall deposit, Mutsugi pyroclastic flow deposit occurs. It consists of compact dacite blocks of essential materials and comparatively small amount of fine-grained materials of the same composition. The age of about 4,000 years was reported by 14C method using carbonized wood. Judged from distribution, lithological similarity, and age, their origin is estimated to be Shichimenzan.

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