Iwate Volcano Research Articles

Brecciation and fracturing by water ingress into the Genbudo basaltic andesitic lava flow, Iwate volcano, northeastern Japan

The Genbudo lava, the late Pleistocene basaltic-andesitic lava flow in the southwestern part of Iwate Volcano, Japan, is a 70 m thick columnar jointed flow that can be divided into three parts from bottom to top: the colonnade, the entablature, and the partly-brecciated uppermost part. Two main types of fractures developed in the entablature: pseudopillow fractures that formed in a branching network-like pattern throughout the entablature, and sheet fractures with curved surfaces that are nearly parallel to each other. At the uppermost part of the flow, finger-like structures of lava extend upward from the coherent lava, and cogenetic autoclastic rocks form between the fingers. This occurrence suggests that hyaloclastites were generated during emplacement in the uppermost part of the flow, apparently when water from a dammed river valley covered the flow. The texture of the lava near the pseudopillow fractures in the entablature is commonly hypocrystalline, while the texture in other parts is holocrystalline. There are two types of pyroxene microlites, large prismatic (average size ~ 30 µm) and dendritic (< 10 µm in length) crystals in the lava near the pseudopillow fractures. These suggest that the cooling rate of the lava was greatest in the vicinity of the pseudopillow fractures. Networks of palagonite-filled micro-fractures (less than 10 µm in width) are found in this part of the flow, and many bubbles are observed along the fractures. This is clear evidence that the rapid cooling of the lava was caused by water infiltration through the pseudopillow fractures. From the measurement of Fe-rich droplet sizes that formed due to liquid immiscibility within the lava, we estimate the cooling rate within the colonnade as about 49 °C/h and within the entablature as 642 °C/h, consistent with much more rapid cooling by water infiltration from above.

Constraining the geotherm beneath the Japanese islands from Curie point depth analysis and comparison with seismicity and drillhole data in the Kakkonda geothermal field

This paper shows results of Curie point depth (CPD) analysis of the Japanese Islands using magnetic data. The analysis method is Bouligand et al. (2009) based on fractal theory. This analysis result was combined with the cutoff depth of ordinary earthquakes, low frequency earthquakes and drillhole data. CPD by magnetic data (Dcmag) performed in the case of 50 km window size ranges 6–9 km deep in volcanic areas and is deeper than 9 km in non-volcanic areas. The results are concordant with the previous study by Okubo et al. (1989). Cutoff depths calculated by JMA unified catalog are 10 km or shallower in the volcanic area, deeper than 20 km in the forearc side and concordant with Dcmags. Dcmags across the Kakkonda GP gradually become shallower from 6.8 km west Akita-Komagatake toward to 4.6 km near Iwate volcano. The depth-to-temperature curves from drillhole database around the Kakkonda GP show that thermal gradients between Iwate volcano and the Kakkonda GP range 0.157 - 0.222 K/m. Assuming that these thermal gradients continue from the bottom of hole to the deeper part, the depths reaching 580 °C (Dchole) are determined. Dchole is concordant with Dcmag in the west side of the Kakkonda GP and much shallower than Dcmag in the east side. This suggests that Dcmags may miss to delineate a local high temperature area smaller than 50 km square in the east because of the average effect of CPD analysis. Dchole is shallower than 3 km west Iwate volcano and deepens at about 5 km west the Kakkonda GP. Combining the low frequency earthquakes data from the JMA unified earthquake catalog and the drillhole data of the Kakkonda geothermal field, we find the Curie point depths are consistent with those data.

岩手火山，一本木原岩屑なだれ堆積物の14Cウィグルマッチング年代と崩壊堆積物の特徴：活火山における小規模な表層崩壊堆積物

岩手火山，一本木原岩屑なだれ堆積物の14Cウィグルマッチング年代と崩壊堆積物の特徴：活火山における小規模な表層崩壊堆積物

Magma generation conditions at the Akita-Komagatake volcano, Northeast Japan arc: Implications of across-arc variations in mantle melting parameters

Akita-Komagatake is an active volcano in the Northeast Japan arc with a slab depth (H) of ∼110 km. In this study, magma generation conditions were estimated for basaltic products from the volcano to examine the thermal structure of the sub-arc mantle and the across-arc variations in the parameters related to magma genesis. The studied basalts have the highest MgO content (∼6 wt%) among the products from the volcano and contain ∼15% plagioclase phenocrysts, ∼5% olivine phenocrysts, and <1% pyroxene phenocrysts. The water content of the melt was estimated to be ∼4 wt% by melt inclusions in the phenocrysts using plagioclase–melt hygrometry. Multi-component thermodynamic analyses suggest that the primary magma was generated by ∼14% melting of the source mantle with ∼0.5 wt% H2O at ∼1275 °C and ∼1.3 GPa. The estimated pressure condition of ∼1.3 GPa coincides well with the low-velocity anomaly observed in the mantle at ∼40 km depth beneath the volcano. By comparing the results from Akita-Komagatake to the published results of the frontal-arc Iwate volcano (H = ∼90 km) and the rear-arc Sannome-gata volcano (H = ∼150 km), which are arranged along a cross-arc line in the Northeast Japan arc, we found that the pressure conditions of the magma generation are ∼1.3 GPa at both Iwate and Akita-Komagatake, which are significantly lower than the ∼1.8 GPa at the rear-arc Sannome-gata volcano. The source mantle temperature of ∼1275 °C at ∼1.3 GPa beneath Akita-Komagatake is higher than the ∼1250 °C at ∼1.3 GPa beneath Iwate, which suggests that the ascending flow of the mantle wedge corner flow may reach the shallowest levels beneath Akita-Komagatake. It was also demonstrated that the depth at which fluid was released from the subducting slab tends to increase for the volcanoes with increasing H.

Re-evaluation of explosive activities of Iwate Volcano in the last 10,000 years:

Re-evaluation of explosive activities of Iwate Volcano in the last 10,000 years:

Report of the Study Meeting for Young Volcanologist 2018 in Iwate Volcano, Japan

Report of the Study Meeting for Young Volcanologist 2018 in Iwate Volcano, Japan

岩手火山 (薬師岳火山ステージ)における最近 3500年間のマグマプロセスの変遷

岩手火山 (薬師岳火山ステージ)における最近 3500年間のマグマプロセスの変遷

P27 岩手火山における短期傾斜・ひずみ変動 : 2009年10月～2010年6月(ポスターセッション)

P27 岩手火山における短期傾斜・ひずみ変動 : 2009年10月～2010年6月(ポスターセッション)

A20 岩手火山,西岩手カルデラ形成に伴うマグマシステムの変化 : 噴出物の同位体組成変化に基づく検討(火山の物質科学(2),日本火山学会2008年秋季大会)

A20 岩手火山,西岩手カルデラ形成に伴うマグマシステムの変化 : 噴出物の同位体組成変化に基づく検討(火山の物質科学(2),日本火山学会2008年秋季大会)

B33 岩手山構造探査から得られた 3 次元地震波速度構造による震源再決定

B33 岩手山構造探査から得られた 3 次元地震波速度構造による震源再決定

A27 岩手火山・火山岩中のホウ素含有量 : 島弧火山マグマ生成モデルの検証

A27 岩手火山・火山岩中のホウ素含有量 : 島弧火山マグマ生成モデルの検証

A19 岩手山大地獄谷火山ガスを形成するマグマ端成分エンタルピーの低さと火山体構造の関係(火山の物質科学(2),日本火山学会2008年秋季大会)

A19 岩手山大地獄谷火山ガスを形成するマグマ端成分エンタルピーの低さと火山体構造の関係(火山の物質科学(2),日本火山学会2008年秋季大会)

P27 熱力学計算に基づくマグマ溜まりの温度-圧力-酸素分圧-含水量の推定 : 岩手火山, 焼け走り溶岩における適用例

P27 熱力学計算に基づくマグマ溜まりの温度-圧力-酸素分圧-含水量の推定 : 岩手火山, 焼け走り溶岩における適用例

A57 岩手火山の活動史 : 特に完新世の火山活動について

A57 岩手火山の活動史 : 特に完新世の火山活動について

A11 岩手火山, 西暦 1732 年噴出物の岩石学

A11 岩手火山, 西暦 1732 年噴出物の岩石学

B2-18 岩手火山におけるマグマプロセスの変遷 : 全岩化学・同位体組成に基づく検討(マグマ供給系,口頭発表)

B2-18 岩手火山におけるマグマプロセスの変遷 : 全岩化学・同位体組成に基づく検討(マグマ供給系,口頭発表)

A1-06 火山活動鎮静化後の住民の火山活動に対する意識変化 : 岩手山を例にして(ジオパークと火山防災,口頭発表)

A1-06 火山活動鎮静化後の住民の火山活動に対する意識変化 : 岩手山を例にして(ジオパークと火山防災,口頭発表)

B37 岩手山の構造と低周波地震 : (2) レシーバー関数から得られた S 波速度構造

B37 岩手山の構造と低周波地震 : (2) レシーバー関数から得られた S 波速度構造

A2-07 岩手火山における水蒸気噴火履歴の再検討(火口湖・熱水系,口頭発表)

A2-07 岩手火山における水蒸気噴火履歴の再検討(火口湖・熱水系,口頭発表)

P04 岩手火山, 江戸時代の噴火活動史 : 古文書・記録による検討

P04 岩手火山, 江戸時代の噴火活動史 : 古文書・記録による検討