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浅海におけるアルカリ玄武岩火砕丘の形成過程（Sallow marine tuff cone)

火道を充填する流紋岩凝灰岩の溶結変形過程（Welding process in eruption conduit）

巨大軽石を大量に含む軽石堆積物の起源と定置機構（Coarse volcanislastic massflow）

新島軽石の噴火様式と定置機構（Subaqueous eruption and emplacement of Shinjima PUmice）

浅海におけるアルカリ玄武岩火砕丘の形成過程−浅海におけるマグマ水蒸気爆発の事例研究

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A shallow-marine alkali-basalt tuff cone in the Middle Miocene Jinzai Formation, Izumo, SW Japan

Kazuhiko Kano

Geology Department, Geological Survey of Japan,
1-3 Higashi 1-chome, Tsukuba , Ibaraki, 305 Japan

Abstract

The Jinzai basalt is an alkali-basalt tephra unit erupted on the Japan Sea side of SW Japan immediately after the opening of the Japan Sea ceased 15 Ma. The Jinzai basalt comprises a large volume of glassy alkali basalt clasts of phreatomagmatic origin, and is interpreted to have constituted a tuff cone. The tuff cone grew to a height of 100 m above the 50-to-150-m-deep submarine substrate, but the summit and a part of the flank were lost due to syn- and post-eruptive sliding, slumping and erosion. The restored tuff cone has a crater 400-500 m wide and 100 m high, and the cone rim reaches 1500 m in width, with bedding dipping outward at a maximum angle of 20-30｡. The primary deposits include thinly bedded lapillistone to lapilli tuff and tuff, and thickly bedded lapilli tuff and tuff, which are thought to be correlated to a small-volume phreatomagmatic explosion and more voluminous ones, respectively. The shallow marine Jinzai basalt tuff cone is essentially similar in constituents, internal structures, configuration, and size to other tuff cones built in shallow surface water. This tuff cone, however, appears dominated by phreatomagmatic products due to dynamic interaction of rapidly ascending magma and water-saturated unconsolidated sediments or vent-filling slurry of water and ejecta, and has been entirely altered through syn- and post-eruptive reactions of hot basalt glass and interstitial seawater.

Keywords: tuff cone; alkali basalt; shallow marine; Middle Miocene; ohreatomagmatic explosion

　神西玄武岩は，15 Maに日本海の拡大が停止した直後，西南日本の日本海側に噴出したアルカリ玄武岩火砕岩である．マグマ水蒸気爆発起源のガラス質アルカリ玄武岩破片を主体とするタフコーンをなしている．このタフコーンは，水深50-150mの海底から海面上100 mの高さまで成長したが，噴火時あるいはその後の斜面崩壊や浸食によって山頂部は失われた．復元されたタフコーンは，マグマ水蒸気爆発の強弱に対応すると思われる火山礫岩ないし火山礫凝灰岩と凝灰岩がなす薄い層と火山礫凝灰岩と凝灰岩とがなす厚い層とで構成されており，火口縁の直径が400-500 m，高さが100 m，タフコーンの外縁の直径は1500 mにも達する．このタフコーンは，構成物，内部構造，輪郭，大きさ，いずれにおいても浅い地表水域に形成されるタフコーンと似ている．しかし，海底の水に飽和した未固結堆積物あるいは火道の水と噴出物が混在しているところにマグマが急激に上昇して見ずとマグマが爆発的に反応する環境にあったためか，マグマ水蒸気爆発によってもたらされた火砕物が卓越している．また，冷え切っていない玄武岩ガラスと海水とが反応して，全体に変質している．

(Journal of Volcanology and Geothermal Research, 1999 in press)

火道を充填する流紋岩凝灰岩の溶結変形過程−火道における凝灰岩の溶結過程モデルの提唱

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Miocene rhyolitic welded tuff infilling a funnel-shaped eruption conduit at Shiotani, southeast of Matsue, SW Japan

K. Kano , H. Matsuura and S. Yamauchi

Abstract

At Shiotani, SW Japan, rhyolitic welded tuff forms a steep-sided funnel-shaped body, confined by Paleogene granitic rocks to an elliptical area 1-1.5 km across. The Shiotani welded tuff is pervasively welded and foliated concordantly with the contact that dips inward at angles of 70-90O. In contrast nearby, contemporary volcaniclastic deposits are non-welded and gently inclined. Near the contact with the granite, the tuff is plastically deformed and shows lineations that plunge inward at angles of 40-65O. Lithic and crystal clasts in the rheomorphic outer part are rotated in a plane normal to the foliations and parallel to the lineations indicating downward flow of the welded tuff. The geometry and internal structures suggest that the Shiotani welded tuff was emplaced and welded in a funnel-shaped eruption conduit. Upon collapse of a plinian or phreatoplinian eruption column, the majority of the pyroclasts that still remain within the conduit or immediately above the vent probably fell back en masse into the conduit. Heat and steam from underlying magma and diffusion of interstitial volatiles into the glass perhaps reduced the viscosity of juvenile pyroclasts and facilitated welding in the conduit, especially at deep levels. The tuff then flowed down the conduit wall in response to welding compaction and retreat of the magma. These processes resulted in increased welding toward the contacts and welding foliations concordant with the steep wall. Emplacement of nearby correlative volcaniclastic mass-flow deposits in a shelf to upper bathyal environment suggests a possibility that when active, the Shiotani conduit was under the sea. Welding compaction would occur even under the sea providing that the steam generated in the upper part of the conduit-fill prevented water access.

Keywords: welded tuff; conduit fill; funnel-shaped eruption conduit; agglutination ; welding; rheomorphism; flow lineation

　松江市南東，広瀬町塩谷には，径が1-1.5 kmで，傾斜の急な漏斗状の流紋岩溶結凝灰岩の岩体が古第三紀花崗岩に囲まれて分布している．周辺の非溶結で水平ないし緩やかに傾斜している周辺の火砕岩とは対照的に，この溶結凝灰岩（塩谷溶結凝灰岩）は全体に溶結しており，その溶結面は花崗岩との接触面に調和的に，内側に70-90O傾斜している．接触部では塑性変形して内側に 40-65O傾斜した線構造が明瞭で，溶結面に直交し線構造に平行な面内で岩石や結晶の破片が下に向かって回転している．このようなことから，塩谷溶結凝灰岩は漏斗状の火道を埋めて溶結したと考えられる．
　プリニー式あるいはフレアトプリニー式噴火で形成された噴煙柱が崩壊すると，火道の近くや火道内にあった火砕物の多くは高温を保ったまま火道内に堆積する．このような状態に置かれた火砕堆積物は自らの重みと熱に加えて，下のマグマから揮発性成分や熱が供給されるため，下の方ほど強く溶結する．そして，圧密が進みマグマが地下深く後退するにつれて下に向かって流動し火道の壁と調和的な流動溶結面をなすはずである．
　ところで，周辺に分布する同時代の堆積物は大陸棚ないし半深海に堆積したと考えられる．とすれば，塩谷にあったこの火道も当時は水面下にあったと予想される．そのような場合，火道の上部では海水と高温の火砕物とが爆発的に反応するが，水蒸気の膜で画されたその下の部分では水が浸透しないために溶結が進むと考えることができる．

(Bulletin of Volcanology, vol.59, p.125-135)

巨大軽石を大量に含む軽石堆積物の起源と定置機構−水底溶岩ドームの爆発に由来する火山砕屑重力流の発生定置モデルの提唱

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A Miocene coarse volcaniclastic massflow deposit
in the Shimane Peninsula, SW Japan: product of a deep submarine eruption ?

Kazuhiko Kano

Abstract

A subaqueous volcaniclastic massflow deposit in the Miocene Josoji Formation, Shimane Peninsula is 15-16 m thick, and comprises mainly blocks and lapilli of rhyolite and andesite pumices and non- to poorly-vesiculated rhyolite. It can be divided into four layers in ascending order. Layer 1 is an inversely- to normally graded and poorly-sorted lithic breccia 0.3-6 m thick. Layer 2 is an inversely to normally graded tuff breccia to lapilli tuff 6-11 m thick. This layer bifurcates laterally into minor depositional units individually composed of a massive, lithic rich lower part and a diffusely stratified, pumice-rich upper part with inverse to normal grading of both lithic and pumice clasts. Layer 3 is 2.5-3 m thick, and consists of interbedded fines-depleted pumice-rich and pumice-poor layers a few centimeters thick. Layer 4 is a well-stratified and well-sorted coarse ash bed 1.5-2 m thick.
The volcaniclastic deposit shows internal features of high-density turbidites and contains no evidence for emplacement at a high temperature. The massflow deposit is extremely coarse-grained, dominated by traction structures, and is interpreted as the product of a deep submarine, explosive eruption of vesicular magma or explosive collapse of lava.

Keywords: submarine eruption; explosive lava-collapse; coarse pumice; volcaniclastic massflow; traction; suspension

　島根半島中新世成相寺層のある水底火山砕屑重力流堆積物は厚さが15-16 mで，火山礫・火山岩塊の大きさの流紋岩と安山岩の軽石とほとんど発泡していない岩片を主体としており，下から四つの層に区分できる．最下部のLyer 1は，厚さが0.3-6 mで，逆級化ないし正級化した淘汰不良の火山角礫岩からなる．その上のLayer 2 は，厚さが6-11 mの逆級化ないし正級化した凝灰角礫岩ないし火山礫凝灰岩で，岩片に富む塊状の部分とこれに重なり軽石に富みかつかすかながら層理を示す部分とからなるいくつかの堆積単位に分かれる．Layer 3は，細粒物に乏しく軽石の富む層と軽石に乏しい層とがなす厚さ数cmの層の繰り返しで，全体の厚さは2.5-3 m．Layer 4は，厚さ1.5-2 m，層理が明瞭で淘汰の良い粗粒凝灰岩である．
　このような特徴は高密度タービダイトに特有で，堆積物自体は高温で定置した証拠は持たない．しかし，構成物は極端に粗粒で，層流で運ばれたときに形成される堆積構造が卓越していることから，深海のマグマあるいは水底溶岩が爆発的に発泡するような噴火によってもたらされた可能性が高い．

(Bulletin of Volcanology, vol.58, p.131-143)

新島軽石の噴火様式と定置機構−水中における噴煙柱の崩壊と火山砕屑重力流の発生定置モデルの提唱

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Subaqueous eruption and emplacement of the Shinjima Pumice, Shinjima (Moeshima) Island, Kagoshima Bay, SW Japan

Kazuhiko Kano, Takahiro Yamamoto and Koji Ono

Abstract

The Shinjima Pumice is a fines-depleted pumice lapilli tuff emplaced several thousands years ago at about 100-140 m below sea level. This deposit about 40 m thick, comprises poorly defined many flow units, which are 1-10 m thick, diffusely stratified and show upward-coarsening of pumice clasts with a sharp to transitional base. Parallel to wavy diffuse stratifications are commonly represented by alignment of pumice clasts, especially in the lower half of the flow units. Pumice clasts of block- to coarse lapilli-size commonly have thermal-contraction cracks best developed on the surfaces, demonstrating that they were hot but cooled down to the ambient temperatures prior to their emplacement. These features are suggestive of the direct origin of the Shinjima Pumice from subaqueous eruptions. A theoretical consideration on the behavior of subaqueous eruption plumes and hot and cold pumice clasts suggests that subaqueous eruption plumes commonly collapse by turbulent mixing with the ambient water and are transformed into water-logged massflows.

Keywords: subaqueous eruption; collumn collapse; eruptin-related massflow; pumice

　新島軽石は数千年前に，鹿児島湾奥の水深100-140 mの海底から噴火した細粒物に乏しい軽石火山礫凝灰岩である．堆積物の厚さはおよそ40 mで，それぞれ基底が明瞭もしくは漸移的で，かすかに成層し，軽石が上部に濃集した厚さ1-10 mがらのいくつかの流動堆積単位からなる．層理は平行ないし波状で，不明瞭ながら軽石の配列から読みとれる．火山岩塊もしくは粗粒火山礫には，高温で噴出した後に定置するまでの間に冷却されてできたと考えられる亀甲状割れ目が発達している．このような新島軽石の特徴は，水底噴火によって直接もたらされた堆積物重力流の堆積物であることを示唆する．水中での噴煙柱と軽石の挙動について理論的に検討したところ，噴煙柱は乱流状態で容易に水と混合して崩壊し，堆積物重力流になり得ることがわかった．

(Journal of Volcanology and Geothermal Research, vol.71, p.187-206)

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