Gravity flows sourced from volcanoes: a review on their flow and emplacement mechanisms

/* (,**/) S,/- S,1, Gravity flows sourced from volcanoes: a review on their flow and emplacement mechanisms Kazuhiko KANO This paper gives a brief review on the gravity flows sourced from volcanoes on land and under water. Pyroclastic flows are supported by internal gas and the air incorporated during flowage and run out a long distance as density currents. Ash-cloud umbrella is a special case of density current and the particle fallout from the umbrella is a transition to a dilute, pyroclastic density current. Subaqueous equivalents of pyroclastic flows are supported by internal gas and/or the water incorporated during flowage and are thus interpreted as either subaqueous pyroclastic flows in the strict sense or eruption-fed density currents. Debris avalanches and lahars are also important elements of volcaniclastic gravity flows both on land and under water. These pyroclastic and volcaniclastic gravity flows are thought to transform into traction-dominated flow, particle dispersion-dominated flow (grain or granular flow), fluid escape-dominated flow, or debris flows during flowage in response to the changes mainly of flow velocity, particle concentration, and shear stress. The details of these processes still remain in debate. The role of the heat in pyroclastic density current and subaqueous eruption-fed density current is a future subject to be solved. Key words : gravity flow, pyroclastic density current, eruption-fed density current, flow dynamics, emplacement mechanism + (pyroclastic flow) ash-cloud surge ground -*/ 2/01 + + + 1 Institute of Geology and Geoinformation, Geological surge +33+ gravity flow gravity current (pyroclastic density current : Druitt, +332) (Branney and Kokelaar,,**,), density flow density current Survey of Japan, AIST, Tsukuba Central 1, + +, Higashi +-chome, Tsukuba, Ibaraki -*/ 2/01, Japan e-mail: kazu.kano@aist.go.jp

S254 (sediment) sediment gravity flow sediment mass flow, + (turbidity current) (underflow) (overflow) (Fig. +) (interflow) (Fig. +) Rayleigh-Taylor (Fig. +) (vertical flow) W i W s W i W s + (Marsh, +322) W i W s (Marsh, +322 ; Fiske et al., +332) W i W s C h a, r i r f r s r f (+) C h a Fig. +. Principal types of eruption-fed density current. r density of the flowing fluid. r a density of the air. r b density of water. g r s r i r f (+) (Carey, +331 ; Fiske et al., +332) (Manville and Wilson,,**.) (Fig.,) (Fig.,) (cleft) (lobe) (Fig.,) (Fig.,) (Fig. -)

S255 Fig. -. Structure of a turbulent density current. Head velocity U head is smaller than body velocity U body, so the main body feeds the flow head. Fig.,. Structure of the head of a density current with streamlines and current directions relative to the ground. Modified from Allen (+32.). U body U body 2 + r f r c ghsinq f D* f D+ +, (,) g q r c r f h f D* f D+ Darcy-Weisbach (,) +, -. (Allen, +32.),, q t c r c, r f ( r c) y (Fig..) t t r c r f gysinq (-) t c (plug) (plug flow) Fig... Structure and velocity profile of a plug flow by basal shear (laminar) flow. (Bingham body) t c t (debris avalanche), m (Ui et al.,,***) plug U y m t t c mdu dy (.) (-) du dy r c r f gysinq t c m (/) du dy

S256 du dy r.c r f gysinq t c m (0) du dy du dy * t c t (r c-r f) gysinq h du dy * y c U c y h U * U U c y c y * (1) U r c r f g h, y, sinq, t c h y m h y y c (2) (2), - (grain flow) P T a T P T P tan a (3) Bagnold (+3/.) : T K +l -, mdu dy (+*) : Fig. /. Two-dimensional model of a grain flow. U y m r s D K + K, l (linear grain concentration) l + C m C + - + (+,) C C m C (Bagnold number) Ba Ba D r st +, l +, m Dr s T lr s +, m (+-) Ba +* /* q U (Fig. /) y h, C, r s, r f T C r s r f g h y cosq (+.) T (+*) (++) T y * U * : U C r s r f gcosq h, h y, K +l -, m (+/) : T K,r sl, D, du dy, (++)

S257 U, - C r s r f gcosq K,r s +, h -, h y -, ld (+0) (upward coarsening) inverse grading reverse grading (kinematic sieve),. (liquefaction) (fluidization) (fluid-escape structure) Fig. 0. Five types of fluidization. Modified from Allen (+32.) and Branney and Kokelaar (,**,). U flow velocity, V fluidization velocity. (Fig. 0) +) Fig. 0a: stationary fluidization,) Fig. 0b: flow-fluidization -) Fig. 0c: bulk self-fluidization.) Fig. 0d: grain self-fluidization /) Fig. 0e: sedimentation fluidization (Allen, +32.) Sparks (+313)

S258 Fig. 2. Mechanism of particle support in sediment gravity flows. Modified from Middleton and Hampton (+31-). Fig. 1. Fluidization and transport velocities for particles withdensity + g/cm - and porosity./ in CO, gas at /** and +***. Modified from Sparks (+310). Particles can be entrained where gas flow velocity exceeds their settling velocities. Branney and Kokelaar (,**,) (particulate fluidization) (aggregative fluidization) (plug) (settling velocity) (Fig. 1) CO, + g cm -./ (Fig. 1) + g cm - (, 0. mm) + +* m s, (, mm) + m s +* /* m s, +* m s +* m s (Sparks, +310) - - + (particle-support mechanism) (Fig. 2) (turbidity current) (liquefied or fluidized sediment flow) (grain flow) (debris flow)

S259 -, (flow transformation) +) (body transformation),) (gravity transformation) -) + supercritical flow : subcritical flow : (hydraulic jump) (surface transformation) (Fisher, +32.) (Frude number) Fr U, L Fr U, gl +, U gl +, (+1) (gl) +, (+1) + (Fr +) (Fr +) F g F i F g r c r f gahcosq (+2) Fi r f AU,, (+3) r c r f A h q F i F g r f U,, r c r f ghcosq (,*) F i F g afr, a : (,+) (,*) (,,) (densiometric Frude number) Fr d Fr d r f r c r f Fr (,,) (Richardson number) Ri Ri r c r f ghcosq r cu, (,-) (Bursik and Woods, +330) (Ri +) (Ri +) (Woods and Bursik, +33.)

S260 Fig. 3. Transition between laminar and turbulent flows in terms of Bingham number B and Rheynolds number Re. Based on Hampton (+31,) and Hiscott and Middleton (+313). (Bingham number) B t cl mu (,.) Re ULr c m (,/) Re +*. B + Re B : Hampton number Re B r cu, t c +*** (,0) (Fig. 3) L r c m t c U B + B Re (Fig. 3) (Hampton, +31,) *.. Fig. +*. Features of sediment gravity flow deposits. Modified from Middleton and Hampton (+31-). (Mohrig et al., +332) (,**/) (*.2 +) Kelvin-Helmholtz - - (Fig. +*) (Fig. +*a) Middleton and Hampton (+31-)

S261 Sohn (+331) (Fig +*a) (imbrication) (Fig. +* b) (Fig. +*c) (Allen, +32.) du dt (,1) U t x U U x du dt U t U U x (,1) U t (steadiness) U U x (uniformity) U t Fig. ++. Domains of erosion by and deposition from a density current, classified in terms of its steadiness and uniformity. Modified from Knellar and Branney (+33/). * U U x * (Fig. ++) du dt U t (waxing) (waning) U U x (accumulative) (depletive) (Fig. +,a)

S262 Fig. +,. Four modes of particles emplacement from a density current under steady conditions, with their profiles of particle concentration and current velocity. Modified from Branney and Kokelaar (,**,). (Fig. +, b) (Fig. +,c) (Fig. +,d) (Fig. +*d).. + (Ui et al.,,***) +33. (Melosh, +321),**, (Voight et al., +32-) +),) +32. +322

S263 m, U, x, g, W d d mu,, gmdx dw (,2) H, L U * * gmh W (,3) m F mmg (-*) W FL mmgl (-+) (-*) (-+) (,3) m H L (-,) Dade and Huppert (+332) t A, L W tal (--),l A ll, (-.) (-*) A l + - gmh t, - l +, t, - gmh, - (-/) Fig. +-. (a) Plots of total spreading area A vs. potential energy for debris avalanche deposits. The curve is given by A (l +/, t),/- (gmh),/- with t/l +/,./ 0 kpa. The correlation coe$cient is R *.3- (n 0/). (b) Plots of area A vs. volume V for debris avalanche deposits. Modified from Dade and Huppert (+332). gmh A (Fig. +-a) (--) gmh t A - l +, (-0) V N f N f rgh t A -, l +, V (-1) r N f gh U, (Re/B r cu, /t c) +,***

S264 Fig. +- N f +,/** 0** A l +, N f V -, (-2) (Fig. +-b) l +, N f +* + +*.., (Takahashi,,**+) (stony debris flow) (hyper concentrated flow) (mud flow) +33+ 0 Pinatubo (Pierson et al., +330) +32. (+322). - (Carey and Sparks, +320) (Carey and Sigurdsson, +32,).. Pyroclastic flow Gilbert, +3-2 (+3/1) (+3/1) +320 ; +33+ +3/1 +320 ; Wright et al., +32* (block and ash flow) ( 0. mm) (, mm)

S265 (ash flow) (pumice flow) (scoria flow) (blast surge) (base surge) (ash cloud surge) (Fisher et al., +321) +32* / +2 (Heiken and Wohletz, +32/) -* /* Fig. +..Standard ignimbrite flow unit, proposed by Sparks et al. (+31-) and modified by Fisher (+313).The depositional system (flow boundary zone) of each layer can be interpreted according to the concept of flow boundary approach of Branney and Kokelaar (,**,). (Layers +,, and - : Sparks et al., +31-) (Layers a, b, c and d: Fisher, +313) (Fig. +.) Layer a Layer b (Layer b,) (Layer b +) (Layer b -) Layer c Layer d Sparks et al. (+31-)

S266 (standard ignimbrite flow unit) (lag breccia) Layer a Layer b+ Fisher et al. (+32*) (Layers A, B and C) Layer A Layer B Layer C layer Layer b, c, d (Fisher and Heiken, +32,) Layer a : ground surge Lag breccia (Druitt and Sparks, +32+) (Sparks and Walker, +31-) Wilson and Walker (+32,) Layer a (ground layer) Layer a (jetted deposit) Layer b Sparks (+310) Layer b (Hildreth, +32+) Layer b (Freundt and Schmincke, +32/, +320 ; Cole et al., +33-) Oregon Bend (Kamata and Mimura, +32- ; Mimura, +32.) (Hiscott and Middleton, +313, +32*) Campanian Ignimbrite (AMS) (Suzuki and Ui, +32, ; Fisher et

S267 Fig. +/. Succession ofthe Kurofuji pyroclastic flow deposits (a), (b), (c) and (d) and a wood log fallen down to the flow direction and burnt in the deposit (b). CA and FA are ash layers, and CA is altered to clay. PF denotes pyroclastic flow deposit. After Mimura and Kobayashi (+31/). al., +33-) (Fig. +/) (b) Layer b (Fisher, +300 ; Branney and Kokelaar,,**,) Layer b + Layer b, Layer b, Layer b - (Fisher, +313) Layer c Layer d (co-ignimbrite ash) Layer d (Fisher, +300) Sparks (+310) Fisher (Fisher, +313) Cole et al. (+33-)

S268 Fisher (+300) Branney and Kokelaar (,**,) Grunewald et al. (,***) Soufriere +33/ +333 / +* mm, *./ +* cm, + m cataclasite pseudotachylite Branney and Kokelaar (,**,) (flow boundary approach). / (Fisher, +32.) (Kano, +33* ;Cole and DeCeles, +33+) (Kokelaar and Köninger,,***) (Head and Wilson,,**-) (Wright et al.,,**-) (Head and Wilson,,**- ;Allen and McPhie,,**,) (Kano, +330) (Kano et al., +330 ;Fiske et al.,,**+ ;Yuasa and Kano,,**- ; Wright et al.,,**-) (Head and Wilson,,**- ; Kano,,**-) (Kano et al., +330) (Kokelaar and Busby, +33,) (Cas and Wright, +33+) (Kano et al., +33.) (subaqueous eruption-fed density current : White,,***)

S269,**-) Fig. +0. Idealized flowunits presumably fed by subaqueous pumice eruption and subaqueous explosive collapse of pumiceous dome, respectively. Modified from Kano et al. (+330) and Kano (+330). (Yamada, +32.) (Kano, +330) +33* (Kano et al., +33.) (Fig. +0) (White,,*** ; Kano,,**-) (Kokelaar and Köninger,,***) (Kano, +33*) (Kano et al., +33.) (Cole and DeCeles, +33+) +31. ; +320 (Kano et al., +330 ; Allen and McPhie,,***) (Kurokawa and Watanabe, +33+) Fiske and Matsuda (+30.) (Cas and Wright, +33+ ; Müeller and White, +33, ; Kano, +33*, +330,,**- ; Kano et al., +33., +330 ; Fiske et al., +332,,**+ ; White et al., / (nue e ardente : Lacroix, +3*.) (Yamamoto et al.,,**/) +33+ 3 +/ (Fujii and Nakada, +333) +320 (Brusik and Woods, +330 ; Takahashi,,**+),** +*** 3* -** +330 +32* (Kiefer and Sturtevant, +322) +33, +* -** m s +33* +33/

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