lltt.:'

:

l. Introduction than severalhundred metersacross as a maximum and ll. GeomorphicCharacteristics lessthan a meter acrossas a minimum. lll. InternalFramework debris-avalanchematrix A debris-avalanchematrix is a mix- lV. Frequencyand Magnitude ture of small volcanicclasts derived from variousparts V. Eruption Processes of the source volcano. This faciesis massive,poorly Vl. Flow and EmplacementProcesses sorted,and made of fragmentsof volcaniclasticforma- Vll. Examples tions and occasionallyof fragments of paleosolsand Vlll. Conclusions plants. hummocksCharacteristic topographic fearures for debris avalanchedeposits. The shapeof hummocksis variable and irregular. No overall trend in the alignment of hum- mocksis found. GLoSSARY jigsawcracks Jigsaw cracks are characteristicjoint patterns within a debris-avalancheblock. Jigsaw cracksare typi- cally more irregular than the cooling joints of massive amphitheaterAn arm-chair-shapedlandscape formed at igneousrocks. The joint planesusually remain closed, the sourceof a sectorcollapse. The depth, width, and but many of them open wide due to deformation during height of an amphithearer are variable. Amphitheaters the transport of the debrisavalanche. associatedwith a major debris avalancheat composite sector collapseA destructivevolcanic process volcanoesmay enclosethe summit vent. during the growth history of a volcano.Debris avalanchedeposits debrisavalanche The product of a large-scalecollapse of are the productsof sectorcollapses. a sectorof a volcanicedifice under water-undersaturated conditions.The depositis characterizedby two deposi- tional facies,"block" and "matrix." An amphitheaterat the sourceand hummoclg, topography on the surfaceof the deposit are characteristicropographic features of a SECTOR COLLAPSE of a volcanicedifice pro- debris avalanche. ducesa debrisavalanche. A debrisavalanche is debris-avalancheblock A fracturedand deformedpiece of triggered typically by intrusion of new magma, a phre- the source volcano included within a debris avalanche atic explosion, or an earthquake. A debris avalanche deposit.The sizesof single blocks are variable,more is generated at water-undersaturatedconditions. The

Copyright O 2000 by AcademicPress !:.u,t clope d i a of VoI can ou 617 All rights of reproductionin any form reserved. 6ta D Bsnrs Av,q.r-,A.NcHBs deposit is characterizedby two depositional facies, the texture, internal stnrcture, or surface morphology. Sci- debris-avalanche block and the debris-avalanchematrix entists from the United States Geological Survey facies. The debris-avalanche block facies is composed (USGS) analyzed the eruptive processes,flowage, em- oflarge, coherentyet fractured and deformed piecesof placement mechanisms, and depositional structure for the source volcano that formed the debris avalanche the 1980 Mount St. Helens debris avalanche. Other deposit. The debris-avalanche matrix facies is a more researchersthen summarized the general characteristics uniform and fine-grained mixture of volcanic fragments of debris avalanchedeposits. Since this critical eruption, derived from various parts of the source volcano. An research related to debris avalancheshas increased sig- amphitheater at the source and hummocky topography nificantly. It is now clear that the formation of debris on the surface of the deposit are characteristic topo- avalanchesis a common phenomenon within the growth graphic features produced by a debris avalanche. history of many composite volcanoes.

I. INTRODUCTIoN I I. GEoMoRPHIc CHARACTERISTIcS

Debris avalanchesare recently recognized volcanic phe- Hummoclly topography, natural levees,a marginal cliff, nomena. One of the recent historical events was when a distal cliff, remnants of temporary river channels, and the northern sector of Mount St. Helens collaosed to an amphitheater at the source are characteristic geomor- be emplacedas a debris avalanchedeposit in thi upper phic featuresof a debrisavalanche deposit (Fig. 1).Hum- tributary of Toude River on May 18, 1980. Similar mocky topography is perhaps the most significant geo- processeswere observedand recorded during the 1888 morphic feature. The shape of individual hummocks eruption of Bandai in Japan and the 1956 eruption of is variable and irregular (Fig. 2A). Although parallel Bezymianny in Kamchatka. These eventswere first clas- alignments of the long axes of hummocks have been sified as ultranrlcanian eruptions until their true nature describedin some deposits,no consistentoverall trend was understood. Before the eruption of Mount St. Hel- has been observed.The volume and height of hummocks ens in 1980, most debris avalanchedeposits were inter- are larger in tJre proximal to medial part of the deposit preted as lahar deposits.Some other debris avalanche and decreasetoward the distal end. Sbme glacial termi- deposits have been interpreted aspy"roclastic flow depos- nal moraines show a similar topographic expression. its, lava flows, or moraines, due to the similarities in Due to irregularity in form and composition, it is often

Al Longltudinalsection

B: Medtattransverse section C: oistaltransverse section

Naturallevee Matrix Marginalclitf Matrixfacies I o"bri"-"ualancheblock t t_ \:\. Debris-avalancheblock

FIGURE I Schematicsection for a debris avalanchedeposit: (A) a longitudinalsection stretchingfrom the source amphitheater to the distalend; (B) a transversesection of the medialregion; (C) a transversesection for the distalregion. Size of hummocksgradually decreasestoward the distalarea. Debris-avalancheblocks are smallerand scarceat the distalarea. A

D

-: - ln .t1* : :i--r'' ,,ti;)-

F lc U R E 2 Photographsof debrisavalanche features: (A) hummockytopography, 466 B.C.Kisakata debris avalanche deposit, Chokai volcano,northern Japan; (B) natural levee in the medialregion of the 1980Mount St.Helens debris avalanche deposit; (C) amphitheater of the 1980Mount St. Helensdebris avalanche; (D) hummockyterrain and a sourceregion already filled with postavalanchevolcanic deposits,Mount Shasta;(E) a sectionof a debris-avalancheblock, 1980Mount St. Helensdeposit; (F) a jigsawcrack, Owarabi debris avalanchedeposit, Chokai volcano, Japan; (G) a sectionof debris-avalanchemalrix, 466 B.C.Kisakata debris avalanche deposit, Chokai volcano, northern Japan;(H) deformed soft soil fragments(arrow) near the bottom contact of the 1797 debris avalanchedeposic, Unzenvolcano, Japan. 620 Dr,enrs Av.A.r,aNcHEs

hard to discriminate debris avalanchedeoosits based on topography alone.Internai depositionalieatores should III. INTERNALFRAMEwoRK be examinedto confirm the origin of a deposit. Similar undulating topography is also known in the case of The internal framework of debris avalanchedeposits small-scale pyroclastic flow deposits. However, their comprises debris-avalancheblocks and a debris-ava- gentle and regularly undulating pattern differs from the lanche matrix. Debris-avalanchebiocks are surrounded hummocky topography of debris avalanchedeposits. by a debris-avalanchematrix. Fracture patternsand fault Naturai levees(Fig. 2B), a marginal cliff, and a distal displacementin debris-avalancheblocks, heterogeneity cliff are other characteristictopographic features in a in the debris-avalanchematrix, and paieomagneticevi- well-preserweddebris avalanchedeposit. Natural levees dence are clues to the mode of emolacement. have been described that rise 40 m above the debris avalanche deposit at Socompa Volcano, in northern Chile. A well-preserued example of a natural levee is the 1792 debris avalanchedeposit of Maluyarna lava A. Debris-Avalanche Blocks dome, Unzen volcano, Japan. Marginal ciiffs (Fig. 1) occur at both the 1792 Ma1,ryamadebris avalanchede- Most debris-avalancheblocks are fragments derived posit and the Zenkoji debris avalanchedeposit of Usu frorn the source volcano (Fig'. 2E). These blocks are volcano, Japan. The height of marginal cliffs is up to fractured and deformed but preseruemany of the pri- 10 m. The cliffs develop at the lower part of a debris mary texturesand geologic structuresof the sourcevol- avalanchecleposit if it is ernplacedon a wide plain.Distal cano. Some debris-avalanche blocks are fragments cliffs are a continuation of the marsinal cliff at the distal eroded from the ground surface during transportation encl of the deposit. The height oflhe distal cliff at the of the avalanche.The size of such debris-avalanche nonvolcanic tslackhawkLandslide, California, is about blocks is generallynot so big as the blocks derived from 20 m. the source volcano. The rnaximum measureddiameter Remnants of temporary river channelsare left on the for a debris-avaiancheblock is 280 m in the Mount surface of the valley-filling debris avalanchedeposits. Shastadebris avalanche deposit. The diameterof smaller Typical exarnplesof this are seenat the Kisakatadebris debris-avalancheblocks may be lessthan 1 m. Fracnrre avalancheof Chokai volcano and the Nirasaki debris patterns called jigsaw cracks are comrnonly observed avalancheof Yatsugatakevolcano in Japan. Such phe- within debris-avalancheblocks (Fig. 2F). Jigsaw cracks nomena lbrm soon afier the emolacementof the debris within a debris-avalancheblock are typically not asregr- avalanche.In the caseof the 1980 Mount St. Helens lar as the cooling joints of massiveigneous rocks. These eruption, stream capture began just after the emplace- joint planes usualiy remain closed, but some of thern ment of the debris avalancheand the river channel had open widely, due to deformation during transport of thr stabilized within several years after emplacement of debris avalanche.Conjugated joint patterns can form in the deposit. the massivepart of debris-avalancheblocks, suggestins The shapes of the source amphitheaters can vary the existenceof iocal compressivestresses. Jigsaw crack\ widely. Tl-re amphitheater formed during the 1980 are common within debris-avalancheblocks formerl Mount St. Helens debris avalancheis a fairly typical from massivelava flows and dikes and are scarcewithin example,showing a U-shaped plan vier'".(Fig. 2C). The debris-avalancheblocks made of volcaniclasticmaterials. floor slopesnorthward toward the oper-ring(Fig. 1).The The frequency of jigsaw cracks depends on both th. initially steepwall of the amphitheaterhas slumped now rock type and travel distance. to form a lower gradient apron. The growth of a central Sorne debris-avalancheblocks show minor fault dis lavadorne partially hasfilled the amphitheater(Fig. 2C). placements(Fig. 1).The paleomagneticorientations t,: Ultimately, lava flows and dome growth will fill the samplescollected from a single debris-avalancheblock amphitheater as seen at Mount Taranaki in New are often nearly uniform. However, the declination \-ru- Zealandand Mount Shasta(Fig. 2D) and Mount Rainier ies between debris-avalancheblocks and differs fron in the United States.Amohitheaters can also be formed that of the source volcano. This implies that initi.r by relatively small-scaledebris avalancheson the outer avaianchematerial split into many pieces and the r,'- slope of a volcanic edifice, as in the case of the 1792 tation of large (>ca. 10 m) debris-avalancheblocks r- May'uyama debris avalancheat Unzen volcano. These only about an axisperpendicular to the ground surfrrcr amphitheatersshow relatively wide anglesand shallow There is little or no rotation paraliel to the grourl depths. surface.

- Doenrs Aver-,rNcrrns 621

B. Debris-Avalanche Matrix 10 . DEBRtsAVALANCHE ^6 O NONVOLCANICLANDSLIDE

The debris-avalanche maffix consists of a mixture of ! smallervolcanic parts F fragments derived fromvarious of ? the source volcano (Frg. 2G). No jigsaw cracks develop g qI within the debris-avalanchematrix. The paleomagnetic I u orientation of clasts in the matrix is random. This sug- o Al gests that the debris-avalanche matrix is formed by the J J collision and fragmentation of o debris-avalanche blocks. o Fluvial gravels, soil layers, and basement rocks are = = eroded and mixed with the primary debris-avalanche = x matrix material during flowage. The abundance of such = exotic material logarithmically increases with distance away from the source. Hummocks are mainly made of 0.1 huge debris-avalanche blocks and surrounded by the 1 10 100 debris-avalanchemauix. Smaller (

.{srffI ozz Desnrs Av,Al,eNcnps

TABLE I Listof SelectedDebris Avalanche Deposits

Volcono Deposit Height (km) Length (km) Volume(km3) T||iPe Source

Unzen t792 0.7 6 0.48 U D Yatsugatake Nirasaki 2.4 51 9 s Tateshina Otsukigawa 1.4 t2.5 0.35 S Asama Tsukahara t.8 20 2 5 Bandai | 888 1.2 tl t.5 Ba s Chokai Kisakata 2.2 25 3.5 Ba? S Tashiro-dake lwasegawa i7 8.8 0.55 s Komagatake t640 t.2 t5 t.l Bz s Usu Zenkoji 0.5 Aq 0.3 Ba? s Taranaki Pungarehu 2.6 3l 7.5 s Papandayan t772 t.5 II 0.l4 lriga t6281 l.l tl t.5 s Banahao Lucenaand Lucban t.7 26 5 s Bezymianny | 956 2.4 t8 0.8 Bz S Shiveluch t964 2 t2 t.5 S St. Helens | 980 2.55 24 2.5 Bz S Shasta 300-360 kyr B.P. 3.55 50 26 s Chaos Crags ca. | 650 0.65 5 0.t5 D Citlalt6petl Teteltzingo 4 85 t.8 L s Colima Nevadode Colima 4.3 t20 22-33 L s Socompa Holocene 3 35 t5 Bz s Rainier Osceola 4.7 t20 3.8 L S

Abbreviations:Bz, Benzymiannytype; Ba, Bandaitype; U, Unzen type; L, long-runout debris avalanche;S, stratovolcano;D, lavadome.

quency of debris avalanchesat each composite volcano amples is variable. No juvenile material is included is lessthan one per l0 kyr. Older debrisavalanche depos- within this type of deposit. The Unzen qpe is not di- its will generally be covered by younger deposits or rectly related to volcanic activity, but triggered by an eroded. Thus the exact frequenry of debris avalanche earthquake. Another origin for debris avalanchesis events is often difficult to confirm. slumping of a calderawall during the processof caldera collapse. Major slope failure of oceanic islands also causessubmarine debris avalanches. The collapse process is not always simple as the case of Mount St. Helens in 1980. Two successivecollapses V. ERUPTIoN PRoCESSES creating two separatedeposits also occurred during the 1640 debris avalancheof Hokkaido-Komagatake,Japan. The processesthat form debris avalanchesare variable. In the caseof the debris avalanchedeoosit of Banahao Three types of debris avalancheshave been proposed: volcano, Philippines, two successi't..ollrpr.r occurred Bezy'rnianny, Bandai, and lJnzen tlpes. Bezymianny- and flowed in different directions in the proximal area type debris avalanchesare associatedwith magmatic but merged into a singleflow at the middle to distal area. eruptions, as in the caseof Mount St. Helens in 1980. A lateral blast accompaniedthe 1980Mount St. Hel- The Bandai type is associatedwith a phreatic eruption, ens debris avalancheand the 1956 debris avalancheof as in the caseof Bandai, Japan,in 1888. The duration Bezyrnianny volcano. The lateral blasts spread over of precursory phenomena associatedwith recorded ex- a much wider area than the debris avalanches and DosnIs Aver-eNcnBs 6,23

desuoyed a large area of forest. A lateral blast did fluidization and sliding on a lubricated substrate model not accompany the 1792 debris avalanche of Unzen can explain the depositionalcharacteristics ofthe Sher- volcano. man Glacier debris avalanche,Alaska. In this caseit was thought that the debris avalanche was caused through mechanical fluidization, derived from the vibration aid energy of the Great Alaska earthquake. This debris ava- lanche behaved as a Bingham plastic and its high viscos- ity prevented turbulence and kept deformation in VI. FLoW AND EMPLACEMENTPRoCESSES the avalanchelargely confined to shear at the baseon glacier ice, so that for t-he most part, the avalanche slid as a More than 20 dlnamic emplacementmodels have been thin flexible sheet. proposed to explain the high mobility of large volcanic The acoustic fluidization model has been proposed and nonvolcanic debris avalanchesand many controver- as a more geologicallyand energeticallyacceptable ver- sies over their emplacement still continue. The air fluid- sion of fluidization than the dispersivegrain flow model. ization theory was proposed based on geologic relations This model suggeststhat acoustic fluidization is pro- of the Saidmarreh, Frank, and Madison debris ava- duced indirectly by the releasedfall energy through the lanches as well as on eyewitness accounts of their em- propagation of strong sound waves of just the right placement. A "hovercraft" mechanism, involving air- frequency through a flowing breccia stream. Acoustic layer lubrication, was proposed to explain the long fluidization requires much less energy than mechani- runout and sedimentological and morphological fea- cal fluidization. tures, for example, preservedintact primary stratigra- For debris avalanchescomposed of limestone,notably phy, bulldozed distal ridges, three-dimensionaljigsaw- Films (Switzerland), another lubricated sliding model puzzle breccia,distal rims, lateral levees,and transverse hasbeen suggested.It is believedthat high temperarures ridges and troughs, in the Blackhawk and Sharman de- developedalong a discrete slip surfacewould continu- bris avalanches,lJnited States. FIowever, the air-layer ously dissociatethe carbonate rock into a mixture of lubrication mechanismhas difficulties in explainins de- lime (CaO) and CO2 gasduring travel and the CO2 gas bris avalancheson Mars and the Moon and-subacu-eous would provide a lubricant along a basalslip surface. debris avalancheson Earth. A basal gaseourpot. pr.r- A mechanical fluidization model in spreading ava- sure model was proposedto explain the rapid velocities Ianches has been proposed. In this model, the debris and low apparent angle of friction of large coherent avalanchedeposits are formed by fluidlike spreadingof debris avalanchesusing pore fluid vaporization along the debrisunder the action ofgravity, and this spreading their basesdue to frictional heating during sliding. A occurs due to fluidization of the debris by high basal basalselflubrication mechanismis an alternativeexpla- shear stressas it moves rapidly acrossrhe ground. The nation of the low angle of friction and other characteris- correlation between volume and the apparent angle of tic features of the Blackhawk debris avalanche. The friction is a result of fluidlike spreading of debris ava- Blackhawk debris avalanchebulldozed a distal heao of lanches during runout into thin sheets: the larger the sandstoneahead ofit asit progresseddownslope at high volume, the larger the area covered by debris, and the speed.The sandstone"breccia" at the front ofthe flow larger the runout length for a given fall height. 'was "smearedout" beneaththe overridins marble brec- A Bingham flow model has been proposed for the cia, providin g a zone of high fluidity beneath the front Mount St. Helens debris avalanchebased on the geo- of the debris lobe (selfJubrication). morphology of the Mount St. Helens deposit.The exis- A mechanical fluidization model based on the revived tence of natural levees(Fig. 28) and marginal and distal idea of grain contact linked with granular flow theory cliffs (Fig. 1) suggestsnon-Newtonian (Bingham) flow has also been suggested.This assumesthe presenceof behavior. The Bingham flow model has also been used an intergranular fluid and proposes that highly energetic to explain the depositional features and eyewitness of interstitial dust could reduce the effective normal pres- the 1984 debris avalanche,Ontake volcano, Japan. A sure on grains and consequently reduce frictional resis- transport model using initial sliding and later plug tance. For wet debris avalanches,such as the 1970 Hu- (Bingham) flow was proposed after quantitative data ascar6n debris avalanche, it is thought that mud could were obtained from the Iwasegawadebris avalanche de- have formed the fluid component between clasts.In the posft at Tashiro-dake volcano, Japan, and the Kaida case of large dry debris avalanches,fine dust might act debris avalanchedeposit at Ontake volcano,Japan. as the interstitial fluid. A combination of mechanical The granular flow model has also been proposed for 624 Drsnrs Aver-eNcnBs the 1980 Mount St. Helens debris avalanche,based on ated around the basallayer, and the shock contributes to detailed field evidence of the deposit. The granular flow a loosening of the avalanchemass into a fluidized state. model has also been used for the debris avalanche at A biviscous flow model has been used to simulate Cantal volcano, France, using anisotropy of magnetic realistic peak velocities and travel times for the 1974 susceptibility (AMS) measurements of the matrix fabric. Mayrnmarca debris avalanche, Peru, the 1980 Mount The data suggesttransport of the avalancheby liquefied St. Helens debris avalanche.United States.and the 1984 nonturbulent granular flows. Distally, imbricated depos- Ontake debris avalanche, Japan. Numerical biviscous its suggest more turbulent flow. Emplacement of the modeling indicates that the avalanche at Mayunmarca avalanche results from progressive aggradation of the and avalanche I at Mount St. Helens in 1980 (H/L particles related to a loss of the nonturbulent liquefied - 0.2) were of relatively high strength, with apparent itage. The mass of the avalanche becomes a plug flow Newtonian viscosities of up to several hundred m2ls. In that exerts strong friction on the basement. contrast, the explosively triggered avalancheIIIIII flow Another model uses basal low-density layers, based at Mount St. Helens in 1980 and the Ontake debris on computer simulation of interacting two-dimensional avalanchein 1984 (H/L - 0.1) were of relatively low disks. The basal low-density layer is a region where the strength, with apparent Newtonian viscosities l-2 or- particles are very active in the sensethat they have large ders of magnitude lower. random velocity components (high granular tempera- Some combination of these models probably applies ture). A basal low-density layer supports an overriding in most cases,and each debris avalanche,depending on high-density, low-mobility debris "plrrg" during travel. its composition, cause,volume, and environment, could Computer simulations of granular avalancheswere per- be simulated by only some of the models listed here. formed using from 5000 up to 1,000,000 two-dimen- sional disks. The model results suggest that the basal friction coefficient varies with shear rate, where higher shear rates give rise to higher friction coefficients. For VII. EXAMPLES debris avalanches with a given fall height to particle diameter ratio, debris avalancheswith larger volumes have lower internal shear rates. and thus. lower basal A. 1980 Mount St. Helens friction coefficients. (Bezyrnianny Typ") Another concept is the mass loss model, which sug- gests the low apparent angle of friction values achieved A swarm of volcanic earthquakes was first detected on by debris avalanchesoccur becausethey selectively de- March 20, 1980.An initial phreatic explosion occurred posit low-velocity material during movement. The ulti- on March 27, after 123 years of dormanry. Severalphre- mate runout length of a debris avalanche traveling by atic explosions followed and normal faults gradually de- such a mechanism would deoend on the rate and timinE veloped at the summit crater. The north side of the of the massloss. In this model, once a debris avalanch! volcanic edifice gradually bulged outward, beginning reaches a depositional area, material at the trailing end in early April. Phreatic explosions were repeated and of the moving avalanche begins to deposit, transferring deformation accelerated. A maximum horizontal dis- its energy forward to the moving part of the avalanche. placement of 120 m was recorded in the middle of May. The toe of the avalanche continues to move downslooe Finally, the northern sector of the volcanic edifice col- propelled by the input of momentum from decelerating lapsed at 8:32 a.m. on May 18. The resulting debris trailing clasts until the stopping wave catches up with avalanche filled the upper tributary of the Toude River the toe. and traveled up to 28 km away from the source. Its A seismic energy fluidization model was given for the volume was 2.5 km3. The velocity of the avalanchewas 1944 Mount Vesuvius block-and-ash avalanchesin Italy. about 50-70 m/s, judging from a seriesof photographs. In this model the relative importance of seismic tremor The collapsing masswas affected by a secondary explo- to maintaining avalanche mobility is stressed.A basal sion producing a lateral blast and destroying over 500 pressure wave model has also been suggested where a km2 of forest area. The maximum velocity of the blast wave propagated along the basallayer at the phaseveloc- was assumedto be more than 80 m/s. Aplinian eruption ity, which is initially greater than the debris avalanche followed just after the emplacement of the debris ava- velocity. With increasing avalanche velocity, the ava- lanche and a pyroclastic flow of 0.1 kmr was also em- lanche mass catches up with the guided wave. Over a placed. The climactic eruption was over in the evening threshold avalanche velocity, a "sonic boom" is gener- of May 18. The total number of fatalities was 57. DBrnrs Av,cr,eNcHps 625

Plinian eruptions with pyroclastic flows were pro- phitheater at the source is also a characteristic geomor- duced on several occasions through October 1980 and phic feature. The internal framework that characterizes then a new lava dome began to grow on the floor of debris avalanche deposits is composed of debris-ava- the amphitheater. Srnall-scale explosive eruptions and lanche blocks and a debris-avalanchematrix. The former intrusion of new lobes were repeated until October comprises large fractured and deformed fragments 1986. mosdy derived from the sourcevolcano, preserving orig- inal internal structures and textures. The latter consists of a mixture of smaller volcanic fragments derived from various parts of the source volcano and mixed B. 1888 Bandai (Bandai Typ") with a minor amount of exotic materials. Three types of link to eruption processesare defined: The first seismicactivity began onJuly 8, 1888.Contin- debris avalanches associated with intrusion of new uous seismicity occurred almost every day until the magma, those associatedwith phreatic explosions, and morning ofJuly 15. Then a strong earthquakewas felt those triggered by earthquakes. The ratio of collapse about7:3 0 a.m.onJuly 1 5. A seriesof strongearthquakes height to runout distance is generally between 0.2 and followed and finally a phreaticexplosion occurred at the 0.06. In some cases,clay-rich debris avalanchesmay summit.After a further 15-20 explosionsoccurred, the transform into lahars and spread over a much wider area. northern sector of the volcano collapsedto produce a The eruption and emplacemenr of debris avalanches debrisavalanche. The debrisavalanche covered anarea are now recognized as common phenomena during the of 3.5 km2 and its volume was 1.5 kmi. Small-scaleexplo- entire growth history of composite volcanoes. However, sionsfollowed 30-40 min after the climacticevent. The the transportation process for debris avalanchesis not eruption wasover by the eveningof the sameday. The yet fully understood. The frequency of this type of volca- total numberof fatalitieswas 46t. nic activity is lorver than the other styles of eruption, but the magnitude of single events is generally large and hazardous. Thus, further work, both theoretical C. 1792 tlnzen (Unzen Typ") modeling and casestudies of deposits,is necessaryfor the mitigation of volcanic risk. A swarm of earthquakesbegan in November 1791.A magmaticeruption beganon February 29, 1792.The volcanoproduced a dacitelava flow from a vent at the nortlern slopeof Unzenvolcano byApril 23.Its volume AcKNowLEDGMENTS wasabout 0.03 km3.A swarmof tectonicearthouakes occurredsimultaneously with the dischargeof thi lava We gready flow. A small-scalecollapse of part of a 4000-year-old appreciatevaluable comments byJonathan Dehn, Vincent E. Neall, and lavadome, Mayuyama, located 4 km awayfrom the vent JamesW. Vallance. of the new lava, occurred on April 9. A secondand climactic collapsewas triggered by a relatively large tectonic earthquake(M : 6.4) on May 21. The ava- lanche flowed into Ariake Bay and generateda major SEE ALSO THE FOLLOWINGARTICLES tsunami.The total volume of the avalanchewas 0.48 km3.The total numberof fatalitieswas I 5,1 90, including CompositeVolcanoes . Ignimbrites and Block-and-Ash 11,000taken by the tsunami. Flow Deposits. Lahars. Plinian and SubplinianEruptions . VulcanianEruotions

VIII. CoNCLUSIoNS FURTHER READING Debris avalanches constitute an only recendy recog- nized form of volcanic phenomena. Hummocky topog- Campbell,C. S., Cleary,P. W., and Hopkins,M. (1995). raphy, natural levees, marginal cliffs, and distal cliffs Large-scalelandslide simulations: Global deformation, ve- characterize young and well-preserved deposits. An am- locities and basalfriction.J. Geophys.Res. 100, 8267-8283. 6'26 Dnenrs Avar-eNcnrs

Davies,T, R. H. (1982).Spreading of rock avalanchedebris Takarada, S., Ui, T., and Yamamoto,Y. (1999). Depositional by mechanicalfluidization. Rock Mech. 15,9-24. features and transportation mechanism of valley-filling Iwa- Goguel, J. 0978). Scale-dependentrockslide mechanisms, segawa and Kaida debris avalanches,Japan. Bull. Vohanol., with emphasison the role of pore fluid vaporization.Pages 60,508-522. 693-706rn "Rockslidesand Avalanches 1" (8. Voight, ed.), Ui, T. (1983).Volcanic dry avalanche deposits-Identification Elsevier,Amsterdam/New York. and comparisonwith nonvolcanicdebris streamdeposits. Gorshkov, G. S. (1959). Gigantic eruprion of the Volcano J. Vohanol.Geotherrn. Res. 18, 135-150. Be4rmianny.Ball, Vohanol.20, 77 -109. Ui, T. (1989).Discrimination betweendebris avalancheand Shreve,R. L. (1968).The Blackhawklandslide. Geol. Soc. Am. other volcaniclasticdeposits. In "Yolcanic Hazards" (f. H. Spec.Pap. LO8,47. Latter, ed.),IAVCEI Proc.Volcano. 1,201-209. Siebert,L. (1984).Large volcanicdebris avalanches: Charac- Ui, T., Yamamoto,H., andSuzuki-Kamata, K. (1986).Charac- teristics ofsorlrce areas,deposits, and associatederuptions. terizationof debrisavalanche deposits in Japan.J. Vohanol. J. Volcanol.Geotherrn. Res. 22, 163-197. Geotberrn.Res. 29, 231-243. Siebert,L. (1996).Hazards of largevolcanic debris avalanches Vallance,J. W., and Scott, K. M. (1997).The OsceolaMud- and associatederuptive phenomena.Pages 541-572 in flow from Mount Rainier:Sedimentology and hazard impli- o'Monitoring and Mitigation of Volcano Hazards" (R. cation of huge clay-rich debris fow. Geol. Soc.Am. Ball. Scarpaand R. I. Tilling, eds.),Springer-Verlag, Berlin/ tog, 143-163. New York. Voight, 8., Janda,R. J., Glicken,H., and Douglass,P. M. Siebert,L., Glicken,H., andUi, T. (1987).Volcanic hazards (1983).Nature and mechanicsof the Mount St. Helens from Bezymianny-and Bandai-typeeruptions. Bull. Voha- roclalide-avalancheof 18 May 1980. Geotechnique33, nol.49.435-459.