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2. Debris Avalanches 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 COLLAPSEof 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
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