Lahars Research Articles

1987年岩手県中部沿岸地震における岩手県内の高密度震度分布と地質構造との関係

A questionnaire survey was carried out to estimate the detailed seismic intensities on the Iwate Chubu-engan Earthquake of magnitude 6.6 of January 9, 1987. 5032 questionnare sheets were delivered to the persons living in and around Morioka City, to investigate the relation between the detailed seismic intensities at every 500m×500m mesh and the subsurface structure based on the boring data. Other 5122 questionnare sheets were delivered to the persons living in another area of Iwate Prefecture. The relation of intensity attenuation with distance was deduced and the relation between the intensity deviation and general geological stucture was investigated.Obtained results are as follows:1) In Morioka City and its suburban districts, the detailed seismic intensity considerably depends on the subsurface structure at the site. For example, deviations from the average intensity are -0.1--1.0 in the district at where base rock of granite and shalstein is exposed or covered by sediment with sand and gravel. Conversely, at place of the soft ground constituted by volcanic mudflow and loam, they are +0.1-+0.72) From a view point of general geological structure in the whole Iwate Prefecture, seismic intensities are 0.1-0.6 greater than those expected from empirical formula at the valley of the Kitakami river and Toono basin where Quaternary fluvial plain deposit is widely distributed. On the contrary, seismic intensities are 0.1-0.4 less than the average at the west of the Kitakami river where Tertiary system and Quaternary volcanic rocks are distributed, and the whole Kitakami mountains where pre-Tertiary is distributed.

Summer Production of Coho Salmon Stocked in Mount St. Helens Streams 3–6 Years after the 1980 Eruption

Abstract We monitored habitat use and summer production of stocked underyearling coho salmon Oncorhynchus kisutch from 1983 to 1986 in three streams affected by the 1980 eruption of Mount St. Helens, Washington. Two streams were in the blast area and one was on a volcanic mudflow terrace, Midsummer water temperatures frequently exceeded presumed stressful thresholds and occasionally surpassed the incipient lethal limit. Temperatures at the study sites (up to 29.5°C) may have been the highest ever recorded in small streams in western Washington. In addition, there was relatively little submerged cover and limited pool habitat. Despite the severe conditions created by the eruption, production rates of stocked coho salmon at all sites ranged from 15.1 to 143.8 mg/m2·d (2.3–21.6 g/m2 over an average 150-d summer period) and were equal to or greater than those measured in other streams of comparable size in the region. Coho salmon production in the streams was more strongly influenced by population biomass and...

Large-scale volcano-ground ice interactions on Mars

The process of volcano-ground ice interaction on Mars is investigated by thermodynamic calculations and observations of Viking Orbiter images. We develop a numerical model of volcano-ground ice interaction that includes heat transport by conduction, radiation from the surface, heat transfer to the atmosphere, and H 2O phase changes in an ice-rich permafrost. We consider eruption of lava flows over permafrost, and intrusion of sills into permafrost. For eruption of lava over permafrost, most of the heat in the flow is lost by radiation and atmospheric effects. The amount of H 2O liquid and vapor produced is small, and its removal would not be sufficient to cause collapse that would lower the surface of the lava flow below the surrounding terrain. For intrusion of a sill, most of the heat in the sill eventually goes into H 2O phase changes, producing much larger amounts of water that could have profound geomorphic and geochemical effects. Approximate meltwater discharge rates are calculated for both extrusive and intrusive interactions. We examine two large regions of large-scale volcano-ground ice interactions. Near Aeolis Mensae, intrusion of a complex of dikes and sills into ice-rich ground has produced substantial melting, with mobilization and flow of material. This interaction probably also produced large quantities of palagonite tuff and breccia. Morphologic evidence for progressive fluidization implies that meltwater was stored beneath the surface for some time, and that most of the release of water and volcanic mudflow took place late in the interaction. Northeast of Hellas, several large channels emanate from the area near the volcano Hadriaca Patera. If genetically related to the volcanic activity, large collapse features at the sources of some channels must have originated due to heat from large buried magma bodies. A channel emerging directly from the base of Hadriaca Patera may have originated from release of heat from thick extruded material. Other small channels in the region results from heat released from surface lava flows. Inferred channel discharges may be compared to discharge rates calculated for lava-ground ice interactions. Such comparisons show that meltwater probably accumulated beneath the surface and then was released rapidly, with a discharge rate limited by soil permeability. Volcano-ground ice interaction has been a widespread and important geologic process on Mars, and may be the primary source of palagonites making up the ubiquitous Martian dust.

Volcanic disasters in Latin America and the 13th November 1985 eruption of Nevado del Ruiz volcano in Colombia

Volcanic eruptions in Latin America have claimed about 61,000 lives since 1600 A.D. and the region's volcanoes are responsible for about a quarter of the world's fatalities from this type of hazard. Nearly all loss of life from volcanism in Latin America is due to pyroclastic surges, pyroclastic flows and lahars or volcanic mudflows. Lahars generated during the 13th November, 1985 eruption of Nevado del Ruiz in Colombia claimed 25,000 lives, underscoring the great hazard from lahars, which can be generated from the fifty-six, active, ice-capped Central and South American volcanoes during even very small eruptions. The probability of specific prediction of the timing of such events is currently low, whereas the probability of a general prediction of volcanic eruption is high, giving sufficient time to install telemetered lahar alarm systems, which could largely avoid the loss of life.

Volcanic Debris Flows: Facies and Sediment Transformations: ABSTRACT

Volcanic Debris Flows: Facies and Sediment Transformations: ABSTRACT

Mesozoic Sedimentary Rocks and Depositional Facies, Vizcaino-Cedros Area, Baja California, Mexico: ABSTRACT

Mesozoic sedimentary rocks in the Vizcaino-Cedros area constitute a 135-m.y. history of arc-related marine sedimentation with a cumulative thickness of 14 km. The Upper Triassic San Hipolito Formation is the oldest sedimentary unit in the region, and is recognized only on the Vizcaino Peninsula. The formation depositionally overlies an ophiolite sequence and consists of 2.4 km of tuffaceous sediment including a limestone megabreccia. The upper two-thirds of the sequence in the type section is now believed to be of Early Jurassic age. On Cedros Island, the basal sedimentary rocks are 1.2 km of the Lower Jurassic Gran Canon Formation. This richly tuffaceous unit depositionally overlies both ophiolite and arc volcanics. Conformably overlying the Gran Canon on Cedros Island is the Coloradito Formation, a spectacular sedimentary olistostrome up to 0.4 km thick, containing Triassic and late Paleozoic metasedimentary blocks. This formation record the first evidence of continental detritus in the region. Conformably overlying the Coloradito and, in part, stratigraphically equivalent to it on Cedros is 0.4 km of volcanogenic-metasedimentary conglomerate designated the Eugenia Formation. The Eugenia is much thicker (to 2.7 km) and widespread on Vizcaino where it unconformably overlies the San Hipolito Formation. On Vizcaino, the lower Eugenia (Upper Jurassic)more » includes spectacular volcanic debris flows interbedded with sandstone, but it is transitional upward into finer facies with increasing plutonic detritus. Time equivalent to the upper Eugenia and locally deposited on tonalite and volcanics is the Lower Cretaceous Asunction Formation (0.8 km thick). This sequence includes calcareous coarse breccias of serpentinized gabbros and tonalite.« less

Downstream Dilution of a Lahar: Transition From Debris Flow to Hyperconcentrated Streamflow

Nearly instantaneous melting of snow and ice by the March 19, 1982, eruption of Mount St. Helens released a 4 × 106m3flood of water from the crater that was converted to a lahar (volcanic debris flow) through erosion and incorporation of sediment by the time it reached the base of the volcano. Over the next 81 km that it traveled down the Toutle River, the flood wave was progressively diluted through several mechanisms. A transformation from debris flow to hyperconcentrated streamflow began to occur about 27 km downstream from the crater, when the total sediment concentration had decreased to about 78% by weight (57% by volume). The hyperconcentrated lahar‐runout flood wave, transporting immense quantities of sand in suspension, continued to experience progressive downstream dilution. Although turbulence was significantly dampened by the extremely high suspended load, very large standing waves and antidune waves were observed. The hyperconcentrated lahar‐runout flow deposited an unusual, faintly stratified, coarse sand which locally contained small, isolated gravel lenses. Very similar deposits in the Quaternary stratigraphy of Mount St. Helens and other Cascades volcanoes suggest that lahars may be more frequent than previously recognized.

The occurrence and mitigation of volcanic hazards in Indonesia as exemplified at the Mount Merapi, Mount Kelut and Mount Galunggung volcanoes

Summary Indonesia is one of the most active volcanic areas in the world and has a population of 150 million. Natural disasters affecting large areas and numbers of people are therefore a common occurrence and the authorities have developed strategies to cope with them. On 5 April 1982 Mt Galunggung in W Java suddenly erupted after 64 years and it is still active. From April to June this activity coincided with the rainy season; the volcanic mud and boulder flows (lahars) which were generated ended in late November. Tens of millions of cubic metres of unconsolidated volcanic ejecta, mostly ash and sand size, were deposited around the newly formed crater on the slopes of the volcano and over a wider area. In anticipation of the next rains check dams and other works were planned, as it was feared that as many as 250 000 people might be affected, and further large areas of agricultural land inundated as a result of rain generated lahar. A civil defence system has been initiated, utilizing the experience gained as result of previous similar disasters. The paper summarizes the geological background, and describes the related types of primary and secondary hazards and the attempts being made to mitigate the effects of these hazards.

Initiation and flow behavior of the 1980 Pine Creek and Muddy River lahars, Mount St. Helens, Washington

Two large, high-velocity lahars (volcanic debris flows) were triggered by a pyroclastic surge during the first few minutes of the May 18, 1980, eruption of Mount St. Helens. The initial surge cloud evolved progressively by gravity segregation from a gas-mobilized, highly inflated density flow to a dense, water-mobilized, basal debris flow (lahar) and accompanying ash cloud as it flowed down the east flank of the volcano. The main source of the water for the lahars was probably from eroded snow and ice incorporated into the flow by turbulent mixing, but ground water, expelled together with the rock debris by the initiating volcanic explosions, also may have contributed. Peak lahar discharge from the Pine-Muddy fan, upper Smith Creek, and Ape Canyon probably exceeded 250,000 m 3 /s initially but decreased exponentially in the downstream direction. Total volume of the lahars was in excess of 1.4 × 10 7 m 3 . Initial peak-flow velocities in excess of 30 m/s also decreased markedly downstream. Where flow was not impeded, velocity was strongly related to the depth-slope term (R 2/3 S 1/2 ) from the Manning uniform-flow equation as a power-law function. During much of the route traveled, lahar flow appears to have been supercritical. Deposits left in channels were generally thin relative to flow depth (0 to 2.5 m). Particles up to small boulder size were randomly distributed in the poorly sorted, nonstratified matrix, indicating complete suspension in a fully developed debris-flow slurry; however, much larger clasts were transported as “bed-load.” Computed sediment concentrations of matrix slurry samples ranged from 84% to 91% solids by weight and were similar for the two lahars. Two indirect methods for computing peak-flow velocity, previously only tentatively applied to debris flows, were tested for accuracy by comparing computed lahar arrival times with recorded arrival times at Swift Reservoir. The computed velocities appear to be ∼15% slower than the recorded velocities, which is consistent with the restriction that the velocity formulas produce only minimum values.

Goodnews Terrane and Kuskokwim Group, Eek Mountains, Southwest Alaska: Open Marine to Trench-Slope Transition: ABSTRACT

In the Eek Mountains, southwest Alaska, four depositional settings record Permian to mid-Cretaceous marine sedimentation and post-Valanginian crustal shortening. Sequence 1: pillowed basalt flows (SiO2 = 44%) and volcaniclastic sediments intercalated with Permian Atomodesma-bearing sandy limestones grade upward through volcanic debris flow deposits and tuffaceous argillites into sequence 2: thin-bedded chert with argillite partings. Sequence 3 rests conformably(?) on sequence 2 and consists of undated argillite grading upward into Early Cretaceous (Valanginian) thin-bedded Tcde/Tde turbidites and thick-bedded conglomeratic grain flow deposits. Sequence 4 consists of late Early Cretaceous (Albian) thick-bedded conglomeratic grain flow deposits and minor Tcde/Tde and Ta/Tab turbidites. Sequence 4 (Kuskokwim Group) overlies sequences 1, 2, and 3 (Goodnews terrane) with an angular unconformity preserving submarine canyon cut-and-fill. Sequence 1 requires an ocean island(?), and sequence 2 an open marine origin. Sequences 3 and 4 are inner-fan turbidite deposits; conglomerate and sandstone compositions indicate recycled orogen, arc orogen, and collision orogen provenances. This suggests preaccretion Valanginian clastic input from the now-adjacent Kilbuck terrane. All four sequences record southeast over northwest imbrication along southeast-dipping thrusts. Deformation and metamorphic intensity increases with age/depth; a maximum is recorded in pervasively foliated Permian greenstone crosscut by prehnite and pumpellyite veins. Sequences 1 and 2 are highly disrupted, with local blocks-in-matrix fabric. Sequence 3 shows a similar, less pervasive style. Sequence 4 is locally disrupted and overturned along faults but lacks penetrative deformation. Deformation began between Valanginian and Albian times, and either progressively penetrated upsection with time or was multiphased with consistent recurring structural style. End_of_Article - Last_Page 672------------

Hazard of lahars to the tongariro power development, New Zealand : Paterson, B R Proc 3rd Australia-New Zealand Conference on geomechanics, Wellington, 12–16 May 1980, V2, P7–14. Publ Wellington: New Zealand Institution of engineers, 1980

The Tongariro hydro-electric scheme intercepts a number of rivers which drain two active volcanoes and diverts them into rivers and natural lakes renowned for trout fishing. One of the active volcanoes, Ruapehu, is subject to unpredictable eruptions of its crater lake, during which lahars or volcanic mudflows are generated in several of the main valleys. As the chemical waters from the crater lake are toxic to fish, a lahar protection system was installed to prevent the spread of volcanic contamination into waters normally unaffected by these eruptions. This system also prevents volcanic debris from silting the power scheme waterways and reservoirs. The lahar protection system was designed to cope with eruptions of the size that occurred in 1969, but proved to be inadequate for the 1975 eruption which was several times greater. Data obtained from the 1975 eruption have been utilized to upgrade the original lahar protection system and extend it into areas previously thought to be free from risk. The 1975 eruption also demonstrated the serious hazard of future eruptions to public safety as well as to the fisheries and the power scheme.

Fire and Other Disturbances of the Forests in Mount Rainier National Park

AbstractThe recent history of catastrophic disturbances in forests was reconstructed at Mount Rainier National Park. Basic data were ages of trees based on ring counts of early seral conifer species and maps of age-class boundaries from field work and aerial photographs. Maps illustrate age classes of the forests and show disturbances from fires, snow avalanches, and lahars (volcanic mudflows). Fires are by far the most important major disturbers, followed by snow avalanches and lahars. Fires over 250 ha in size are called fire events. Burns over 1000 ha, which may have been one fire or a series of fires within a short time, are called fire episodes. Important fire events or episodes occurred in the years 1230, 1303, 1403, 1503, 1628, 1688, 1703, 1803, 1825, 1856, 1858, 1872, 1886, 1894, 1930, and 1934 A.D. The largest fire episode was in 1230; it affected approximately 47% of the forests in the park. The majority of the forests are over 350 yr old, and several stands are over 1000 yr old. Stands 350 yr and 100 to 200 yr in age are the most extensive age classes in the park. Three fire frequency indices are compared. None describe fire frequency at Mount Rainier well. Natural fire rotation was estimated at about 434 yr. All but two episodes of major fires since 1300 A.D. correspond well with major droughts reconstructed for locations east of the Cascade Range crest. Impacts of humans on the disturbance regime may have increased the frequency of fire in the 1850–1900 period, followed by a decrease in frequency after 1900. Fuel build-up as a result of fire suppression should have no significant impact on fire frequency, since fires are relatively infrequent and fuels accumulate naturally.

El Lahar Tinguiririca: su significado entre los Lahares chilenos

The Tinguiririca lahar is an example of many similar landforms, which coverlarge surfaces 60th in andine valleys and the Chilean longitudinal depression.I t is smaller than other ones in Chile, but - at the same time - better conserved,offering good conditions for genetic, kinematic and geochronologic studies,respecting volcanic mudflows.From the genetic and kinematic point of view, the longitudinal lahar profile maybe in two parts: a) An upper and middle accumulative area, with concavetransverse profile; b) A low accumulative area; whose transverse profile is convexe.These two profiles could be explained by means of braking process inlateral parts, related to central parts, as a function of longitudinal distoncesfrom the origin.From the geochronologic point of view, the lahar was between theLast Glaciation and Late Glacial times. Therefore, i t is more recent than otherones.

富士山の地質と変ぼう

The geological formations of Mt. Fuji are composed of Pre-Miocene, Miocene, Pliocene and Komitake, Fuji volcanic ejecta.The following table shows the geological formations of this region.Pre-Miocene, Miocene and Komitake volcanic ejecta lie beneath Mt. Fuji. These rocks are the foundation of Fuji volcano.The first building of Fuji volcano started by Kofuji volcanic mudflow which covered the Komitake volcano and older rocks. The second eruption is made up of a sequence of Younger volcanic ejecta, which cover on alluvium sequence.The sequence of ejecta is L1-T1-L2-T2-L3-L4-T3 layers, except for L4 and T3 layers they all erupted from summit fire crater. L-layer is composed of the lava and tephra alternation and T-layer, which has the most of tephra. These limits make up the stratified volcano.The flanks of mountain are sculptured by a number of radial valleys but they not have surface streams. The largest valley is “Osawa” and the second is “Yosidaosawa”, both integrated these valleys are now growing. These valley are eroded by streams of the heavy rainfall, flow of underground water and by snowslides. Osawa is a V-type erosion valley and Yoshidaosawa is a U-type wallerosion valley.On the flank of the mountain, rainwater sinks into the ground and is stored on the surface of the unpermeable layers (Kofuji volcanic mudflow) as the underground water. It runs downward and flows out at the mountain foot. From the cliff, which outcrips at the top of Yoshidaosawa, large rock fragments fall and roll on the valleyfloor. Summer in 1980, twelve persons were killed by such rockfall. This rockfall happened by the increase of the undergroundwater pressure which made the rock fragments on the near cliff unstable and caused the disasterous rockslides.

ACCUMULATION RATES OF ALLUVIAL CONES, CONSTRUCTED BY DEBRIS-FLOW DEPOSITS, IN THE DRAINAGE BASIN OF THE TAKAHARA RIVER, GIFU PREFECTURE, CENTRAL JAPAN

The Takahara River and its tributaries dissect rugged mountains in central Honshu Island, forming steep mountain slopes along the rivers. Small precipitious streams deeply dissecting the mountain slopes usually attain gradients more than twenty degree. Debrisflows have occasionaly occurred from these streams, bringing heavy damages to inhabitants. The authors selected 271 streams from those in the study area and investigated the number of debris-flows occurred from each stream by the method of interviews to the inhabitants. It is revealed that the debris-flows occurred sixty times from the selectedd streams during the period from 1913 to 1979. The estimated average probability value of debris-flow occurrence from a stream, is accordingly calculated to be 3.3 × 10-3/year. The estimated return period of debris-flow occurrence is, therefore, determined to be 3.0 × 102 year. The distribution of stream numbers classified by the number of debris-flow occur rence are conformed to the poisson distribution with the same probability, except four large streams (Table 2) . Accordingly, it is considered that the occurrence of debrisflow has a character of a randam phenomena as a whole. The authors also measured the volumes of fifteen debris-flow deposits accumulated during the last seventy years, and found that the volume of each debris-flow deposit is explained as a function of the catchment area of each stream (eq. 2; Fig. 6). Six steps of fluvial terraces are distributed along the Takahara River and its tributaries; namely, Hongo, Miyahara, Nakagoe, Tono, Sakamaki and Miza terraces, from higher to lower. Hongo terrace was formed by the deposition of volcanic mudflows mainly transported by the fluvial process. The volcanic mudflow deposit covers the underlying Sukusaka terrace deposit with slight unconformity (Koike, 1978) . Sukusaka terrace is a dissected alluvial fan formed along the Jinzu River, the main river of the Takahara, and correlated with fluvial terraces distributed scatteredly around the margin of Northern Japan Alps (Fukai, 1957; Machida, 1979). As the age of these terraces is considered to be ca. 50, 000_??_60, 000y. B. P., the age of Hongo terrace is estimated to be slightly younger than 50, 000years. Miyahara terrace was mainly formed by the erosion of Hongo terrace deposit. An alluvial cone deposit intercalating the volcanic ash bed(DKP, 45, 000_??_47, 000y. B. P.) covers the terrace surface. Therefore, the age of Miyahara terrace is estimated to be ca. 47, 000y. B. P. Etchu Akatsuchi covers both Hongo and Miyahara terraces (Fig. 4). Nakagoe terrace was formed by fluvio-volcanic mudflow deposit, derived from Kachigane mudflow. The age of the terrace was estimated to be 11, 000y. B. P., from the 14C age of wood intercalated in Kachigane mudflow (Koike, 1978). This terrace is not covered by Etchu Akatsuchi, but by brown forest soil with clear B-horizon. Tono terrace was formed slightly eroding Nakagoe terrace deposit. This terrace is formed immediately after the formation of Nakagoe terrace. Sakamaki terrace is a strath terrace formed by usual fluvial process. This terrace is correlated with the buried terrace developed along the Ashiaraidani. The age of terrace deposit was measured to be ca. 5, 000y. B. P. with 14C-dating method by Fujii et al. (1974). Miza terrace is another strath terrace. The terrace deposit unconformably covers Nakao pyroclastic flow deposit, the age of which is reported to be 2, 500y. B. P. by Fujii et al. (1974). The age of this terrace is, therefore, estimated to be 2, 300y. B. P. (Table 1).

Damage on Slope due to the Near Izu-Oshima Earthquake 1978

The main shock (M=7.0), occurred in near Izu-Oshima Island on January 14, 1978, caused the violent disaster in central part of Izu peninsula.Disasters were found out in roads, railways and buildings. Slope failures occurred remarkably in the area.A total number of slope failure was awfully counted as about six hundred and twenty, and the greater part of slope failure occurred on near mountain ridge, that is, near slope changing line. Half number of failures was 20-40 meter in height and 10-20 meter in width as shown in Fig. 1 and 2. Geology of failure site was mainly tertiary and andesitic lava, and a few failure occurred in scoria and volcanic mud flow as shown in Fig. 10.Failures taked place along the earthquake fault by the shock and coincided with after shocks as shown in Fig. 12.The ratio of failure area is about 6% near the earthquake fault and about 0.1% farther than 5 kilometer away from the fault.

Quaternary of Chile: The state of research

The aim of this paper is to present a summary of current knowledge about Quaternary climatic changes, sea level fluctuations, tectonic deformation, and volcanic activity in Chile. In the Andean highlands of the hyperarid desert, glaciers and lakes fluctuated repeatedly. Evidence of pluvial periods is not well documented in the marginal desert. On the contrary, pronounced climatic changes are recorded in semiarid Chile. In central Chile two or three major glacial advances have been identified, but they remained confined within the high Cordillera. In the longitudinal valley of the Lake Region geomorphic remnants of four glaciations have been described; 14C dates are available for the last glaciation. The glacial history of the Fjord Region is still obscure. Whether Pleistocene climatic changes in the northern and southern part of Chile were synchronous or not is a problem which requires further investigation. Sea level fluctuations along the Chilean coast are in part ascribed to glacio-eustatic effects. They left striking sets of step-like marine terraces in northern and central Chile. From a paleontologic point of view the Pleistocene corresponds to a pronounced move toward isolation and endemic development of the marine fauna. The Quaternary tectonic tendency seems to be toward extension and not compression expected as sea-floor spreading compensation. Normal faults limiting uplifted, downwarped, and tilted blocks are common. Folds are rarely found. Northern Chile is characterized by an imposing chain of about 600 stratovolcanoes. They rest on Tertiary ignimbrites which cover the altiplano. Quartz-bearing latite-andesites are predominant. Present volcanic activity is sporadic and weak. South of a conspicuous gap between 27 and 33°S, Quaternary volcanism reappears in the high Cordillera, and many volcanoes have erupted violently within historic times. Rocks are fundamentally andesite or basaltic andesite. Poorly sorted ashes including pumice clasts in the Central Valley south of Santiago are interpreted as volcanic mudflows of late Pleistocene age.

Lahars as major geological hazards

Major loss of life caused by lahars (volcanic mudflows) in historical times has been largely restricted to the Circum-Pacific region and more particularly to Japan (>11,650 killed), Indonesia (>9,300 killed) and Central America (>1,300 killed). In addition to such losses of life, widespread damage may occur to buildings, bridges, communication networks and arable land. A review of the causal mechanisms of lahars, flow behaviour and protective measures, with selected case histories, is therefore appropriate to an understanding of this major geological hazard. The potentially most destructive lahars are those involving sudden release of very large quantities of water from crater lakes or from subglacial lakes. The Icelandic jokulhlaups, although not strictly lahars, give some idea of the huge discharges of water that can be released — ephemeral maximum discharge rates have been estimated up to 100,000 m3/sec, or temporarily equivalent to the flow of the River Amazon. Other potentially destructive lahars are those resulting from pyroclastic flows becoming admixed with running or ponded waters. Of more common but less devastating occurrence are lahars generated by heavy rainfall on the slopes of volcanoes, more particularly on recently ejected pyroclastics. Historical lahar disasters of this type occur most frequently in tropical regions. Other initiating mechanisms include melting of snow and ice directly accompanying eruptions, earthquake triggered collapse, phreatic explosions and directed blasts. Historical lahars generated by these mechanisms have not been responsible for any considerable loss of life, with the exception of the Shimbara Catastrophe in Japan where a lahar entered the sea producing tsunamis. Upon initiation of a lahar, mud, sand and gravel combine with available water to form a high bulk density (>1,400 kg/m3) flow. In some lahars the flow behaviour may approximate to a Newtonian liquid, whilst in others a high concentration Non-Newtonian liquid is formed with the capability of transporting very large clasts which may each weigh over 200 tonnes. The formation of a laminar boundary layer at the base of the flow is responsible for a low friction factor that enables some lahars to travel very large distances (>100 km). It also explains how lahar deposits often overlie completely undisturbed yet easliy erodible materials. This boundary layer can often be identified in many lahar deposits by a fine-grained layer at the base. The continuous phase of such lahars exhibits strength which retards the sinking of boulders and is responsible for the unsupported framework and poor sorting of lahar deposits. Protective measures against loss of life and damage to property are discussed with particular reference to case histories in Indonesia and New Zealand. Indonesian measures have included siphoning water from the crater lake of Mt. Kelut, effective warning systems, and preparation of maps showing regions that may be destroyed by lahars. In New Zealand, two principal centres of Post-glacial lahar activity are Mt. Ruapehu and Mt. Egmont. Since 1861 A.D. eight lahar episodes have been generated from the crater-lake on Mt. Ruapehu, the 1953 lahar being responsible for the ”Tangiwai Disaster”, when 151 persons were killed. Existing and future protective measures against Mt. Ruapehu lahars are discussed. Mt. Egmont has a long record of Post-glacial lahar activity. The causal mechanism of some Egmont lahars has been heavy rains, but the existence of a former crater lake in the summit area cannot be discounted. Based on detailed geological, pedological and botanical investigations a geological hazards map of the Mt. Egmont region has been prepared.

妙高火山群の地質記載 ＩＩＩ 飯づな火山の地質

Iizuna volcano (1871m) is formed at the southern end of the Myoko volcanoes which are situated in the northern part of the Fossa Magna. It is a Quaternary stratovolcano built on an anticlinal axis of basal Neogene strata, and has a horseshoe caldera with 2.5×2.0km in diameter at its summit. The volcanic cone is considerably dissected in contrast to the Myoko and Kurohime volcanoes. At the western to northern midslope of the cone, the five domes of Kenashiyama, Nakanomine, Takadekki, Tengudake, and 1340 m-ridge are arranged in NE-SW direction (Fig. 1).The growth-history of the Iizuna volcano may be divided broadly into the Ist and IInd stages, and the latter may be subdivided into the stratovolcano, caldera, and lava dome stages (Table 1). The eruptives from this volcano are full of variety such as lavas, pyroclastic (scoria, pumice, lapilli, and others) falls, pyroclastic flows, and volcanic mud flows. The rocks consist of basalt, pyroxene andesite, and hornblende andesite (Fig. 2). Augite-hypersthene andesite is predominant in volume.During the activity of the IInd stage, the mode of eruption and rock nature generally tend to change as follows, respectively: mode of eruption; eruption of scoria fall→alternated eruption of lava flows and pyroclastics→eruption of pyroclastic flows and falls, and viscous lave flows: rock nature; basalt→pyroxene andesite→horhblende andesite. The activity of this volcano began at the middle Pleistocene, and had already closed at least 32, 000 years ago.

妙高火山群の地質記載 Ｉ 妙高火山・第ＩＶ期の活動とその噴出物

The Myoko volcano, one of the stratovolcanoes in the northern part of the Fossa Magna, Central Japan, has the complicated history (Table 1).The last activity of the Myoko volcano is called the IV stage, and is divided into precaldera, caldera, and central cone substages. The strata formed during this stage are collectively called the Myokosan group which consists of lavas, pyroclastic flow deposits, pyroclastic fall deposits, volcanic mud flow deposits, and lake deposits. They are described in detail and their stratigraphic relations are tabulated in Table 2 and Fig. 4.The rocks of the essential eruptives are basalt, pyroxene andesite and hornblende andesite. They change from basalt to pyroxene andesite and hornblende andesite keeping step with the eruptive order. Quantitatively, the hornblende andesite is predominant.