Magnetic monitoring at Merapi volcano, Indonesia

Merapi volcano, located 30 km north of the heavily populated city of Yogjakarta, Java, is one of the most active of the 129 volcanoes in Indonesia. About every 2 years a new phase of activity is observed. Depending on the past activity the unrest gives rise either to an endogenous dome which partly collapses in the southwest direction or to pyroclastic flows which travel as far as 15 km. The 1990–1997 period has involved a plume emission on 30 August 1990, an extrusion on 20 January 1992, and a pyroclastic eruption on 22 November 1994. The intensity of the Earth magnetic field has been measured simultaneously and digitally recorded at four stations since 1990. Two Overhauser magnetometers with resolution of 0.01 nT have been installed in the summit area to strengthen the volcano monitoring. Outstanding magnetic changes appear to correlate with volcanic activity. Three types of volcanomagnetic signals can be identified: long-term trends up to 15 nT with period >10 years; medium-term cyclic variations, at most 3 nT in amplitude and with 1–2 years period; and small events, reaching 1.5 nT, lasting a few months, and associated with any remarkable volcanic activity. Merapi volcano began a new cycle of activity in 1995 leading to a dome growth in July 1996, and accompanied by 27 nuées ardentes in August. The comparison between magnetic data, seismicity, and surface phenomena suggests that some long-term trends of decade periods could be of thermomagnetic origin, while mid-term volcanomagnetic variations associated with the cycles of Merapi activity could be of piezomagnetic origin. Short-term variations of a few weeks duration, less than 1.5 nT, are well correlated with the 1995–1996 seismic activity.     

Emplacement of small-volume pyroclastic flows at Laacher See (East-Eifel,Germany)

We distinguish three eruptive units of pyroclastic flows (T1, T2, and T3; T for trass) within the late Quaternary Laacher See tephra sequence. These units differ in the chemical/mineralogical composition of the essential pyroclasts ranging from highly differentiated phonolite in T1 to mafic phonolite in T3. T1 and T2 flows were generated during Plinian phases, and T3 flows during a late Vulcanian phase. The volume of the pyroclastic flow deposits is about 0.6 km³. The lateral extent of the flows from the source vent decreases from > 10 km (T1) to < 4.5 km (T3). In the narrow valleys north of Laacher See, the total thickness of the deposits exceeds 60 m.At least 19 flow units in T1, 6 in T2, and 4 in T3 can be recognized at individual localities. Depositional cycles of 2 to 5 flow units are distinguished in the eruptive units. Thickness and internal structure of the flow units are strongly controlled by topography. Subfacies within flow units such as strongly enriched pumice and lithic concentration zones, dust layers, lapilli pipes, ground layers, and lithic breccias are all compositionally related to each other by enrichment or depletion of clasts depending on their size and density in a fluidized flow. While critical diameters of coarse-tail grading were found to mark the boundary between the coarse nonfluidized and the finer fluidized grain-size subpopulations, we document the second boundary between the fluidized and the very fine entrained subpopulations by histograms and Rosin-Rammler graphs. Grain-size distribution and composition of the fluidized middle-size subpopulations remained largely unchanged during transport.Rheological properties of the pyroclastic flows are deduced from the variations in flow-unit structure within the valleys. T1 flows are thought to have decelerated from 25 m/s at 4 km to < 15 m/s at 7 km from the vent; flow density was probably 600–900 kg/m³, and viscosity 5–50 P. The estimated yield strength of the flows of 200– > 1000 N/m² is consistent with the divergence of lithic size/distance curves from purely Newtonian models; the transport of lithics must be treated as in a Bingham fluid. The flow temperature probably decreased from T1 (300°–500°C) to T3 (<200°C).A large-scale longitudinal variation in the flow units from proximal through medial to distal facies dominantly reflects temporal changes during the progressive collapse of an eruption column. Only a small amount of fallout tephra was generated in the T1 phase of eruption. The pyroclastic flows probably formed from relatively low ash fountains rather than from high Plinian eruption columns.     

The geological evolution of Merapi volcano, Central Java, Indonesia

Merapi is an almost persistently active basalt to basaltic andesite volcanic complex in Central Java (Indonesia) and often referred to as the type volcano for small-volume pyroclastic flows generated by gravitational lava dome failures (Merapi-type nuées ardentes). Stratigraphic field data, published and new radiocarbon ages in conjunction with a new set of ⁴⁰K–⁴⁰Ar and ⁴⁰Ar–³⁹Ar ages, and whole-rock geochemical data allow a reassessment of the geological and geochemical evolution of the volcanic complex. An adapted version of the published geological map of Merapi [(Wirakusumah et al. 1989), Peta Geologi Gunungapi Merapi, Jawa Tengah (Geologic map of Merapi volcano, Central Java), 1:50,000] is presented, in which eight main volcano stratigraphic units are distinguished, linked to three main evolutionary stages of the volcanic complex—Proto-Merapi, Old Merapi and New Merapi. Construction of the Merapi volcanic complex began after 170?ka. The two earliest (Proto-Merapi) volcanic edifices, Gunung Bibi (109?±?60?ka), a small basaltic andesite volcanic structure on Merapi’s north-east flank, and Gunung Turgo and Gunung Plawangan (138?±?3?ka; 135?±?3?ka), two basaltic hills in the southern sector of the volcano, predate the Merapi cone sensu stricto. Old Merapi started to grow at ~30?ka, building a stratovolcano of basaltic andesite lavas and intercalated pyroclastic rocks. This older Merapi edifice was destroyed by one or, possibly, several flank failures, the latest of which occurred after 4.8?±?1.5?ka and marks the end of the Old Merapi stage. The construction of the recent Merapi cone (New Merapi) began afterwards. Mostly basaltic andesite pyroclastic and epiclastic deposits of both Old and New Merapi (<11,792?±?90 ¹⁴C years BP) cover the lower flanks of the edifice. A shift from medium-K to high-K character of the eruptive products occurred at ~1,900 ¹⁴C years BP, with all younger products having high-K affinity. The radiocarbon record points towards an almost continuous activity of Merapi since this time, with periods of high eruption frequency interrupted by shorter intervals of apparently lower eruption rates, which is reflected in the geochemical composition of the eruptive products. The Holocene stratigraphic record reveals that fountain collapse pyroclastic flows are a common phenomenon at Merapi. The distribution and run-out distances of these flows have frequently exceeded those of the classic Merapi-type nuées ardentes of the recent activity. Widespread pumiceous fallout deposits testify the occurrence of moderate to large (subplinian) eruptions (VEI 3–4) during the mid to late Holocene. VEI 4 eruptions, as identified in the stratigraphic record, are an order of magnitude larger than any recorded historical eruption of Merapi, except for the 1872?AD and, possibly, the October–November 2010 events. Both types of eruptive and volcanic phenomena require careful consideration in long-term hazard assessment at Merapi.     

Nuées ardentes of 22 November 1994 at Merapi volcano, Java, Indonesia

Nuées ardentes associated with dome collapse on 22 November 1994, at Merapi volcano traveled to the south–southwest as far as 6.5 km, and collectively accumulated roughly 2.5–3 million cubic meters of deposits. The damaged area comprises 9.5 km² and is covered by two nuée ardente facies, a conventional “Merapi-type”, valley-fill block-and-ash flow facies and a pyroclastic surge facies. The proximal deposits reflect the accumulation of dozens of nuées ardentes, with many subsidiary flow units. The distal deposits are more simply organized, as only a few individual events reached to distances >3.5 km. The stratigraphic relationships north of Turgo hill indicate that the surge deposits are a facies of particularly mobile nuées ardentes that also deposited channeled block-and-ash flow facies. They further suggest that the surge facies beyond the channel margins correlate laterally with a finer-grained sublayer locally developed at the base of the block-and-ash flow facies. Eyewitness reports suggest that the emplacement of the block-and-ash flow facies in the distal part of the Boyong river may have followed, by a short time interval, the destruction and deposition of the surge facies at Turgo village. The stratigraphy is in accord with the eyewitness reports. The surge facies was emplaced by a dilute surge current, detached from the same dome-collapse nuée ardente that, as a separate flow unit, subsequently emplaced the distal block-and-ash deposit in the Boyong valley. The detachment occurred at higher elevations, likely at or above the slope break at about 2000 m elevation. This flow separation enabled the surge current to shortcut over the landscape and to emplace its deposit even as the block-and-ash flow continued its tortuous southward movement in the Boyong channel. Dome-collapse nuée ardente activity formed the bulk of the eruption, which was accompanied by virtually no significant vertical summit explosive activity.     

Merapi volcano (Java,Indonesia) and Merapi-type Nuée ardente

A particular nuée ardente type (Merapi-type avalanche nuée) has been defined at the Merapi volcano because of its prominent role in the recent activity of the volcano: gravity plays a significant role during the eruption. However, some other eruption styles occur too producing surges and ashfalls. Three types of tephra, deposited in a very short time-span (15 years) are compared: chemistry and mineralogy are similar, but grain-size analyses are different. There is no vesicular glass, and it is concluded that there is an absence of new magma. This example shows clearly the variety of volcanic styles, with similar chemistry in a very short period. Avalanche nuées from collapsed domes or flows are separated into two types:

1. Merapi-typesensu stricto, without any fresh glass, derived from a wholly solidified dome.
2. Arenal-type, containing pumiceous glass, derived from a dome, the interior of which is still liquid.

     

Lahars at Merapi volcano, Central Java: an overview

Merapi volcano, in Central Java, is one of the most active volcanoes in the world. At least 23 of the 61 reported eruptions since the mid-1500s have produced source deposits for lahars. The combined lahar deposits cover about 286 km² on the flanks and the surrounding piedmonts of the volcano. At Merapi, lahars are commonly rain-triggered by rainfalls having an average intensity of about 40 mm in 2 h. Most occur during the rainy season from November to April, and have average velocities of 5–7 m/s at 1000 m in elevation. A wide range of facies may be generated from a single flow, which may transform downvalley from debris flow to hyperconcentrated streamflow.Because of the high frequency and magnitude of the lahar events, lahar-related hazards are high below about 450–600 m elevation in each of the 13 rivers which drain the volcano. Hazard-zone maps for lahar were produced by Pardyanto et al. (Volcanic hazard map, Merapi volcano, Central Java (1/100,000). Geol. Surv. of Indonesia, Bandung, II, 4, 1978) and the Japanese–Indonesian Cooperation Agency (Master plan for land conservation and volcanic debris control in the area of Mt Merapi, Jakarta, 1980), but these maps are of a very small scale to meet modern zoning requirements. More recently, a few large-scale maps (1/10,000- and 1/2000-scale) and risk assessments have been completed for a few critical river systems.     

Explosive activity and generation mechanisms of pyroclastic flows at Arenal volcano, Costa Rica between 1987 and 2001

Explosive activity at Arenal and associated tephra fall that has occurred over the 14-year period from 1987–2001 is described. Explosions have been notably variable in both frequency and size. A marked decrease in both frequency and quantity of tephra fallout occurred in early 1998 until the end of 2001. Grainsize distributions of cumulative tephra samples collected once a month are typically bimodal. Aggregation causing premature fallout of fine ash and possibly fallout from ash plumes produced by pyroclastic flows are considered responsible for the bimodality of fallout. Scanning electron microscopy of the glass component of tephra from single explosions show predominantly blocky and blocky/fluidal clast types, interpreted as being the product of vulcanian type explosions. Fragmentation of a mainly rigid, degassed magma body, and a minor molten component is inferred for these explosions. Pyroclastic flows were produced either associated with the larger explosions by a mechanism of column collapse (1987–1990), or unrelated to explosions by partial collapse of the crater wall (1993, 1998, 2000, 2001). Pyroclastic flow activity has migrated from west to north during the period reported. Pyroclastic flow deposits are variable in the quantity of juvenile material and any associated surge component. Large juvenile blocks were partially molten on emplacement and many have a typical cauliform texture. Blocks with both juvenile and lithic textures indicate that at the summit magma was in intimate contact with the pre-existing edifice, rather than as a simple open crater or lava pool. Crater wall collapse may have been promoted by the reduction in explosive activity, which has increased the lava accumulation at the summit and in turn increased instability of the summit region. Thus although explosive activity has waned, if the lava output is maintained, the hazard of pyroclastic flows is likely to continue.Editorial responsibility: R. Cioni     

Correlation of seismic activity and fumarole temperature at the Mt. Merapi volcano (Indonesia) in 2000

In this paper we present densely sampled fumarole temperature data, recorded continuously at a high-temperature fumarole of Mt. Merapi volcano (Indonesia). These temperature time series are correlated with continuous records of rainfall and seismic waveform data collected at the Indonesian–German multi-parameter monitoring network. The correlation analysis of fumarole temperature and precipitation data shows a clear influence of tropical rain events on fumarole temperature. In addition, there is some evidence that rainfall may influence seismicity rates, indicating interaction of meteoric water with the volcanic system. Knowledge about such interactions is important, as lava dome instabilities caused by heavy-precipitation events may result in pyroclastic flows. Apart from the strong external influences on fumarole temperature and seismicity rate, which may conceal smaller signals caused by volcanic degassing processes, the analysis of fumarole temperature and seismic data indicates a statistically significant correlation between a certain type of seismic activity and an increase in fumarole temperature. This certain type of seismic activity consists of a seismic cluster of several high-frequency transients and an ultra-long-period signal (<0.002 Hz), which are best observed using a broadband seismometer deployed at a distance of 600 m from the active lava dome. The corresponding change in fumarole temperature starts a few minutes after the ultra-long-period signal and simultaneously with the high-frequency seismic cluster. The change in fumarole temperature, an increase of 5 °C on average, resembles a smoothed step. Fifty-four occurrences of simultaneous high-frequency seismic cluster, ultra-long period signal and increase of fumarole temperature have been identified in the data set from August 2000 to January 2001. The observed signals appear to correspond to degassing processes in the summit region of Mt. Merapi.     

Toward a revised hazard assessment at Merapi volcano, Central Java

Of 1.1 million people living on the flanks of the active Merapi volcano, 440,000 are at relatively high risk in areas prone to pyroclastic flows, surges, and lahars. For the last two centuries, the activity of Merapi has alternated regularly between long periods of viscous lava dome extrusion, and brief explosive episodes at 8–15 year intervals, which generated dome-collapse pyroclastic flows and destroyed part of the pre-existing domes. Violent explosive episodes on an average recurrence of 26–54 years have generated pyroclastic flows, surges, tephra-falls, and subsequent lahars. The 61 reported eruptions since the mid-1500s killed about 7000 people. The current hazard-zone map of Merapi (Pardyanto et al., 1978) portrays three areas, termed ‘forbidden zone’, ‘first danger zone’ and ‘second danger zone’, based on successively declining hazards. Revision of the hazard map is desirable, because it lacks details necessary to outline hazard zones with accuracy, in particular the valleys likely to be swept by lahars, and excludes some areas likely to be devastated by pyroclastic gravity-currents such as the 22 November 1994 surge. In addition, risk maps should be developed to incorporate social, technical, and economic factors of vulnerability.Eruptive hazard assessment at Merapi is based on reconstructed eruptive history, on eruptive behavior and scenarios, and on existing models and preliminary numerical modeling. Firstly, the reconstructed eruptive activity, in particular for the past 7000 years and from historical accounts of eruptions, helps to define the extent and recurrence frequency of the most hazardous phenomena (Newhall et al., 2000; Camus et al., 2000). Pyroclastic flows traveled as far as 9–15 km from the source, pyroclastic surges swept the flanks as far as 9–20 km away from the vent, thick tephra fall buried temples in the vicinity of Yogyakarta 25 km to the south, and subsequent lahars spilled down the radial valleys as far as 30 km to the west and south. At least one large edifice collapse has occurred in the past 7000 years (Newhall et al., 2000; Camus et al., 2000). Secondly, four eruption scenarios are portrayed as hazardous zones on two maps and derived from the past eruptive behavior of Merapi and from the most affected areas in the past. Thirdly, simple numerical simulation, based on a Digital Elevation Model, a stereo-pair of SPOT satellite images, and one 2D-orthoimage helps to simulate pyroclastic and lahar flowage on the flanks and in radial valley channels, and to outline areas likely to be devastated.Three major threats are identified: (1) a collapse of the summit dome in the short-to mid-term, that can release large-volume pyroclastic flows and high-energy surges towards the south–southwest sector of the volcano; (2) an explosive eruption, much larger than any since 1930, may sweep all the flanks of Merapi at least once every century; (3) a potential collapse of the summit area, involving the fumarolic field of Gendol and part of the southern flank, which can contribute to moderate-scale debris avalanches and debris flows.     

Implications of long-term changes in valley geomorphology on the behavior of small-volume pyroclastic flows

Stratigraphic mapping in the lower 3km of the Vazcún Valley on the NE flank of Volcán Tungurahua (Ecuador) provides insight into the effects of long-term geomorphic changes on pyroclastic flow behavior. Exposures of deposits in the Vazcún Valley record activity over the last 2000years, during which time significant changes in the geomorphology of the valley have occurred. Two sets of terraces are present in the lower 2–3km of the valley, the older of which grades into a small debris fan at the mouth of the valley. Each terrace formed during a period of frequent activity that was separated by a long period of quiescence during which the Río Vazcún eroded a channel as deep as 40m reaching the previous base level. The pyroclastic flows from historical eruptions appear to have been largely contained within the channel that is cut through the higher terraces and debris fan. Their surface forms the lower terrace located upstream from the head of the debris fan. Thin pyroclastic deposits exposed within the city of Baños are mostly related to ash cloud surges that detached from the main flows as they slowed down within the channel. The lower reach of the present channel of the Rio Vazcún is very sinuous and deeply incised into the two sets of terraces. The winding channel would severely impede the mobility of future pyroclastic flows resulting in the deposition of thick deposits. Detachment of an overriding ash cloud surge could also occur in this region. Such a surge could be more likely to surmount the channel banks and travel over the surface of the terraces and debris for up to several kilometers from the channel.     

Basaltic pyroclastic flows of Fuji volcano, Japan: characteristics of the deposits and their origin

Fuji volcano is the largest active volcano in Japan, and consists of Ko-Fuji and Shin-Fuji volcanoes. Although basaltic in composition, small-volume pyroclastic flows have been repeatedly generated during the Younger stage of Shin-Fuji volcano. Deposits of those pyroclastic flows have been identified along multiple drainage valleys on the western flanks between 1,300 and 2,000 m a.s.l., and have been stratigraphically divided into the Shin-Fuji Younger pyroclastic flows (SYP) 1 to 4. Downstream debris flow deposits are found which contain abundant material derived from the pyroclastic flow deposits. The new¹⁴C ages for SYP1 to SYP4 are 3.2, 3.0, 2.9, and 2.5 ka, respectively, and correspond to a period where explosive summit eruptions generated many scoria fall deposits mostly toward the east. The SYP1 to SYP4 deposits consist of two facies: the massive facies is about 2 m thick and contains basaltic bombs of less than 50 cm in size, scoria lapilli, and fresh lithic basalt fragments supported in an ash matrix; the surge facies is represented by beds 1 to 15 cm thick, consisting mainly of ash with minor amount of fine lapilli. The bombs and scoria are 15 to 30% in volume within the massive facies. The ashes within the SYP deposits consist largely of comminuted basalt lithics and crystals that are derived from the Middle-stage lava flows exposed at the western flanks. SYP1 to SYP4 were only dispersed down the western flanks. The reason for this one-sided distribution is the asymmetric topography of the edifice; the western slopes of the volcano are the steepest (over 34 degrees). Most pyroclastic materials cannot rest stably on the slopes steeper than 33 degrees. Therefore, ejecta from the explosive summit eruptions that fell on the steep slopes tumbled down the slopes and were remobilized as high-temperature granular flows. These flows consisted of large pyroclastics and moved as granular avalanches along the valley bottom. Furthermore, the avalanching flows increased in volume by abrasion from the edifice and generated abundant ashes by the collision of clasts. The large amount of the fine material was presumably available within the transport system as the basal avalanches propagated below the angle of repose. Taking the typical kinetic friction coefficient of small pyroclastic flows, such flows could descend the western flanks where scattered houses are below 1,000 m a.s.l. A similar type of pyroclastic flow could result if explosive summit eruptions occur in the future.Editorial responsibility: R Cioni     

Magma eruption rates of Merapi volcano,Central Java,Indonesia during one century (1890–1992)

The magma eruption rates of Merapi volcano form 1890 to 1992 are re-examined chronologically. For this volcano, movements of extruded lavas and domes as well as their extrusions are important because they control the modes of the subsequent activities and cause nuées ardentes and lahars. The monthly eruption rates varied widely, but the cumulative volume of lavas has increased linearly and is expressed as 0.1x10⁶ m³/month. The magma production rate of this volcano may have been constant for these 100 years. Recurrent excessive effusion of lavas is tentatively interpreted by assuming a magma reservoir. The averaged eruption rate is small in comparison with other volcanoes such as Nyramuragia, Kilauea and Vesuvio. However, it is remarkable that the activity has been continuous for these 100 years and the total amount of lava discharged during this period reached more than 10⁸ m³. A simple model for the formation of the 1992 lava dome is presented. The viscosity of the lavas is probably between 10⁶ and 10⁷ P and the length of the magma conduit is probably less than 10 km.     

Characterization of pyroclastic fall and flow deposits from the 1815 eruption of Tambora volcano,Indonesia using ground-penetrating radar

Ground-penetrating radar (GPR) is used to image and characterize fall and pyroclastic flow deposits from the 1815 eruption of Tambora volcano in Indonesia. Analysis of GPR common-mid-point (CMP) data indicate that the velocity of radar in the sub-surface is 0.1 m/ns, and this is used to establish a preliminary traveltime to-depth conversion for common-offset reflection profiles. Common-offset radar profiles were collected along the edge of an erosional gully that exposed approximately 1–2 m of volcanic stratigraphy. Additional trenching at select locations in the gully exposed the contact between the pre-1815 eruption surface and overlying pyroclastic deposit from the 1815 eruption. The deepest continuous, prominent reflection is shown to correspond to the interface between pre-eruption clay-rich soil and pyroclastics that reach a maximum thickness of 4 m along our profiles. This soil surface is distinctly terraced and is interpreted as the ground surface augmented for agriculture and buildings by people from the kingdom of Tambora. The correlation of volcanic stratigraphy and radar data at this location indicates that reflections are produced by the soil-pyroclastic deposit interface and the interface between pyroclastic flows (including pyroclastic surge) and the pumice-rich fall deposits. In the thickest deposits an additional reflection marks the interface between two pyroclastic flow units.     

The coalescence and organization of lahars at Semeru volcano, Indonesia

We present multi-parameter geophysical measurements of rainfall-induced lahars at Semeru Volcano, East Java, using two observation sites 510 m apart, 11.5 km from the summit. Our study site in the Curah Lengkong channel is composed of a 30-m wide box-valley, with a base of gravel and lava bedrock, representing an ideal geometry for high density measurements of active lahars. Instrumentation included pore-pressure sensors (stage), a broad-band seismograph (arrival times, vibrational energy, and turbulence), video footage, and direct bucket sampling. A total of 8 rainfall-induced lahars were recorded, with durations of 1–3 h, heights 0.5–2 m, and peak velocities 3–6 m/s. Flow types ranged from dilute to dense hyperconcentrated flows. These recorded flows were commonly composed of partly coalesced, discrete and unsteady gravity current packets, represented by multiple peaks within each lahar. These packets most likely originate from multiple lahar sources, and can be traced between instrument sites. Those with the highest concentrations and greatest wetted areas were often located mid-lahar at our measured reach, accelerating towards the flow front. As these lahars travel downstream, the individual packets thus coalesce and the flow develops a more organised structure. Observations of different degrees of coalescence between these discrete flow packets illustrate that a single mature debris flow may have formed from multiple dynamically independent lahars, each with different origins.     

Volcanology of trachytic and associated basaltic pyroclastic deposits at Roccamonfina volcano, Roman Region, Italy

The 274 ka “Basalt-Trachytic Tuff of Tuoripunzoli” (TBTT) from Roccamonfina volcano (Roman Region, Italy) consists of a basaltic scoria lapilli fall (Unit A) overlain by a trachytic sequence formed by a surge (Unit B), repetitive pumice lapilli and ash-rich layers both of fallout origin (Unit C) and a pyroclastic flow deposit (Unit D). The TBTT is widespread (40 km²) in the northern sector of the volcano, but limited to a small area on the southern slopes of the main cone. Interpolation between the northern deposits and the latter one yields a minimum depositional area of 123 km², and an approximate bulk volume of 0.2-0.3 km³. Isopach and isopleth maps are consistent with a source vent within the main caldera of Roccamonfina.Unit A shows a fairly good sorting and a moderate grain size; glass fragments are cuspate and vesicular. Unit B is fine grained and poorly sorted; shards are blocky and nonvesicular. Pumice lapilli of Unit C are moderately sorted and moderately coarse grained. Glass shards are equant and vesicular. Lithic clasts are strongly comminuted to submillimetric sizes. By contrast, the ash-rich internal divisions are very fine grained and poorly sorted. They consist of a mixture of equant shards which are prevailingly blocky and poorly vesicular. Unit D is a massive, poorly sorted, moderately coarse-grained deposit. Glass fragments are nearly equant and slightly or nonvesicular.The TBTT is interpreted as due to eruption of a basaltic magma followed in rapid succession by one trachyte magma. Unit A formed by Subplinian fallout of a moderate, purely magmatic column. Interaction between a trachyte magma and water resulted in eruption of surge Unit B. A high-standing eruption column erupted alternating fallout pumice lapilli and fallout ashes. Pumice lapilli originated prevailingly from the inner part of the eruption column, whereas magma-water interaction on the external parts of the column resulted in ash fallout. The uppermost pyroclastic flow Unit D is interpreted as due to final collapse of the eruption column.     

Generation of pyroclastic flows and surges by hot-rock avalanches from the dome of Mount St. Helens volcano,USA

Several hot-rock avalanches have occurred during the growth of the composite dome of Mount St. Helens, Washington between 1980 and 1987. One of these occurred on 9 May 1986 and produced a fan-shaped avalanche deposit of juvenile dacite debris together with a more extensive pyroclastic-flow deposit. Laterally thinning deposits and abrasion and baking of wooden and plastic objects show that a hot ash-cloud surge swept beyond the limits of the pyroclastic flow. Plumes that rose 2–3 km above the dome and vitric ash that fell downwind of the volcano were also effects of this event, but no explosion occurred. All the facies observed originated from a single avalanche. Erosion and melting of craterfloor snow by the hot debris caused debris flows in the crater, and a small flood that carried juvenile and other clasts north of the crater. A second, broadly similar event occured in October 1986. Larger events of this nature could present a significant volcanic hazard.     

Longitudinal structure of pyroclastic-flow deposits,revealed by GPR survey,at Merapi Volcano,Java, Indonesia

In 2006 Merapi volcano, Indonesia, erupted for a few months, producing several block-and-ash flows reaching a maximum distance of 7.5 km from the main vent. During the eruption, we conducted a survey on those flow deposits in the Gendol Valley at Kaliadem village, about 4.5 km from the Merapi submit, using a Ground Penetrating Radar (GPR). The upper deposit was studied in its distal reaches, whereas the one below was studied in its medial reaches. The field study was carried out with a commercial RAMAC® GPR coupled with 100 MHz antennas, and the data treatment conducted with Reflex™ software. From this survey, we determined both deposits' local (1) thickness – reaching a maximum of 15 m – and (2) internal architecture. This last one is governed by long reflecting horizons extending over 20 to 30 m that delimit layers showing progradation patterns in their distal reaches. Within these layers we could also observe an internal architecture of still unknown origin. The layers are interpreted as the result of the flow pulses that progressively deposited downstream-ward by progradation. However the interpretation of those GPR profiles is a bit hazardous, because of the absence of outcrops, and we can only proceed by analogy with other studies. Nevertheless, despite numerous limitations, GPR is a helpful tool to understand pyroclastic deposits' structure when no visual observations are available.