Tephra Thickness Research Articles

A new interpolation method to model thickness, isopachs, extent, and volume of tephra fall deposits

Tephra thickness distribution is the primary piece of information used to reconstruct the histories of past explosive volcanic eruptions. We present a method for modeling tephra thickness with less subjectivity than is the case with hand-drawn isopachs, the current, most frequently used method. The algorithm separates the thickness of a tephra fall deposit into a trend and local variations and models them separately using segmented linear regression and ordinary kriging. The distance to the source vent and downwind distance are used to characterize the trend model. The algorithm is applied to thickness datasets for the Fogo Member A and North Mono Bed 1 tephras. Simulations on subsets of data and cross-validation are implemented to test the effectiveness of the algorithm in the construction of the trend model and the model of local variations. The results indicate that model isopach maps and volume estimations are consistent with previous studies and point to some inconsistencies in hand-drawn maps and their interpretation. The most striking feature noticed in hand-drawn mapping is a lack of adherence to the data in drawing isopachs locally. Since the model assumes a stable wind field, divergences from the predicted decrease in thickness with distance are readily noticed. Hence, wind direction, although weak in the case of Fogo A, was not unidirectional during deposition. A combination of the isopach algorithm with a new, data transformation can be used to estimate the extent of fall deposits. A limitation of the algorithm is that one must estimate “by hand” the wind direction based on the thickness data.

Vegetation structure influences the retention of airfall tephra in a sub-Arctic landscape

Vegetation cover mediates a number of important geomorphological processes. However, the effect of different vegetation types on the retention of fine aeolian sediment is poorly understood. We investigated this phenomenon, using the retention of fine, pyroclastic material (tephra) from the 2011 eruption of the Grímsvötn volcano, Iceland, as a case study. We set out to quantify structural variation in different vegetation types and to relate structural metrics to the thickness of recently deposited volcanic ash layers in the sedimentary section. We utilised a combination of vegetation and soil surveys, along with photogrammetric analysis of vegetation structure. We found that indices of plant community composition were a poor proxy for vegetation structure and were largely unrelated to tephra thickness. However, structural metrics, derived from photogrammetric analysis, were clearly related to variations in tephra layer thickness at a landscape scale and tephra layers under shrub patches were significantly thicker than those outside the shrub canopy. We therefore concluded that: a) vegetation cover was a critical factor in the retention of fine aeolian sediment for deposit depths up to few centimetres; b) structural variation in vegetation cover played a major role in determining the configuration of tephra deposits in the sedimentary section. These findings have implications for the analysis of ancient volcanic eruptions and archaeological/palaeoenvironmental reconstructions based on the interpretation of tephra deposits. Furthermore, they present the possibility that the detailed form of tephra layers may be used as a proxy for palaeo vegetation structure.

Impacts to agriculture and critical infrastructure in Argentina after ashfall from the 2011 eruption of the Cordón Caulle volcanic complex: an assessment of published damage and function thresholds

The 2011 Cordon Caulle (Chile) was a large silicic eruption that dispersed ashfall over 75,000 km2 of land in Central Argentina, affecting large parts of the Neuquen, Rio Negro, and Chubut provinces, including the urban areas of Villa la Angostura, Bariloche and Jacobacci. These regions all received damage and disruption to critical infrastructure and agriculture due to the ashfall. We describe these impacts and classify them according to published damage/disruption states (DDS). DDS for infrastructure and agriculture were also assigned to each area using the tephra thickness thresholds suggested by previous studies reported in the volcanological literature. The objective of this study was to evaluate whether the impacts were as expected based on the DDS suggested thresholds, and to determine whether other factors, apart from ashfall thickness, played a part. DDS thresholds based on tephra thickness were a good predictor of the impacts that occurred in the semi-arid steppe area around Jacobacci. This was unexpected as the more severe impacts were related to the challenging environmental conditions (low precipitation levels, high levels of wind erosion) and the daily wind remobilisation of ash that occurred, rather than the ashfall thicknesses received. The temperate region, including Villa la Angostura and Bariloche, performed better than the DDS assigned by ashfall thickness suggested. Despite deposits as thick as 300 mm, full recovery occurred within months of the ashfall event. The DDS scales need to incorporate a wider range of system characteristics, and environmental and vulnerability factors, as we propose here.

Estimation of tephra volumes from sparse and incompletely observed deposit thicknesses

We present a Bayesian statistical approach to estimate volumes for a series of eruptions from an assemblage of sparse proximal and distal tephra (volcanic ash) deposits. Most volume estimates are of widespread tephra deposits from large events using isopach maps constructed from observations at exposed locations. Instead, we incorporate raw thickness measurements, focussing on tephra thickness data from cores extracted from lake sediments and through swamp deposits. This facilitates investigation into the dispersal pattern and volume of tephra from much smaller eruption events. Given the general scarcity of data and the physical phenomena governing tephra thickness attenuation, a hybrid Bayesian-empirical tephra attenuation model is required. Point thickness observations are modeled as a function of the distance and angular direction of each location. The dispersal of tephra from larger well-estimated eruptions are used as leverage for understanding the smaller unknown events, and uncertainty in thickness measurements can be properly accounted for. The model estimates the wind and site-specific effects on the tephra deposits in addition to volumes. Our technique is exemplified on a series of tephra deposits from Mt Taranaki (New Zealand). The resulting estimates provide a comprehensive record suitable for supporting hazard models. Posterior mean volume estimates range from 0.02 to 0.26 km 3. Preliminary examination of the results suggests a size-predictable relationship.

Agricultural impact assessment and management after three widespread tephra falls in Patagonia, South America

Agricultural production is often concentrated in volcanically active or previously active areas where weathered volcanic products form fertile soils. However, this proximity means agriculture is exposed to tephra fall hazards. The type and severity of impacts to agricultural systems from tephra fall are dependent on both the hazard intensity metrics (tephra fall characteristics, such as thickness, grain size) and the vulnerability characteristics of the exposed agricultural system(s). Understanding the relationship between significant intensity metrics of tephra fall hazard and farm-scale and region-scale vulnerabilities is key to impact assessment and informing management and recovery strategies. Several large silicic eruptions have occurred over the past 20 years in the Patagonian region of South America, including the 1991 Hudson, 2008 Chaitén, and 2011 Cordón Caulle eruptions. These events deposited varying thicknesses of tephra on thousands of farms distributed across a variety of climates and production styles. Drawing on impact assessment data collected from interviews undertaken on post-event impact assessment reconnaissance trips, and other reports, this study evaluates the importance of tephra thickness as a hazard intensity metric, and vulnerability characteristics, when assessing impacts in the short and long term and, compares the effectiveness of response and recovery strategies. Whilst tephra thickness was the best single indicator of agricultural production losses, other factors, notably climate, farm type, and access to mitigation measures such as irrigation and/or cultivation, were also important indicators of damage. The climatic zone and associated precipitation level was found to be one of the most important characteristics of vulnerability, with higher damage occurring at lower tephra thicknesses in the semi-arid regions compared to farms in the temperate zone.

The Opala IV: An explosive caldera-generating super eruption, which was the largest in Kamchatka in the last 50000 years

It is shown that the youngest (~40000 14C years BP) caldera of the Opala caldera complex, the Opala IV, was formed by a catastrophic explosive supereruption, the largest in Kamchatka during the last 50000 years of the seven dated similar-type eruptions that occurred during the Late Pleistocene paroxysm of explosive volcanism. It is thought that the ejected volume was on the order of 250 km3 of pyroclastic material. A smaller part of it went to form pumice-rich pyroclastic flows, with the greater part being transported as tephra. The principal axis of the ash fallout was oriented NNE (azimuth ~30°) where the tephra thickness was 20–30 cm at a distance of 300–320 km from the eruptive center. The uncontaminated juvenile material is rhyolite, the concentration of SiO2 was 75–76%, the total alkali content 7.3–8.3%, and the K2O/Na2O ratio 0.83–0.96. It was concluded that no such eruption can occur in the Opala caldera complex in the future for hundreds or thousands of years.

Deposition of the 2011-2012 Cordón Caulle tephra (Chile, 40°S) in lake sediments: Implications for tephrochronology and volcanology

Tephras preserved in lake sediments are commonly used to synchronize sedimentary archives of climate and environmental change and to correlate them with terrestrial environments. They also provide opportunities to reconstruct volcanic explosive activity, e.g., eruption frequency and tephra dispersal. Although sedimentary processes may affect the record of tephras in lakes, lake sediments are generally considered as one of the best archives of tephra stratigraphy. The 2011-2012 eruption of Cordon Caulle volcano (Chile, 40 degrees S) offered an ideal opportunity to study the processes affecting tephra deposition in lakes. Although the prevailing westerlies transported the erupted pyroclastic material away from nearby Puyehue Lake, the tephra was identified within this relatively large lake with a thickness ranging from 1 to >10cm. This is in contrast with smaller lakes, where tephra thickness was in agreement with ashfall distribution maps. Geomorphological observations and sedimentological analyses provide evidence that the tephra deposited in Puyehue Lake entirely consists of material reworked from the upper watershed, transported by rivers, and distributed by lake currents according to particle size and density. Our results have important implications for tephrochronology and volcanology. They suggest that (1) lakes do not act as passive tephra traps; (2) lakes with large watersheds record more eruptions than smaller lakes, which only register direct ashfalls, affecting conclusions regarding the recurrence of volcanic eruptions; and (3) using lakes with large watersheds for isopach mapping systematically leads to an overestimation of erupted tephra volumes. Smaller lakes with limited drainage basins are generally better suited for volcanological studies.

Constraining tephra dispersion and deposition from three subplinian explosions in 2011 at Shinmoedake volcano, Kyushu, Japan

Constraining physical parameters of tephra dispersion and deposition from explosive volcanic eruptions is a significant challenge, because of both the complexity of the relationship between tephra distribution and distance from the vent and the difficulties associated with direct and comprehensive real-time observations. Three andesitic subplinian explosions in January 2011 at Shinmoedake volcano, Japan, are used as a case study to validate selected empirical and theoretical models using observations and field data. Tephra volumes are estimated using relationships between dispersal area and tephra thickness or mass/area. A new cubic B-spline interpolation method is also examined. Magma discharge rate is estimated using theoretical plume models incorporating the effect of wind. Results are consistent with observed plume heights (6.4–7.3 km above the vent) and eruption durations. Estimated tephra volumes were 15–34 × 106 m3 for explosions on the afternoon of 26 January and morning of 27 January, and 5.0–7.6 × 106 m3 for the afternoon of 27 January; magma discharge rates were in the range 1–2 × 106 kg/s for all three explosions. Clast dispersal models estimated plume height at 7.1 ± 1 km above the vent for each explosion. The three subplinian explosions occurred with approximately 12-h reposes and had similar mass discharge rates and plume heights but decreasing erupted magma volumes and durations.

Physical characteristics of tephra layers in the deep sea realm: the Campanian Ignimbrite eruption

Abstract Tephra deposits in the deep sea can survive undisturbed for long periods of time and, on regional scales, tend to be much better preserved than their subaerial counterparts. In this study, grain size distributions and thicknesses of tephra deposits from the Campanian Ignimbrite (CI) eruption (39 000 yr BP; magnitude c. 7.7) preserved in thirty-three deep sea cores are analysed to infer key eruption parameters. Distal deep sea tephra thickness data show an exponential decrease with distance from source. Such trends are difficult to identify in distal subaerial data owing to reworking and limited exposure. We find that tephra grain size distributions are much less affected by depositional environment than thickness, with trends that are consistent across distal subaerial, lacustrine and deep sea environments. The CI layer exhibits bimodal grain size distributions to distances of c. 1000 km, after which it becomes unimodal. Such trends can be related to different mechanisms of tephra transport in the atmosphere, whereby at proximal to medial distances the Plinian and co-ignimbrite phases produce distinct plumes. Within 150 and 900 km from source, Plinian tephra constitutes 40±5% of the deposit volume. Beyond this region where coarse particles are deposited, the plumes merge and fines derived largely from co-ignimbrite elutriation are spread in the atmosphere at velocities greater than the settling velocities of the particles.

First documented deep submarine explosive eruptions at the Marsili Seamount (Tyrrhenian Sea, Italy): A case of historical volcanism in the Mediterranean Sea

The Marsili Seamount (MS) is an about 3200m high volcanic complex measuring 70×30km with the top at ~500mb.s.l. MS is interpreted as the ridge of the 2Ma old Marsili back-arc basin belonging to the Calabrian Arc–Ionian Sea subduction system (Southern Tyrrhenian Sea, Italy). Previous studies indicate that the MS activity developed between 1 and 0.1Ma through effusions of lava flows. Here, new stratigraphic, textural, geochemical, and 14C geochronological data from a 95cm long gravity core (COR02) recovered at 839mbsl in the MS central sector are presented. COR02 contains mud and two tephras consisting of 98 to 100 area% of volcanic ash. The thickness of the upper tephra (TEPH01) is 15cm, and that of the lower tephra (TEPH02) is 60cm. The tephras have poor to moderate sorting, loose to partly welded levels, and erosive contacts, which imply a short distance source of the pyroclastics. 14C dating on fossils above and below TEPH01 gives an age of 3kaBP. Calculations of the sedimentation rates from the mud sediments above and between the tephras suggest that a formation of TEPH02 at 5kaBP MS ashes has a high-K calcalkaline affinity with 53wt.%<SiO2<68wt.%, and their composition overlaps that of the MS lava flows. The trace element pattern is consistent with fractional crystallization from a common, OIB-like basalt. The source area of ashes is the central sector of MS and not a subaerial volcano of the Campanian and/or Aeolian Quaternary volcanic districts. Submarine, explosive eruptions occurred at MS in historical times: this is the first evidence of explosive volcanic activity at a significant (500–800mbsl) water depth in the Mediterranean Sea. MS is still active, the monitoring and an evaluation of the different types of hazards are highly recommended.

Quantifying uncertainties in the measurement of tephra fall thickness

The uncertainties associated with tephra thickness measurements are calculated and implications for volume estimates are presented. Statistical methods are used to analyse the large dataset of Walker and Croasdale J Geol Soc 127:17-55, 1971 of the Fogo A plinian deposit, Sao Miguel, Azores. Dirichlet tessellation demonstrates that Walker and Croasdale’s measurements are highly clustered spatially and the area represented by a single measurement ranges between 0.5 and 10 km2. K-means cluster analysis shows that lower thickness uncertainties are associated with closely spaced measurements. Re-examination and analysis of Fogo A fall deposits show thickness uncertainties are about 9% for measured thickness while uncertainty associated with natural variance ranges, between 10 and 40%, with an average error of 30%. Correlations between measurement uncertainties and natural variance are complex and depend on a unit’s thickness, position within a succession and distance from source. Normative error increases as tephra thickness decreases. The degree to which thickness measurement error impacts on volume uncertainty depends on the number of measurements within a given dataset and their associated uncertainty. The uncertainty in volume associated with thickness uncertainty calculated herein for Fogo A is 1.3%, equivalent to a volume of 0.02 km3. However uncertainties associated with smaller datasets can be much larger; for example typically exceeding 10% for less than 20 data points.

Modeling thickness variability in tephra deposition

The attenuation of tephra fall thickness is most commonly estimated after contouring isolated and often irregular field measurements into smooth isopachs, with varying degrees of subjectivity introduced in the process. Here, we present an explicit description of the variability introduced into a semiempirical tephra attenuation relation. This opens the way to fitting models to actual tephra observations through maximum likelihood estimation, rather than using weighted least-squares estimation on the isopachs. The method is illustrated for small-scale basaltic explosive eruptions using a simple, but typical, data set of the actual tephra thickness data published from the 1973 Heimaey eruption. Of the distributions considered to describe variability in these measurements, the lognormal performed poorly, due to its tendency to predict a small number of greatly over-thickened deposits. The Weibull and gamma distributions fitted the data to a very similar degree and produced very similar estimates for the “effective volume,” mean wind direction, and mass/thickness attenuation rate. The latter can be inverted to obtain an estimate of the mean column height. The estimated wind direction, and the column height derived from the estimated thickness attenuation parameter, agreed very well with the direct observations made during the eruption. Augmented by a mixture framework allowing for the incorporation of multiple lobes and/or vents, the model was able to identify the source and direction of tephra deposition for the 1977 Ukinrek Maars eruptions from only the tephra thickness data.

Probabilistic assessment of tephra fallout hazard at Changbaishan volcano, Northeast China

Tephra fallout is an important type of hazard caused by volcanic eruption, and is also one of the main hazards at Changbaishan volcano, Northeast China. Numerical simulation is an effective approach to assess the dispersion of tephra fallout. According to the theory of dispersion model, we developed a simple and practical diffusion program that can be run on a personal computer. The input parameters for the simulation of tephra fallout from the Millennium Eruption of Changbaishan volcano, such as the size, density and shape of the tephra, the bulk volume and column height, the diffusion parameter P(z), wind direction and intensity, were obtained by field investigation and laboratory analysis. The simulated results in the intermediate scope when the parameter β > 0.3 are in good agreement with the results from measurement in situ, indicating that the model is reliable and the parameters used in the model are reasonable. We carried out more than 20,000 tephra fallout simulations using a statistical dataset of wind profiles which are obtained from China Meteorological Data Sharing Service System (CMDSSS). Tephra fallout hazard probability maps related to high- and low- magnitude eruption scenarios in Changbaishan volcano, are constructed for several tephra thickness thresholds, such as 70, 20, 10 and 1 cm. The results from this study can give support to the risk mitigation plans in Changbaishan area.

Potential impacts from tephra fall to electric power systems: a review and mitigation strategies

Modern society is highly dependent on a reliable electricity supply. During explosive volcanic eruptions, tephra contamination of power networks (systems) can com- promise the reliability of supply. Outages can have signifi- cant cascading impacts for other critical infrastructure sectors and for society as a whole. This paper summarises known impacts to power systems following tephra falls since 1980. The main impacts are (1) supply outages from insulator flashover caused by tephra contamination, (2) dis- ruption of generation facilities, (3) controlled outages during tephra cleaning, (4) abrasion and corrosion of exposed equipment and (5) line (conductor) breakage due to tephra loading. Of these impacts, insulator flashover is the most common disruption. The review highlights multiple instan- ces of electric power systems exhibiting tolerance to tephra falls, suggesting that failure thresholds exist and should be identified to avoid future unplanned interruptions. To ad- dress this need, we have produced a fragility function that quantifies the likelihood of insulator flashover at different thicknesses of tephra. Finally, based on our review of case studies, potential mitigation strategies are summarised. Specifically, avoiding tephra-induced insulator flashover by cleaning key facilities such as generation sites and trans- mission and distribution substations is of critical importance in maintaining the integrity of an electric power system.

Ultra-distal tephra deposits from super-eruptions: Examples from Toba, Indonesia and Taupo Volcanic Zone, New Zealand

Voluminous rhyolitic eruptions from Toba, Indonesia, and Taupo Volcanic Zone (TVZ), New Zealand have dispersed volcanic ash over vast areas in the late Quaternary. The ∼74 ka Youngest Toba Tuff (YTT) eruption deposited ash over the Bay of Bengal and the Indian subcontinent to the west. The ∼340 ka Whakamaru eruption (TVZ) deposited the widespread Rangitawa Tephra, dominantly to the southeast (in addition to occurrences northwest of vent), extending across the landmass of New Zealand, and the South Pacific Ocean and Tasman Sea with distal terrestrial exposures on the Chatham Islands. These super-eruptions involved ∼2500 km3 and ∼1500 km3 of magma (dense-rock equivalent; DRE), respectively.Ultra-distal terrestrial exposures of YTT at two localities in India, Middle Son Valley, Madhya Pradesh, and Jurreru River Valley, Andhra Pradesh, at distances of >2000 km from the source caldera, show a basal ‘primary’ ashfall unit ∼4 cm thick, although deposits containing reworked ash are up to ∼3 m in total thickness. Exposures of Rangitawa Tephra on the Chatham Islands, >900 km from the source caldera, are ∼15–30 cm thick. At more proximal localities (∼200 km from source), Rangitawa Tephra is ∼55–70 cm thick and characterized by a crystal-rich basal layer and normal grading. Both distal tephra deposits are characterized by very-fine ash (with high PM10 fractions) and are crystal-poor.Glass chemistry, stratigraphy and grain-size data for these distal tephra deposits are presented with comparisons of their correlation, dispersal and preservation. Using field observations, ash transport and deposition were modeled for both eruptions using a semi-analytical model (HAZMAP), with assumptions concerning average wind direction and strength during eruption, column shape and vent size. Model outputs provide new insights into eruption dynamics and better estimates of eruption volumes associated with tephra fallout. Modeling based on observed YTT distal tephra thicknesses indicate a relatively low (<40 km high), very turbulent eruption column, consistent with deposition from a co-ignimbrite cloud extending over a broad region. Similarly, the Whakamaru eruption was modeled as producing a predominantly Plinian column (∼45 km high), with dispersal to the southeast by strong prevailing winds. Significant ash fallout of the main dispersal direction, to the northwest of source, cannot be replicated in this modeling. The widespread dispersal of large volumes of fine ash from both eruptions may have had global environmental consequences, acutely affecting areas up to thousands of kilometers from vent.

Ablation of debris-covered ice: some effects of the 25 September 2007 Mt Ruapehu eruption

Melt rates of ice surfaces are strongly influenced by the existence of debris-cover. Dependent on thickness, climate and physical properties of debris, a debris layer can enhance or reduce ablation, compared to bare ice conditions. Ice ablation on bare ice and under varying thicknesses of tephra was measured using a network of 12 ablation stakes drilled into the Summit Plateau ice field, Mt Ruapehu, from 17 January to 2 February 2008 in order to study how tephra from the 25 September 2007 eruption was affecting ablation. Observation of air temperature allowed the application of a simple positive-degree-day approach to the calculation of ablation rate factors, which for bare ice was 7.8 mm d−1 °C−1, with a rate of 6.0 mm d−1 °C−1 mm for ice under 10 cm of tephra. Mean daily ablation rates varied from 116 mm d−1 on bare ice to 78 mm d−1 under tephra cover. Debris covers thicker than 70 mm reduced ablation.

Multistage volcanic events: Tephra hazard simulations for the Okataina Volcanic Center, New Zealand

While volcanic events are commonly characterized by multiple eruptive stages, most probabilistic tephra hazard analyses only simulate the major (paroxysmal) stage. In this study, we reconsider this simplified treatment by comparing hazard outcomes from simulated single‐ and multistage eruption sequences, using the Okataina Volcanic Center (OVC) in New Zealand as a case study. Our study draws upon geological evidence particular to the OVC as well as generalized patterns of eruptive behavior from other analogous volcanic centers. Exceedance probabilities of simulated tephra thickness, the cumulative duration of explosive behavior, and the duration of the entire eruptive sequence were all compared. Multistage simulations show an increased hazard with the greatest differences lying close to the vent for long duration and high thickness thresholds and at intermediate distances between the vent and the maximum extent of the deposit for lower thickness and duration thresholds. Multiple explosive stages increase the likelihood of an event lasting longer than 1 month by up to sevenfold and, for given low‐probability events, accumulated tephra thicknesses in some locations may increase by 1 order of magnitude and impact up to 22% more of New Zealand's North Island. Given our understanding of the eruptive history of the Okataina Volcanic Center, multistage simulations provide a better understanding of the potential hazard from this source.

Quantifying volcanic ash fall hazard to electricity infrastructure

Quantifying the potential ash fall hazards from re-awakening volcanoes is a topic of great interest. While methods for calculating the probability of eruptions, and for numerical simulation of tephra dispersal and fallout exist, event records at most volcanoes that re-awaken sporadically on decadal to millennial cycles are inadequate to develop rigorous forecasts of occurrence, much less eruptive volume. Here we demonstrate a method by which eruption records from radiocarbon-dated sediment cores can be used to derive forecasting models for ash fall impacts on electrical infrastructure. Our method is illustrated by an example from the Taranaki region of New Zealand. Radiocarbon dates, expressed as years before present (B.P.), are used to define an age-depth model, classifying eruption ages (with associated errors) for a circa 1500–10 500 year B.P. record at Mt. Taranaki (New Zealand). In addition, data describing the youngest 1500 years of eruption activity is obtained from directly dated proximal deposits. Absence of trend and apparent independence in eruption intervals is consistent with a renewal model using a mix of Weibulls distributions, which was used to generate probabilistic forecasts of eruption recurrence. After establishing that interval length and tephra thickness were independent in the record, a thickness–volume relationship (from [Rhoades, D.A., Dowrick, D.J., Wilson, C.J.N., 2002. Volcanic hazard in New Zealand: Scaling and attenuation relations for tephra fall deposits from Taupo volcano. Nat. Hazards, 26:147–174]) was inverted to provide a frequency–volume relationship for eruptions. Monte Carlo simulation of the thickness–volume relationship was then used to produce probable ash fall thicknesses at any chosen site. Several critical electrical infrastructure sites in the Taranaki Region were analysed. This region, being the only gas and condensate-producing area in New Zealand, is of national economic importance, with activities in and around the area depending on uninterrupted power supplies. Forecasts of critical ash thicknesses (1 mm wet and 2 mm dry) that may cause short-circuiting, surges or power shutdowns in substations show that the annual probabilities of serious impact are between ~ 0.5% and 27% over a 50 year period. It was also found that while large eruptions with high ash plumes tend to affect “expected” areas in relation to prevailing winds, the direction impacts of small ash falls are far less predictable. In the Taranaki case study, areas out of normal downwind directions, but close to the volcano, have probabilities of impact for critical thicknesses of 1–2 mm of around half to 60% of those in downwind directions and therefore should not be overlooked in hazard analysis. Through this method we are able to definitively show that the potential ash fall hazard to electrical infrastructure in this area is low in comparison to other natural threats, and provide a quantitative measure for use in risk analysis and budget prioritisation for hazard mitigation measures.

Impact of the eruptive activity on glacier evolution at Popocatépetl Volcano (México) during 1994–2004

Distribution, rate and magnitude of the glacier changes caused by eruptive activity over 1994–2001 at Popocatépetl Volcano were determined. Digital-elevation models computed by scanned photographs within a digital photogrammetric workstation and the subsequent DEM comparison allowed the determination of glacial changes. The results show that drastic changes occurred in glacier geometry and morphology over 1996–2001: 53% of the glacier surface area was lost due to melting processes. The most important loss occurred over 2000–2001 when 19% of the glacier-covered area disappeared. The glacier volume losses increased over time; during 1999–2000 the largest loss was observed. However, in February 2001, the glacier showed an apparent small increase in volume. Volcanic processes disturbed the mass balance of the glacier, thereby accelerating the ablation processes. The eruptive behavior of Popocatépetl was characterized during this time by alternating periods of explosive events and low activity. Pyroclastic flow generation, ejection of incandescent material, and tephra fall affected the glacier. The tephra volume, distribution and remobilization produced considerable changes on glacier evolution. Tephra deposition on the glacier surface was the most frequent process. The irregular distribution and thickness of tephra provoked differential ablation, and tephra remobilization processes took place on the glacier surface. Together, these processes incised the glacier surface over time. Based on these results and observations, a model of glacier evolution is proposed, involving an adjustment phase, thinning phase, areal-retreat phase, and fragmentation phase. A complex interplay of factors and processes took place between eruptive activity and the glaciers at Popocatépetl Volcano. The glacier evolution and subsequent extinction were induced by the eruptive behavior over the years. While not the only process at work, eruptive activity played the primary role in accelerating retreat and as a consequence in glacier extinction.

Landslides in the Tandayapa Valley, northern Andes, Ecuador: implications for landform development in humid and tectonically active mountain ranges

Landslides are common throughout the Ecuadorian Andes, but their causal and controlling factors and their roles in landform development have not yet been systematically investigated. This paper reports observations and hypotheses arising from a reconnaissance study of the Tandayapa Valley in the Cordillera Occidental, approximately 30 km west of Quito. This study area is characterised by high local relief (ca. 800 m) associated with dissected mountainous terrain, high annual rainfall (>2,000 mm), and secondary-succession wet montane ‘cloud forest’. Regolith cover is extremely thin on the very steep (45 to >60°) upper main valley slopes, but there are thick accumulations of tephra on the slope crests and ridgetops. These deposits show periods of soil development separated by deposition events. Natural landslides in this environment comprise rare large deep-seated bedrock failures and occasional shallow failures on the steep upper slopes where potentially unstable thicknesses of tephra have accumulated. Landslides associated with construction of roads and forest trails are more common. Back-analysis of one road-cut landslide, using field and laboratory data to characterise the in situ weathered material where possible, indicated that natural shallow failures are unlikely in the absence of a surficial cover of tephra. By analogy with another mountainous tropical landscape, it is suggested that long-term landform development can be explained in terms of channel incision, driven by uplift, producing steep lower valley slopes that eventually exceed bedrock failure thresholds. Therefore, large deep-seated landslides appear to control valley slope form development, whilst the shallow landslides contribute to general denudation.