V33B-01 INVITED
Volcano-Ice Interactions in Mexico: Extinction of Glaciers at Popocatépetl and the Fate of the Glaciers of Iztaccíhuatl and Citlaltépetl Volcanoes
In spite of the state of activity of the Mexican volcanoes (erupting, fumarolic, or dormant) study of volcano-ice interactions is of key importance because one of the main consequences of these interactions is the generation of lahars. Study of glaciers and volcanic activity help to prevent the possibility of devastating events at volcanoes in Mexico, especially because some explosive events at Popocatépetl volcano generated lahars during the past twelve years. Small-sized glaciers exist or existed at Iztaccíhuatl, Popocatépetl and Citlaltépetl whose volcanic activity is characterized by a different level of activity. The extinction of glaciers of Popocatépetl volcano was eruption-forced. 40% of climatic-related shrinkage occurred in 4 decades whereas 32% of eruption-related shrinkage occurred in 4 years. Long-term effects of glacier extinction include an imbalance between recharge and extraction of groundwater at surrounding aquifers provoked by disappearance of glacier-related melt water. The volcano started to erupt in 1994. Impact of the eruption on the glaciers was in several ways: immediate thermal effect of hot falling ash on snow and ice.; ballistic projectiles are also hot and yield high kinetic energy producing melting restricted to the impact areas; every event depositing 1cm of ash represents a load of ~102-104 tons; fumaroles of <100°C on cracks beneath the glacier provoked continuous melting at the base of the glacier even during the winter; possibly, volcanic gases indirectly affected the glaciers, but greenhouse effect is difficult to assess. Iztaccíhuatl volcano's glaciers have been influenced by the same factors as at Popocatépetl, except the eruption. Citlaltépetl volcano's glaciers have been just affected by climatic changes. Both volcanoes show fumaroles or diffuse degassing. An eruptive event in the short term may cause the same effect as occurred at Popocatépetl volcano. The large ice masses of the world are claimed to be affected by global warming and local climatic variations. Tropical glaciers as those of Mexico are more vulnerable because of their size and exposure to eruptive processes. Even though their extinction might not have a global impact, their disappearance deprives us of important climatic "gauges" at the ~20° north latitude and impacts strongly the local environment. Relations between the volcanic activity and the presence of glaciers in Mexico have still several aspects to study, among them: ice melting provoked by eruptive products and heat flux, glacier extinction, and debris flow generation.
V33B-02 INVITED
Rapid Loss of Andean Alpine Glaciers: A Reflection on Cotopaxi´s Long-Distance Historical Lahars and Future Lahar Scenarios
Andean alpine glaciers are in rapid retreat, as witnessed by actual measurements, comparative imagery and popular memory. Overall glacier losses will diminish future water availability for human consumption as well as for lahar generation, the product of mixing incandescent eruptive materials with glacial ice and snow. The field study and modeling of long-distance historical lahars from Cotopaxi volcano, Ecuador has shown them to be some of the most voluminous and longest reported. Based on back calculations, peak discharges were commonly between 45,000-60,000 m3/sec, velocities reached 70 km/hr, and run outs attained 325 km. The last "super" debris flow was produced at Cotopaxi in 1877. Observations made after the 1877 eruption reported that the glacier had suffered about 10 meters of ice stripped off the top and the incision of deep gullies from melting and erosion by the scoria block-rich pyroclastic flows. Average reductions of 45% and 60%, respectively, of the area and volume of Cotopaxi´s 19 alpine glaciers during the last 30 years have left an ice cap of only 13 km2 and a volume of 0.60 km3. Descriptions by astute 18th and 19th century observers lead us to conclude that Cotopaxi glaciers were much more robust then, surpassing a total area of about 30 km2, a fact which contributed to generating large volume lahars and high discharges, during the waning "Little Ice Age". If an eruption similar to that of 1877 occurs at Cotopaxi in the future, reduced glacier sizes and the glaciers´ preferential distribution upon the cone will likely attenuate volcano-ice interactions and will lower the probability of "super" lahars being produced during eruptive periods. However, in the last 2000 years of eruptive activity, explosive eruptions display a large size span-- from weakly explosive events (VEI= 2) to highly explosive eruptive cycles (VEI= 4-5). Given the uncertainty of the size of the next explosive eruption of Cotopaxi, several scenarios for lahar generation must be envisioned, which include the magnitude of the explosive event as well as the retreat of the glacier. These scenarios all have implications for the populations living in adjacent valleys, where future lahars may pass.
V33B-03
Unusual Ice-Rich Lahars From the 23 September 1995 Eruption of Mt. Ruapehu, New Zealand
Interactions between explosive eruptions and summit ice or snowpack are among the most hazardous lahar- forming events at snow-capped volcanoes, as demonstrated by the 1985 disaster at Nevado del Ruiz, Colombia. A case of contrasting impact is the 23 September 1995 eruption at Mt. Ruapehu, New Zealand, in which an ice- rich lahar traveled <3 km before coming to rest on relatively steep slopes (10-16°). Addressing the diverse thermal and fluid dynamic consequences of such events is therefore critical to understanding the threat to surrounding areas. In the 1995 Ruapehu eruption, a series of explosions through the Crater Lake ejected jets of hot tephra and warm lake water upon the snow cover, generating two bifurcating lahars that took paths directly down a ski-field. The late hour and limited runout precluded any loss of life or property. Fieldwork immediately after the event acquired samples and multiple thickness profiles across the deposit. Componentry and geochemical analyses showed that the deposit consisted of 10-30 wt% tephra and lake sediments, 1-4 wt% frozen lake water, and 65-85 wt% snow and ice. We infer that the evolving thermal balance within the lahar induced it to come to rest as the liquid component (lake water and melted snow/ice) progressively froze with distance from the source. We will present progress in development of a treatment of the heat transfer and fluid dynamics tephra-water-ice mixtures to constrain conditions leading to the widely varying runout distances and associated hazards of such events.
V33B-04
Subglacial Silicic Eruptions: Wet Cavities and Moist Cavities.
Comparing the deposits of subglacial eruptions with those of subaerial and subaqueous eruptions enables the influence of magma-water-ice interactions to be explored. In this presentation, the Icelandic subglacial rhyolite tuyas of Kerlingarfjöll and Prestahnúkur are compared with subaerial and subaqueous rhyolite formations at Sierra La Primavera, México. Prestahnúkur formed by the subglacial lava effusion and thick lava flows with steep termini are products of confinement by ice walls. Basal deposits of perlitised obsidian lobes suggest a water-saturated environment, and the extremely abundant microvesicular lava blocks surrounding these lobes and throughout the edifice are broadly similar to the carapaces of silicic lava domes at La Primavera known to have a subaqueous origin. Although bedded and sorted deposits are present at Prestahnúkur, they are trivial compared to the thick and extensive caldera-lake deposits of La Primavera, which even contain a "giant pumice" marker bed formed by the lake-wide deposition of once-bouyant blocks. The Kerlingarfjöll rhyolite tuyas formed during explosive subglacial eruptions. The first-erupted material forms structureless beds of phreatomagmatically-fragmented ash; ash from subaqueous eruptions at La Primavera is similarly fine grained, but in contrast is well-bedded (due to lacustrine deposition). Later-erupted material at Kerlingarfjöll typically consists of massive unconsolidated lapilli-tuffs. The lapilli themselves are similar to those within the well-sorted subaerially-formed pumice cones La Primavera, however Kerlingarfjöll's lapilli- tuffs have grain-size characteristics of proximal pyroclastic flows. These observations suggest that although similar fragmentation mechanisms operated in both locations, transport and consequent sorting was limited at Kerlingarfjöll. The different products of the two Icelandic subglacial tuyas are related to their different eruption rates and magma volatile contents. Melting of ice produces water, however in the Kerlingarfjöll eruption (which is thought to have been relatively brief and the vesicular magma is likely to have contained less heat per unit volume) the volumes were small and the subglacial cavity could be appropriately described as 'moist'. The Prestahnúkur eruption occurred in a 'wet' cavity but 'lacustrine' conditions were never developed and the ice was always close to the edifice. Poor sorting and structure in the subglacial deposits are due to a lack of time and space for sorting to occur. In contrast to more mafic eruptions, which are characterised by very strong meltwater-ice interactions, the main influence of the ice during subglacial rhyolite eruptions is reflected in the confinement of eruptive products.
V33B-05
Evidence for a new Type of Basaltic Subglacial Sheet-Like Sequence and Implications for the Inferred Thickness of Associated ice
Basaltic volcanic sequences erupted subglacially have been classified empirically into two major types, thought to correspond to eruptions under thick and thin ice, respectively. The latter can be called subglacial sheet-like sequences from the appearance of the outcrops. Only one type of sheet-like sequence has been described so far. However, there is now evidence that there are at least two types of subglacial sheet-like sequence, with significantly different implications for interpretations of associated palaeo-ice sheet thicknesses. The first type, which is relatively well known, is a diagnostic product of eruptions associated with relatively thin glaciers (< c. 150 m) of any thermal regime (temperate, polar, polythermal). It is called the Mount Pinafore type after sequence holotypes described in Antarctica. A second type of sheet-like sequence is now proposed based on minor but significant differences in lithofacies and lithofacies associations. Eruptions of the second type commenced with the injection and inflation of a sill along the ice:bedrock interface. Such interface sills have been predicted theoretically but had no known geological example, until now. Subsequent evolution involves floating of the ice cover, catastrophic meltwater drainage and emplacement of widespread sheets of hyaloclastite in major subglacial mass-flow and hyperconcentrated-flow flood events. However, floating of the glacier may not always occur and truncated sequences should also exist, with predictable lithofacies characteristics. In fully developed sequences, the water-saturated hyaloclastite is commonly and distinctively intruded by apophyses of the underlying interface sill. Eruptions giving rise to the new sequence type are apparently not explosive, unless the ice over the vent thins sufficiently. Like the Mount Pinafore sequences, the deposits can be characterised as outflow sequences in that they do not form part of a recognisable vent structure. They are probably rooted in subglacial volcanic edifices of tuya, tindar or pillow volcano types and all known examples are associated with fissure eruptions. Whilst the new sequence type would have formed under a thicker glacial cover than Mount Pinafore sequences, it is usually only capable of providing an indication of minimum thicknesses of associated overlying ice.
V33B-06
Glacial fragmentation induced by eruptive activity: Popocatépetl Volcano (México)
Ice-volcano interactions at volcanoes depend on eruptive behavior, glacier characteristics and time scales. High- intensity eruptions occurred in a short span of time can provoke dramatic glacial changes whilst intermittent eruptive behavior of variable intensity over years can generate gradual glacial changes. Popocatépetl volcano hosted a small glacial area when it started to erupt in December 1994. Over 12 years of intermittent and fluctuating eruptive activity gradual glacial changes occurred. Based on results obtained by digital photogrammetry using a collection of aerial photographs, daily eruptive activity reports and observations a model of glacial evolution is proposed. 1) Adjust phase, no important volume loss occurred in spite of intense eruptive activity, although upheaval at the terminus was observed. 2) Thinning phase, through 1999 the glacier experienced considerable thinning due to differential ablation due to the irregular distribution of the pyroclastic material deposited on its irregular glacial surface with stair-like pattern. Flows and rilling-related to tephra remobilization incised the glacier surface repeatedly. 3) Areal retreat phase, the greatest glacial area loss occurred in 2000 and some elongated blocks of ice were identified. 4) Fragmentation phase, started on 2001, as a combination of differential ablation and recurrence of remobilization processes of tephra reduced the glacier to a set of blocks, the upper part of them was covered by a thick layer of tephra and their sides exposed the remnant ice to ablation processes. Ever since isolated blocks are spread over the pre-eruptive glacial area. Glacier evolution was fundamentally influenced by the fluctuating eruptive behavior over the years but was not the single factor.
V33B-07
Energy balance at North Glacier's surface of Citlaltépetl volcano (Mexico): understanding glacier behavior at a non-eruptive volcano.
Mexican glaciers are located on Iztaccíhuatl and Citlaltépetl volcanoes and at both sites they are retreating.
The glaciers of Popocatépetl volcano are currently considered extinct after the combination of climate forcing
and eruptive activity. It is difficult to distinguish at eruptive volcanoes how important the climatic factors conduct to
the disappearance of glaciers.
In order to recognize the importance of the regional climatic factors, previous to reactivation of any of the other two
volcanoes, and better understand the volcano/glacier interactions, it is needed to study both parts of the system
independently: volcanic activity and the glacier behavior.
In this work we present the methodology used to calculate the energy balance at the glacier's surface at the north
face of Citlaltepetl volcano and the results of this calculation. We use the data collected over a period of 10
months by two meteorological stations installed on the glacier's surface and on a rock buttress at 5000 meters
above sea level. This volcano was chosen because it permits to study the glacier behavior without the influence of
eruptive activity.
http:www.geofisica.unam.mx/popoc/colaboracion/GTNH/