V23A-01
Origin and age of the Volcanic Rocks of Tláloc Volcano, Sierra Nevada, Central Mexico
The Tláloc volcano (TV) is a 4125 m high stratovolcano of the Trans Mexican Volcanic Belt (TMVB) and is located in the northern end of the N-S trending Sierra Nevada, 30 km NE of Mexico City. Few data on the petrological and temporal evolution of TV have been published to date. Recently dated deposits gave ages between 32'000 and 34'500±500 years BP (Huddart and Gonzalez, 2004). Mapping and sampling of extrusive rocks in the summit region of TV revealed a dome structure with radiating lava flows consisting of dacitic rocks containing plagioclase and hornblende phenocrysts. Some flows, however, seem to be associated with a collapse structure E of the main summit. Crossing relationships indicate that this structure is older (“Paleo Tláloc”). A stratigraphy of the pyroclastic deposits was established along the northern slope of TV. From the numerous pyroclastic flows, separated by paleosoils and fluviatile deposits, only two pumice and one block and ash flow (BAF) have regional extent. Their thickness - distance relationship and their granulometry point to major explosive events. A carbonized wood sample from the BAF deposit gave ages similar to the previous ages (33'180±550 yr BP and 23'170±270 yr BP), a sample from a pyroclastic flow gave even a younger age (16'620±110 yr BP), suggesting that TV remained active also after the volcanoes Iztaccíhuatl and Popocatépetl further to the South started their activity. Based on these preliminary data it may be necessary to reconsider the accepted scenario of the temporal evolution of the central section of the TMVB, which assumes that the activity migrates from North to South with time. Huddart, D. and Gonzalez, S., 2004. Pyroclastic flows and associated sediments, Tláloc-Telapón, piedmont fringe of the eastern basin of Mexico. In: G.J. Aguirre-Diaz, Macías, J.L., and Siebe, C., (Editor), Penrose Conference. UNAM, Metepec, Puebla, Mexico, pp. 35.
V23A-02
Holocene and Late Pleistocene Tephra Stratigraphy of the Mono Craters
The Mono Craters consists of ~30 high-silica rhyolitic domes and flows and one rhyodacite dome. The exposed domes were emplaced during eruptions from the late Pleistocene to the most recent eruption 600 y BP. The ages of the eruptions with exposed domes were constrained by obsidian hydration rind dating by previous workers to two distinct phases; between 20,000 and 13,000 y BP and from 6,000 y BP to 600 y BP. The first eruptive phase consists of a biotite bearing assemblage while the more recent suite of eruptions began with alternating orthopyroxene and fayalite bearing rhyolites, which gave way to sparsely porphyritic and most recently aphyric rhyolites. By studying the Holocene tephra stratigraphy of the Mono Craters, this work aims to confirm the apparent gap in late Pleistocene and Holocene activity from 13,000 to 6,000 y BP and better constrain the local volcanic chronology. Excavation of a number of sites of tephra deposition from the Mono Craters were conducted and the various tephras were analyzed for petrology geochemistry and radiocarbon. The tephras include the full range of petrologic assemblages of the Mono Craters and range from the most recent eruptions to the biotite bearing eruptions of the late Pleistocene. Results of radiometric carbon dating are consistent with the cessation of the emplacement of a biotite bearing high silica porphyritic suite of eruptions prior to the 13,300 y BP eruption of the Black Point basalt, and the resumption of activity with alternating orthopyroxene and fayalite bearing assemblages. Between a pair of proximal radiocarbon dated layers (1672 +/- 36 and 1688 +/- 47 14C y BP) are four volcanic units, representing three of the petrological assemblages (aphyric, sparsely porphyritic, and porphyritic orthopyroxene-bearing); indicating that the different assemblages were, at least in this eruptive sequence, erupted contemporaneously. This indicates that the chamber beneath the Mono Chain is likely weakly zoned. Major and trace element chemistry of the younger volcanics from this and previous work indicate that the orthopyroxene and fayalite bearing rhyolites are less evolved than their biotite bearing predecessors. The tephras contain abundant mafic xenoliths, which are particularly pronounced in some of the orthopyroxene bearing units. Fayalites in the fayalite bearing assemblage show varying degrees of skeletal texture. These data point toward a more mafic magma interacting with the Mono source at depth.
V23A-03
COMMENTS ON SOME PHANEROZOIC BATHOLITHS, IGNIMBRITES, F- RHYOLITES
Mesozoic and Paleozoic ignimbrites and tuffs might support rejuvenation of batholiths as zircon dating caused in many cases a discrepancy between the inferred eruption age derived from zircon ages and the age of +/- time- equivalent fossils. Such age discrepancies exist in the range of several to more than 10 my i.e. tuffs within Permian beds (Königer et al.. 2002) and Blasseneck-Porphyroid ignimbrites (BPIs) - (Söllner et al. 1997). The BPIs (ca. Ord. VI) are exposed along ca. 300 km in the E Alps. Compacted thicknesses are max. 600 m at lower greenschist-facies. It is not determined yet if the BPIs represent a silicic LIP (1 pulse volcanism(?)). They are underlain by sediments and km-thick (flood?) basalts of lower Ord. age. The BPIs predate the Hirnantian glaciation. The end of zircon growth could also be interpreted as the age of the batholith emplacement. Rejuvenation did not cause further zircon growth. Zircons of Trias. ignimbrites (Obenholzner et al. 1999) showed a broad set of preservation from euhedral to highly corroded during diagenesis (see also Reddy et al. (2006)). Within the realm of Variscean batholiths of Cornwall and Erzgebirge F-rich rhyolites exist which are interpreted as late-stage fractionation or as products of an independent magmatic event. These rhyolites are ca. 20 m.y. younger than the associated granites and intruded or erupted at the very end of the subduction. The Cret. Lost River granite is characterized by distinctive greisenization events (Sainsbury, 1960) and intra-intrusive breccias (Dobson, 1982). In the Erzgebirge explosion breccias exist within the granite (Kempe et al., 2004). Does it represent an episode of rejuvenation including zircon growth? If the time gap of ca. 20 m.y. can be considered an average the F-rhyolites of the Sierra Madre Occidental (Orozco-Esquirel et al., 2002) might be related to batholith emplacement older than the ignimbrite-forming batholiths. The Variscean batholiths of the E Alps host many features indicative for a caldera-root system. Basalt dikes dissected tonalites and pillowed into aplites (Frasl et al., 1995). Permian ignimbrite sequences on Carb. volcanics are known from the S Alps. The F- and B-rich Plio./Quart. granite at Larderello (Italy) is a cooling intrusion with melt batches (Fulignati, pers. comm.). Thermal anomalies of the E Alps had been interpreted as surface manifestation of plutons. Atmospheric 210Po data exist from Bad Gastein (Austria; Wallner, 2001). Could such dormant "volcanoes" become sites of future F-rhyolite eruptions? Pure gas sparging cannot rejuvenate a non-solid batholith (Bachman et al., 2003). Heating of Na fluoride and Na borate in a glass vial (ca. 900°C) revealed surface modification accompanied by vesicle nucleation even inside the glass. Can F- and B- rich fluids operate as fluxing agents? Past super-eruptions need to be correlated with past mantle and core dynamics and maybe F- and B-rich fluid events. "3 magnetic superchrons precede the 4 largest Phanerozoic extinction events (K/T, G/T P/T doublet, end Ord.)".(Courtillot et al., 2006). This hypothesis needs to identify the Ord. LIPs (BPIs of the E Alps?). Bachman et al. 2003, Geology, 31, 789-792. Courtillot et al., 2006. EOS Trans. AGU, 87(52), Fall Meet. Suppl.. Frasl et al., 1995. Geol. Paläont. Mitt. Innsbruck, 20, 121-151. Kempe et al., 2004. Min. Depos., 39, 646-669. Königer et al., 2002. Int.J. Earth Sci., 91, 341-356. Reddy et al., 2006. Geology, 34, 4, 257-260. Obenholzner et al., 1999. Ann. Naturhist. Mus. Wien, 100A, 13-38. Vienna. Orozco-Esquirel et al., 2002. JVGR, 118, 37-56. Sainsbury, 1960. Econ. Geol., 55, 1478-1509. Söllner et al., 1997. Z. dt. geol. Ges., 148/3-4, 499-522. Wallner, G. (2001): Radiochim. acta, Bd. 89, 791-798. .
V23A-04
Volcanic Centers of the Northern McCullough Range, Southern Nevada USA: a View of Pre- Extensional Volcanism in the Colorado River Extensional Corridor
The northern McCullough Range, just south of Las Vegas, Nevada, is being developed by the US Bureau of Land Management as the Sloan Canyon National Conservation Area to preserve its natural history. Compared to adjacent ranges, the northern McCullough Range was relatively undeformed by Miocene extension in the Colorado River Extensional Corridor. Therefore, the well preserved volcanic centers within the McCullough Range provide an excellent opportunity to study pre-extensional volcanism. There are at least seven volcanic centers in the northern McCullough Range; this study focuses on the Cactus Hill, McCullough Wash, and Eldorado Valley Volcanoes in the central McCullough Range, and the Henderson Caldera in the northern McCullough Range. The Cactus Hill volcano is a 200 m thick section of flows and agglomerates that form a broad basalt-andesite cone, nearly 2 km in diameter. This cone is cut by two (2-3 m wide) basalt dikes and at least 8 dacite domes. Each of the domes is associated with a broad debris apron. The McCullough Wash volcano is composed of at least 6 dacite domes and carapace breccias that reflect periods of dome growth and collapse. The Eldorado Valley Volcano, another series of dacite domes and flows, is the source of a 250 m thick breccia unit (Eldorado Valley breccia). The breccia is a block and ash deposit (with beds up to 1.5 m thick) containing spectacular blocks (1 cm - 3 m in diameter) and bombs (10 cm - 6 m in diameter) that are interbedded with flows from the McCullough Wash and Cactus Hill volcanoes. Interbedding of dacite breccia of the Eldorado Valley Volcano with dacitic, andesitic and basaltic dome debris from the Cactus Hill volcano reflect coeval mafic and felsic volcanism. The Henderson caldera at the northern tip of the McCullough Range is formed by a arc of domes that erupted a series of biotite dacite flows. The caldera is also filled by domes and flows of hornblende andesite, ash-flow tuff and mesobreccia deposits. The tuff of the Henderson caldera (~20 m thick) grades from a pumice poor base to a pumice rich top and contains lithic fragments of andesite that reflect the explosive truncation of the central McCullough Range stratovolcano (2500 to 3000 m thick section of andesite flows). Mesobreccia occurs along the southern margin of the caldera and contains andesite clasts (< 25 cm) within a matrix of the tuff of the Henderson caldera. The ongoing study the volcanic centers of the McCullough Range provide geologic data for the development of the Sloan Canyon National Conservation Area as well as providing insight into the evolution of the Colorado River extensional corridor.
V23A-05
Reconstruction of lava fields based on 3D and conventional images. Arenal volcano, Costa Rica.
Conventional air photographs, multi-spectral images and a map scale 1:10 000 were used to upgrade Arenal
volcano's lava field.
Arenal volcano located in NW Costa Rica has been active for 39 years. Fifty two days after the initial explosive
events that opened three craters on the west flank, lava flows were erupted from crater A (1050 m) in September,
1968 and continued flowing until November, 1973. These lavas were the most voluminous of the eruption and the
effusion rate of lava was relatively high in this period. In April, 1974 lava flows were erupted from crater C (1460
m) and continue to present time. Younger lava flows extended over uncovered ground to the south and southwest
in the 1980s and early 1990s and onto the northern slopes in the 1990s and 2000s. Lava flows are becoming
shorter and narrower with time. Therefore, the centre of mass of the whole lava flow-field has migrated closer to
the vent. Above crater C a cone has been growing steadily, reaching a height of 1670 m, 36 m higher than the
prehistoric Arenal cone by 2004. After 39 years of continuous emission of lava flows, the profile of Arenal volcano
consists of a duplet of cones whose summits are separated by less than 500 meters. Most of the build up around
the new cone comes from varied lava flows.
For near 30 years volcano monitoring staff (from OVSICORI-UNA) has recorded field observations of regular and
extraordinary events, in paper. Several drafts maps have been used for teaching, academic presentations and for
graphic explanations to specific audiences and to the general public. An upgraded version was needed.
The purpose of this work is to present the most recent lava flows giving a visual presentation of them by computer
methods.
Combined SIG techniques (Arc View 3.3) and ERDAS produced a base map in which layers containing the
recorded lava flows from the recent 16 years, were depicted.
Each lava flow has its own characteristics: direction, year of origin, width, length, surface texture, chemical
composition, type of lava, velocity, etc. With all this information and photographs; real, visual and topographic
images of the position and characters of the 1990s and 2000s lava flows, were obtained .
An illustrative poster will be presented along with this abstract to show the construction process of such tool.
Moreover, 3D animations will be present in the mentioned poster.
http:www.ovsicori.una.ac.cr
V23A-06
The 13 ka Pelée-Type Dome Collapse at Nevado de Toluca Volcano, México.
The Nevado de Toluca is an active volcano located in the central sector of the Trans-Mexican Volcanic Belt, 80 km southwest of Mexico City. Activity at this andesitic to dacitic stratovolcano began ca. 2.6 Ma ago. During the last 42 ka, the volcano has been characterized by different eruptive styles, including five dome collapses dated at 37, 32, 28, 26, and 13 ka and five Plinian eruptions at 42 ka, 36 ka, 21.7 ka, 12.1 ka and 10.5 ka. The 13 ka dome collapse is the youngest event of this type, and originated a 0.11 km3 block-and-ash flow deposit on the north-eastern sector of the volcano. The deposit consists of two facies: channel-like, 10 m thick, monolitologic, that is composed of up to five units, with decimetric dacitic clasts set in a sandy matrix; and a lateral facies that consists of a gray, sandy horizon, up to 4 m thick, with a 30 cm-thick surge layer at the base. The main component is a dacitic lava, with different degree of vesciculation, with mineral association of Pl-Hbl-Opx. Plagioclases show two different textures: in equilibrium, with normal zoning (core = An37-64.3, rim = An30.7-45.8) or with spongy cellular texture with inverse zoning (core = An38-43.5, rim = An45-51.2). Hornblende is normally light green, barren of oxidation. The rock matrix contains up to 53 perc. of glass with abundant microlites, indicating over-pressure on the crystallizing magma and a rapid expulsion. All these stratigraphic and petrographic features indicate that the dome was quickly extruded on the summit of the volcano, probably triggered by a magma mixing process, and its collapse was accompanied by an explosive component, being classified as a Pelée-type event.
V23A-07
Explosive and Phreatomagmatic Activity from San Salvador Volcanic Complex (El Salvador) and Their Effects on El Cambio Archaeological Site: a Review of the Last 3000 yrs. Based on Volcanic Stratigraphy Data
El Cambio archeological site (ECAS; Zapotitán Valley), 4 km NW from the San Salvador Volcanic Complex comprises 3000 yrs. of pyroclastic record. Sheets (1983) identified different levels rich in cultural remains intercalated within the volcanic deposits, indicating that different prehistoric settings were affected by San Salvador volcano eruptions, and giving information on the reoccupation frequency in the area. Accordingly, ECAS was occupied since the Late Pre-Classic period until before the last plinian eruption of Ilopango Caldera (425AD) reference, that originated the Tierra Blanca Joven (TBJ), pyroclastic deposits generally used as key-layer in stratigraphic reconstructions. Within the next two centuries, there is no evidence of human occupation at ECAS until the end of Late Classic which was a period of maximum splendor in the valley. During this time the area was affected by at least 3 eruptions from the San Salvador volcanic complex that produced the: Laguna Caldera volcanic fall deposits (which affected Joya de Cerén archeological site in 625AD), "Talpetate" surge deposits or Toba de San Andrés (600-900AD), and fall deposits of El Playón volcano (1658). We report new data on volcanic stratigraphy and archeological history including the following: a) the phreatomagmatic nature of eruptions that affected the area, the new excavations allowed the detailed study of surge deposits indicating magma-water interaction at Laguna Caldera and El Playón, previously considered strombolian eruptions; b)document the occupation of ECAS during Middle Pre-Classic period, new surge deposits below TBJ have been identified (with Middle Pre-Classic artifacts and pottery), that had not been documented before, extending the historic record up to 3000 yrs. BP. and c) detailed study of the "Talpetate" deposits, this sequence consists of fall, pyroclastic flow and surge deposits, present in the rim and slopes of San Salvador Volcano, which can be correlated with surge deposits in a wide area in the northwest and west part of the main volcano. Dating of organic material collected recently will help to better constrain the timing and sequence of the eruptive and human settlement events.
V23A-08
Costa Rica's Chain of laterally collapsed volcanoes.
From the NW extreme to the SW end of Costa Rica's volcanic backbone, a number of laterally collapsed
volcanoes can be observed. Due to several factors, attention has been given to active volcanoes disregarding the
importance of collapsed features in terms of assessing volcanic hazards for future generations around inhabited
volcanoes.
In several cases the typical horseshoe shape amphitheater-like depression can be easily observed. In other
cases due to erosion, vegetation, topography, seismic activity or drastic weather such characteristics are not
easily recognized.
In the order mentioned above appear: Orosi-Cacao, Miravalles, Platanar, Congo, Von Frantzius, Cacho Negro and
Turrialba volcanoes.
Due to limited studies on these structures it is unknown if sector collapse occurred in one or several phases.
Furthermore, in the few studied cases no evidence has been found to relate collapses to actual eruptive
episodes. Detailed studies on the deposits and materials composing dome-like shapes will shed light on
unsolved questions about petrological and chemical composition. Volume, form and distance traveled by
deposits are part of the questions surrounding most of these collapsed volcanoes.
Although most of these mentioned structures are extinct, at least Irazú volcano (active volcano) has faced partial
lateral collapses recently. It did presented strombolian activity in the early 60s. Collapse scars show on the NW
flank show important mass removal in historic and prehistoric times. Moreover, in 1994 a minor hydrothermal
explosion provoked the weakening of a deeply altered wall that holds a crater lake (150m diameter, 2.6x106 ).
A poster will depict images of the collapsed volcanoes named above with mayor descriptive characteristics. It will
also focus on the importance of deeper studies to assess the collapse potential of Irazú volcano with related
consequences.
Finally, this initiative will invite researchers interested in such topic to join future studies in these Costarrican
volcanoes.
http:www.ovsicori.una.ac.cr
V23A-09
Towards Improving Ash Monitoring; Combining and Comparing IR Satellite Data and Forward Trajectory Models
Techniques used to observe and monitor volcanic ash in order to mitigate aviation hazard require testing to determine their reliability and limitations. Currently, the most used techniques rely on satellite remote sensing and trajectory modeling. Infrared data from such sensors as the Moderate Resolution Imaging Spectroradiometer (MODIS), and the Advanced Very High Resolution Radiometer (AVHRR) are used to determine the location and abundance of ash within eruptive clouds in as close to real-time as is possible. The other technique that has been developed uses a volcanic ash dispersion model (e.g. PUFF, HYSPLIT) to predict potential areas of high ash content based on wind fields, settling velocities and an initial location in three dimensional space. Used jointly, these two techniques could further improve ash detection by allowing us to compare the reliability of infrared data when applied to the detection of ash clouds. Using eruptions from various volcanoes, we can compare the location of the clouds based on satellite data to the location of the cloud based on PUFF simulation models quantitatively. In most cases where discrepancies do exist, they are attributed to ash cloud - atmosphere interaction. Ash clouds that are erupted into wetter atmospheric environments (latitude <40) are more difficult to monitor using satellite IR data due to the presence of water vapor in the atmosphere. An appropriate example of this phenomenon occurred with the May 10, 2003 eruption of Anatahan Volcano in the Mariana Islands (16.35 North). Satellite IR observations of this eruption show limited agreement with PUFF dispersion models of this eruption and this may be due to the presence of water vapor in the atmosphere which either 1.) masks the negative "split-window" signal indicative of ash in the atmosphere or 2.) causes the aggregation and faster fallout rate of ash than what is modeled. Further analysis of this eruption cloud and other eruption clouds in various atmospheric settings will provide further insight into what causes discrepancies between satellite IR observations of volcanic ash clouds and dispersion models.
V23A-10
Continuous in-situ Measurements of Gases (H2, H2S, CH4, N2, O2, Ar, He, and CO2) at the Fumarole "Soffionissimo" (Solfatara volcano, Southern Italy)
From November 29th to December 1st 2006, a gas monitoring experiment was carried out at the Solfatara volcano (Pozzuoli, Italy). The primary objectives were to prove that monitoring is possible with the experimental set-up described below, and to compare the new data obtained with those from earlier continuos gas monitoring carried out in November 2001. Temperature measurements and gas extraction were done at the fumarole "Soffionissimo" very close to the "Bocca Grande". The temperature measurements were performed with a temperature probe (K-type thermocouple), which was let about 30 cm into the fumarole. For better comparison of temperature and gas data, the gas tube was directly connected with the temperature probe. After having adjusted a continuous gas flow with a diaphragm pump and a needle valve, the gas was piped through a 10 m Teflon© tube for more than 40 hours. The gas phase primary consists of water gas, which was condensed in a trap, installed in a refrigerator. The amount of water in the trap was determined regulary every 3-4 hours. At the beginning of the monitoring experiment, the Teflon© tube was heated in order to avoid condensation of the water in the tube before getting trapped. Although the tube was not heated for the whole time of the experiment, it turns out that the amount of water, condensed in the water trap per hour, and does not significantly change when the tube was not heated. Hence, the amount of water, condensing in the tube before getting trapped, seems negligible. The remaining, almost water-free gas phase was finally dried over Fe wool in a filter, and then continuously analysed with a quadrupole mass spectrometer (Balzers Omnistar ©) for the following components: H2, H2S, CH4, N2, O2, Ar, He, and CO2. To make sure that the final drying process does not influence the gas composition in particular for H2 and H2S, a comparison measurement was done without the filter, which only revealed somewhat higher water content. During the second half of the monitoring, additionally CO2 gas concentration measurements were performed using a laser spectrometer based on a semiconductor laser source emitting around 2 m connected on-line with the gas line. Gas samples were taken from the gas line for laboratory gas-chromatographic analysis and noble gas analyses. Although data processing is still ongoing, the residual (=water-free) average gas composition can be preliminary described as follows: CO2 >97 vol%, H2S ~0.16 vol%, H2 ~0.15 vol%, N2 <2 vol%, O2 <0.5 vol%, Ar <0.02 vol%, CH4 ~0.05 vol%, He <20ppmv. O2 and most of the N2 and Ar are due to atmospheric contamination of the system. Besides the varying air contribution, the gas composition shows no significant variations over time within the analytical uncertainties of the experiment.
V23A-11
Explosive eruption at Bezymianny Volcano, Russia, captured by satellite data
Bezymianny (55.9°N, 160.6°E, ~2900 m elevation) is an active, explosive volcano within the Central Kamchatka Depression (CKD), Kamchatka Peninsula, Russia. Based on information from the Kamchatka Volcanic Eruption Response Team (KVERT 2006), an explosive eruption occurred at Bezymianny Volcano, Kamchatka, Russia at 09:17 UTC on 24 December 2006. This produced an ash cloud up to ~10 km ASL. We investigate the 24 December 2006 eruption using rapid-response data from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), combined with field photographs of the deposit. Furthermore we consider thermal and textural observations on the deposits. Satellite images agreed with aerial photographs taken on 27 December 2006 showing the warm deposit (relative to the snow) concentrated in the south-eastern channel. We present data from rapid response images collected in conjunction to the Alaska Volcano Observatory (AVO) and suggest inferences toward future activity.
V23A-12
Andaman Mud Volcanoes: Low Temperature Geochemistry of Eruptive Fluids and Potential Hydrocarbon Source
Mud volcanoes are a common feature, reported by several workers, in the Baratang Island of Middle Andamans, India. The association of methane gas and adsorbed hydrocarbons in the mud has been cited well by scientists working in Andamans and also by others working on other eruption areas around the world. Our intended work centres around delineating the nature of such mud volcanoes in the above terrain, their chemical composition with special reference to the clay minerals formed under this sedimentary volcanism environment and exploiting such volcanic deposits towards potential hydrocarbon reserves (if any). The analysis suggests that, at any instant of time, gas vents at variable rates in different gas channels at the same site, and that the compositional differences in these vent gases are nearly as large as can be produced by hydrate crystallization. Almost two orders of magnitude differences in venting rate between individual gas channel ways are suggested. Our hypothesis is compatible with geologic generalizations that venting evolves from fast (mud volcano), to intermediate (hydrate crystallization), to slow (carbonate precipitation) if venting organized into more discrete vents with time. The most realistic agent that explains the observed effects is a rapid local emission of mud and/or water. Stable isotopes of the separated clay minerals (smectite- and illite-rich extruded mud) from the mud volcanoes will be utilized towards the knowledge of nature of volcanism in the Andaman areas. The most likely mechanism believed is re-hydration of shales by both hydrocarbons and a geochemically mature fluid from greater depth within the wedge. Deep fluid source studies supported by our results from gas analyses, includes He-3, thermogenic C-13 in methane as well as 'ultraheavy' C-13 in CO2. Trace element and REE studies of the muds are being conducted to attest to the above hypothesis. The overall results attest active local flow of geochemically different fluids along deep-seated faults penetrating the wedge, with the waters as well as the gases coming from below. Pore water chemistry of mud volcanoes in Andamans reveal that a distinct correlation between the chloride and the isotopic composition of the water exists. O2 gets heavier and H gets lighter with decreasing chloride concentration and increasing gas hydrate content. Data of pore water isotopic composition from gas hydrate-bearing volcanoes might testify, in addition to isotopic composition by gas hydrate formation, that there are other geological processes affecting the isotopic composition.
V23A-13
Comparison of Eruption Effects on Humans at Four North American Cinder Cones
Cinder-cone eruptions are typically low in volume and explosivity, but they can affect local populations profoundly. Comparisons of three prehistoric eruptions (Xitle, Sunset Crater, and Little Springs Volcano) and one historic eruption (Parícutin) in southwestern North America show that eruption effects vary dramatically due to eruption style, tephra blanket extent, climate, types of land use, the culture and complexity of the affected group, and resulting governmental action. Lava flows have long-term effects that extend little beyond the flow edges. These flows, however, are used for defensive purposes, providing refuges from invasion for those who know them well. In arid lands, tephra blankets serve as mulches, decreasing runoff and evaporation, increasing infiltration, and regulating soil temperature. Management and retention of these cinder mulches, which can open new areas for agriculture, becomes a priority for farming communities. In humid areas, though, the tephra blanket impedes plant growth and increases erosion. Cultural responses to eruptions vary, from collapse of an already declining culture at Xitle, fragmentation of society at Parícutin, dramatic changes and development of new technologies at Sunset Crater, to little apparent change at Little Springs volcano. Eruptions may be viewed as retribution for poor behavior and attempts are made to mollify angry gods. At Sunset Crater and Little Springs volcano, offerings (corn and pot sherds, respectively) were made at hornitos, rootless vents on the lava flows. Spatter containing the corn impressions and sherds was then carried away and incorporated into the walls of structures. This indicates ritual significance to the offerings.
V23A-14
DEM Data for Lahar Modeling: Lahars From Popocatépetl Volcano (Mexico)
Modeling of lahars has become important for related hazards assessment, particularly in regions with scarce geologic record. Digital terrain data is a crucial element for mass-flow modeling but limited by availability of DEM's. Remote sensing technology offers perspectives for DEM generation. DEMs derived from ASTER stereo satellite images and SRTM from Popocatépetl Volcano, taken on 2001, were evaluated using two simple, non-dynamic lahar models LAHARZ and MSF. Laharic volumes were based on the 1995-2001 events. Results of model runs for the Huiloac gorge based on the ASTER-DEM show that only lahars with a volume of 5x106 m3 or larger reach towns 11km away. Lahars modeled with SRTM-DEM travel 1.8 km further downstream. The ASTER-based model tends to a stronger lateral distribution of the flow volume, thus limiting the longitudinal distribution. In comparison to the observed lahar of corresponding volume in 1997, the modeled lahars fit in travel distance by ca. 2 and 4 km for SRTM and ASTER, respectively. The flow paths for the ASTER and SRTM DEMs are consistent except for the middle section where an erroneous flow-routing of the ASTER model can be observed, caused by an error of the respective DEM. ASTER and SRTM DEMs, are suitable for use with LAHARZ and MSF. Flow path prediction is more reliable with SRTM data but with coarser spatial resolution. Errors of the ASTER DEM affecting the prediction of flow paths are mainly identified in deeply incised gorges with north-facing slopes due to the sensor geometry. LAHARZ is more sensitive to errors of the ASTER DEM than the MSF model. Lahar modeling with the ASTER-DEM results in a finer spaced predicted inundation area but does not add any significant information in comparison with the SRTM DEM. The verification and sensitivity of the DEM used is fundamental when deriving hazards maps to forecast inundation areas.
V23A-15
Volcanic hazard map for Telica, Cerro Negro and El Hoyo volcanoes, Nicaragua
A volcano hazard study was conducted for Telica, Cerro Negro and El Hoyo volcanoes, Nicaragua, based on geological and volcanological field investigations, air photo analyses, and numerical eruption simulation. These volcanoes are among the most active volcanoes of the country. This study was realized 2004-2006 through technical cooperation of Japan International Cooperation Agency (JICA) with INETER, upon the request of the Government of Nicaragua. The resulting volcanic hazard map on 1:50,000 scale displays the hazards of lava flow, pyroclastic flows, lahars, tephra fall, volcanic bombs for an area of 1,300 square kilometers. The map and corresponding GIS coverage was handed out to Central, Departmental and Municipal authorities for their use and is included in a National GIS on Georisks developed and maintained by INETER.
V23A-16
Comparative Spectrograms Between the Popocatepetl Volcano Magnetic Station and the Teoloyucan Magnetic Observatory, Mexico.
We present a comparative spectrogram analysis for the Popocatepetl Volcano magnetic station (70.943° N CoLat, 261.363° E, 4029 m) and the Teoloyucan Magnetic Observatory (70.254° N CoLat, 260.807º E, 2280 m) time series between 1997 and 2003. Instrumentation at both sites include a Geometrics G856 proton-precession magnetometer operating at a 60 second sampling rate and is complemented with the magnetic record from a dF fluxgate variograph at the Teoloyucan Magnetic Observatory (TEO). Popocatepetl's total magnetic field record is reconstructed using a harmonic analysis technique, and subtracted to the TEO record, which is considered as a reference site. The resulting difference shows a significant diurnal component, presumably from a local magnetic induction or from ionospheric origin. This is in sharp contrast with our initial considerations for geomagnetic volcano monitoring that considered the distance between both sites to be close enough and assumed similar ionospheric conditions at both sites. This diurnal component influence can be removed using a normalized difference approach or by cancellation during the harmonic reconstruction process. This analysis will improve previously used techniques such as normalized differences or correlation in magnetic data analysis for short, middle and long term active volcano magnetic monitoring.
V23A-17
Using GIS tools to visually represent complex volcanic issues. Poas volcano, Costa Rica.
GIS software was used to visualize a series of extraordinary events that have occurred recently at Poas Volcano,
Costa Rica. Maps, images, graphics and other visual instruments were produced during the course of an
academic internship at OVSICORI-UNA. Spatial and temporal concepts related to volcano monitoring
conventional observations are introduced.
Poas (10 11 15 N, 84 13 48 W, and 2708 m.a.s.l) is one of the main massifs on the Central Volcanic range, Costa
Rica. It is located NW of the Central Valley, where the most developed and populated cities are located. The
volcanogenic pollution and its impact on the environment, as well as the implications over socio-economic
activities developed at the surroundings of the volcano, are more severe during increases in subaerial fumarolic
outgassing and these points to the important role played by the presence of the crater lake as a buffering system.
This is an important aspect to consider due to the frequency of the events and to the prolonged periods of impact
over the same areas.
The work consisted in putting together a large amount of data varying from notebooks, maps, documents,
conventional, multispectral and aerial photographs, in a format readable by the GIS interface, in this case ArcGIS
9.1. As most data refers to geographic locations, a great deal of the data could easily be represented in a GIS.
Using the built-in analysis functionalities in ArcGIS, complex issues, concerning volcanogenic pollution and its
impact on the surrounding environment, were allowed to be visually represented, using both two and three
dimensions.
Being able to visualize these issues, do not only help volcanologists to understand the processes involved, but
also aids in the communication of these issues to concerned actors in the society. This is of importance due to
the fact that volcanogenic pollution does address several areas handled by various authorities. GIS offers a
platform for analysis and visualization that could aid in joint efforts to address the problems described.
We will present a poster fully illustrated with images and 3D visual aids containing gas and solid plumes derived
from recent activity at Poas volcano.
http:www.ovsicori.una.ac.cr