Claus Siebe

Repeated volcanic disasters in Prehispanic time at Popocatépetl, central Mexico: Past key to the future?

The Holocene eruptive history of Popocatepetl volcano is characterized by recurrent voluminous Plinian eruptions every 1000 to 3000 yr, the most recent of which destroyed human settlements. Major eruptions occurred between 3195 and 2830 B.C., 800 and 215 B.C., and A.D. 675 and 1095. The three eruptions followed a similar pattern and started with minor ash fall and ash flows. The eruptions reached their peak with a main Plinian pulse that produced deposition of a pumice fall, the emplacement of hot ash flows, and finally extensive mudflows. Each time the area of devastation had become repopulated, before being devastated once again. During the last eruption several settlements, including Cholula (a major urban center), were inundated by lahars. A scenario of the possible recurrence of an eruption of similar magnitude, which would have disastrous consequences for the now highly populated areas around Popocatepetl, should be considered seriously in any volcano emergency contingency plan. This is especially important because more than one million people are living within a radius of 35 km around the volcano (the outskirts of Mexico City are at a distance of 40 km), and Popocatepetl resumed emitting ash on December 21, 1994, after decades of dormancy.

Passive infrared spectroscopy of the eruption plume at Popocatépetl volcano, Mexico

Volcanic gases provide important insights into deep-Earth processes, and gas composition and flux variations show promise as predictors of eruptive activity. But data correlating gas composition with eruptions are sparse, largely because such studies have traditionally involved direct sampling inside a volcanic crater — a hazardous operation that has resulted in numerous deaths,. Crater-rim-based spectroscopy, closed-path spectroscopy of gases sampled from aircraft, and time-averaged studies using volatile traps allow measurements to be taken from safer distances. But when a full-scale explosive eruption threatens, even these methods become dangerous as the hazard radius expands to many kilometres. Previously, only sulphur dioxide has been reliably measurable at such large distances, using correlation spectroscopy. Here we describe techniques that extend the useful range of passive infrared spectroscopy to monitor many gases at distances of over 17 km. We demonstrate the use of these techniques in a high-temporal-resolution study of short-term compositional variations associated with an explosive eruption at Mexicos Popocatépetl volcano on 25–26 February 1997. We observed a steady increase in SiF4/SO2 over several days preceding the eruption, followed by a tenfold decrease in this ratio over a few hours immediately afterwards.

Passive infrared remote sensing evidence for large, intermittent CO2 emissions at Popocatépetl volcano, Mexico

Abstract Passive infrared (FTIR) and correlation spectrometer (COSPEC) measurements were conducted at Popocatepetl volcano during February 10 to 26, 1998 from sites 4 to 17 km distant from the summit. Volcano behavior was relatively quiet and SO 2 flux averaged 1670±1420 t/day (51 measurements), relatively small for Popocatepetl. Concurrent HCl/SO 2 and HF/SO 2 ratios were 0.17±0.01 and 0.031±0.003, respectively, about the same as ratios measured from 1994 to 1997. The amount of CO 2 in the volcanic plume was quantified using FASCODE in which atmospheric CO 2 is numerically subtracted from the total infrared spectrum to obtain the residual magmatic CO 2 . Surprisingly, CO 2 /SO 2 mass ratios rose dramatically to values as high as 140, about 30 times higher than typical values of 2 to 8 measured from 1994 to 1996. These excursions in high CO 2 /SO 2 ratios were short-lived, lasting no longer than about 0.5 to 3.0 h but CO 2 flux occasionally exceeded 100,000 t/day. We estimate that the average CO 2 /SO 2 ratio for the period was about 23, yielding an average CO 2 flux of roughly 38,000 t/day. Chemical and petrographic analyses of lava and pumice erupted during explosions on June 30, 1997 and January 1, 1998 show conclusively that Popocatepetl produces mixed products formed by injection of mafic magma into a more silicic chamber at temperatures and pressures of roughly 1040°C and 5 kbar. In addition, Popocatepetl eruptive products include xenoliths of metamorphosed carbonate rocks containing wollastonite and other calc-silicate minerals indicating reaction of magma with Cretaceous limestone underlying the volcano. Using a normal CO 2 /SO 2 ratio of 4 for reference, we calculate an average excess CO 2 production of 32,000 t/day for 17 days. This would require assimilation of only 5×10 −4 km 3 of limestone, an amount easily accessible in the 3-km-thick Cretaceous section beneath the volcano. We also examine two scenarios in which excess CO 2 is produced by degassing of subjacent basalt magma, but these explanations seem less plausible to us. Because many other volcanoes are underlain by carbonate sequences, short-duration bursts of CO 2 flux, and increased CO 2 /SO 2 ratio, might be observed at other sites, if simultaneous, real-time measurements of major gas species are made.

Geochemistry, Sr–Nd isotope composition, and tectonic setting of Holocene Pelado, Guespalapa and Chichinautzin scoria cones, south of Mexico City

Holocene Pelado, Guespalapa and Chichinautzin monogenetic scoria cones and associated lava flows located within the Sierra del Chichinautzin Volcanic Field (SCVF) at the southern margin of Mexico City were mapped and sampled for mineralogical and chemical analyses. With the exception of Paricutin volcano in western Mexico, few scoria cones in the Trans-Mexican Volcanic Belt have ever been sampled in greater detail. Chemical analyses of rocks indicate that mafic products (e.g. Guespalapa and Chichinautzin) from individual volcanoes in the Sierra del Chichinautzin are characterized by substantial chemical variability, whereas high-silica andesite volcanoes (e.g. Pelado) are very uniform in composition. These findings have important bearings for regional tephrochronology. As a whole, rock compositions form a continuous coherent calc–alkaline suite, explicable by polybaric fractional crystallization±assimilation associated with successive stagnation at different depths along the ascent path. Trace element and Sr–Nd isotope analyses point toward a <1-km-scale heterogeneous (enriched/depleted) mantle wedge underneath the SCVF. The recently proposed plume-origin for these rocks is not in accord with our data. Instead, magma origin is discussed in relation to the tectonically complex subduction process of the oceanic Cocos Plate underneath the continental North American Plate.

Geochemical surveillance of magmatic volatiles at Popocatépetl volcano, Mexico

Surveillance of Popocatepetl volcanic plume geochemistry and SO 2 flux began in early 1994 after fumarolic and seismic activity increased significantly during 1993. Volatile traps placed around the summit were collected at near-monthly intervals until the volcano erupted on December 21, 1994. Additional trap samples were obtained in early 1996 before the volcano erupted again, emplacing a small dacite dome in the summit crater. Abundances of volatile constituents (ppm/day of Cl, S total , F, CO 2 , Hg, and As) varied, but most constituents were relatively high in early and late 1994. However, ratios of these constituents to Cl were highest in mid-1994. δ 34 S-S total in trap solutions ranged from 1.5‰ to 6.4‰; lowest values generally occurred during late 1994. δ 13 C-CO 2 of trap solutions were greatly contaminated with atmospheric CO 2 and affected by absorption kinetics. When trap data are combined with SO 2 flux measurements made through November 1996, Popocatepetl released about 3.9 Mt SO 2 , 16 Mt CO 2 , 0.75 Mt HCl, 0.075 Mt HF, 260 t As, 2.6 t Hg, and roughly 200 Mt H 2 O. Near-vent gas concentrations in the volcanic plume measured by correlation spectrometer (COSPEC) and Fourier transform infrared (FTIR) commonly exceed human recommended exposure limits and may constitute a potential health hazard. Volatile geochemistry combined with petrologic observations and melt-inclusion studies show that mafic magma injection into a preexisting silicic chamber has accompanied renewed volcanism at Popocatepetl. Minor assimilation of Cretaceous wall rocks probably occurred in mid-1994.

Age and archaeological implications of Xitle volcano, southwestern Basin of Mexico-City

The Pedregal lavas are fresh, well-exposed basaltic flows erupted from the Xitle scoria-and-cinder cone in the southwestern part of the Basin of Mexico. These lavas cover an area of 70 km 2 and were emplaced over pyramids and other buildings (e.g. Cuicuilco and Copilco archaeological sites). Today, a part of Mexico-City (including the National University) is built on the flows. Initial strombolian activity produced an ash fallout layer, which was immediately followed by effusive emplacement of lava flows. The Xitle cone grew on the north-facing slope of Ajusco volcano, and lava flowed down to the N‐NE until it reached the basin floor. More than 30 radiocarbon dates have been obtained by several workers on charcoal samples from beneath the lava, and several ages for the eruption have been proposed from these dates. Most dated samples were not directly produced by Xitle’s eruption but instead are artifacts of human activity that predates the eruption. Thus, these ages (mostly about 2000 bp) are older than the eruption. A new age of 1670 ^ 35 years bp (AD 245‐315) obtained on charcoal samples collected just beneath the lavas is favored for the Xitle eruption. These samples originated by ignition of vegetation during the emplacement of hot scoriaceous tephra. The new age is within the Classic period of Mesoamerican archaeology, whereas the earlier reported ages are at the end of the Preclassic. The new age carries important implications for the timing of population shifts within the Basin of Mexico. q 2000 Elsevier Science B.V. All rights reserved.

Surface ocean iron fertilization: The role of airborne volcanic ash from subduction zone and hot spot volcanoes and related iron fluxes into the Pacific Ocean

Surface ocean iron (Fe) fertilization can affect the marine primary productivity (MPP), thereby impacting on CO2 exchanges at the atmosphere-ocean interface and eventually on climate. Mineral (aeolian or desert) dust is known to be a major atmospheric source for the surface ocean biogeochemical iron cycle, but the significance of volcanic ash is poorly constrained. We present the results of geochemical experiments aimed at determining the rapid release of Fe upon contact of pristine volcanic ash with seawater, mimicking their dry deposition into the surface ocean. Our data show that volcanic ash from both subduction zone and hot spot volcanoes (n = 44 samples) rapidly mobilized significant amounts of soluble Fe into seawater (35–340 nmol/g ash), with a suggested global mean of 200 ± 50 nmol Fe/g ash. These values are comparable to the range for desert dust in experiments at seawater pH (10–125 nmol Fe/g dust) presented in the literature (Guieu et al., 1996; Spokes et al., 1996). Combining our new Fe release data with the calculated ash flux from a selected major eruption into the ocean as a case study demonstrates that single volcanic eruptions have the potential to significantly increase the surface ocean Fe concentration within an ash fallout area. We also constrain the long-term (millennial-scale) airborne volcanic ash and mineral dust Fe flux into the Pacific Ocean by merging the Fe release data with geological flux estimates. These show that the input of volcanic ash into the Pacific Ocean (128–221 × 1015 g/ka) is within the same order of magnitude as the mineral dust input (39–519 × 1015 g/ka) (Mahowald et al., 2005). From the similarity in both Fe release and particle flux follows that the flux of soluble Fe related to the dry deposition of volcanic ash (3–75 × 109 mol/ka) is comparable to that of mineral dust (1–65 × 109 mol/ka). Our study therefore suggests that airborne volcanic ash is an important but hitherto underestimated atmospheric source for the Pacific surface ocean biogeochemical iron cycle.

Submarine eruption near Socorro Island, Mexico: Geochemistry and scanning electron microscopy studies of floating scoria and reticulite

Abstract Products of an underwater eruption near Socorro Island in the NE Pacific were observed directly on January 29, 1993, ten days after precursors were first recorded by SOFAR (Sound Fixing and Ranging) hydrophones located in Hawaii and Tahiti. Eruptive activity was noticed from boats and ships as small steam plumes rising from the sea at an area centered at 18 °48′N, 111 °05′W, 2.4 km NW of Punta Tosca and 4.6 km SSW of Cape Henslow on Socorro Island. The observed steam was produced by 1–3-m-large blocks of hot, dark-grey, highly vesiculated basalt rising buoyantly to the surface from two submarine shallow vents at 210 and 30 m depth. Tens of blocks accompanied by bubbles could be observed rising to the surface in irregular pulses. These scoriaceous blocks remained floating at the surface until they would crack into smaller pieces by thermal contraction, emitting hissing noises from vapourizing seawater in contact with the hot interior of the blocks. Steam jets several metres in height were produced and occasionally blocks were propelled laterally by the steam jet. Depending on vesicularity and permeability, blocks remained floating and drifting with the surface current for 1–15 minutes before sinking back. Floating rocks covered an area of about 6000 m2. This intermittent activity has been observed ever since and has not stopped as of April 1994. Buoyant scoria and reticulite are indicative of volatile (mostly CO2) supersaturation and exsolution in the magma prior to rapid quenching, which inhibits loss of volatiles by bubble escape. A high-velocity ascent of low-viscosity magma in a relatively narrow conduit is also required to prevent substantial gas escape and allow formation of reticulite. The buoyant scoria is most probably ejected by intermittent lava fountaining at fixed vents as a result of changes in eruption velocities due to changes in the exsolved gas content of the lava. Between January and July 1993 floating blocks of scoria and reticulite were collected on several occasions from the surface of the sea for chemical and mineralogical analyses. Major- and minor-element analyses (including REE), as well as electron microprobe analyses of different phases revealed that the composition of the emitted lava has not changed through time. Blocks of basalt (SiO2 = 45–47%) are highly vitric and vesicular with tabular anorthite phenocrysts up to 3 mm in length and minor grains of forsteritic olivine. REE and trace-element composition of these rocks suggest an anomalous mantle source for the erupted alkali basalt lava. The ongoing eruption will either continue in a similar fashion as described and eventually cease or built up a mound that reaches the surface and forms an island with accompanying change in eruptive style.

Volcanic hazards in the Mexico City metropolitan area from eruptions at Popocatépetl, Nevado de Toluca, and Jocotitlán stratovolcanoes and monogenetic scoria cones in the Sierra Chichinautzin Volcanic Field

Tephrochronological studies carried out over the past decade in the area surrounding Mexico City have yielded a wealth of new radiocarbon ages from eruptions at Popocatepetl, Nevado de Toluca, and Jocotitlan stratovolcanoes and monogenetic scoria cones in the Sierra Chichinautzin Volcanic Field. These dates allow us to constrain the frequency and types of eruptions that have affected this area during the course of the past 25,000 yr. They have important implications for archaeology as well as future hazard evaluations. Late Pleistocene and Holocene volcanic activities at the stratovolcanoes are characterized by recurrent cataclysmic Plinian eruptions of considerable magnitude. They have affected vast areas, including zones that today are occupied by large population centers at Puebla, Toluca, and Mexico City. During Holocene time, Nevado de Toluca and Jocotitlan have each experienced only one Plinian eruption, ca. 10,500 yr B.P. and 9700 yr B.P. respectively. During the same period of time, Popocatepetl had at least four such eruptions, ca. 8000, 5000, 2100, and 1100 yr B.P. Therefore, the recurrence interval for Plinian eruptions is less than 2000 yr in this region. The last two Plinian eruptions at Popocatepetl are of particular interest because they destroyed several human settlements in the Basin of Puebla. Evidence for these disasters stems from pottery shards and other artifacts covered by Plinian pumice falls, ash-flow deposits, and lahars on the plains to the east and northeast of the volcanic edifice. Several monogenetic scoria cones located within the Sierra Chichinautzin Volcanic Field at the southern margin of Mexico City were also dated by the radiocarbon method in recent years. Most previous research in this area was concentrated on Xitle scoria cone, whose lavas destroyed and buried the pre-Hispanic town of Cuicuilco ca. 1665 ± 35 yr B.P. The new dates indicate that the recurrence interval for monogenetic eruptions in the close vicinity of Mexico City is also <2000 yr. The longest lava flow associated with a scoria cone was erupted by Guespalapa and reached 24 km from its source; total areas covered by lava flows from each monogenetic eruption typically range between 30 and 80 km2, and total erupted volumes range between 0.5 and 2 km3/cone. An average eruption rate for the entire Chichinautzin was estimated at ~0.5 km3/1000 yr. These findings are of great importance for archaeological as well as volcanic hazard studies in this heavily populated region.

Holocene plinian eruption of La Virgen volcano, Baja California, Mexico

A plinian eruption occurred approximately 6500 yr ago at La Virgen Volcano, the youngest volcano of the Tres Virgenes Volcanic Complex (TVVC), located in Baja California, Mexico. Deposits of the eruption suggest a sequence of events that started with the opening of the volcanic conduit, and development of a plinian eruption column up to 18 km in height. This eruption column produced a fallout deposit with a dispersal axis toward the southwest, an areal extent of about 500 km 2 , and a minimum volume of 1.14 km 3 . Vulcanian activity (hydromagmatic) followed the plinian phase, producing pyroclastic surge and fallout deposits. The eruptive activity ceased after a basaltic-andesite lava flow was emplaced closing the eruptive activity. Petrological and geochemical evidence indicates that the eruption was triggered by magma mixing processes. Our studies confirm that La Virgen is a dormant volcano with the potential for future violent eruptions. The present study provides important information for the construction of a volcanic hazards map. Significant hazards are presented to the population living within a distance of 30 km from the volcano, together with the interstate road connecting the entire peninsula of Baja California, which runs at a distance of only 3 km from the volcano.