Ilopango Caldera Research Articles

Ash aggregate-rich pyroclastic density currents of the 431 CE Tierra Blanca Joven eruption, Ilopango caldera, El Salvador

The VEI 6, Tierra Blanca Joven pyroclastic sequence (30–90 km3 DRE volume), erupted from Ilopango caldera, El Salvador, in 431 CE, is the product of one of the largest eruptions of the last two millennia. The eruption devastated Central America's Mayan civilization. The eruption began with a short-lived phase of ash and pumice fall deposition and transitioned to a ‘wet’ explosive phase during which pyroclastic density currents flowed >40 km from the caldera. Detailed field and sedimentological analyses are provided for the deposits of ash-aggregate-rich pyroclastic density currents generated during early phases of the eruption. The first phase of pyroclastic density current inundation incinerated forests and deposited up to 30 m of, non-welded, ash-rich ignimbrite in proximal regions, along with ash fall layers of co-ignimbrite origin. Following fallout of a thin layer of pumice and lithic lapilli, a second phase of pyroclastic density current inundation and co-ignimbrite ash fall commenced. A range of ash aggregate types is present in the pyroclastic density current deposits and interbedded co-ignimbrite ash fall layers. Whole and broken concentrically layered ash aggregates (accretionary lapilli) reach >50 vol% in some horizons within some beds in the pyroclastic density current deposits. The evidence indicates that the ash aggregates grew within overriding co-ignimbrite ash plumes and subsequently fell into ground-hugging currents. Our findings suggest that the aggregate-rich nature of the pyroclastic density current deposits originated through incorporation of lake water into eruptive plumes, which in turn triggered rapid, pervasive aggregation within ash clouds and co-ignimbrite plumes.

2D FEM Numerical Prediction of Local Seismic Effects at San Salvador Municipality (El Salvador) Induced by 2001 Earthquakes

San Salvador Metropolitan Area (MASS) is an urbanized territory in the country of El Salvador, located between the San Salvador volcano and the Ilopango caldera, in a sub-flat area called “Valle de las Hamacas” (Valley of the Hammocks). The high seismicity of this area is due to the subduction zone of the Cocos plate that causes strong seismic events such as the earthquakes that occurred on 13 January (7.6 Mw) and 13 February 2001 (6.6 Mw). As part of the international cooperation project between Italy (AICS) and El Salvador University, the CASTES project focussed on natural hazards in the territory of El Salvador was launched. Therefore, 2D simulations are carried out along two sections to evaluate the Local Seismic Response (LSR) in the Southeast part of MASS territory. Results show spatially variable amplifications (from 3 to 6.5) in the period ranging 0.1–0.7 s and evidence of lateral FA variations that can be calculated only through 2D numerical analyses. Two amplified periods are recognised, 0.1–0.5 s and 0.4–0.8 s, due to the presence of two types of subsoil volcanic deposits: the shallow and soft Tierra Blanca deposits and the deeper and stiffer Volcanic ash and Tuff.

Growth of complex volcanic ash aggregates in the Tierra Blanca Joven eruption of Ilopango Caldera, El Salvador

Aggregation processes control both the residence time and dispersal of volcanic ash during eruptions yet remain incompletely understood. The products of aggregation vary from simple ash clusters to large, complexly layered accretionary lapilli. Here we detail the micro-stratigraphy of a single population of accretionary lapilli that grew during the ∼431 CE Tierra Blanca Joven eruption from Ilopango Caldera, El Salvador. The accretionary lapilli were sampled 10 km from the caldera source within a sequence of ash-rich pyroclastic density current deposits and intercalated fall material, known as unit D, which is traceable >40 km from Ilopango. Scanning electron microscopy and image analysis reveal common facies that form distinct layers within the accretionary lapilli. Each facies is distinguished by quantitative and qualitative variations in particle size distribution, porosity, and particle fabric. We infer that these textures resulted from aggregation conditions that differed in terms of liquid water availability, particle concentration and grain size distributions. In our proposed model, a characteristic sequence of facies accreted from core to rim in the accretionary lapilli during passage through ash clouds generated by vent-derived plumes and pyroclastic density currents. The accretionary lapilli are mostly composed of smaller aggregates (ash clusters, ash pellets) and grew predominantly by accretion of already-formed aggregates, rather than by grain-by-grain accretion of individual particles. This finding is consistent with observations of rapid aggregate growth in volcanic plumes, suggesting a common evolutionary pathway for accretionary lapilli formation across diverse eruptions.

Effusive and explosive eruptive history of the Ilopango caldera complex, El Salvador

The Ilopango caldera in central El Salvador is the source of at least 13 voluminous rhyolitic eruptions over the last two million years, including the 431 CE Tierra Blanca Joven (TBJ) which blanketed areas inhabited by the Mayan civilization, and the 1.707 ± 0.022 Ma Olocuilta ignimbrite, one of the largest Quaternary eruptions in Central America. These rhyolitic deposits have been the subject of numerous detailed studies. In contrast, the abundant effusive and monogenetic eruptions at Ilopango have not been studied. These small-volume eruptions range from basalt to rhyolite (49.7–72.8 wt% SiO2) and occur both within the caldera margin and on the south flank of the Antiguo Ilopango stratovolcano. Here we present 28 new 40Ar/39Ar ages and geochemical data from domes, lavas, dikes, and ignimbrites from the Ilopango caldera and the Antiguo Ilopango stratovolcano as well as from adjacent volcanic systems. Collectively, these new data are used to construct a revised eruptive sequence for the Ilopango caldera that spans almost three million years. The new chronostratigraphy for Ilopango is highlighted by two volcanic cycles. Each cycle starts with rhyolite dome effusions, and then a prolonged period of explosive rhyolitic volcanism followed by a ~120–175 kyr period of less evolved eruptions. The spatio- and chemo-temporal patterns in the Ilopango eruptive sequence are similar in many ways to those observed in other well-studied Quaternary silicic centers.

Zircon and Melt Extraction From a Long‐Lived and Vertically Extensive Magma System Underneath Ilopango Caldera (El Salvador)

AbstractThe Tierra Blanca (TB) eruptive suite comprises the last four major eruptions of Ilopango caldera in El Salvador (≤45 ka), including the youngest Tierra Blanca Joven eruption (TBJ; ∼106 km3): the most voluminous event during the Holocene in Central America. Despite the protracted and productive history of explosive silicic eruptions at Ilopango caldera, many aspects regarding the longevity and the prevailing physicochemical conditions of the underlying magmatic system remain unknown. Zircon 238U‐230Th geochronology of the TB suite (TBJ, TB2, TB3, and TB4) reveals a continuous and overlapping crystallization history among individual eruptions, suggesting persistent melt presence in thermally and compositionally distinct magma reservoirs over the last ca. 80 kyr. The longevity of zircon is in contrast to previously determined crystallization timescales of <10 kyr for major mineral phases in TBJ. This dichotomy is explained by a process of rhyolitic melt segregation from a crystal‐rich refractory residue that incorporates zircon, whereas a new generation of major mineral phases crystallized shortly before eruption. Ti‐in‐zircon temperatures and amphibole geothermobarometry suggest that rhyolitic melt was extracted from different storage zones of the magma reservoir as indicated by distinct but synchronous thermochemical zircon histories among the TB suite eruptions. Zircon from TBJ and TB2 suggests magma differentiation within deeper and hotter parts of the reservoir, whereas zircon from TB3 and TB4 instead hints at crystallization in comparatively shallower and cooler domains. The assembly of the voluminous TBJ magma reservoir was also likely enhanced by cannibalization of hydrothermally altered components as suggested by low‐δ18O values in zircon (+4.5 ± 0.3‰).

Active deformation in the San Salvador extensional stepover, El Salvador from an analysis of the April–May 2017 earthquake sequence and GPS data

Modern and historic seismic observations indicate that earthquake swarms, often with similar characteristics, have occurred within a few tens of kilometers of the San Salvador volcano, which poses a major active hazard to the San Salvador metropolitan area. Here, we evaluate whether the April–May 2017 earthquake sequence near the San Salvador volcanic complex was tectonic or volcanic and more broadly consider the implications of up-to-date GPS observations and seismic data for earthquake and volcano hazards in the San Salvador metropolitan area. Based on magnitudes calculated for 532 earthquakes in the April–May sequence, we report a Gutenberg-Richter law b-value of 0.95 ± 0.12, consistent with b values for tectonic earthquakes, and based on temporal distribution of aftershocks we report an Omori law p-value of 1.5 ± 0.1. Focal mechanisms estimated for the mainshock and 14 largest foreshocks and aftershocks are all strike-slip with NNE-striking, left-lateral-slip and ESE-striking right-lateral-slip nodal planes. The best located foreshocks and aftershocks unequivocally indicate that the earthquake sequence accommodated left-lateral slip along a NNE-striking fault, compatible with the local tectonic setting. An absence of seismic and GPS evidence for any unrest of the San Salvador volcano during the years after the 2017 earthquake sequence is consistent with its tectonic origin. Continuous GPS measurements at a site 5 km south of the volcano reveal a previously unknown inflationary episode from mid-2010 to mid-2012, but no evidence for volcanic unrest in 2017 or later. Updated GPS velocities indicate that ~4–5 mm yr−1 of E-W-dominated stretching occurs between the San Salvador volcano and Ilopango caldera. That the 2017 earthquakes and Mw = 5.7 1986 San Salvador earthquake both accommodated left-lateral strike-slip along NNE-striking faults suggests that the GPS-measured E-W stretching is accommodated by bookshelf faulting between blocks that rotate clockwise in response to 10 ± 1.4 mm yr−1 (95%) of dextral motion across the volcanic arc at the location of San Salvador.

Relation between surface dynamics and remote sensor InSAR results over the Metropolitan Area of San Salvador

InSAR information to record dynamic changes on the surface using remote sensor has been obtained for the Metropolitan Area of San Salvador and compared with surface dynamic situation and fixed GPS surface movement information. The city is located in an active tectonic depression between the tectonic remains of ancient volcanoes and active volcanic centers. This area has been historically affected by earthquakes, volcanic activity, erosion, mass movements, and flooding. The morphology of the area has been changed by human activities, most of the time without any respect to natural processes thus increasing the problems. There are sectors that show subsidence close to Ilopango Caldera that could be connected to diastrophic block tectonic movements connected to post-seismic episodes after 2012 earthquake, and aquifer level changes due effects of climate change and overexploitation of aquifers. These movements may increase the huge problems of erosion and mass wasting through las Canas basin. In addition, after 2017 seismic swarm a horizontal movement was recorded with the satellite radar interferometry (InSAR) in two areas, which reflects the possible movement of faults that crosses the area of study. This type of information provides important data for the decision-making of land, protection and mitigation zones, monitoring of governmental institutions, and for regional planning.

Water and dissolved gas geochemistry at Coatepeque, Ilopango and Chanmico volcanic lakes (El Salvador, Central America)

Coatepeque (on the E slope of Santa Ana Volcano), Ilopango (inside Ilopango Caldera) and Chanmico (associated with San Salvador Volcano) are volcanic lakes in El Salvador, showing a thermocline at a relatively shallow depth (from 30 to 40 m, from 20 to 40 m and from 5 to 15 m depth, respectively) and anoxic conditions below 33, 24 and 4 m depth, respectively. The Na+–Cl− composition of the Coatepeque and Ilopango lakes, displaying TDS values up to 1226 and 1216 mg/L, respectively, is likely due to hydrothermal fluids that feed these two lakes, as also confirmed by Cl−/Br− molar ratios ≤ 650, high As, B, Li and Si contents and Cl−/SO42− ratio > 1. The Mg2+–HCO3− water composition of Lake Chanmico, whose TDS values were between 566 and 856 mg/L, suggests water-rock interaction processes with mafic/ultramafic rocks variably affected by serpentinization processes, which produced high Mg2+, Si and B concentrations. Waters at depth were characterized by the presence of CO2 from an extra-lacustrine source, as suggested by δ13C-CO2 values significantly less negative than those typically related to biogenic processes, albeit this gas was found in smaller quantities when compared to those recorded in other meromictic lakes hosted in quiescent volcanic systems (e.g. Lake Kivu in DRC, Monticchio, Albano and Averno lakes in Italy, Hule and Rio Cuarto lakes in Costa Rica, Lake Pavin in France). The occurrence of CH4, whose concentrations in Lake Chanmico were up to two orders of magnitude higher than those recorded in Coatepeque and Ilopango lakes, suggests bacterial methanogenesis.

The Ilopango Tierra Blanca Joven (TBJ) eruption, El Salvador: Volcano-stratigraphy and physical characterization of the major Holocene event of Central America

The Ilopango caldera is the source of the large Tierra Blanca Joven (TBJ) eruption that occurred about 1.5 ka years ago, between ca. AD270 and AD535. The eruption dispersed volcanic ash over much of the present territory of El Salvador, and pyroclastic density currents (PDCs) extended 40 km from the volcano. In this study, we document the physical characteristics of the deposits from all over El Salvador to further constrain the eruption processes and the intensity and magnitude of the different phases of the eruption. The succession of deposits generated by the TBJ eruption is made of 8 units. The eruption started with PDCs of hydromagmatic origin (Unit A0), followed by fallout deposits (Units A and B) that are <15 cm thick and exposed in sections close to the Ilopango caldera (within 10–15 km). The eruption, then, transitioned into a regime that generated further PDCs (Units C–F), these range from dilute to dense and they filled the depressions near the Ilopango caldera with thicknesses up to 70 m. Deposits from the co-ignimbrite plume (Unit G) are the most widespread, the deposits are found in Guatemala, Honduras, Nicaragua, Costa Rica and the Pacific Ocean and cm-thick across El Salvador. Modelling of the deposits suggests that column heights were 29 km and 7 km for the first two fallout phases, and that the co-ignimbrite phoenix plume rose up to 49 km. Volumes estimated for the fallout units are 0.15, 0.8 and 16 km3 dense rock equivalent (DRE) for Unit A, B and G respectively. The PDCs deposits volumes were estimated to be ~0.5, ~3.3, ~0.3 and ~9.1 km3 DRE for Units C, D, E and F, respectively. The combined volume of TBJ deposits is ~30 km3 DRE (~58 km3 bulk rock), indicating that it was one of largest Holocene eruptions from Central America. This eruption occurred while Mayan populations were living in the region and it would have had a significant impact on the areas within tens of kilometres of the vent for many years to decades after the eruption.

Predicting the landslides triggered by the 2009 96E/Ida tropical storms in the Ilopango caldera area (El Salvador, CA): optimizing MARS-based model building and validation strategies

The main topic of this research was to evaluate the effect in the performance of stochastic landslide susceptibility models, produced by differences between the triggering events of the calibration and validation datasets. In the Caldera Ilopango area (El Salvador), MARS (multivariate adaptive regression splines)-based susceptibility modeling was applied using a set of physical–environmental predictors and two remotely recognized landslide inventories: one dated at 2003 (1503 landslides), which was the result of a normal rainfall season, and one which was produced by the combined effect of the Ida hurricane and the 96E tropical depression in 2009 (2237 landslides). Both the event inventories included shallow debris—flow or slide landslides, which involved the weathered mantle of the pyroclastic rocks that largely outcrop in the study area. To this aim, different model building and validation strategies were applied (self-validation, forward and backward chrono-validations), and their performances evaluated both through cutoff-dependent and -independent metrics. All of the tested models produced largely acceptable AUC (area under curve) values, albeit a loss in the predictive performance from self-validation to chrono-validation was observed. Besides, in terms of positive/negative predictions, some critical differences arose: using the 2009 extreme landslide inventory for calibration resulted in higher sensitivity but lower specificity; conversely, using the 2003 normal trigger landslide calibration inventory led to higher specificity but lower sensitivity, with relevant increase in type-II errors. These results suggest the need for investigating the extent of such effects, taking multitrigger intensities inventories as a standard procedure for susceptibility assessment in areas where extreme events potentially occur.

The Ilopango caldera complex, El Salvador: Stratigraphic revision of the complete eruptive sequence and recurrence of large explosive eruptions

Ilopango caldera erupted episodically at least 13 tuff-forming eruptions with a minimum estimate volume of 1–5 km3 DRE per eruption, reaching up to 150 km3 DRE for the first caldera-forming eruption. All tuffs are of dacitic-rhyolitic composition. The complete pyroclastic sequence spans a range in time from 1.785 to 0.0015 Ma, and based on stratigraphy and geochronology constraints can be divided into three formations: the Comalapa, Altavista and Tierras Blancas formations. In this work, we focus on the members of the newly described Altavista Formation (middle part of Ilopango caldera volcanic sequence), which consist of six consolidated pyroclastic deposits or tuffs. Each tuff corresponds to a specific eruption followed by a period of quiescence during which soil beds were developed on the deposits. The ages of the Altavista Formation ranges from 918 to 257 ka, based on new 40Ar/39Ar, U/Pb-zircon, and U/Th-zircon analyses. The tuffs of this formation show similar characteristics in mineralogy and composition. They are calcalkaline, rhyodacitic tuffs, with plagioclase, clinopyroxene, and hornblende. From field mapping and descriptions of the deposits, we have inferred the eruptive styles that include pumice fallouts, pyroclastic density currents and also hydromagmatic explosions. The common vent in all tuffs was the Ilopango caldera and each member of the Altavista Formation could correspond to a caldera collapse event, except for one of the six eruptions. The volume of each member was estimated to be >30 km3 DRE, which is the same order of magnitude than that estimated for the Tierra Blanca Joven (TBJ) eruption at about 1,500 B. P, and smaller than those of the ignimbrites of the Comalapa Formation, the first three members of the Ilopango caldera reported previously. The tuffs of the Altavista Formation are visible up to 15–20 km away from the caldera's topographic margin. The recurrence interval of large explosive events at the Ilopango caldera was established by integrating the stratigraphic and geochronologic data of all 13 ignimbrites and pumice fallouts erupted from Ilopango caldera since the first one at 1.78 Ma to the last explosive event (TBJ).

The Ilopango caldera complex, El Salvador: Origin and early ignimbrite-forming eruptions of a graben/pull-apart caldera structure

The Ilopango caldera is located in the central part of El Salvador, within the right-lateral El Salvador Fault System (ESFZ) and adjacent to the capital city of San Salvador. The caldera has a polygonal shape of 17 × 13 km and hosts an intra-caldera lake. Ilopango caldera had multiple collapse eruptions that formed widespread and voluminous silicic ignimbrites. Volcanic activity of the caldera has been controlled by strike-slip faults of the ESFZ. In this work we present the geological characteristics of the first three ignimbrite-forming eruptions of Ilopango caldera, providing an interpretation of the origin and initial stages of the volcanic evolution of this caldera complex. An initial extensional regime of the ESFZ possibly developed a graben at or near the actual Ilopango caldera, where the graben's master faults worked as fissure vents during the first caldera collapse. The Olocuilta Ignimbrite was emplaced at 1.785 ± 0.01 Ma BP, with a Dense Rock Equivalent (DRE) volume > 50 km3 (probably ~300 km3). The ESFZ stress gradually changed from extensive to transtensive, inducing the second collapse associated with a pull-apart caldera, producing the Colima Ignimbrite at 1.56 ± 0.01 Ma BP, with a DRE volume of >11 km3. The transtensive regime increased along the ESFZ, producing the third collapse in the pull-apart graben caldera apparently affected by the newly formed strike-slip San Vicente Fault. This phase corresponds to the explosive eruption that formed the Apopa Ignimbrite at ~1.34 Ma BP, with >9 km3 DRE volume. The latter ignimbrite marks a change in the eruptive style producing hydromagmatic pyroclastic flows followed by a dense ignimbrite with coignimbrite lithic breccias. These features suggest the involvement of water that could come from a paleoIlopango lake within the caldera depression associated with the second caldera collapse at 1.56 Ma BP. Ilopango is thus a multistage caldera system associated with the largest explosive events registered in El Salvador so far.

Magma storage in a strike-slip caldera.

Silicic calderas form during explosive volcanic eruptions when magma withdrawal triggers collapse along bounding faults. The nature of specific interactions between magmatism and tectonism in caldera-forming systems is, however, unclear. Regional stress patterns may control the location and geometry of magma reservoirs, which in turn may control the spatial and temporal development of faults. Here we provide new insight into strike-slip volcano-tectonic relations by analysing Bouguer gravity data from Ilopango caldera, El Salvador, which has a long history of catastrophic explosive eruptions. The observed low gravity beneath the caldera is aligned along the principal horizontal stress orientations of the El Salvador Fault Zone. Data inversion shows that the causative low-density structure extends to ca. 6 km depth, which we interpret as a shallow plumbing system comprising a fractured hydrothermal reservoir overlying a magmatic reservoir with vol% exsolved vapour. Fault-controlled localization of magma constrains potential vent locations for future eruptions.

Neotectonic deformation within an extensional stepover in El Salvador magmatic arc, Central America: Implication for the interaction of arc magmatism and deformation

Dominantly westward movement of the El Salvador forearc at rates of 11mm/yr is accommodated by a series of E-W to WNW oriented, dextral, strike-slip fault zones herein referred to as the El Salvador Fault System (ESFS). The geometry of the ESFS defines a series of extensional step-overs. Along the arc, basaltic volcanism in the stepovers is associated with NNW-oriented normal faults, whereas rhyolitic volcanism is associated with strike-slip fault zones of the ESFS.On the ESFS, the San Salvador Extensional Stepover (SSES) is bound to the south by the San Vicente fault zone, where the rhyolitic Ilopango caldera is located. In the SSES, tephras from Ilopango –the Tierra Blanca (TB) sequence– track long-term elongation. Older TB units (TB5–8) contain abundant normal faults; lying unconformably above these older TB units, younger TB members (TBJ, TB2–4) are generally unfaulted. Analyses of faults in TB5–8 indicate NE- to ENE-oriented elongation in the SSES. Deformation occurred between deposition of the TB4 and TB5 units, during quiescence of the Ilopango eruptive center. Using this temporal constraint, minimum elongation rates of 3.50×10−15s−1, 2.06×10−14s−1 and 4.42×10−14s−1 were calculated for three traverses. From regional geodetic data and fault kinematics throughout El Salvador, we interpret the SSES as part of a series of pull-apart structures along the arc axis. The calculated paleostress orientations are consistent with a pull-apart geometry resulting from forearc movement.The extensional deformation occurs during a ~50k.y. lull in rhyolitic activity, suggesting an interplay between magmatism and deformation within the arc. During significant rhyolitic volcanic activity, only minor elongation is observed in the SSES, despite ongoing translation of the Salvadoran forearc. We speculate that rhyolitic magmatism along upper crustal faults may facilitate strike-slip movement on the ESFS, rather than distributing deformation throughout the extensional stepover.

Monitoring and behavior of unsaturated volcanic pyroclastic in the Metropolitan Area of San Salvador, El Salvador.

Field monitoring and laboratory results are presented for an unsaturated volcanic pyroclastic. The pyroclastic belongs to the latest plinian eruption of the Ilopango Caldera in the Metropolitan Area of San Salvador, and is constantly affected by intense erosion, collapse, slab failure, sand/silt/debris flowslide and debris avalanche during the rainy season or earthquakes. Being the flowslides more common but with smaller volume. During the research, preliminary results of rain threshold were obtained of flowslides, this was recorded with the TMS3 (a moisture sensor device using time domain transmission) installed in some slopes. TMS3 has been used before in biology, ecology and soil sciences, and for the first time was used for engineering geology in this research. This device uses electromagnetic waves to obtain moisture content of the soil and a calibration curve is necessary. With the behavior observed during this project is possible to conclude that not only climatic factors as rain quantity, temperature and evaporation are important into landslide susceptibility but also information of suction–moisture content, seepage, topography, weathering, ground deformation, vibrations, cracks, vegetation/roots and the presence of crust covering the surface are necessary to research in each site. Results of the field monitoring indicates that the presence of biological soil crusts a complex mosaic of soil, green algae, lichens, mosses, micro-fungi, cyanobacteria and other bacteria covering the slopes surface can protect somehow the steep slopes reducing the runoff process and mass wasting processes. The results obtained during the assessment will help explaining the mass wasting problems occurring in some pyroclastic soils and its possible use in mitigation works and early warning system.

The influence of gigantic eruption from Ilopango Caldera on mangrove forest in Jiquilisco Bay, El Salvador, Central America

The influence of gigantic eruption from Ilopango Caldera on mangrove forest in Jiquilisco Bay, El Salvador, Central America

Stratigraphic relationship between microlandform in the southern coastal plain of El Salvador, Central America, and the TBJ tephra erupted from Ilopango Caldera in the 3rd to 6th century

Stratigraphic relationship between microlandform in the southern coastal plain of El Salvador, Central America, and the TBJ tephra erupted from Ilopango Caldera in the 3rd to 6th century

Unsaturated Volcanic Tephra and Its Effect for Soil Movement in El Salvador

In El Salvador during the rainy season or when major earthquakes affect the area, processes like mass move-ments, liquefaction, erosion and floods affect the territory leaving environmental, social and economic damages. The vol-canic tephras called "Tierra Blanca Joven" (TBJ) which is a product of the last plinian eruption of the Ilopango Caldera are the more problematic region in the Metropolitan Area of San Salvador. In El Salvador during the summer and rainy season the groundwater level remains low in most of the areas, this means that the soils in the country are unsaturated and there are capillary forces that act on the soil structure making that an “apparent cohesion” (suction) improve the strength of the soil; this circumstance makes that the slopes are almost vertical and temporally stable, but will collapse when wetted or during seismic events. To that problematic is possible to add that the majority of the water pipelines are in bad condition and when they fail is possible to expect the collapse of entire streets or houses. Wetting of the soil is likely to occur following construction development, because evaporation is reduced by covering the ground surface with a structure. For unsaturated soils the use of two independent stress variables: net stress and suction are needed. Suction values of TBJ “G” unit of the proximal facie were obtained using pressure plate, tensiometer and filter paper to obtain a Soil-Water Retention Curve (SWRC); the use of this curve into finite elements software can help to obtain the shear strength and permeability properties. For monitoring, the use of Quickdraw tensiometers (suction) and TMS3 (soil moisture content) is proposed, because it could be a way to build a field methodology for early warning system for the slopes for urban planning and risk assessment.

THE IMPACT OF THE CLIMATE CATASTROPHE OF 536–537 AD IN ESTONIA AND NEIGHBOURING AREAS

Introduction In 536-541 AD a short-term and sudden cooling took place in the northern hemisphere which has caught the attention of researchers only quite recently. In 1983, Richard Stothers and Michael Rampino published a list of volcanic eruptions prior to 630 AD known from historical sources (Stothers & Rampino 1983). Their list included a veil of dust or dry fog that darkened the sky for almost a year in 536-537 AD and caused crop failure. Dendrochronologist Mike Baillie found physical evidence of the event studying the tree rings of Irish oak (Baillie 1994). During the last decades, numerous publications (e.g. Randsborg 1997; Axboe 1999; 2001a; 2001b; Baillie 1999; Keys 1999; Gunn 2000; Hoilund Nielsen 2006; Graslund 2008; Graslund & Price 2012; Arrhenius 2013) have discussed the historical significance and impact of the 536-537 event as well as its archaeological manifestations and written sources. The emergence of this new research topic is due to recent advancements in climate reconstructions based on natural science. Having access to much higher-resolution climate records makes it possible to discuss the demographic, economic, and cultural impacts of climate change more precisely (Widgren 2012, 126). The event in question appears clearly in the growth rings of trees in the northern hemisphere, namely in the common oak (Quercus robur) and families of pine (Pinus). Tree rings show abnormally little growth in 536 and the following years. A similar pattern has been found in tree rings from 540 in the southern hemisphere, for example in southern Chile and Argentina (Baillie 1999; 2007; Gunn 2000; Jones 2000; Young 2000 and citations therein). Tree rings of the northern hemisphere show that growth was hampered in two periods. After recovery a new, even sharper drop emerged in 540-541 (D'Arrigo et al. 2001, 240). According to tree rings, extraordinarily cold weather continued in the northern hemisphere until the year 545 (Graslund & Price 2012, 430 and citations therein). Traces of the event can be found in ice cores from Greenland and Antarctica. The earliest studies referred to the high sulphuric acid content of ice deposits in Greenland from around 540 which indicate the volcanic origin of the event (see Stothers & Rampino 1983; Stothers 1999). Later researchers have also found evidence of substantial sulphate deposits in ice layers from Greenland and Antarctica, supporting the notion of volcanic dust (e.g. Traufetter et al. 2004; Larsen et al. 2008; Ferris et al. 2011). Most scientists who have studied the causes of the event of 536 have concluded that it was caused by an immense volcanic eruption in the tropical zone of Earth (see Stothers & Rampino 1983; Stothers 1999; Larsen et al. 2008). Several volcanoes and places have been proposed (see Stothers 1984; Keys 1999; Wohletz 2000). The most convincing evidence so far refers to the Tierra Blanca Joven eruption of the Ilopango caldera in central El Salvador (Dull et al. 2001; 2010; Oppenheimer 2011, 254 ff.). Others believe that a comet or a meteorite explosion caused the event (Baillie 1999; 2007; Rigby et al. 2004). Magnetite and silicate spherules found from the ice layers of 536-537 in Greenland support this alternative explanation (Abbott et al. 2008). Similar sphelures have been found in northern Australia from a supposed metorite crater in the Gulf of Carpentaria (Abbott et al. 2008; Subt et al. 2010). Thus, natural scientists have not agreed on what caused the climate anomaly of 536-537. Nevertheless, according to tree growth rings it was the worst shock to the ecosystem within the last 2000 years (Baillie 2007, 106). Antti Arjava (2006) has studied written evidence from Mediterranean sources of the extraordinary event of 536-537. In several of these sources it appears that a darkening of the sun was observable in the Mediterranean region during more than a year. Bishop Michael the Syrian writes in his 12th century chronicle, quoting the 6th century ecclesiastical historian John of Ephesos: Each day it shone for about four hours, and still this light was only a feeble shadow. …

Volcanic history of the Conchagua Peninsula (eastern El Salvador)

New results of detailed geological mapping, K–Ar dating and geochemical study of the Conchagua Peninsula in eastern El Salvador are presented. Volcanism in the area was controlled by intersection of three tectonic structures, the trenchparallel Central Graben, perpendicular Comayagua Graben, and the Guayape Fault Zone. The age of the volcanic activity spans from Miocene to Quaternary, however, the volcano itself is extinct. The basement is built of the welded rhyolitic Playitas ignimbrite, which extends as far as to the Island of Zacatillo. The pyroclastic rocks of La Union unit (mean K–Ar age: 13.3 ± 3.7 Ma) display signs of mingling between basaltic and dacitic magmas (banded pumice, deposits containing both mafic scoria and felsic pumice fragments), and this is interpreted as a result of eruptions triggered by injection of a basaltic magma into a dacitic magma chamber. Lavas and pyroclastic flow deposits of the subsequent Pozo unit are poorly exposed and strongly altered. Following effusive activity hereby defined as Pilon Lavas was dominated by andesite and basaltic andesite lavas. Pleistocene volcanic activity is represented by the Pre-Conchagua edifice (1.6 ± 0.6 Ma), Cerro Montoso, El Bable and Juana-Pancha. Regarding the trace-element composition, some lavas of the PreConchagua – Juana-Pancha are distinct from common volcanic front products (lower Zr/Nb, Th/Nb, Ba/Nb), resembling the lavas of Tegucigalpa volcanic field to the north, which is located behind the volcanic arc in the Comayagua Graben. Behind-arc extensional tectonics could have facilitated the magma genesis via decompression melting of the mantle wedge. The current Conchagua Volcano consists of two cones, Ocotal and Banderas, built by repeated Strombolian eruptions associated with effusions of basaltic lavas. The uppermost unit consists of a white tuff preserved in the sedimentary fill of several tectonic valleys west of Conchagua. The white tuff was interpreted as distal fall-out of the Tierra Blanca Joven eruption of the Ilopango Caldera.