Richard E. Stoiber

Quaternary volcanic and tectonic segmentation of Central America

Central America is divided into seven segments based on differences in the strikes and positions of volcanic lineaments in the historically active volcanic chain. This block-like pattern is also seen in the contrasting volcano morphology, recent fault patterns and the distribution of shallow earthquakes. Discontinuities in the deep seismic zone can be identified at some segment margins.The boundary areas between the segments are characterized by faulting transverse to the volcanic lineaments, frequently accompanied by basalt cinder cone fields, by concentrations of shallow earthquakes and by catastrophic historic eruptions. This volcanological, structural and seismological data can be explained by a segmented converging plate margin. At shallow depths the underthrust lithosphere is broken along tear faults into 100 to 300 km long segments. The separate segments descend into the mantle with different strikes and dips and give the geology its segmented character. Evidence for similar segmented converging plate margins is found in Mexico and in other parts of the circumpacific and alpine belts.

Sulfur Dioxide Contributions to the Atmosphere by Volcanoes

The first extensive measurements by remote-sensing correlation spectrometry of the sulfur dioxide emitted by volcanic plumes indicate that on the order of 103 metric tons of sulfur dioxide gas enter the atmosphere daily from Central American volcanoes. Extrapolation gives a minimum estimate of the annual amount of sulfur dioxide emitted from the worlds volcanoes of about 107 metric tons.

Annual contribution of sulfur dioxide to the atmosphere by volcanoes

Abstract The average contribution of SO2 to the atmosphere by volcanoes, estimated largely by extrapolation from direct measurements of volcanic SO2, is calculated to be 18.7 Tg per year. Of this, an estimated 6.8 Tg per year (36%) is from non-erupting degassing volcanoes. Erupting volcanoes yield an average annual flux estimated to be 11.9 Tg SO2 per year (64%). The estimated total annual SO2 output is about 23% larger than the most recent estimate based on direct measurements. Our calculations suggest that 64% of the total comes from erupting volcanoes, as compared to 7% in the earlier estimate. Of the total worldwide annual SO2 flux, 9% is found to be from volcanoes.

Fumarole incrustations at active central american volcanoes

In 11 yr of sampling at 14 volcanoes in Guatemala, El Salvador, Nicaragua and Costa Rica, we have identified 47 minerals in incrustations depositing at approximately 100 different high temperature fumaroles. Most of these minerals are sulfates. The most abundant and most frequently found minerals are: sulfur, hematite, halite, sylvite, gypsum, ralstonite, anhydrite, thenardite and langbeinite. Incrustation suites deposit around fumaroles to produce a zonal pattern which is a response to the rapidly changing temperature and oxygen pressure at the mouth of the vent. The observed zoning pattern can be explained by the reaction of a volcanic gas composed of H2O, SO2, CO2, HC1 and HF, along with trace amounts of volatile cations, which interacts with the atmosphere and the fumarole wallrock. This interaction is aided at lower temperatures by the formation of sulfuric acid. The mineralogies and descriptions of incrustations at fumaroles at a large number of other volcanoes from every part of the world are similar to what we have found in Central America. Thus we believe our conclusions have general applicability.

Studies of volcanic ash from two recent Central American eruptions

The 1971 eruptions of Cerro Negro volcano in Nicaragua and Fuego volcano in Guatemala produced ash blankets with minimum volumes of 7 × 107 m3 and 6 × 107 m3, respectively. Seven new chemical analyses show that ash produced by both eruptions was basaltic and similar to previously — erupted lavas of both volcanoes. Both ash blankets were sampled areally and stratigraphically before they were affected by rain. Chemical analyses of soluble materials leached from these ashes show that near the end of both eruptions the concentrations of soluble materials increased by about an order of magnitude. These changes are believed to reflect changes in the composition of eruptive gases.

The Geochemistry of Central American Volcanic Gas Condensates

Results of analyses for Cl, F, SO 4 , Ca, Mg, Na, and K in 132 condensates from fumaroles of the Central American volcanoes Santiaguito, Fuego, Pacaya, Izalco, Cerro Negro, and Arenal are given. Each fumarole is of high temperature, greater than 150°C, and is located within 500 m of an active volcanic vent. Condensates were collected over a 5-yr period, August 1964-July 1969. The variation in chemistry of the condensates is shown to depend upon the fumarole location and the time of collection relative to the eruptive and cooling history of the volcano. Ground-water dilution also affects concentration because it is variable and cumulative. The data show that Cl and F in condensates decrease as the magma source is depleted and as the ground-water component in the fumarole increases. F is believed to be depleted by reaction with wallrock, as well. The alkalis, Na and K, are shown to come in part from the wallrock, and probably in part from the magma. Wallrock is suspected to be the chief source of Ca and Mg, as the Ca/Mg ratio in condensates is that of the wallrock. Fumaroles at main active vents are shown to be SO 4 rich, with comparatively low CI/SO 4 ratios. Fumaroles on flows are Cl-rich. The ratio CI/SO 4 decreases at fumaroles in or near a main vent at the beginning of an eruptive period. Regular monitoring of the condensate at such fumaroles should enable prediction of eruptive periods months in advance, although the exact timing would vary with location. The practicality of the method depends upon the existence of accessible fumaroles.

Soluble Material on Ash from Active Central American Volcanoes

Eruptions of six active volcanoes in Central America provided 57 volcanic ash samples in the past 7 yrs which were unaffected by rain. The ash samples were leached with water which was analyzed for Cl, F, SO4, Na, Ca, Mg, K, Mn, Zn, and Cu, all of which were usually present. The analytical data were examined in terms of time of eruption, eruptive cycle, and physical character of the vent. It was found that the leachate material came primarily from the surface of ash particles, consisted of chemicals commonly found in volcanic gases and sublimates, and was more abundant in ash from a deep vent. It was concluded that much of the soluble material was deposited on the ash surface from volcanic gas during eruption. Isopach mapping indicated that the total amount of ash falling within the 2.5-mm-thickness contour during the 48-day eruption of Cerro Negro in 1968 was 17 million tons; more than 21,000 tons of soluble material were contained in this ash. If projected to the world scale and over geologic time, the soluble material on volcanic ash appears to account for the Cl and SO4 which Rubey (1951) found to be in excess in the oceans.

Recent volcanic and fumarolic activity at Santiaguito volcano, Guatemala

Santiaguito, an extrusive domal complex in southwestern Guatemala has recently exhibited renewed domal growth, as much as 50 meters vertically in the first twelve months of growth. Synchronous with this activity, near the other end of the complex 1.3 km distant, there are pyroclastic eruptions. Ninety percent of the extensive hot lumarolic emission at Santiaguito is concentrated near the active pyroclastic vent, although numerous other lumaroles are found including a group of particularly hot vents associated with the current dome extrusion. Description and tabulation of chemical and mineralogical characteristics of sublimates and condensed gas from fumaroles at eight localities on the mountain illustrate the essential similarity and also the minor differences. The differences are in many instances related to volume of gas, the amount of rainwater dilution, and amount of oxidation of the gases along their path.

Lahars initiated by the 13 November 1985 eruption of Nevado del Ruiz, Colombia

The eruption of Nevado del Ruiz in Colombia on 13 November 1985 was accompanied by the formation of four lahars (mud flows) triggered by the melting of glacial ice near the summit of the volcano. The lahars began as flows of water, sand and gravel, but they incorporated clay by eroding the soil along the steep valleys through which they passed. The largest flow was a cohesive debris flow, more than 45 m deep and moving at ∼12 ms−1 when it debouched from the canyon of the Rio Lagunilla 2.5 km from Armero, where it killed 25,000 people. The continuing volcanic activity and abundant remaining glacial ice create an extremely high risk of future destructive flows.

Organic compounds in volcanic gas from Santiaguito Volcano, Guatemala

Gas samples collected at Sapper fumarole, Santiaguito, Guatemala, on December 5, 1969, were analyzed by gas chromatography-mass spectrometry. A number of compounds were found, including saturated and unsaturated hydrocarbons, aldehydes, ketones, alcohols, aromatics, halogenated hydrocarbons, and inorganic sulfur compounds. The compounds are probably produced by heating of fossil soil or sedimentary layers by the magma.