Preliminary map of the volcanic hazards of Fuego Volcano. A new report is available: J. W. Vallance, S. P. Schilling, O. Matías, W. I. Rose, and M. M. Howell, 2001, Volcano Hazards at Fuego and Acatenango, Guatemala USGS Open File Report 01-431

I. Ashfall

This hazard is significant in view of the high (5-15 km) columns which are typical of Fuego eruptions. The ashfalls can be spread distances of well over 100 km (Rose et al., 1973; 1978). The most notable problems that result from ashfall are destruction of crops and damage to roofs from excessive weight. Contamination of drinking-water supplies is a possible problem for Fuego, which has had significant soluble F on some of its modern tephra. Ballistic fall of blocks sometimes has occurred at distances of as much as 8 km from the summit. We have not heard of burning of roofs by Fuego's ashes, but we have found charcoal in some fall deposits.

Fallout is strongly influenced by prevailing winds. Although both the 1971 and 1974 fallout was to the West, and upper level winds blow consistently westward during the rainy season (May-October), we know that some eruptions will carry ash in other directions. In Fuego's 1932 eruption significant ashfall occurred in Guatemala City, and Radiosonde data from Guatemala City demonstrate that upper level winds frequently blow to the East and NE during the dry season.

The record of fall deposits around Fuego suggest that ash is carried Westward more frequently. Yepocapa has received fallout from Fuego especially frequently, but fall deposits were encountered East of the volcano also.

Ash fall at Yepocapa, about 8.5 km WNW of Fuego's summit, following the 1974 eruption of Fuego. Many roofs collapsed from the weight of basaltic ash. Photo by Sam Bonis.

The possibility of ash fallout in Antigua Guatemala, Escuintla and even Guatemala City must be considered. Ashfall in cities is a special problem because the continual distrubances associated with human activity and remobal of the ash cause it to be dispersed into the air. Respiratory problems and general dustiness result, especially for fine ash. Only a miniscule fallout can be associated with enough ash in the air to affect aircraft, especially jets. In Anchorage, Alaska in March 1986, the airport was closed for three days due to minor ash fallout from Augustine Volcano, 300 km away. Complete obscuring of sunlight, and strong electrical fields which prevent radio communications are additional problems which should be anticipated in areas up to 30 km downwind from Fuego during eruption.

Preliminary map of potential ashfall zones, Fuego Volcano.

II. Potential Extreme Hazard from Falling Bombs, Blocks, Avalanches, Lava Flows, Glowing Avalanches, and Mudflows

This is a zone surrounding the summit for a radius of about 3.5 km. It includes mostly places with approximately 30° slopes. It extends down farther along the principal barrancas. This area is dangerous even when the volcano is inactive, but during activity it is subject to extremely grave hazards to all living things. The area is uninhabited.

Detail of hazard map of Fuego, showing the zone of extreme hazard due to falling bombs, blocks, avalanches, lava flows, and glowing avalanches. It includes areas within 3 km of the summit, on steep slopes and within barrancas that have channeled recent avalanches and block and ash flows. Photo by Rose et al., 1987.

West flank of Fuego following the 1974 eruption. Note the debris on Fuego's slopes and the barrancas which channel the pyroclastic flows and avalanches.

III. Potential Extreme Hazard from Glowing Avalanches, Avalanches, and Mudflows

This zone occurs along principal barrancas where the slope breaks to less than 25°. Often the profile of the barranca widens and the block and ashflow materials deposit in a fan. In some barrancas, like the Río Ceniza, the channel remains constricted and the block and ashflow deposits fill the deep channel. The elevation of this zone varies, it reaches elevations as low as 800 m along the Río Ceniza, but only to 1,500 m at Quebrada San Jose. This zone appears uninhabited, except for daytime gathering and grazing.

Pyroclastic flow descending Fuego's west flank, 14 October 1974. Photo by W.C. Buell.

IV. Potential Hazard from Mudflows, Debris Flows, and Floods.

This zone occurs downslope from III, along the same barrancas. The slope is often about 10° or less, and the lower range of elevation is variable, probably depending on the amount of material deposited in the channels upstream. The mudflows do not necessarily occur at the same time as the eruption. If the eruption occurs in the dry season, mudflows may not begin until heavy rains ensue. In the period following an eruption, the amount of block and ashflow material in a channel will indicate the magnitude of hazard down the valley. Aerial photography is suggested as a way to measure the degree of mudflow hazard in each valley. This hazard may continue for a year or more after eruptions, longer for very large eruptions.

Fuego (right) and Acatenango (left) as seen from the highway along the coastal plain SSW of the volcano. The foreground is volcanic alluvial fan material derived from floods and lahars from Fuego. Photo by Bill Rose, 1978.

There are large areas of laharic fans around Fuego which have not received mudflows from the most recent activity. The deposits have well-developed soils and vegetation cover. The likelihood of hazard in these areas is less than the others, but we have included them as hazard zones because only slight changes inteh pattern of pyroclastic flows could affect these areas. This is most notable in the area North of Siquinala where there is a complex fan of river valleys. It is difficult to know exactly where laharic deposition wil occur next in that area.

Steaming hot mudflow materials in Barranca Honda, east of Fuego's summit. Photo by Sam Bonis, October 1974.

V. Potential Hazard from Floods in Rivers Affected by III and IV

This includes the floodplains of rivers and the rivers they flow into. Bridges crossing these rivers are subject to wash out. The period of more frequent flooding may lat for 10 years or more following an eruption, but will eventually decline again. In some places areas may be flooded beyond floodplains and terraces, especially where one river dams another at their confluence (El Palmar Effect). Affected rivers should be periodically examined after eruptions to anticipate such problems before it is too late.

(Rose et al., ????)

Hazards due to Flooding

The meteorology of the area, thoroughly discussed in Flood Control Project No. 69, indicates some specific potential damage areas/trouble spots. These apply to the Achiguate and Pantaleon river basins.

I. Achiguate River
*Loss of bridges of the vital road and the railway affected by floods.
*Inundation along the Achiguate River main course for the whole stretch from the river mouth to the bridge point due to the limited flow capacity attributable to the rising of riverbeds caused by sedimentation.
*Inundation along the other river situated close to Achiguate River caused by the overtopping flows from the river at the section between 28.0 km and 30.0 km.
*Inundation in the downstream area from 20.0 km caused by the overtopping flow at the said point.
*Inundation in the estuary due to river mouth closure caused by sedimentation.
II. Pantaleon River
*Loss of bridges of the vital road and the railway affected by floods.
*Inundation along the Pantaleon River main course due to the limited flow capacity attributable to the rising of riverbed caused by sedimentation.
*Inundation along the other rivers situated close to Pantaleon River caused by overtopping flows from the river at the points between 14.0 km and 18.0 km and between 6.0 km and 8.0 km.

Epiclastic Processes

The most comprehensive study to date on epiclastic processes is by Vessell and Davies (1981) on the cone of Fuego. They divided the deposits of Fuego into four facies associations:

(a) Volcanic core facies of lavas, air-fall deposits and colluvium breccias.

(b) Proximal volcaniclastic facies of volcanic breccias (block and ash-flow deposits, colluvium breccias) and air-fall deposits.

(c) Medial volcaniclastic facies of debris-flow deposits (lahars) and fluvial conglomerates with some air-fall deposits.

(d) Distal volcaniclastic facies dominated by fluvial sands, breccias and conglomerates, which interfaces with the coastline near Fuego.

Facies model of a stratovolcano based on studies of Fuego. in (a), Tvc are massifs of Tertiary volcaniclastics which seperate elongate troughs filled with modern volcaniclastic sediment. X to X' is the cross section shown in (b). (Vessell and Davies, 1981).

The succession examined by Vessel and Davies (1981) was deposited in the past 20,000-30,000 years, and repeated cycles of deposits and sedimentary processes have been recognized. Three cycles, each triggered by an eruption, could be divided into four phases. These phases are presented in the following table.

Sedimentary cycles triggered by larger eruptions of Fuego volcano (Vessel and Davies, 1981).

From this table it is apparent that there is much more happening on volcanoes than simply volcanic eruptions. On Fuego, only eruptions producing greater than 6 x 10^7 m³ of ejecta were found to be capable of triggering large-scale sedimentary events, and the repose period between these eruptions is 80-125 yeaers. Minor eruptions with a shorter repose period do not significantly affect the sedimentary system, which proceeds as a series of short pulses. Since major eruptions betwen 1972 and 1974, at least 6 x 10^6 tonnes of volcaniclastic debris have been removed from the cone of Fuego, which at present is in a Phase 4 stage.

(Cas and Wright, 1988)