The eruption of 1974 produced a substantial ash blanket whose volume can be estimated to exceed 0.3 km³ (equivalent to 0.1 km³ dense rock). Because the 1974 ash blanket is really four superimposed fall units, each deposited under diffeerent atmostpheric conditions, the volume estimation is not as straightforward as it is in many examples (Rose et al., 1973), and a four-step integration is needed. Uncertainty in the volume of fine ash scattered to great distances inhibits a maximum volume estimate, but at least one line of evidence suggests that the maxiumum bulk volume is unlikely to be more than 0.4 km³. For the total volume of ash to exceed 0.4 km³, more than 50% by weight of the ash would have fallen at distances exceeding 100 km and this ash would be very fine-grained with a mean grain size of less than 20 µ (Davies et al., 1978). Since the ash is thought to originate primarily by vesiculation, the size of particles produced is influenced strongly by the size of vesicles. The vesicle sizes in Fuego ash average about 75 µ, ranging from a minimum of about 1µ to a maximum of 250 µ. Ash samples taken within a few tens of kilometers of Fuego consist of a mixture of multi-vesicled ash, crystal fragments and shards, whereas distal samples are greatly enriched in shard particles.
- Map of ash thicknesses measured in the field following the 1974 Fuego eruption. Map below was measured on October 15 and 16 following the initial phase of the eruption, but prior to later activity. Map based on data collected by the Instituto Geográfico Nacional, Guatemala.
(Rose et al., 1978)
The Fuego tephra of 1974 are, like other historic tephra, olivine-bearing high-alumina basalts, made up of phenocrysts (generally > 0.1 mm), microphenocrysts (10-60 µ), glass (or devitrified glass) and round, subspherical vesicles (1-400 µ). Phenocrysts are dominantly plagioclase and olivine. Magnetite, augite, and oxyhornblende are minor phenocrysts.
(Rose et al., 1978)
- Aerial view of Fuego summit from the SW, showing weak ash eruptions. Photo by Bill Rose, 1978.
Grain size analysis of samples representing all sampleable portions of the airfall deposit produced by the Fuego volcano in Guatemala on 14 October 1974 form the basis for estimating the total grain size distribution of tephra from this eruption. The region enclosed by each isopach has a particular average grain size distribution which can be weighted proportionally to its percentage volume. The grain size of pyroclastic avalance deposits produced during the eruption are also included. The total grain size distribution calculated as a sum of weighted distributions has a median grain size of 0.8 Ø (0.6 mm) and a sorting coefficient of 2.3. The size distribution seems to approximate Rosin and Rammler's law of crushing and this observation allows us to estimate that no more than 15% volume of the fine tail of the total size distribution is likely to be missing. The ash composed of these fine particles did not fall in the region of the volcano as part of the recognizable tephra blanket. The eruption column reached well into the stratosphere: heights estimated from the ground were 10-12 km above sea level but estimated heights based on mass flux rates are higher (18-23 km). The proportion of ash smaller than 2 µ, which could remain for substantial periods in the stratosphere, is no more than 0.8% volume of the total. It seems probable that acid aerosol particles from vulcanian type eruptions are more important to stratospheric aerosol perturbation than fine silicate ash particles by at least an order of magnitude.(Murrow et al., 1980)