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114 CHAPTER 8

Table 8.1 Values of the magma eruption speed, the corresponding magma water content, the mass eruption rate,
the eruption cloud height, and the factor by which the wind speed during the eruption exceeded the current annual
average wind speed for the vent location, for a series of prehistoric (Avellino, Toluca and Fogo A) and historic (Pompeii,
Askja and Fogo 1563) eruptions.
Eruption name and Gas eruption Magma water Mass eruption Eruption cloud Wind speed
−1
−1
location speed (m s ) content (wt%) rate (kg s ) height (km) factor
Avellino (Vesuvius) 210 1.1 2.9 × 10 8 31 1.4
Pompeii (Vesuvius, AD 79) 230 1.3 5.8 × 10 8 37 1.1
Askja (Iceland, AD 1875) 400 2.9 1.7 × 10 8 27 0.6
Toluca (Mexico) 500 4.3 2.3 × 10 8 29 2.3
Fogo A (Azores) 520 4.6 1.1 × 10 8 24 0.8
Fogo (Azores, AD 1563) 415 3.2 6.5 × 10 6 12 1.9

Data for Avellino, Fogo A, Pompeii and Fogo 1563 provided by G.P.L. Walker; data for Askja taken from Sparks, R.S.J., Wilson,
L. and Sigurdsson, H. (1981) The pyroclastic deposits of the 1875 eruption of Askja, Iceland. Philos. Trans. Roy. Soc. Ser. A,
299, 242–273; and data for Toluca taken from Bloomfield, K., Sanchez Rubio, G. & Wilson, L. (1977) The plinian pumice-fall
eruptions of Nevado de Toluca Volcano, Central Mexico. Geol. Rundsch., 66, 120–146.

A third possibility is the failure of the model to take find the differences between the maximum down-
account of the detailed weather conditions pre- wind and cross-wind ranges of clasts of a given
vailing during the eruption. For various eruptions, size and density. This difference provides a good
Table 8.1 gives values that have been deduced in approximation to the downwind distance that the
this way for the magma eruption speed, the corre- clast was transported by the wind while falling, and
sponding magma water content, the mass erup- so dividing the transport distance by the fall time
tion rate, and the eruption cloud height. The cloud gives the average wind speed. A separate average
heights were found from the mass eruption rates wind speed is obtained from each clast size used
using eqn 6.7. in the analysis, and so some idea of the variation of
wind speed with height can be deduced from these
values. The last column of Table 8.1 shows some
8.3.3 Finding the wind speed examples of average wind speeds deduced in this

Clearly the elongation of the isopleths and isopachs way. The values are given in terms of the amount
in Fig. 8.5 must be an indicator of the wind speed by which the value deduced for the average wind
during the eruption. In fact the wind speed – and speed exceeds the current annual average for the
even the wind direction – commonly varies with location of the vent. This serves as a check on
height under normal conditions on Earth, and so whether the analysis is sensible. There is no par-
whatever is deduced from the deposit will rep- ticular reason to expect eruptions to occur in un-
resent some kind of average of conditions between usually windy or unusually calm conditions, so we
the ground and the level of the top of the eruption might expect the average value of these wind
cloud. speed factors to be unity; in fact for the six erup-
Application of the analysis described in the pre- tions given it is 1.35, which seems a reasonable
vious section will provide an estimate of the erup- result given the small sample and all of the potential
tion cloud height and also the maximum height errors involved.
above the ground from which any given size of pyro-
clast has fallen. This means that the terminal veloci-
8.3.4 Finding the fall deposit volume and
ties discussed in section 8.2.3 (taking account of
the eruption duration
the way they vary with height) can be used to find
the times taken by each clast size to reach the The isopachs in Fig. 8.5a are the key to finding the
ground from its release height. The next step is to total volume of magma erupted. Imagine a deposit