Page 134 - Fundamentals of Physical Volcanology

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PYROCLASTIC FALLS AND PYROCLASTIC DENSITY CURRENTS 111

deposit produced by the eruption must be charac- of Askja volcano in Iceland. Notice how the iso-
terized in a way that makes it possible to compare pachs converge towards a point, in this case just
the actual dispersal pattern with the theoretical inside the north end of a lake called Öskjuvatn. This
predictions. We now describe how geologists go convergence point is the approximate location of
about doing this. Needless to say, it helps if the the vent. The lake formed soon after the eruption
deposit is exposed and well-preserved at as many finished when the ground above the magma reser-
locations as possible. voir that fed the eruption collapsed to form a small
caldera.
An isopleth is a line joining points of equal clast
8.3.1 Analyzing a fall deposit
size. An isopleth map will typically be drawn using
At each location the thickness of the deposit is mea- the maximum clast sizes in a deposit and will there-
sured and the sizes and densities of the largest clasts fore show contours of the maximum distance from
(up to 10 of each type present) are obtained. This the vent reached by clasts of a given size. Figure
is done for each recognizable horizon within the 8.5b shows the isopleth map for the pumice clasts
deposit, and wherever possible these horizons are from the 1875 Askja eruption. The isopleths also
traced from location to location based on any dis- converge on the location of the vent. Note how the
tinctive properties (color, unusual grain size, etc.). isopachs and isopleths indicate the direction of the
For very large clasts the sizes are found in the field wind; the more elongate the contours the stronger
using a measuring scale, whereas smaller clasts the wind that was blowing.
are taken back to a laboratory and passed through We can now discuss how the isopleth and
standard-sized sieves. Note that the dimension that isopach maps are used to quantify the eruption
determines if a clast can pass through a sieve is nei- conditions.
ther its longest nor shortest dimension, so that typ-
ical shapes of clasts need to be recorded as well as
8.3.2 Estimating the eruption rate and
size. In many cases a sample of the clasts will also be
the eruption speed
taken so that the overall size distribution of clasts in
the whole deposit can be determined. The clast Recall the comments made earlier about Figs 8.1 &
densities can be found in various ways: sawing 8.2: pyroclasts leaving the column at a given height
clasts into cubes to measure their volumes and and hence at a given distance from the vent are
weighing them for their masses; or coating their blown downwind, but the wind does not greatly

surfaces with a waterproof layer and weighing change their distance from the vent measured at
them first in air and then under water. Recall that right angles to the wind direction, in other words
many pyroclasts will be very vesicular, and the their cross-wind range. This is the key to using fall
vesicles (the holes previously occupied by vol- deposits to analyze eruptions, and the critical steps
canic gases) may be interconnected and open at in the whole process can be stated as follows. For
the surface with many sharp edges or they may be a given set of eruption conditions (magma gas
closed off and trapped within the relatively smooth content and erupted mass flux) there is a specific
surface of a clast. The fact that there can be a range height that the top of the resulting eruption column
of clast types, even within a single eruption, can will reach, and there is a specific variation of gas
make measuring densities a complicated and often ascent velocity and gas density with height in the
inaccurate procedure. eruption column. This means that there is a max-
When all of the information on clast size and den- imum height to which a pyroclast of a given size and
sity and on deposit thickness has been collected, density can be carried in the column. But this also
two maps are typically drawn for a deposit – an means that there is a well-defined maximum lateral
isopach map and an isopleth map. An isopach distance from the vent at which that pyroclast can
map shows the contours of deposit thickness. For be released and hence an equally well-defined max-
example, Fig. 8.5a shows the isopachs for the main imum cross-wind distance at which that pyroclast
part of the pumice deposit from the 1875 eruption will be found. There is thus a direct relationship