Jökull - 01.12.1994, Page 51

Examples of various particle shapes and their shape
indices are given in Schwartz (1980) and on Fig. 9.
The irregularity or ruggedness of the particle cir-
cumference is defined as the relation of the convex
perimeter (CPERIM) to the actual perimeter (PERIM)
expressed as the ratio CPERIM/PERIM. The rugged-
ness index decreases with increasing complexity of
the particle outline.
The circularity index is defined as (4pAREA)/
(PERIM) 2. The circularity index equals unity for cir-
cles, for all other shapes it is less than unity. The index
increases with increasing roundness and smoothness
and thus does not distinguish between elongated parti-
cles and more rounded rugged particles.
The elongation index is defined as the ratio be-
tween the minimum and maximum diameters (DMIN/
DMAX). The elongation index takes a value of unity
for equiaxial particles (e. g. circles and squares) and
values less than unity for all other shaped particles, the
smallest values representing the longest particles.
RESULTS
The results of the image analysis are shown in
Table 3. Mean values for the three parameters are
plotted in Fig. 10, where the samples have been sorted
by increasing circularity. It is obvious that the three
shape parameters, ruggedness, elongation, and circu-
larity show a similar trend within the group of sam-
ples selected for the present study. In fact the respec-
tive Pearson correlation coefficients for the parameter
pairs ruggedness and circularity, ruggedness and
elongation, and elongation and circularity have been
calculated as 0.993, 0.871, and 0.883.
In the first instance it is appropriate to analyze the
results with respect to field evidence and historical
accounts of the respective eruptions. The overall pat-
tem for all parameters is frorn low mean values for
strombolian tephra (samples 1267 and 1287) to high-
er values for surtseyan and phreatomagmatic tephra.
This indicates that basaltic, magmatic eruptions pro-
duce more rugged, more elongate, and less circular
tephra particles than hydrovolcanic eruptions of simi-
lar chemical composition.
The shape of hydrovolcanic pyroclasts was dis-
cussed by Wohletz (1987), who presented a classifica-
tion table of pyroclast morphology and texture.
Extensive vesiculation and fragmentation caused
by the expansion of exsolving magmatic gases is likely
to generate rugged edges and a complex particle out-
line affected by the viscosity and surface tension of the
cooling magma. Subaerial emptions of the strombolian
type, such as the Heimaey eruption of 1973, and the
later stages of the Laki eruption in 1783, have pro-
duced tephra with relatively low values for ruggedness
and circularity. On the other hand, rapid quenching of
magma through contact with cooling water, will gener-
ally result in abortive vesiculation and outlines that are

S u r t s e Surtsey Surtsey - 1 1 1 >
R 3 ■ R 3 R 3 - >
Surtsej, - ■ Surtsey ■ Surtsey - >
R G 3 / L - ■ RG3 /L ■ RG 3 / L - >
R 3 - ■ R 3 ■ R 3 - >
RG2 /L - ■ RG2 / L ■ RG2 / L >
RG3 /U - ■ RG3 /U ■ R G 3 / U > -
R 7 > R 7 ■ R 7 > -
R 9 ■ R 9 ■ R 9 ■ -
RG2 /U ■ RG2/D ■ R G 2 / U . -
B L - B L > B L > ■
H e i m a e y ■ Heinaey > Heinaey > -
Laki/P ■ Laki/P ■ Laki/P r ■ -
B L - ■ B L B L - > -
Laki/S ■ Laki / S - Laki/S - > •
0.620 .652 0.684 0.716 0.748 0. 7 8 0 0 . 8 0 0.82 0.84 0.86 0.88 0 9 0 0 4 0 . 5 0 . 6 0 .7 0 . 8
Elongation Ruggedness Circularity
Fig. 10 Mean values for ruggedness, elongation, and circularity of all samples. The upward sample order is based on increas-
ing circularity.
10. mynd. Graf yfir meðalhrjúfleika, meðalílengd og meðalhringlögun allra sýnanna. Sýnunum er raðað eftir vaxandi
hringlögun upp á við.
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