Jökull - 01.12.1989, Page 60

Nevertheless, several workers have included com-
ments on grain shape in their descriptions or used
shape to differentiate tephra units (Walker and
Croasdale, 1972; Larsen and Thorarinsson, 1978).
Tephra studies in Iceland were initiated by the
tephrochronological studies of Thorarinsson. The
term tephra was defined by him (Thorarinsson,
1944) to encompass all airborne pyroclastics.
Because the tephra grains are formed by volcanic
processes and subsequently transported and depo-
sited by sedimentary processes, the study of tephra
joins the methodologies of volcanology and sedi-
mentology. Any tephra unit is defined by its chemi-
cal composition as well as textural properties such
as mineralogy, grain size, shape, fabric, colour, den-
sity etc. These properties may be used to interpret
the origin of the tephra beds and in tephra stratigra-
phy. The chemical range of volcanic eruptions in
Iceland and the variation in physical conditions
from submarine to subglacial and subaerial erup-
tions make it possible to build up a database for
analyzing form parameters and their relationship to
eruption types.
Within many fields of geology the concept of
shape has been approximated by measuring the three
principal axes in the geological body or grain. This
method is used here as a first step in the analysis of
form, although it does not reflect much of the varia-
tion in tephra grain shape caused by varying vesicle
shape and fluidal structure. The present study
employs shape parameters commonly used in the
analysis of pebble morphology. These parameters
are based on measurements of the long (L), inter-
mediate (I), and short (S) axes of tephra grains. Two
classification diagrams for particle shapes are com-
monly used by geologists, the Zingg diagram and
the Sneed-Folk diagram. Zingg (1935) plotted the
I/L ratio against the S/I ratio (Fig. 1). Four main
shape fields are indicated on the Zingg diagram:
equant (spherical), tabular (discoidal, oblate),
bladed, and rod-shaped (prolate). Sneed and Folk
plotted the S/L ratio against the ratio (L - I)/(L - S)
on a triangular diagram (Fig. 2) subdivided into 10
shape classes ranging from compact (equant) to
platy (discoidal), bladed or elongate (prolate) shapes
Tabular . • • »t\ Equant •
• • Bladed • • X Prolate
2/3
S/l
Fig. 1. A Zingg diagram with sample means plotted
as black circles. L, I, and S refer to the three major
(long, intermediate, and short) axes of each particle.
Mynd 1. Zingg graf. Meðaltalsgildi fyrir hvert sýni
eru sýnd með svörtum deplum. L, I og S tákna lang-
ás, miðás og skammás í hverju korni.
(Sneed and Folk, 1958).
Heiken (1972), Fisher and Schmincke (1984), and
Heiken and Wohletz (1985) have outlined the
characteristics of volcanic ash produced in various
types of eruptions. The following remarks on the
formation of tephra grains and their shape charac-
teristics are based on their introductory overview of
the formation of volcanic ash. Only basaltic, andesi-
tic and dacitic to rhyolitic magmatic and phreato-
magmatic eruptions are considered relevant to the
present study.
The genesis of tephra grains in magmatic erup-
tions is controlled by the process of magma vesicu-
lation which is dependent upon composition, tem-
perature, and volatile content of the magma. These
factors control magma viscosity and surface tension.
Generally, the melt viscosity increases with
increased silicity.
Basaltic magmatic (Hawaiian) eruptions produce
58 JÖKULL, No. 39, 1989