Jökull - 01.12.1989, Blaðsíða 59

The Morphometry of Selec^ed Tephra Samples
from Icelandic Volcanoes
JÓN EIRÍKSSON
BÖRGE JOHANNES WIGUM
Department of Geosciences, Science lnstitute,
University oflceland, 101 Reykjavík, Iceland
abstract
Three dimensional morphometry of tephra sam-
ples from a variety ofvolcanic eruptions in Iceland
has been analyzed with methods that are commonly
applied to the form of geological bodies regardless
of size. These studies were motivated by the need to
quantify the descriptive shape classification of
tephra and to test the significance of shape varia-
tions and the relation to phreatomagmatic, mag-
matic, or mixed eruption types. 1760 sand sized
tephra particles from 22 samples were measured
und the results were statistically tested. Cluster
analysis was used to investigate dijferentiation of
samples and parameters. Variations in the chemical
and physical factors in the eruptive environment are
reflected in variations in form parameters such as
elongation and sphericity. The tendency to elongate
grains increases with increasing SiO^ content or
acidity of the magma. The highest variance values
for elongation were obtained for samples from mag-
matic eruptions, while the lowest values were from
Phreatomagmatic eruptions.
INTRODUCTION
The main purpose of this paper is to present data
°n the shape of tephra samples from various Ice-
landic volcanoes. The objective of the research pro-
ject was to apply quantitative methods to the
classification of sand sized tephra grains according
to their three dimensional form. A distinction
between tephra units which is simply based on a
visual comparison of shape or other textural proper-
ties may yield rapid results. It has been demon-
strated, however, that individuals tend to have a
biased perception of particle form (Folk, 1955), and
a quantitative approach is therefore preferable.
There are at least three reasons why the study of
form (morphometry) might be important. Firstly, the
particle form characteristics of tephra layers is
presumably controlled by environmental and chemi-
cal factors during an eruption, and the form parame-
ters couid give clues to these factors in past erup-
tions. Secondly, quantifiable variation in form from
one tephra layer to another, or within single layers,
may be a useful stratigraphical and tephrochrono-
Iogical tool in the identification of individual layers.
Thirdly, form is a routine element in the description
and interpretation of sediments, including volcani-
clastic sediments, and data on the original form of
the grains are therefore necessary as a reference.
This is important because any transport of tephra
particles during an eruption or through reworking
after deposition may lead to collisions between
grains or between grains and any obstacles in the
transport path. The collisions may cause breakage or
abrasion of the grains and the original form will
probably be changed.
The literature survey of Olafsson (1985) revealed
a limited number of publications on the textural pro-
perties of Icelandic tephra layers. Most of these deal
with grain size and mineralogy, but no quantitative
studies of tephra morphometry were found.
JÖKULL, No. 39, 1989 57