Jökull - 01.12.1994, Síða 43

Image Analysis and Morphometry of Hydromagmatic and
Magmatic Tephra Grains, Reykjanes Volcanic System, Iceland.
Jón Eiríksson,
Science Institute, University of Iceland, Jarðfræðahús Háskólans, IS-101 Reykjavík, Iceland
Magnús Á. Sigurgeirsson,
National Energy Authority, Grensásvegur 9, IS-108 Reykjavík, Iceland
& Torsten Hoelstad.
Geological Survey of Denmark, Thoravej 8, DK-2400 Copenhagen NV, Denmark
Abstract - The paper describes two-dimensional shape oftephra grains from the Reykjanes vol-
canic system in SW-Iceland. The Reykjanes volcanic system offers numerous well mapped tephra
units that have been formed in submarine and subaerial eruptions during the Holocene. Samples
from Surtsey, Heimaey, and Laki were studied for comparison. All the tephra samples were ob-
tainedfrom dispersed, unlithiftéd units ofbasaltic composition with minimal signs of alteration.
Morphometric parameters were obtained by computerized image analysis. SEM images were
usedfor visual evaluation ofthe grain shape and as an aidfor interpreting the parameters. Ini-
tially, tephra grain shape is controlled by eruptive mechanisms and by the chemical and physical
properties ofthe magma. The primary shape may subsequently become modified during transport
awayfrom the eruptive vent before deposition, e. g. in surges. Image analysis enables rapid quan-
tification of tephra grain shape by calculating shape parameters from digitized silhouette out-
lines. The project was motivated by the need to obtain quantitative tephra grain shape charcicter-
istics in various eruption types. This is necessary for the interpretation ofancient tephra layers
andfor maturation studies of volcaniclastic sediments in general. In the present study, the main
emphasis is on the shape of phreatomagmatic tephra grains from the Reykjanes volcanic system,
with additional phreatomagmatic samples from the Surtsey and Lakagígar volcanoes. Magmatic
tephra samples from Heimaey and Laki were studiedfor comparison. Hydromagmatic tephra is
generally morefine grained and shows much greater areal distribution. Slightly vesicular, blocky
glass shards with distinct, partly straight-edged outlines characterize the hydromagmatic tephra,
while the magmatic tephra is more vesicular with outlines that are partly jagged, partly smoothly
undulated. Fifteen samples of sand-sized tephra grains were analyzed and the shape parameters
ruggedness, elongation, and circularity were calculated for a random subset of 160 grains from
each sample. The parameter values enabled a statistically significant differentiation between
strombolian and hydromagmatic tephra. The strombolian tephra is more rugged and elongate
than the hydromagmatic one.
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