Jökull - 01.12.1989, Side 61

Compact
1
Fig- 2. A Sneed-Folk form 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 2. Sneed-Folk graf. Meðaltalsgildi fyrir hvert
syni eru sýnd með svörtum deplum. L, I og S tákna
langás, miðás og skammás í hverju korni.
a great range of shapes but most grains have a
smooth surface skin formed by surface tension
within droplets of melt ejected from lava fountains.
These may take the form of spheres, teardrops,
dumbbells or ovoids. Other less common shapes
include delicate pele’s hairs, irregular clots of glass,
and scoria.
Strombolian eruptions are slightly more viscous
magmatic eruptions producing a variety of pyro-
clasts indicating the variety of conditions that may
extst in such eruptions. Tephra grains are commonly
either sideromelane or tachylyte glass. The
stderomelane grains vary in shape from either irreg-
ular droplets, many of which reflect spherical to
ovoid vesicle walls, to blocky sideromelane dro-
plets. Angular, blocky tachylyte grains tend to have
rough surfaces and the shapes include irregular,
open-vesicle networks and blocky grains. Smooth-
skinned droplets or ovoids are absent.
In andesitic to rhyolitic eruptions the grain shapes
are dependent upon the vesicles developed within
the highly viscous magmas. These eruptions include
some Vulcanian and most Peléan and plinian erup-
tions producing mainly pumice. Many glass parti-
cles consist of fragmented walls of elongate vesicles
and may form Y-shaped curved plates or needle-like
forms. In dacitic and rhyolitic ash, most of the fine
grained glass particles consist of comminuted pum-
ice.
Volcanic eruptions are classified as phreato-
magmatic if the rising magma comes into contact
with bodies of water such as groundwater (phreatic
water), lakes, the sea, snow or ice. Such eruptions
are dominated by steam explosions and produce
mainly blocky, equant tephra grains with relatively
few vesicles.
Fisher and Schmincke (1984) stated that pyroclas-
tic particles are shaped initially by the forces of
explosion, vesiculation and surface tension and by
drag effects on semi-molten juvenile liquid. Accord-
ing to them, pyroclastic fragments tend to be rela-
tively equant or somewhat elongate, but platy or
bladed fragments may be common, using Zingg’s
classification.
Walker and Croasdale (1972) presented a fairly
extensive comparison of two types of tephra. They
distinguished between a strombolian/hawaiian type
and a surtseyan type. The former type is typically
coarse grained and well sorted, being piled up in
scoria cones during lava fountain eruptions on land.
The surtseyan type derives its name from the subma-
rine Surtsey eruption off South Iceland in 1963-
1967, and the tephra is typically fine grained and
poorly sorted. Walker and Croasdale compared the
grain shape in these two types of tephra. They
claimed that the form of the strombolian/hawaiian
grains is controlled by surface tension and the
interaction with air during lava spraying. This leads
to the formation of achneliths (derived from the
Greek word achne, which means spray). This type of
tephra is characterized by great variation in form,
ranging for instance from pele’s hairs to pele’s
drops, but according to Walker and Croasdale, the
JÖKULL, No. 39, 1989 59