Page 35 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Unmodified pyroclastic pumice or scoria fragments any original bedding-parallel foliation.
have equant, elongate, platy or irregular shapes,
bounded by rough, ragged surfaces. Pumice and scoria Achneliths, bombs and blocky juvenile clasts (6)
lapilli in pyroclastic flow and surge deposits can be
appreciably rounded due to abrasion during transport In explosive eruptions of low viscosity magmas, some
(21.4, 22.7). Autoclastic pumice fragments are blocky pyroclasts are ejected in a molten condition and drawn
or prismatic with planar to curviplanar surfaces. out into elongate ribbons or aerodynamically-shaped
Quench-fragmented tube pumice commonly breaks achneliths and bombs (Macdonald, 1972; Walker and
along surfaces normal to the elongation of the tube Croasdale, 1972; Williams and McBirney, 1979) (6.8,
vesicles (woody pumice). Autoclastic scoria fragments 39.7-8). These may solidify before deposition and retain
associated with a'a lava have ragged, twisted, spinose their distinctive shapes, or else be flattened into
shapes. Transport and reworking of pyroclastic or irregular rounded disks on impact. More viscous,
autoclastic pumice and scoria by wind or water result in degassed and/or chilled magma breaks up into ragged or
well-rounded shapes. angular, poorly or non-vesicular, blocky pyroclasts that,
in some cases, can be difficult to distinguish from non-
Pumice and scoria fragments commonly have densities juvenile accessory lithic fragments or juvenile clasts
3
less than that of water (1.0 g/cm ) and may float. If produced by non-explosive autoclastic fragmentation.
pumice fragments from subaerial eruptions are Explosive magmatic and phreatomagmatic eruptions
transported to shorelines or deposited on water, they can that accompany extrusion of silicic lava domes and
be transported by flotation in surface currents for flows generate non- to moderately vesicular, angular,
thousands of kilometers prior to becoming waterlogged blocky pyroclasts, some of which may be flow banded.
and sinking. Lava domes erupted underwater, for In some cases, the interiors of bombs and juvenile
example in caldera lakes or the sea, sometimes have a blocks continue to vesiculate after deposition, causing
pumiceous carapace that breaks up into very large the chilled outer surfaces to crack in a breadcrust
blocks, which are buoyant, at least temporarily pattern (6.8).
(Reynolds et al., 1980; Clough et al., 1981; Wilson and
Walker, 1985) (40.3-4). Experiments conducted by Bombs, achneliths and juvenile blocks are important
Whitham and Sparks (1986) suggested that, following components of welded and non-welded, proximal,
an initial rapid uptake of water, cold pumice clasts subaerial fallout deposits, especially those involving
absorb water slowly, the rate depending on the pumice basaltic magma. Bombs are, however, not restricted to
clast size, initial density, the size and distribution of subaerial settings. They occur in shallow marine
vesicles, and the extent to which vesicles are volcaniclastic deposits, as a result of direct fallout from
interconnected. Conversely, hot pumice clasts can sink mildly explosive pyroclastic eruptions in shallow water
immediately, even though they are less dense than (Staudigel and Schmincke, 1984; Dimroth and
water. At low temperatures (<150°C), gas in vesicles Yamagishi, 1987) (13.1), or as a result of downslope
contracts and water is drawn in. At higher temperatures, resedimentation of primary deposits from littoral or
gas within vesicles is flushed out, as absorbed water is shallow-water fire-fountain eruptions (Dolozi and
converted to steam. On cooling, the steam condenses, Ayres, 1991). Basaltic "welded frothy agglutinate" (Gill
more water is absorbed, and eventually the pumice et al., 1990), "bombs" and "glassy spatter" (Smith and
sinks. Batiza, 1989) apparently also occur in modern, deep
submarine (>1700 m) settings, and have been
Pumice and scoria are prone to alteration and textural interpreted as proximal deposits from submarine, mildly
modification, even in young deposits. Glass, especially explosive lava fountaining, associated with especially
vesicular glass, is rapidly devitrified, crystallized and/or high effusion rates.
altered. The new minerals may faithfully preserve the
vesicular texture or else destroy it completely. Pumice Non-vesicular to moderately vesicular juvenile clasts
and scoria fragments in strongly welded pyroclastic are generated in abundance by autobrecciation and
deposits are compacted to discs of dense glass that may quench fragmentation of active lava flows and domes
be subsequently devitrified or altered (24.1-2). Pumice (10, 19.1). Clasts produced by quench fragmentation are
and scoria clasts in non-welded pyroclastic deposits and characteristically blocky and bounded by curviplanar
in volcanogenic sediments are commonly flattened surfaces; margins of such clasts are usually glassy and
parallel to bedding, during diagenesis and lithification cut by "tiny normal joints" (Yamagishi, 1987) (9.6).
(Branney and Sparks, 1990) (45.1-4). If the vesicles are Clasts produced by autobrecciation are typically flow-
infilled and the glass replaced by silica or feldspar soon banded slabs with uneven, angular ends, and/or massive,
after emplacement, then the primary texture can be irregular blocks. The clasts may remain in situ, be re-
preserved. Weathering and/or alteration of compacted, incorporated into the lava, or be reworked and
matrix-poor pyroclastic deposits, rich in porphyritic redeposited by sedimentary processes.
pumice or scoria clasts, can obscure clast boundaries
and result in an apparent evenly porphyritic texture, Juvenile magmatic clasts in resedimented syn-eruptive
similar to that displayed by porphyritic coherent lavas deposits commonly retain their original shape
and intrusions (45.5-6). Tectonic deformation sufficiently well for the clast-forming processes to be
commonly results in cleavage-parallel flattening and established. However, non-welded juvenile magmatic
alignment of relict pumice or scoria clasts, overprinting clasts that are reworked and transported by traction

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