Page 37 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

degassed crusts on lava ponded in the vent, or magma ash cores of accretionary lapilli. Armoured or cored
chilled against the conduit walls, or portions of the lapilli consist of crystal-, pumice-, or lithic-fragment
magma already crystallized in the magma chamber. nuclei coated by fine to coarse ash (Waters and Fisher,
1971) (7.8). Large accretionary structures, termed
In ancient primary and resedimented pyroclastic rocks, armoured mud balls (Dimroth and Yamagishi, 1987),
the three types can be difficult to distinguish. Cognate occur in some Miocene submarine volcaniclastic mass-
and accessory lithic pyroclasts are typically angular, flow deposits in Japan. These have a mudstone
whereas the roundness of accidental lithic clasts varies intraclast nucleus that is surrounded by concentric shells
widely and depends on their source and prior of pumiceous ash, and are 6-10 cm in diameter.
transportation history. Accessory lithic pyroclasts are
commonly, but not invariably volcanic. Cognate lithic The formation of accretionary lapilli usually involves
pyroclasts are co-magmatic with other juvenile suspended ash and moisture. Suspended ash particles
pyroclasts but differ texturally, depending on their begin to aggregate as a result of electrostatic attraction
source. Those derived from early-crystallized portions and particle collision, and are held together by surface
of the magma consist of aggregates of interlocking, tension of condensed moisture, electrostatic forces,
medium- or coarse grained crystals (25.4A). Those particle interlocking and growth of new minerals as the
derived from chilled and degassed portions of the condensed moisture evaporates (Reimer, 1983;
magma are non- or poorly vesicular, and glassy or very Schumacher and Schmincke, 1991; Gilbert et al., 1991).
fine grained. Accidental clasts can be composed of any Electrostatic attraction is especially important in the
rock type or of unconsolidated cohesive sediments. formation of fine-grained outer rims. Accretionary
lapilli also form when rain falls through an otherwise
Surface processes (mass-wasting, physical and chemical "dry" ash cloud (Walker, 1971; Macdonald, 1972).
weathering, erosion) also generate lithic clasts that may Some accretionary lapilli are thought to develop when
subsequently be incorporated into volcanogenic raindrops, moist lithic clasts or crystal fragments fall on
sedimentary deposits, or into pyroclastic flow and surge and roll across freshly deposited ash (Walker, 1971;
deposits, or into lava flows. In subaerial volcanic Reimer, 1983).
terranes, surface processes are a very important source
of lithic clasts. In subaqueous settings, the principal Most accretionary lapilli form in subaerial
non-volcanic lithic-clast-forming process is mass- environments. They are common in a wide variety of
wasting; for example, gravitational collapse of unstable primary pyroclastic deposits, especially those from
parts of lava domes and flows, and rockfall adjacent to phreatomagmatic eruptions; for example, surge deposits
active fault scarps. of tuff rings; pyroclastic flow and fall deposits from
large silicic, phreatomagmatic eruptions (phreatoplinian
Lithic fragments generated by mass-wasting are in most eruptions); fall deposits from ash clouds that accompany
cases angular and may exhibit in situ fracture patterns. pyroclastic flows and surges (co-ignimbrite and co-
Reworking of epiclastic lithic fragments generally surge ash). Accretionary lapilli have also been recorded
results in appreciable rounding. Autoclastic fragments in gas segregation pipes in ignimbrites (Self, 1983)
and volcanic lithic pyroclasts that are subsequently (22.6) and in subvolcanic intrusive breccia complexes
reworked and rounded become texturally (Wormald, 1992). The best guide to interpreting the
indistinguishable from volcanic lithic epiclasts. origins of accretionary lapilli is the field relationships.
Precisely constrained facies relationships may be the Details of their internal structures and grain size
only means of recognizing the primary volcanic origin characteristics may help in determining their mode of
of such clasts once they are incorporated in formation (Schumacher and Schmincke, 1991).
volcanogenic sedimentary deposits.
Accretionary lapilli in fall deposits (co-surge, co-
Accretionary lapilli (7) ignimbrite, phreatoplinian) are commonly concentrated
in layers within discrete, widespread ash beds (39.6).
Accretionary lapilli are spheroidal lapilli-sized The lapilli are well-sorted, may be flattened parallel to
aggregates of ash, recorded sizes of which range from 3 bedding, and are either whole or broken in situ.
or 4 mm to more than 10 cm (7.6-7). There are two Relatively loose packing of ash particles in the lapilli
textural types of accretionary lapilli (Moore and Peck, results in high porosity and low density. In surge
1962; Reimer, 1983; Schumacher and Schmincke, deposits, accretionary lapilli may be concentrated on the
1991): lee-side of obstacles and dune crests (Fisher and Waters,
(1) those with a core of coarse-grained ash, surrounded 1970) and armoured lapilli are common. Accretionary
by a rim of finer grained ash (rim-type); rims may be lapilli may be generated in relatively dilute ash clouds
graded, with grain size decreasing towards the margin, that are associated with pyroclastic flows, and deposited
or else consist of alternating layers of fine- and very in ash-rich beds at the tops of the related flow deposits.
fine-grained ash; in some cases, lapilli have multiple Accretionary lapilli within pyroclastic flow deposits are
rims but lack a well-defined core; usually widely dispersed and may be broken or abraded
(2) aggregates of relatively coarse ash without a finer (McPhie, 1986). Packing of ash particles in accretionary
grained rim (core-type). lapilli found in pyroclastic surge and flow deposits is
relatively tight and results in somewhat higher densities.
Small (<1 mm) vesicles frequently occur in the coarser Although formed principally in subaerial environments,

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