Page 123 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Lahars (35) show reverse grading, reflecting the importance of
dispersive pressure, and the matrix comprises poorly
Lahars are rapid water-supported flows of volcaniclastic sorted sand- to clay-size sediment (Fig. 51B; 34.2-3).
particles generated on volcanoes (Fisher, 1984). In Volcaniclastic deposits from flows of this type are probably
many cases, lahars move partly or entirely as debris important, given the common coincidence of
flows (Smith and Lowe, 1991), and their deposits show abundant coarse clasts and steep slopes in volcanic
the characteristics of debris-flow deposits outlined terranes. Reversely graded, monomict, clast-supported
above (35.1-7). Other important flowage mechanisms beds of hyaloclastite breccia, associated with
exhibited by lahars are hyper-concentrated flow and subaqueous coherent lava domes and the margins of
dilute streamflow (Pierson and Scott, 1985; Smith, lava flows, may be resedimented hyaloclastite
1986) (35.5-7). Some lahars are generated directly by emplaced by grain-flow processes.
or are synchronous with volcanic eruptions, and result
from, for example, an explosive eruption through a
crater lake, or primary pyroclastic flows entering rivers,
or interaction between freshly erupted lavas or
pyroclastic deposits and snow or ice (Major and
Newhall, 1989). These lahars contain hot juvenile
magmatic clasts and their deposits can be texturally very
similar to primary volcaniclastic deposits. In other
cases, the connection with volcanic activity is indirect,
and lahars are triggered by the effects of earthquakes or
heavy rain on unconsolidated volcaniclastic deposits,
or follow on from slope failure events that produce
slides and debris avalanches (Siebert et al., 1987).

Volcaniclastic grain-flow deposits (36)
Fig. 51 Grain-flow deposits. (A) Thinly bedded grain-flow
Grain flow affects cohesionless grains on steep slopes. deposit with characteristic reverse grading and steep
Grains roll and slide downslope under gravity (Lowe, primary dip. (B) Thick, reversely graded, density-
1976, 1979) and, ideally, transportation is independent modified grain-flow deposit. Modified from Lowe (1982).
of the interstitial fluid (air or water). Particle collisions
generate dispersive pressure that contributes to particle Volcanic slides, volcanic debris avalanches and
support during flowage. Grain flows have a frictional their deposits (36)
yield strength that must be exceeded in order for
flowage to occur; thereafter, their behavior is laminar. Slides and debris avalanches are mass movement
Deposition from grain flows involves frictional freezing processes, primarily driven by gravity and not
of the particles and restores the particulate aggregate to a significantly influenced by or dependent on interstitial
more stable "angle of repose" configuration. Angle of fluid. Slides involve the downslope, gravity-driven
repose for sand-size particles is about 30°-35° for displacement of a coherent rock or sediment mass along
subaerial settings and 18°-28° for subaqueous settings. a basal shear plane (Stow, 1986). Slides may be small-
Angle of repose is achieved by multiple grain flows scale, local events or involve collapse of entire sectors of
cascading down steep, unstable slopes. As a result, grain- volcanoes that incorporate large volumes of rock. Slides
flow deposits are characterized by thin (centimeters to a affect subaerial, island and subaqueous volcanoes, and
few tens of centimeters), commonly lenticular beds with volcaniclastic deposits in unstable positions on steep
appreciable primary dip. The beds are typically offshore shelves and deltas. Sector collapse of Socompa
reversely graded and clast-supported (Fig. 51A; 36.1). volcano, northern Chile, generated slide blocks up to 2.5
Foresets of subaerial and subaqueous dunes, fronts of km long and 400 m high, which in total constitute a
3
prograding deltas, and talus aprons are common minimum volume of 2.5 km (Francis et al., 1985)
situations where grain flows operate. Volcaniclastic (36.4). Avalanche and several slide deposits have been
grain flows are important in many subaerial and recognized on the submarine flanks of Hawaiian shield
shallow subaqueous volcanic environments where there volcanoes (Moore, 1964; Lipman et al., 1988).
is an abundant supply of loose particles and steep
slopes. Primary non-welded fallout deposits on steep The deposits of slides consist of relatively coherent
subaerial slopes, especially pumice or scoria lapilli fall masses of the source volcanic edifice, within which
deposits, are especially prone to downslope movement there may be evidence of internal deformation such as
by grain-flow processes. faults, folds and shearing. A strongly sheared contact
Lowe (1976, 1982) has interpreted some pebble and typically separates slide blocks from the substrate. In
cobble conglomerate beds to be deposits from a modern settings, the source areas of slides are marked by
cohesionless particulate flow type termed a density- major arcuate topographic scars (Moore, 1964).
modified grain flow. These involve sediment mixtures in
which larger and denser clasts are buoyant in a silt-sand There are very few detailed or convincing descriptions
suspension. As a result, deposits are coarser (cobbles) of volcanic slide deposits in ancient volcanic sequences.
and in thicker beds (>0.4 m) than normal for true grain- Intra-caldera, heterolithic megabreccia in some Tertiary
flow deposits. Clast-supported pebbles and cobbles subaerial caldera sequences in the western USA
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