Page 124 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

(Lipman, 1976) probably include deposits from true Two distinct facies are typically present within volcanic
slides. The paucity of examples is a reflection of the debris-avalanche deposits: block facies consists of
difficulty of recognizing these deposits, rather than their relatively intact pieces (debris avalanche blocks) of the
abundance. Slide blocks in ancient sequences are likely source volcano, whereas mixed facies consists of an
to be misinterpreted as faulted parts of otherwise in situ unsorted, unstratified, thorough blend of all the rock
and intact volcanic stratigraphy. types present in the source (Glicken, 1991). Debris
avalanche blocks can be gigantic and usually measure at
Slides are commonly associated with debris avalanches, least several metres to tens of metres across (36.5-6).
rapid, gravity-driven flows of unsorted mixtures of rock Blocks up to 500 m across occur in the Nirasaki
and sediment (Ui, 1983; Siebert, 1984; Siebert et al., avalanche deposit of Yatsugatake volcano, Japan (Ui,
1987). Water or ice may be present within and between 1983). Block size commonly decreases with distance of
debris avalanche blocks but they do not contribute to outflow from the source. Blocks dislodged at the time
particle support or transportation. Deposition occurs of failure are fractured as a result of dilation and
en masse by frictional freezing. Volcanic debris shattered during transport (Glicken, 1991). Blocks
avalanches commonly evolve from slides initiated by that comprise weaker lithologies disintegrate rapidly
sector collapse and may be synchronous with explosive and contribute to mixed facies in the deposit, whereas
eruptions, the best known recent example being that of stronger lithologies survive and form the block facies
Mount St Helens, Washington, in 1980 (Voight et al., (Fig. 52). Even relatively intact blocks are usually
1981; Siebert, 1984; Siebert et al., 1987). Slope failure fractured, and adjacent but separate blocks may show
and initiation of volcanic debris avalanches also occur on jigsaw-fit texture (36.3).
inactive volcanoes (dormant and extinct). Although
debris avalanches affect volcanic islands and totally
subaqueous volcanoes, the best studied examples of
volcanic debris-avalanche deposits are confined to
subaerial volcanoes (e.g. Voight et al., 1981; Crandell et
al., 1984; Ui and Glicken, 1986; Palmer et al., 1991).
Youthful submarine debris-avalanche deposits
associated with slides have been mapped on the flanks of
Hawaiian volcanoes, using detailed bathymetry and
GLORIA side-scan sonar imaging (Lipman et al., 1988).
Volcanic debris avalanches are highly mobile and may
override significant topographic features. An avalanche
from Socompa volcano traveled more than 35 km from
source and reached up to 250 m above its lowest point
(Francis et al., 1985) (36.4-6). Velocities of 160 km/hr
were calculated for a volcanic debris avalanche at Chaos
Crags (Crandell et al., 1974), and the Mount St Helens
debris avalanche (18 May 1980) had velocities in the

range of 180-288 km/hr (Voight et al., 1981). Many
avalanche deposits exceed a cubic kilometre in
volume. The Socompa debris-avalanche deposit exceeds
3
15 km (Francis et al., 1985), and a 360 ka debris-
avalanche deposit at Mount Shasta, California,
3
amounts to at least 26 km (Crandell et al., 1984).
3
Sundell (1983) calculated a volume of 292 km for a
Tertiary debris-avalanche deposit in Wyoming.

Volcanic debris-avalanche deposits are characteristically Fig. 52 Detail of the internal texture of the 1980 Mount
non-graded and very poorly sorted, with clasts ranging St Helens volcanic debris-avalanche deposit, showing
in grain size from centimeters to tens of meters (36.2-3). both debris avalanche block facies and mixed facies.
They show small-scale variation from clast- to matrix- Block facies consists of chunks of the source volcano
supported fabric, and are unstratified. Although overall transported more or less intact in the debris
the deposits are polymict, the range of compositional avalanche. Mixed facies is a thorough blend of
variation depends on the heterogeneity of the source, lithologies and commonly dominated by matrix derived
and small exposures can be dominated by only one clast from poorly consolidated material and solid, separate
type. In some young examples, it is possible to match clasts derived from break-up of blocks. Modified from
material in the deposits with different parts of the Glicken (1991).
volcanic edifice and subvolcanic basement (Francis et
al. 1985) (36.5). Syn-eruptive volcanic debris-avalanche The surface morphology of debris-avalanche deposits is
deposits may include a small proportion of juvenile hummocky (36.4, 36.7), with relief ranging from
magmatic clasts (36.6), although in many cases, these metres to tens of metres, and the deposits are
are difficult to distinguish from abundant other commonly lobate in plan, with steep margins marked
volcanic lithic clasts. by levees, reflecting appreciable yield strength (Siebert,

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