Page 128 - Caldera Volcanism Analysis, Modelling and Response

Case Study of the Abrigo Ignimbrite, Tenerife, Canary Islands 103

Alternatively, the distribution of each lithic type can be shown on separate maps
(Figure 1c; Druitt, 1985). The vertical variation in lithic component abundances, or
just in their occurrence, can be plotted against stratigraphic logs (Figure 1d; Druitt,
1985; Suzuki-Kamata et al., 1993; Bryan et al., 2000; Allen, 2001) and combined
with spatial variation maps to show three-dimensional variations (Rosi et al., 1996).

2.3. Lithic clast provenance

Lithic clasts are either: (a) juvenile (cognate) dense fragments of the erupting magma
chamber, (b) accessory country rock fragments eroded from the walls of the magma
chamber, conduit and vent, and (c) accidental fragments of the exposed surface
eroded and entrained into pyroclastic flows (Cas and Wright, 1987). To ascertain
the provenance of specific lithic types, it is necessary to understand the regional
geology of the volcanic terrain, which can often be complex. Conversely, where
lithic types are not represented by any known surface exposure, the nature of the
subsurface geology can be inferred.
Accessory and accidental lithic clasts in volcanic regions are often texturally and
compositionally similar, and difficult to distinguish. Plinian fall deposits contain
solely vent-derived lithologies and can help to establish vent-derived lithic clasts in
ignimbrites from the same eruption (e.g. Potter and Oberthal, 1987; Bryan et al.,
2000; Calder et al., 2000), although later ignimbrites may contain additional
accessory lithologies if the vent location changes. Lithic fragments embedded in
juvenile pumice clasts must also be vent-derived (e.g. Bryan et al., 2000). Both
accidental and accessory lithic clasts are present in variable proportions within
ignimbrite deposits. Vent-derived lithologies generally decrease in size and
proportion away from the vent, whereas local substrate-derived lithologies increase,
especially on the lee side of topographic highs (Suzuki-Kamata, 1988; Buesch,
1992; Calder et al., 2000).
Accessory lithic clasts represent samples of a thick subsurface rock succession
(4–7 km in the case of the Las Can ˜adas edifice, Tenerife; Bryan et al., 2000). Cole
et al. (1998) presented a detailed petrological and geochemical provenance study of
ignimbrite lithic clasts from the Taupo volcanic centre, New Zealand, attempting to
correlate these to local outcrop exposures and inferring a complex subsurface
geology and constraining caldera locations. Fresh volcanic lithic clasts, with no
equivalent outcrop exposure, have been interpreted to represent pre-caldera

Figure 1 Examples of representations of quantitative spatial lithic assemblage variations. (a)
Pie diagrams showing relative lithic proportions within a continuous lithic-rich layer in the
WineglassWeldedTu¡,W, and underlying lithic-rich layers,Wu, around the rim of Crater Lake
caldera (modi¢ed from Suzuki-Kamata et al.,1993). (b) Bars showing relative lithic assemblage
variations of the Soncor ignimbrite, Lascar volcano, Chile, across three down £ow transects
within quebradas (valleys) (modi¢ed from Calder et al., 2000). (c) Qualitative lithic distribution
maps for each lithic type identi¢ed in lag breccias from the Cape Riva eruption, Santorini
(modi¢ed from Druitt,1985). (d) Qualitative vertical variations in lithic clast assemblage
through the Granadilla pumice fallout deposit,Tenerife (modi¢ed and simpli¢ed from Bryan
et al., 2000). Abbreviations: cog. cl., cognate clast; phen., phenocryst; phon., phonolite; weld.,
welded.