Page 159 - Caldera Volcanism Analysis, Modelling and Response

134 A. Pittari et al.

fragmentation progessed to greater depths (c.f. Suzuki-Kamata et al., 1993; Rosi
et al., 1996; Thouret et al., 1999, 2002; Bryan et al., 2000; Allen, 2001). The
upper Sur-A lithic concentration zone contains a relatively higher proportion of
shallow fresh crystalline volcanic and clastic lithic clasts than the underlying and
overlying deposit, suggesting shallower fragmentation levels during the southeastern
piecemeal collapse event.

3.7.4. Implication for caldera evolution
Many caldera-forming eruptions proceed through two stages: an early central vent
plinian stage and a later collapse, ring vent stage (e.g. Druitt and Sparks, 1984;
Druitt, 1985; Druitt and Bacon, 1986; Hildreth and Mahood, 1986; Suzuki-
Kamata et al., 1993; Rosi et al., 1996; Bryan et al., 2000). Remnants of a possible
30 cm thick Abrigo plinian fall deposit occurs beneath the ignimbrite near Poris de
Abona; however, its distribution is very localised (Pittari et al., 2006). Thus, an early
plinian phase may have been very short-lived due to early collapse of the eruption
column.
Multiple vents are suggested by the lateral lithic clast variations within the Sur-A
unit, and a similar inference can be made for the Sur-C unit. Timing of onset of
caldera collapse is poorly constrained. However, a significant conduit-vent wall
fragmentation event, likely to have been triggered by partial subsidence within the
southeastern sector of the caldera, is thought to have occurred during the latter
stages of the Sur-A phase, as represented by the deposition of the upper Sur-A lithic
concentration zone.
After an eruption hiatus, a renewed collapsing eruption column fed a pyroclastic
flow during the Sur-C phase. The latter stage of the Sur-C phase was characterised
by large-scale conduit erosion as represented by the coarse maximum lithic clast size
(typically up to 14 cm), high lithic content (B40%) and large variety of vent-derived
lithic clasts. It represents the final climactic stage of the eruption where large-scale
caldera collapse occurred at least along the southern margin of the caldera.
Northward- and westward-directed pyroclastic flows were sourced from vents
on the north and possibly the west side of the caldera rim, although their timing
with respect to those directed to the south is poorly constrained. Relatively high
lithic contents within the north coast deposits and the Sur-C unit reflect high
eruption intensities, and both may have been deposited during the final climactic
stage of the eruption.

3.8. Implications for pyroclastic flow processes
In the downslope transects near both San Miguel de Tajao (Localities 6, 62 and 66;
Figures 2b, 9) and the Orotava Valley (Localities 119, 124 and 131; Figures 2b, 11),
the proportion of altered lithic clasts, relative to other fresh lithic clast types,
decreases with increasing distances from the source caldera. In a granular flow
mixture of hard fresh volcanic clasts and relatively softer altered clasts, the latter will
tend to be ground to smaller grainsizes more quickly. The distance available for
study along both downslope transects is small (1.5–2 kms) relative to the total