Page 157 - Caldera Volcanism Analysis, Modelling and Response

132 A. Pittari et al.

Ablay et al., 1995) may be a smaller-scale analogy to the pre-Abrigo plinian
eruptions (Edgar et al., 2007 also likens the Mon ˜tana Blanca eruption to early
explosive activity at the start of the Diego Herna ´ndez cycle). Effusive volcanism has
continued since the Mon ˜tana Blanca eruption, and it is possible that intracaldera
effusive eruptions also occurred between the pre-Abrigo plinian eruptions although
there are no interbedded lava flow units within this succession at the caldera wall.
Abundant, fresh, volcanic lithic clasts in the Abrigo ignimbrite, much of which
are likely to be shallow conduit-derived accessory or near-to-vent accidental clasts,
include basaltic to phonolitic compositions and a large variety of textures. This
suggests that the conduit system and near-extra-vent ground surface consisted of a
laterally and vertically variable stratigraphic architecture constructed during
multiple effusive eruptions. This is consistent with the idea of a possible
stratovolcano-building phase preceding the Abrigo eruption (Figure 14), although
its size, morphology and position within the caldera is unknown.

3.7. Destruction of the pre-Abrigo volcanic system

The Abrigo eruption is considered to be the final caldera-forming eruption of the
Diego Herna ´ndez cycle (Martı ´ et al., 1994), which probably included earlier
incremental collapse events (Edgar et al., 2007). The Icod landslide, which occurred
around the same time, removed part of the northern caldera wall although its
significance in determining the final shape of the caldera is uncertain (Watts and
Masson, 1995, 2001; Ablay and Kearey, 2000; Ablay and Hu ¨rlimann, 2000).
Evidence for caldera collapse associated with the Abrigo eruption includes (a) the
3
large deposit volume (WW1.8 km ) of which about 60% is juvenile material, and
which represents the withdrawal of a large volume of magma; (b) the occurrence of
a co-ignimbrite lithic lag breccia on the caldera rim (see Section 3.2; Pittari et al.,
2006); and (c) clast component variations. The latter provides further insight into
caldera-forming processes, which are discussed below.

3.7.1. Relationship between caldera dynamics and hydrothermal activity
The abundance of altered lithic clasts within the Abrigo ignimbrite suggests that an
extensive hydrothermal alteration zone existed above the Abrigo magma chamber,
probably concentrated along pre-existing structures associated with older caldera-
collapse events (Martı ´ et al., 1994; Edgar et al., 2007). During the hypothetical pre-
Abrigo constructive phase, continued pervasive hydrothermal alteration and an
increase in overburden by the addition of pyroclastic material and possible lavas,
would have contributed to an increasing instability in the rock pile above the
magma chamber (c.f. Calvache and Williams, 1992;Lo ´pez and Williams, 1993;
Calvache et al., 1997). A hydrothermal system may have, at least in part,
contributed to the explosive activity associated with the caldera-forming eruption
(c.f. Criswell, 1987; Scandone, 1990; Mellors and Sparks, 1991; Calvache and
Williams, 1992; Rosi et al., 1996), although the configuration of the aquifer system
and availability of water to the magma chamber at the time of the eruption is not
known.