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edifice features (e.g. stratovolcanoes or dome complexes), which were destroyed
by the subsequent caldera-forming eruption (Suzuki-Kamata et al., 1993; Cole
et al., 1998; Allen, 2001; Jurado-Chichay and Walker, 2001); or buried beneath
later caldera in-fill deposits (e.g. Krippner et al., 1998; Bryan et al., 2000). Altered
volcanic, plutonic and metasedimentary lithic clasts are often derived from deeper
source rocks (e.g. Eichelberger and Koch, 1979; Suzuki-Kamata et al., 1993; Bryan
et al., 2000) representing the pre-volcanic basement, earlier volcanic edifice
products, and the intruded plutonic–hypabyssal complex and surrounding
metamorphic aureole associated with ongoing magmatism. At some volcanoes,
hydrothermal alteration may be extensive at the surface (e.g. Yellowstone and Valles
calderas) and, in these cases, not all altered lithic clasts were necessarily derived from
deep source rocks.

2.4. Hydrothermally altered lithic clasts
Hydrothermally altered lithic clasts often comprise a significant fraction of the
lithic population in explosive pyroclastic deposits and have been used to infer
the presence of ancient hydrothermal systems in the subsurface (Heiken and
McCoy, 1984; Rocher and Westercamp, 1989; Suzuki-Kamata et al., 1993; Rosi
et al., 1996; Cole et al., 1998; Bryan et al., 2000; Thouret et al., 1999, 2002;
Adams et al., 2001). In many volcanic systems, they are the only samples of
hydrothermally altered rocks that are otherwise not exposed at the surface, yet they
are rarely used to understand hydrothermal systems. Common alteration processes
affecting volcanic edifices and surrounding basement rock include: hydrothermal
bleaching; glass devitrification; rock dissolution, fracturing and brecciation;
and replacement or cementation by secondary minerals (e.g. Rocher and
Westercamp, 1989; Lo ´pez and Williams, 1993; Suzuki-Kamata et al., 1993).
Secondary alteration minerals (e.g. clay minerals, quartz, carbonates, sulphides,
oxides, zeolites, serpentine, epidote, albite, chlorite and micas) are dependent on
the chemistry of the original rock, and the composition and temperature of the
hydrothermal fluids.
Hydrothermal alteration zones are likely to be discontinuous, concentrated
around volcano–tectonic structures (Rocher and Westercamp, 1989) and interact-
ing with contact metamorphic aureoles (Eichelberger and Koch, 1979). Increasing
alteration of the rock pile above magma chambers, especially along pre-existing
structures, destabilises volcanic edifices and increases the potential for caldera
collapse (Calvache and Williams, 1992; Calvache et al., 1997; Lo ´pez and Williams,
1993). Furthermore, interactions between magma chambers and large volumes of
hydrothermal fluids may contribute to the explosivity of volcanic eruptions
(Criswell, 1987; Scandone, 1990; Mellors and Sparks, 1991; Calvache and
Williams, 1992; Rosi et al., 1996), increasing country rock fragmentation and the
capability to expel lithic debris. However, although the abundance of altered lithic
clasts in a pyroclastic deposit implies the existence of active hydrothermal systems
before an explosive eruption, their presence alone cannot ascertain the role of
hydrothermal systems in initiating or fuelling the eruption itself.