Page 124 - Caldera Volcanism Analysis, Modelling and Response

Case Study of the Abrigo Ignimbrite, Tenerife, Canary Islands 99

(e.g. Hausen, 1956; Aran ˜a, 1971; Martı ´ et al., 1994; Martı ´ and Gudmundsson,
2000) versus a landslide origin (e.g. Bravo, 1962; Anchochea et al., 1990, 1999;
Cantagrel et al., 1999; Huertas et al., 2002). The Abrigo eruption, representing the
last major explosive eruption on Tenerife, produced a moderate volume ignimbrite,
which is consistent with a caldera-forming eruption. Studies of lithic clasts from the
Abrigo ignimbrite, described here, confirm that caldera-collapse processes occurred
during this eruption.
This chapter firsts reviews the use of lithic clast compositional variations in
pyroclastic deposits to understand volcanic edifices and calderas, then provides a
case study of the Abrigo ignimbrite to further constrain the subsurface geology and
dynamic caldera processes of the Las Can ˜adas edifice. Although this chapter is
mainly concerned with lithic clast variations, reference is made to geochemical
variations among juvenile pumice clasts, which complement the results of lithic
clast analyses.

2. Review of Lithic Component Studies and Inferred
Caldera Processes
2.1. Lithic-rich pyroclastic deposits

Plinian fall deposits and ignimbrites, which are the deposits of gas-supported,
high particle concentration pumice and ash granular density currents or pyroclastic
flows (e.g. Iverson, 1997; Iverson and Vallance, 2001; Sulpizio et al., 2007),
constitute the main products of explosive caldera-forming eruptions, and are the
focus of this review. Pyroclastic deposits from eruptions of minor intensity (e.g.
subplinian, vulcanian) also occur at calderas and may contain lithic populations that
are useful for understanding eruption dynamics, although they are generally not
associated with caldera-collapse events. Lithic clast component studies rely on a
sound pre-existing knowledge of the distribution, stratigraphy and facies
architecture of lithic-rich pyroclastic deposit packages. The following is a summary
of the different types of lithic-rich deposits associated with plinian fall deposits and
ignimbrites.

2.1.1. Fall deposits
Most plinian fallout deposits consist predominantly of juvenile pumice lapilli or ash.
Lithic clasts, at any one location, are usually a minor component and finer-grained
than juvenile clasts as a result of hydraulic sorting (Cas and Wright, 1987), although
they are well-documented in many fall deposits (e.g. Aramaki, 1984; Heiken and
McCoy, 1984; Fierstein and Hildreth, 1992; Suzuki-Kamata et al., 1993; Allen
et al., 1999; Rosi et al., 1999; Thouret et al., 1999, 2002; Allen, 2001; Adams et al.,
2001). Some fall deposits contain graded or discrete lithic-enriched horizons,
which could reflect intermittent episodes of vent wall rock instability, collapse and
erosion or phreatomagmatic pulses (e.g. van den Bogaard and Schmincke, 1984;
Criswell, 1987; Macedonio et al., 1994; Perrotta et al., 1996; Bryan et al., 2000;
Jurado-Chichay and Walker, 2001; Schumacher et al., 2001).