Page 134 - Caldera Volcanism Analysis, Modelling and Response

Case Study of the Abrigo Ignimbrite, Tenerife, Canary Islands 109

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volume could be at least an order of magnitude greater than the calculated 1.8 km
proposed on land.
Two widespread depositional units of massive lithic-rich ignimbrite (Abrigo
Sur-A and C, Pittari et al., 2006) occur across the southern to eastern flanks
of the Las Can ˜adas edifice commonly separated by a discontinuous accretionary
lapilli-bearing fine ash layer (Sur-B) (Figure 3). The Sur-A unit commonly
contains cobble-sized lithic concentration zones, which are either (a) mono/
bilithologic and locally derived, occurring in the lower part of the unit, or (b)
heterolithologic and largely vent-derived, near the top of the unit. The Sur-C unit
generally has a lower relatively fine-grained, massive to stratified zone and an upper,
coarser-grained, massive, lithic-rich zone. Lateral facies variations in both the Sur-
A and –C units are strongly controlled by the underlying palaeotopography (e.g.
thinner, stratified ignimbrite packages occur on palaeotopographic highs, Pittari et
al., 2006). Along the northern and western flanks of the Las Can ˜adas edifice, the
Abrigo ignimbrite consists of multiple depositional units of massive, lithic-rich
ignimbrite.

Figure 3 Composite stratigraphic log of the Abrigo ignimbrite on the SW to NE £anks of the
Las Can ì adas edi¢ce. Abbreviations: HLCZ and M/BLCZ, heterolithologic and mono-/bilithologic
lithic pebble to boulder concentration zones, respectively; PCZ, pumice concentration zone.