Page 160 - Caldera Volcanism Analysis, Modelling and Response

Case Study of the Abrigo Ignimbrite, Tenerife, Canary Islands 135

distance from the source-vent region to the end of each transect (at least 10–
15 kms). If a relatively high rate of communition of altered lithic clasts had been
ongoing along the entire distance to the source region, then a significantly higher
proportion and size of altered clasts would have been ejected from the vent(s) than is
represented in the preserved coastal ignimbrite. A slight increase in the proportion
of mafic crystalline volcanic clasts (46–56%, Figure 11) downslope within the
Orotava Valley may be due to entrainment of basalt lithic clasts from the ground
surface. Additionally or alternatively, this may be an artefact of the pulverisation and
decrease in the proportion of vent-derived altered clasts.
Pittari et al. (2006) provide evidence that the massive ignimbrite facies of the
Abrigo ignimbrite was deposited by progressive or step-wise aggradation of
material from the base of the pyroclastic flow (Fisher, 1966; Branney and Kokelaar,
1992). The vertical variation in the proportion of different lithic types, especially
mafic crystalline volcanic and altered clasts (Figure 10), further supports the upward
aggradation of the ignimbrite deposit with a changing influx of lithic clast
componentry over time.

4. Conclusions

The detailed qualitative and quantitative lithic clast study of the Abrigo
ignimbrite highlights the diverse applications of lithic clasts in understanding syn-
and inter-eruptive processes within caldera systems. The abundance of syenite to
gabbroid and altered lithic clasts within the Abrigo ignimbrite show that, prior to
the Abrigo eruption, the active magma chamber was, at least partially hosted within
a deep mafic to syenitic, intrusive complex surrounded by a widespread zone of
hydrothermal alteration (Figure 14). Increasing hydrothermal alteration over time
may have destabilised the edifice and contributed to caldera collapse. A diverse
variety and abundance of fresh volcanic clasts, along with similarities between the
pre-Abrigo plinian eruptions and the recent subplinian eruption of the active Pico
Viejo-Teide stratovolcano suggest that a stratovolcanic complex may also have
existed prior to the Abrigo eruption (Figure 14). Differences in the proportions of
lithic clast types between the northern, western (San Juan) and southern flanks of
the Las Can ˜adas edifice and additional variations across the southern flanks, along
with geochemical variations in juvenile clasts, also provide unequivocal evidence
for syn-eruptive piecemeal caldera collapse and eruption from multiple vents.
Conduit fragmentation progressed to greater depths, along with increased eruption
intensity, during the latter stages of the Sur-C phase.
Lithic clasts, along with juvenile components, if studied within the stratigraphic
and facies framework of the pyroclastic deposits that host them, offer a unique
window into subsurface volcanic and hydrothermal processes. In many cases,
especially when structural components of calderas have been eroded or buried,
lithic clast analyses may provide the only evidence for caldera collapse events and
associated caldera dynamics. Component analyses of pyroclastic deposits provide
important data, which would constrain numerical and experimental caldera models.