Page 156 - Caldera Volcanism Analysis, Modelling and Response

Case Study of the Abrigo Ignimbrite, Tenerife, Canary Islands 131

cause of a magnetotelluric anomaly up to 1 km below the current caldera floor
associated with the structural boundary of the caldera (Pous et al., 2002).
A currently active hot hydrothermal system on Tenerife is evident from
fumaroles on top of Teide, thermal groundwater anomalies (Albert-Beltra ´n et al.,
1990; Valentin et al., 1990), and from seismic (Jime ´nez and Garcı ´a-Ferna ´ndez,
1996) and magnetotelluric (Ortiz et al., 1986; Pous et al., 2002) data. The aquifer
system on Tenerife consists of a series of groundwater bodies/compartments within
fractured volcanic rocks and minor pyroclastic and alluvial deposits, which are
bound by relatively impermeable barriers (e.g. dykes, baked palaeosoils and
geological contacts), and are interconnected by permeable pathways, such as
fractures (Ecker, 1976; Pous et al., 2002).
Ancient hydrothermal activity on Tenerife may have varied with magmatic
activity during each phonolitic magmatic cycle associated with the Upper Group
(o1.6 Ma, Martı ´ et al., 1994; Wolff et al., 2000), reaching peak activity during the
evolution of each phonolitic magma chamber, then reduced to background levels
by climactic caldera-forming eruptions, subsequent removal of the plutonic heat
source and reconfiguration of the central aquifer system. Hydrothermal activity
may also have varied between individual eruptions associated with the same magma
chamber. Multiple generations of secondary minerals in altered lithic clasts record
variations in temperature and chemistry of the hydrothermal fluids during long-
term magmatic cycles and shorter periods of eruption and quiescence.
The pervasiveness of hydrothermal alteration is reflected in the strongly altered
lithic clasts. The higher proportion of reddish altered lithic clasts (A1) compared to
lighter-coloured altered lithic clasts (A3) within the Abrigo ignimbrite on the south
coast and the converse situation on the north coast reflects the asymmetry of the
hydrothermally altered wall rocks around the magma chamber/conduit. Hydro-
thermally altered basaltic lithologies probably made up over half of the altered wall
rock portion of the southern chamber/conduit wall. On the northern, and possibly
western, chamber/conduit wall, the hydrothermally altered rocks were predomi-
nantly of more felsic lithologies.
The Abrigo magma chamber had been active for some 36,000 years prior to the
Abrigo eruption and was the source of as many as 15 preceding plinian eruptions
(Nichols, 2001; Edgar et al., 2007). This was sufficient time to develop a mature
hydrothermal system. However, the state or intensity of hydrothermal activity at the
time of the eruption cannot be inferred from the presence of hydrothermally altered
lithic clasts within the Abrigo ignimbrite alone.

3.6.3. Evidence for a ‘‘Teide–Pico Viejo-like’’ stratovolcano
A constructive volcanic phase may have occurred in at least the 36,000 years before
the Abrigo eruption, represented in part by the preceding 15 or more plinian fall
deposits (Nichols, 2001; Edgar et al., 2007). Construction of the present
intracaldera Teide-Pico Viejo stratovolcano has been ongoing since the Abrigo
eruption (i.e. for a maximum of 202 ka) evolving from early basanitic to
predominantly phono-tephritic to phonolitic compositions (Ablay et al., 1998;
Ablay and Martı ´, 2000). A recent subplinian eruption (Mon ˜tana Blanca, B2 ka,