Explosive volcanism has dominated the large phonolitic shield volcano
of Tenerife, the Las
Cañadas edifice, for the last 1.5 m.y. Pyroclastic deposits of the
Bandas del Sur Formation are exposed
along the southern flanks, and record the last two of at least three long-term
cycles of caldera-forming
explosive eruptions. Each cycle began with flank fissure eruptions of alkali
basalt lava, followed by
minor eruptions of basanite to phonotephrite lavas. Minor phonotephritic
to phonolitic lava effusions
also occurred on the flanks of the edifice during the latter stages of
the second explosive cycle. Non-welded
plinian fall deposits and ignimbrites are the dominant explosive products
preserved on the
southern flanks. Of these, a significant volume has been dispersed offshore.
Many pyroclastic units of
the second explosive cycle exhibit compositional zonation. Banded pumice
occurs in most units of the
third (youngest) explosive cycle, and ignimbrites typically contain mixed
phenocryst assemblages,
indicating the role of magma mixing/mingling prior to eruption. At
least four major eruptions of the
third cycle began with phreatomagmatic activity, producing lithic-poor,
accretionary lapilli-bearing
fallout and/or surge deposits. The repeated, brief phase of phreatomagmatism
at the onset of these
eruptions is interpreted as reflecting an exhaustive water supply, probably
a small caldera lake that was
periodically established during the third cycle. Accidental syenite becomes
an increasingly important
lithic clast type in ignimbrites up-sequence, and is interpreted as recording
the progressive development
of a plutonic complex beneath the summit caldera.Successive eruptions during each explosive cycle increased in volume,
with the largest eruption
occurring at the end of the cycle. More than ten major explosive eruptions
vented moderately large
volumes (1−[ges ]10 km3) of phonolitic magma during the
last two cycles. Culminating each explosive
cycle was the emplacement of relatively large volume (>5−10 km3)
ignimbrites with coarse, vent-derived
lithic breccias, interpreted to record a major phase of caldera collapse.
In the extracaldera
record, explosive cycles are separated by ∼0.2 m.y. periods of non-explosive
activity. Repose periods
were characterized by erosion, remobilization of pyroclastic deposits by
discharge events, and pedogenesis.
The current period of non-explosive activity is characterized by the construction
of the Teide-Pico
Viejo stratovolcanic complex within the summit caldera. This suggests that
eruptive hiatuses in
the extracaldera record may reflect effusive activity and stratovolcano
or shield-building phases within
the summit caldera. Alternating effusive and explosive cycles have thus
been important in the volcanic
evolution of the Las Cañadas edifice.
Facies analysis of pyroclastic deposits within basaltic tuff‐rings of the Auckland volcanic field, New Zealand
