MECHANISMS CONTROLLING EXPLOSIVE-EFFUSIVE TRANSITION OF TEIDE-PICO VIEJO COMPLEX DOME ERUPTION
Volcanoes are among the most amazing and yet
dramatic natural phenomena. Their beauty and, more than anything, their natural
richness have captivated the humans since historical times. But the same forces
that allow this natural development are responsible of one of the most catastrophic events in planet earth: the volcanic eruptions.
Tenerife island is a highly populated and touristic área. Teide and Pico Viejo stratovolcanoes are located in the center of the island but their return periods are way much longer that the mafic eruptions located in the dorsal vents. That’s why historically they haven’t been considered as a potential risk on the island. But, the truth is that they are active volcanoes and that they are capable of much violent and explosive phonolitic eruptions. A better knowledge of this type of eruptions and the main factors that controls these changes in eruptive styles are required to undertake a comprehensive volcanic hazard assessment of Tenerife island. In order to achieve that, we need to study the geology of the volcanic deposits to understand how the volcanic system have worked in the past.
Tenerife island is a highly populated and touristic área. Teide and Pico Viejo stratovolcanoes are located in the center of the island but their return periods are way much longer that the mafic eruptions located in the dorsal vents. That’s why historically they haven’t been considered as a potential risk on the island. But, the truth is that they are active volcanoes and that they are capable of much violent and explosive phonolitic eruptions. A better knowledge of this type of eruptions and the main factors that controls these changes in eruptive styles are required to undertake a comprehensive volcanic hazard assessment of Tenerife island. In order to achieve that, we need to study the geology of the volcanic deposits to understand how the volcanic system have worked in the past.
Phonolitic eruptions in Tenerife can occur in
two different vents: central vents (ex. Teide volcano) and numerous lateral
dome vents (around T-PV stratovolcanoes). Generally, eruptions originated in
central vents tend to be essentially effusive while dome eruptions present a
much more explosive behaviour. This explosive dynamic is typical of the
beginning of the eruption and then is followed by an effusive phase, with the emission
of lava flows. But, which are the driving
forces of the abrupt transitions between explosive and effusive activity during
an eruption?
Reality is very complicated: there
are a lot of factors that can interplay between each other to determine
eruptive style. Different studies have suggested, for example, compositional
and/or volatiles zonation in the magma chamber, magma ascent rate, degassing
processes, influx of new magma into the chamber, changes in the pre-eruptive
conditions, etc. As we need to simplify this, we chose to focus this study in the
pre-eruptive conditions, that is, the temperature, pressure and volatile
content (basically water content) within the differents parts of the magma
chamber related to each eruptive phase.
With that in mind, we have conducted a
petrological and mineral characterization of the different eruption phases of
Pico Cabras dome eruption with the objective
of identifying the pre-eruptive parameters that control these changes in the
volcanic activity.
For
that, we sample rocks from the explosive (pumices) and effusive (lava flows)
phases. Using both petrographic and scanning electron microscopes we can identify the minerals and glass that form that rocks. Also, we can know their chemical
composition by analysing them with an electron microprobe. With that data (chemical
composition of clinopyroxenes and their magma in equilibrium) we can use a
geothermobarometer to obtain the temperature and the pressure at which the
minerals were form. Also, using a geohygrometers (that uses the composition of
the feldspars and the magma in equilibrium) we can obtain how much water was dissolved in the magma. However, the reality is much more complicated because it is
really, really difficult, to find the exact composition of the minerals parental
magma. For that, we had to calibrate our results comparing them with experimental
petrology data.
Once the rocks, the minerals and the chemical
composition have gave us all the information about each eruptive phase, we can start
to understand how the magma chamber was and what were the processes occurring prior and during the eruption. Our results suggest the presence of a compositionally
stratified magma chamber at 1 kbar±0.5kbar in which the differences in the eruptive styles are controlled by the temperature
and the amount of volatiles dissolved in the melt. The explosive phase is
related to the upper part of the magma chamber at 725ºC±25ºC and 3,5-5 wt% H2O
and the effusive phase with the main body of the chamber at 880ºC±30ºC and
2,5-3 wt% H2O.
Feldspar zonation also show us that the
minerals travelled inside different parts of the magma chamber with slightly
different compositions in a process defined in the literature as self-mixing.
They also suggest that the eruption was triggered by an injection of mafic
magma right beneath the magma chamber (underplating). This injection doesn’t
imply magma mixing, but only the contribution of temperature could have
increased the energy inside the magma chamber and trigger the eruption.
Finally, we have also found evidences of
halogen volátiles (principally Cl and Br) in the pumice samples originated from the explosive phase: we have identified a sodalite crystal, a Cl-rich
mineral that hadn’t been found yet in recent T-PV magmas. The release of this kind
of volatiles into the atmosphere have a direct impact on the ozone layer
depletion and it's other factor that should be taken into account in future
risk studies.
Further reading and references in: http://hdl.handle.net/2445/141802
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