Post-caldera eruptions and pyroclastic density current hazard in the Main Ethiopian Rift
Date
03/07/2020Author
Clarke, Benjamin Andrew
Metadata
Abstract
The eruption of a peralkaline rhyolite magma has never been observed, yet these eruptions
are amongst the most common in the Main Ethiopian Rift Valley since 1 Ma and
dominate the eruption record of volcanoes that have undergone caldera collapse. The unusual
rheological properties of peralkaline rhyolites, in combination with the lack of direct
observations of eruptions, means that the style and hazards associated with them is essentially
unknown. With 1 million people living within 10 km of a volcano in Ethiopia,
and numerous geothermal power stations being built directly on these volcanoes, understanding
their eruptive style and hazards is timely, and essential to robustly assess risk.
This thesis aims to evaluate the eruptive styles and pyroclastic density current (PDC)
hazards of post-caldera peralkaline rhyolite eruptions. I focus on Aluto volcano, a restless
caldera system which has seen a multitude of peralkaline rhyolite eruptions over at least
the past 16 Ka. By studying the deposits from these eruptions, I attempt to evaluate the
styles of eruptive activity these magmas undergo; whether they generate PDCs, how these
PDCs are generated, and how mobile they might be. I find that eruptions at Aluto occur
across the edifice, akin to a monogenetic field, and that each eruption tends to undergo
a very similar eruption sequence, albeit over a range of magnitudes. Eruptions typically
begin with the formation of an eruption column, generating tephra fall deposits. Whilst
investigating these deposits we have discovered, described and investigated a largely unrecognised
type of pyroclast which I term a ‘pumiceous achnelith’. Thermal and ballistic
modelling of these pumiceous achneliths indicates that pumice cones are generated by pyroclastic
material falling from the sides of these columns, accumulating around the vent.
Towards the end of the eruption, the eruption-column becomes unsteady, repeatedly collapsing
and re-establishing, generating multiple PDCs. These PDCs usually have a high
particle-concentration at their base, and tend to be confined to drainages. In most cases,
the final stage of the eruption is marked by the effusion of a silicic lava flow, though it is
uncertain how explosive this phase is. Using these insights, I have selected analogue PDC
data, combined with modelled collapse heights and a kernel-density vent-susceptibility
model, to inform a simple energy-cone model which estimates the inundation footprints of
hypothetical PDCs. I employ a Monte-Carlo approach to evaluate a full range of probable
eruption scenarios, and I find that the caldera, and its NW, N, and SE flanks, are particularly
prone to inundation by PDCs. I combine this with geospatial data of people and
infrastructure around Aluto to evaluate the collective risk, and the risk to individuals posed
by PDCs. In terms of collective risk, I find that although most PDCs are constrained to
within a few kilometers of the edifice, though it is still possible for more distal settlements
to be inundated during rarer high-magnitude events. The population of these distal settlements
is much denser than in local settlements, meaning that the collective risk is often
similar between proximal and distal locations. For the individual, PDC risk is much higher
closer to the edifice. I frame this risk amongst ‘everyday’ risks experienced by individuals
in Ethiopia, and establish that for residents on the volcano, the time-averaged yearly risk
of death by PDC is comparable to that of death by malaria, house-fire, malnutrition or
road traffic accident. Using current best-practices, I have produced PDC hazard maps for
different stakeholder groups at Aluto. Though there is still great uncertainty surrounding
the styles and hazards associated with peralkaline rhyolites globally, this work shows that
they’re capable of producing intense eruptions (intensity 7-10); generating moderate to tall
eruption columns (3-16 km), which can collapse to form pyroclastic density currents with
the potential to devastate local settlements and infrastructure.