THE RELEVANCE OF PHREATOMAGMATISM IN THE VOLCANIC FIELD: THE IMPLICATIONS OF THE SOFT SUBSTRATE

Javier Agustín-Flores1, Karoly Németh1, Shane Cronin1, Jan Lindsay2 1Volcanic Risk Solutions, Massey University, Private Bag 11222, Palmerston North, 4442 2School of Environment, The University of Auckland [email protected]

Phreatomagmatism in the What is phreatomagmatism? Factors that control phreatomagmatism

- physical/chemical properties (ascent rates, temperature, viscosity, etc) Pyroclastic density currents (base surges)

Tuff ring

water table level

Thermohydraulic explosions Crater -Substrate rheology and hydrogeology (compressive strength, porosity, permeability; Dike folding, fracturing, jointing; water table level) Sensu stricto: magma encounters phreatic water Magma fragments as a consequence of thermo hydraulic explosions, the substrate is disrup- ted and a broad crater and a tuff ring is formed

Rooster-like jets

Base surges

The most common hazard associated to this Whent magma encounters a body of water instead eruptions is base surges that are clouds compo (not a phreatomagmatic eruption strictly speaking), sed of ash, steam, and liquid water which travel a tuff cone may form (example, North Head). Rooster- at velocities from tens of m/s to >100 m/s, up to tail-like jets and base surges are generated. 6 km from the vent at temperatures >100°C

(associated to phreatomagmatism) The phreatomagmatic phase may shift into a dry activity (forming a cone and/or a lava flow). A renewed phreatomag- m a t i c p h a s e i s p o s s i b e . T h e -Other: paleotopography, weather whole activity may start as a dry eruption Approximately 39 out of >50 vents show (example, Pupuke Moana) evidence of explosive magma/water interaction

FIRST The approach However Reconstruction of eruptive processes Research topic: The role of the geology/ (Stratigraphic and sedimentary characteristics There is a complex interrelationship hydrology of the substrate (Auckland of deposits physical and chemical nature between variables Volcanic Field) o f c o m p o n e n t s ) SECOND Reconstrucion of current and past geological Unconsolidated and hydrogeological conditions of the substrate

Northen AVF nested within Waitemata sediments Poorly consolidated Southern AVF nested within the Waitemata sediments + Plio-Pleistocene sediments

STUDIED VENTS PRELIMINARY MODELS OF PHREATOMAGMATISM IN THE AVF

Motukorea/Browns Island

Scoria cone Tuff ring 1)

Through the Waitemata sediments (Hayward et al, 201 al, et (Hayward

Deposition of moderate energy, wet/dry base surges with inter-bedded fall. Slight changes in magma composition during the phreatomagmatic phase which may have lasted few days to weeks. Variable temperature and velocity of base surges. Tuff ring formed by shallow seated explosions. Possible moderate, downward excavation into the poorly consolidated /Elletts Mountain Waiteamata sediments

Airport

Tuff ring 1)

Through the Waitemata sediments

+ Plio-Pleistocene sediment (Hayward et al, 201 al, et (Hayward

Deposition of low energy, wet base surges with minor subordinated fall. No changes in magma composition during the phreatomagmatic phase which may have lasted hours to days. Temperature and velocity of base surges <150°C and Irregular tuff ring formed by shallow seated explosions. Broad crater and shallow diatreme. Possible little excavation into <100 km/h respectively. the Waitemata sediments with most explosive activity within the unconsolidated Plio-Pleistocene sediments North Head/Maungaunika

1) Through the Waitemata sediments

+ body of water (Hayward et al, 201 al, et (Hayward

Deposition of low energy,wet/dry rooster-like jets, minor wet base surges, and fall. No changes in magma composition during the phreatomagmatic (sensu lato) phase which may have lasted from hours to few days. Relatively low temperature jets with short runout (<1 km). Tuff cone formed possible in shallow marine environments with little excavation into the Waitemata sediments (emergent-type-like volcanism) CONCLUSIONS

The occurrence of even a small dimension phreatomagmatic eruption in the present or near future in the AVF is of great concern, since the city of Auckland is located within the AVF. Therefore, the models for phreatomagmatism in the AVF need to be reinforced by doing the following : - Reconstruction of the eruptive history of other phreatomagmatic vents. But, good exposures are absent. Then geophysical methods need to be applied and systematic drilling must be carried out. - Generation of past, current, and future hydrogeological models. Strongly related to climatic studies and the understanding of the substrate structure which are poorly known. - Dating of vents and the understanding of generation, transport and emplacement of magma.