Transport and deposition processes of the hydrothermal blast of the 6 August 2012 Te Maari eruption, Mt. Tongariro

Abstract

The 2012 eruption of Tongariro volcano (New Zealand) produced highly mobile, low-temperature, blast-derived pyroclastic density currents after partial collapse of the western flank of the Upper Te Maari crater. Despite a low volume (340,000 m³), the flows traveled up to 2.5 km from source, covering a total area of 6.1 km². Along one of the blast axes, freshly exposed, proximal-to-distal sedimentary structures and grain-size data suggest emplacement of the fining upward tripartite depositional sequence (massive, stratified, and laminated) under a dilute and strongly longitudinally zoned turbulent density current. While the zoning formed in the deposit in the first 1500 m of runout, the current progressively waned to the extent where it transported a nearly homogenous grain-size mixture at the liftoff position. Our data indicate that after the passage of an erosive flow front, massive unit A was deposited under a rapid-suspension sedimentation regime. Unit B was deposited under a traction-dominated regime generated by a subsequent portion of the flow moving at lower velocities and with lower sediment transport capacity than the portion depositing unit A. The final and slowest flow zone deposited the finest particles under weakly tractive conditions. Transport and emplacement dynamics inferred in this study show strong similarities between hydrothermal explosions, magmatic blasts, and high-energy dilute PDCs. The common occurrence of hydrothermal fields on volcanic flanks points to this hazard being an under-appreciated one at stratovolcanoes worldwide.

Appendix

Method for estimation of the total blast grain-size distribution

The total grain-size distribution was computed for each PDC unit (A, B, and C) and ballistic grain-size distributions using a total of 24 sampling locations inside a 5° fan bracketing the WNW axis (Fig. 1d). An axisymmetric geometry was chosen because of the radial spreads of both the PDC flow directionality vectors (Lube et al. 2014; Fig. 1c) and the ballistic block fans (Breard et al. 2014), and because along the WNW axis significant drainage and channeling into topographic lows and deviating from the axisymmetric spreading trend is absent (Lube et al. 2014). PDC deposit grain-size distributions are those introduced in this paper, while ballistic grain-size distributions are those reported in Breard et al. (2014) and Fitzgerald et al. (2014).

The volume of ballistic blocks was estimated as follows:

• 1. We mapped ballistic craters in the 5° fan and obtained a crater-size distribution.

• 2. We found an empirical law that relates block and crater diameters (Fig. 13).

  Fig. 13

  

  Crater diameter versus ballistic block diameter

• 3. Ballistic blocks were approximated as spheres, allowing us to estimate the total volume of ballistics from the block-size distribution within the fan.

The integration of each grain-size fraction of the PDC deposit was done as follows:

• 1. We numerically integrated the particle volumetric concentration C of a given size fraction (for a given unit X) noted “ _i ” with distance “x” from source: ${C}_i\left(\mathrm{unit}\;X\right)=\frac{1}{r}*{\displaystyle {\int}_{x=0}^rf\left({C}_i\right)}\;dx$, where r is the final flow runout.

• 2. The arc length, s, with distance is given by: ${s}_x=\frac{\theta \ast \pi }{180}\ast x$, where θ is the spreading angle (5°) in degrees.

• 3. C _i of each unit can then be rewritten as:

$$ {C}_i\left(\mathrm{unit}\kern0.5em X\right)=\frac{1}{A_{\mathrm{PDC}}}{\displaystyle {\int}_{x=0}^rf\left({C}_i\right)*{s}_xdx} $$

where the area of the deposit (A _PDC) in the fan is expressed as follows: ${A}_{\mathrm{PDC}}=\frac{0.5*\theta *\pi }{180}*{r}^2$.

By calculating the C _i for all fractions, we obtained the grain-size distribution of the three units. The total distribution of the PDC has been calculated using the respective volume of each unit A, B, and C. The volume of a certain unit X (V _{unit X} ) is estimated from the area of the fan and from the thickness of the unit T _{unit X} :

$$ \begin{array}{c}\hfill {V}_{\mathrm{unit}\kern0.5em X}={A}_{\mathrm{PDC}}*\frac{1}{r}{\displaystyle {\int}_{x=0}^r{T_{\mathrm{unit}}}_{\kern0.5em X}\kern0.5em dx}\hfill \\ {}\hfill {C}_i\left(\operatorname{PDC}\right)=\frac{C_i\left(\mathrm{unit}\kern0.5em \mathrm{A}\right)*{V}_{\mathrm{unit}\kern0.5em \mathrm{A}}+{C}_i\left(\mathrm{unit}\kern0.5em \mathrm{B}\right)*{V}_{\mathrm{unit}\kern0.5em \mathrm{B}}+{C}_i\left(\mathrm{unit}\kern0.5em \mathrm{C}\right)*{V}_{\mathrm{unit}\kern0.5em \mathrm{C}}}{V_{\mathrm{PDC}}}\hfill \end{array} $$

The full distribution of the blast was estimated from combining total distributions of A, B, C and ballistic blocks. Fine ash, which lifted off as a co-ignimbrite plume, was neglected in the calculation as the fallout had been deposited at further distances.