Fieldwork and physical parameters

The pyroclastic deposits that represent the largest explosive eruptions known from the Mt. Ruapehu geological record are mapped within the Bullot Formation, dated between 27,097 ± 957 cal years BP and ~10,000 cal years BP (Donoghue 1991; Donoghue et al., 1995b). The best exposures of these units are found on the eastern flanks of the volcano and upper Ring Plain, where detailed description of 158 stratigraphic profiles was carried out (Appendix A). Bed geometry, sedimentary structures, including contact types, bed forms and stratification, paleochannels, dunes, lenses, grain-size grading, and syn-depositional deformation structures (e.g., impact sags) were carefully recorded. Textural parameters, such as clast framework (matrix supported vs. clast-supported), particle sorting, roundness, sphericity, colours, pumice vesicularity and crystal content, size, and shape, as well as accidental clast types were initially macroscopically described in the field with the aid of a 10x hand lens.

This detailed description provided the key criteria for the interpretation of the particle transport and deposition mechanisms, as well as the identification of the range in types of eruptive units and time breaks bounding pyroclastic deposits. Pauses between eruption events are based on the recognition of paleosols and inter-eruptive fluvial architectural elements, such as incised channels filled with fluvial deposits, or sheet-like overbank/flood- plain units (Fisher and Schmincke 1984; Miall 1996). Paleoclimate is an important issue to consider with respect to development of paleosols and the preservation of primary pyroclastic deposits in the geological record at Mt. Ruapehu. During emplacement of the Bullot Fm., conditions varied from harsh periglacial climates during the last glacial period, along with warmer and wetter conditions in interglacial periods (Lowe et al., 2008). Hence, pedogenetic processes were highly limited, affecting the types of soils developed (Cronin et

al., 1996b), and the preservation potential of tephra units during periods of extensive

snow/ice cover was low, with extensive tephra remobilization by meltwaters and “snow slurry lahars” (Cronin et al., 1996a; 1997a). In addition, complex interactions occurred

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between tephras, snow, ice, and water, affecting the form and nature of deposits (Manville

et al., 2000). Here, syn-eruptive lahars or fluvial deposits are referred to as those generated

synchronously with the eruptive activity (i.e. included in the eruptive unit); those following tephra accumulation (i.e. bounding eruptive units) will be referred to as inter-eruptive units. The first tasks in the field were to characterise the unique lithofacies for each stratigraphic unit, necessary for stratigraphic correlation. The deposits corresponding to the five largest eruptions were identified and selected according to their significantly contrasting lithofacies that may be taken as representing end-member conditions and triggering controls in similar eruptive styles. A total of 100 detailed (mm resolution) stratigraphic profiles were drawn and analyzed, and 58 more were used for further correlation and mapping. In each location, the maximum thickness and the three maximum axes of the largest five pumice and lithic clasts (c.f., Rosi et al., 2001) of the corresponding pyroclastic fall deposits were measured to construct isopach and isopleth maps. Contours were digitally drawn on a 20 m resolution DEM, using ArcMap 9, to facilitate calculation of distances, areas, and circularity (expressed here as a shape factor: Sh = ((4πArea)/ Perimeter2)). During each procedure, the manual results were compared to contours derived from automatic interpolations (e.g. Natural neighbour/Kriging) generated within Surfer 8 (Golden Software) to better determine the dispersal axis. Long axes were measured from the vent and following the distortion of the corresponding contour; maximum short axes were measured in three different points perpendicular to the longest axis within an error of ±6 %. The fallout tephra volume (V) was estimated from both irregularly shaped, whole deposit isopachs and from the approximated ellipses following Pyle (1989) modified by Fierstein

and Nathenson (1992), Bonadonna et al., (1998) and Sulpizio (2005), to compare the effect

of thickness extrapolation to an infinite area. Plots of thickness (Tk) logarithm vs. the square root of the area (A1/2) enclosed by the corresponding isopach were produced. From the resulting straight-line graph, the extrapolated thickness (To) at the vent (A1/2 = 0) was determined, as well as the slope (k) of the line considering the thickness at two points a and b:

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Following Pyle (1989), V was calculated independently of the isopach shape, as: V = 2To/k2 [2] Equation 2 was valid in the case of a single line segment in the graph. However, in most of the cases tephra-fall deposits did not show a simple exponential or power-law decay in thickness with distance, and instead show multiple (often two to three) segments with different slopes in ln (thickness) vs (area)1/2 plots. When two segments are distinguished, the break-in-slope is referred as Aip and the relationship is described as:

ܶ௜௣ = ܶ଴݁(ି௞ ඥ஺೔೛) [3]

Bonadonna and Houghton (2005) obtained a general formula for volume calculations where

the field data are arranged in n segments, expressed as:

ଶ்ଵబ

௞_భమ + 2ܶ1଴ ቒ௞మ஺೔೛భ_௞మమା ଵ− ௞భ஺೔೛భ_௞భమା ଵቓ ݁൫ି௞భ஺೔೛భ൯+ 2ܶ2଴ቂ௞య஺_௞೔೛మ_యమା ଵ− ௞మ஺೔೛మ_௞మమା ଵቃ ݁൫ି௞భమ஺೔೛మ൯+ ⋯ + 2ܶ (݊ − 1)଴൤௞೙య஺೔೛(೙షభ)_௞య೙మ ା ଵ− ௞(೙షభ)_௞஺೔೛(೙షభ)ା ଵ

(೙షభ)మ ൨ ݁

൫ି௞(೙షభ)஺೔೛(೙షభ)൯ _[4]

where T ݊_଴, -݇_௡ and ܣ_௜௣௡ are the intercept, slope and break-in-slope, respectively, of the segment n. Bonadonna et al., (1998) explained the segmentation of these plots as a reflection of varying thinning rates with distance related to the accumulation of particles of different settling behaviour according to their sizes. Then, ܣ_௜௣ corresponds to the point where the sedimentation from the umbrella cloud changes from high to intermediate Reynolds number particles, and ܣ_௜௣ increases as column height increases. Research by

Sulpizio (2005) further elucidated the different methods to obtain erupted volumes

according to the properties of the deposit, in terms of preservation and data availability. For Mt. Ruapehu, the proximal-intermediate distance data are available, and then ݇_ଵ and ܣ_௜௣ should be re-calculated considering that high-Reynolds Number (Re) particles have a narrow range of settling velocities compared to low Reynolds Number particles. Since the exact proportion of the proximal (Vp) vs. total volume (Vt) ratio, minimum volumes are best obtained assuming ௏೛

௏೟ < 0.3 and:

25 ݇ଵ= 1.61 ටܣ௜௣ିଵ.଴ଶ , [6]

and the volume can be calculated applying Bonadonna et al. (1998; see equation 4).

Following Pyle’s (1989) method, the distance at which the thickness decreases to one half of its maximum value is referred to as “thickness half-distance” (bt), which describes the morphology of the deposit. Likewise, the distance at which the maximum clast diameter halves with respect to its maximum value is termed the “clast half-distance” (bc), and reflects the corresponding column height. The bc/bt ratio gives information about the particles dispersion, considering both the column height and wind effects. High bc/bt indicates rapidly thinning deposits with most of the particles accumulated close to the vent, while low bc/bt should correspond to widely dispersed deposits (Pyle 1989). In this context, bc/bt gives an estimation of the fragmentation index and allows the classification of the eruptive style. A traditional calculation of the fragmentation index (D), by extrapolating the area enclosed by the 0.01 Tmax isopach (Walker 1973), was also obtained.

The eruptive column height (Ht) was determined using the crosswind range and

downwind range obtained from the isopleths data and following the model of Carey and

Sparks (1986). The column heights obtained by plotting the downwind and crosswind

ranges of 8 mm and 16 mm clasts of known density were then averaged to estimate the mean column height and wind speed. In parallel, the level of neutral buoyancy (Hb) was

obtained following Pyle (1989) by obtaining the parameter bc, since:

–bc≈ 0.41 Hb/(Hb1/2 -7.3) , [7] And the total height Ht can be estimated following Sparks’s (1986) model:

Ht ≈ Hb/0.7 , [8] Finally, total column height was also calculated following Sulpizio (2005), considering that Ht is proportional to the proximal thinning rate (k) and:

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Once Ht was estimated, the volume discharge rate (Q), given in m3/s, was obtained following Sparks (1986) as:

ܳ = ቀ_ଵ.଺଻ு௧ቁଷ.଼଺, [10]

The mass discharge rate (MDR) is the mass (in kg) erupted through the vent per time unit

(seconds), and it was obtained graphically using the model of Sparks (1986) and Sparks et

al. (1997). By measuring the weight (in grams) of overnight dried (at 1000C) samples with

a KERN-HCB SOK20 high precision balance for a known volume of bulk-deposit (in cm3), a rough idea of the deposit density was obtained (cf. Arana-Salinas et al., 2010), from which whole-deposit mass was calculated:

ܶ݋ݐ݈ܽ ܯܽݏݏ = (݀݁݌݋ݏ݅ݐ ݀݁݊ݏ݅ݐݕ) ∗ (ܧݎݑ݌ݐ݁݀ ݒ݋݈ݑ݉݁) [11] The total mass is also a measure of erupted magnitude (M), expressed by Pyle (2000) as:

ܯ = ܮ݋݃ (ܶ݋ݐ݈ܽ ܯܽݏݏ) − 7 [12] The resulting eruptive parameters obtained for the five largest eruptive units identified within the Bullot Fm., are shown in Appendix B and Chapter 4.