Supplemental Material, Sahetapy-Engel et al., Bulletin of Volcanology, 2014: Deposition and
generation of multiple widespread fall units from the c. AD 1314 Kaharoa rhyolitic eruption,
Tarawera, New Zealand
Table S1 Locations of selected measured sections and photographs for Kaharoa study that are
mentioned in the Bulletin of Volcanology article.
Table S1: Grid references and latitude/longitude of locations mentioned in text
o'
Location
Sector
NZ 1:50,000 map
NZ 1:50,000 map grid ref. E Long; , ,"
K46
SE
260-V16 Tarawera
1907 1606
176 32 25
K48
SE
260-V16 Tarawera
1709 1505
176 31 11
K66
SE
260-V16 Tarawera
1705 1906
176 30 45
K73A
SE
260-V16 Tarawera
2604 1102
176 36 15
K270
SE
260-V16 Tarawera
1706 1800
176 30 58
K291
SE
260-V16 Tarawera
1605 2300
176 30 08
K321
SE
260-V16 Tarawera
2608 1800
176 37 70
K437
SE
260-V17 Murupara
3080 9921
176 40 02
K496
SE
260-V17 Murupara
3940 8400
176 46 24
K708
SE
260-V16 Tarawera
1808 1505
176 31 56
S Latit ; o,',"
38 17 53
38 18 36
38 16 23
38 20 36
38 17 13
38 14 36
38 16 57
38 27 02
38 34 11
38 18 24
K21 / K305
K185
K413
K456
K600
K605
K607
S32
NW
NW
NW
NW
NW
NW
NW
NW
260-V16 Tarawera
260-V16 Tarawera
260-V16 Tarawera
260-V16 Tarawera
260-V16 Tarawera
260-V15 Edgecumbe
*260-U15 Ngongotaha
260-U16 Rotorua
1904 2802
1907 3108
2408 3204
1808 3509
3105 2302
1207 4900
0104 6603 approx
9005 3708
176 31 58
176 31 60
176 35 32
176 31 15
176 40 24
176 26 49
176 18 33
176 11 52
38 31 58
38 09 47
38 09 20
38 07 34
38 14 09
38 00 34
37 51 28
38 07 06
K402 1
NW
260-U15 Ngongotaha
9306 5009
176 13 49
37 59 52
InfoMap, I:50,000 series, 260-V16 Tarawera, Edition 1, 1988
1
* Published in 2000
near Kaharoa settlement
Supplementary Material S1:
Atmospheric conditions and timing of eruption
Assuming that New Zealand wind patterns and velocities have not changed significantly over the last 700
years, dispersal directions of Kaharoa fall units provide clues to the season of the eruption, a check on
wind speeds estimated from clast dispersal, and, possibly, a way of independently constraining the
duration of major explosive activity. The North Island of New Zealand has a weather system dominated
by westerly upper atmosphere winds, with more variable, but still predominantly westerly, winds at lower
elevations (the convention is that west winds blow towards the east). The frequency of near-surface wind
directions over the North Island show that 40% of the time the winds are westerly, 20% are each
northwesterly and southwesterly, ~ 7% are southeasterly, and less than 5% of the time winds are northerly
and southeasterly (Scott and Nairn 1998). These conditions also apply at higher atmospheric levels but
with more persistent westerly bias around the tropopause and above, at the height of influence on eruption
columns and plumes. Tarawera volcano is in the mid-latitudes (~ 38o S) and the tropopause height over
North Island is typically about 10 (winter) – 12 km (summer). Many years of high-level (up to ~ 16 km
altitude) wind data for New Zealand were tabulated by Reid and Penney (1982) for Auckland Airport, the
nearest North Island measurement site to Tarawera but ~ 200 km to the northwest. These data show that
westerly winds dominate throughout the year around the tropopause (9-14 km heights) and above, and
that velocities range from 22 through 34 m/s (27 m/s average) with the highest wind velocities at all levels
generally during June to September, the austral winter months.
In a study of probabilistic modeling of tephra fall dispersal from Tarawera volcano, based on an
eruption like the Kaharoa and performed for assessing future hazards, Bonadonna et al. (2005) used 3
years of 20th Century gridded zonal and meridional wind fields (after Kalnay et al., 1996), again assuming
that the same wind scheme existed in the early 14th Century. This resulted in an easterly dispersal for the
modeled fallout because it is the most common wind direction at all altitudes up to ~ 25 km in the data set
used; 86% of the wind levels sampled had westerly winds (blowing to easterly directions); of these 40%
were to the SE. This is also consistent with the frequency of surface winds cited above, and, in fact, many
NZ tephra deposits such as those from Taupo have dispersal axes to the east of source (e.g., Shane 2000).
Only 3% of the wind data has at least 15 levels of wind blowing between towards the W (180-360o) and
this occurs during the austral spring-summer (i.e., September through March); these levels are frequently
at heights of ~ 23-30 km. Data presented in Bonadonna et al. (2005) are also consistent with maximum
winds speeds being around the tropopause, a normal atmospheric condition.
We note that wind speeds estimated from the fallout of clasts in units F and J (20-30 m/s (mean of
97 km/hr) are consistent with wind at the tropopause seen in the modern data (22-34 m/s). These fairly
high velocities are consistent with the narrow and elongate isopachs for most of the Kaharoa fall units,
including those in the SE sector, which also have very limited upwind dispersal. The limited range in
dispersal axis azimuths within these units suggests that deposition occurred during a short period of stable
weather with consistently strong northwesterly winds. Such a regional wind pattern is fairly common
throughout the year (~ 40 % of the time) but has a weak dependency for being more common in the
austral fall (autumn) to winter (March-July), with the strongest winds at the tropopause and above during
winter (Reid and Penny, 1982). A strong northwesterly wind impacting the rising eruption columns to
maximum heights of 24-27 km followed by downwind sinking of the suspended clasts into a stronger
wind-field at the level of neutral buoyancy around tropopause levels, attenuating the tephra load to the
SE, explains the dispersal of units A-HSE. It is not possible to put a characteristic duration to such periods
of northwesterly winds but the timescale of about a week or more for episodes A-H the Kaharoa eruption
(see above) is not inconsistent. The summed estimate of duration of venting and deposition for each
lapilli-fall unit and generation of the capping ash layers, plus the lack of evidence for erosion between the
units, except for that between G and H (Fig. 2) which might have been brief, could be accounted for
during a short period of upper northwesterly winds (Figure 14)
During or after phase HSE, and while H PDCs were depositing, dispersal axis azimuths for fall
units HNW to L record a remarkable wind shift (see dispersal axes in Fig. 5a versus b). This must have
occurred rapidly in response to the onset of rare atmospheric conditions for this region (found in 3 % of
the atmospheric levels, and only at high altitude, during the 3-years of modern wind data shown by
Bonnadonna et al. 2005). Consistent with the brief minimum calculated duration of I-L episodes (~ 4
days), and the lack of evidence for erosion features between units of the NW sector, a wind system
dominated by southerly to southeasterly flow at tropospheric levels was established for a short while.
Atmospheric conditions dominated by southeasterly winds at altitude, with speeds consistent to those
estimated for unit J, occur most frequently during austral spring to summer months (Reid and Penny,
1982). This suggests that the later episodes (I-L) were in spring or summer (August to December)
following the winter eruption of episodes A-H, and lasted a few days, which agrees fairly well with the
estimated duration of the later episodes (HNW– L) (Figure 13).
References not cited in main text
Kalnay, E., et al. (1996), The NCEP/NCAR 40-year reanalysis project, Bull. Am. Meteorol. Soc., 77(3),
437–471.
Scott BJ, Nairn IA (1998) Volcanic hazard map of Okataina Volcanic Center. Environment Bay of Plenty
resource planning publication, 97/4.