Volcanoes - Ministry of Civil Defence and Emergency Management

In about 1315AD Tarawera volcano erupted with enormous
force, depositing rhyolitic ash over much of the northern
part of the North Island. This was the Kaharoa eruption.
Columns of ash may have continued for days or weeks, and
these were followed by slow extrusion of lava domes, with
intermittent ash eruptions, which may have continued for
four years (Nairn et al., 2001). If such an event occurred
today it would cause major disruption to the surrounding
area, as indicated in Table 1. Evacuation of some areas will
be inevitable.
The distribution of ash will largely depend on
the wind direction at the time of the eruption, and in an
eruption of this magnitude wind direction will change
several times. In the 1315 Kaharoa eruption, ash first
travelled southeastwards, and then northwestwards,
covering an area from Northland to Mahia Peninsula
with a layer of ash, with the greatest thickness occurring
between Tauranga and Murupara (Fig 1). The dominant
high level wind direction is from the west, therefore
areas to the east of the volcano are more likely to be
covered with ash, even small amounts of which will
cause severe disruption to all forms of transportation
(Table 2).
EVACUATION ZONES
The next eruption of this type in the Bay of
Plenty will most probably be from Tarawera or from
Haroharo volcano to the north. When this happens,
Thickness
of ash
0-2mm
2-20mm
Effects on roads
Road markings obscured, traction reduced (wet and
dry ash), visibility reduced as dry ash is remobilised
by traffic and wind. Steep hills difficult for 2WD
vehicles to climb.
Moderate hills become difficult for 2WD vehicles
to climb, steep hills impossible. Drifts cause larger
humps in road. Once dampened and compacted it
becomes firmer, easier to drive on.
Fig 1. Proximal distribution of Kaharoa ashfall deposits; isopach
values in cms (from Johnston et al. 2002)
the highest risk is within a 10km zone around the vent
area, and this becomes a Primary Evacuation Zone (Fig
2), including the Rotorua Lakes district (bordering Lakes
Rotokakahi, Tikitapu, Okareka, Tarawera and Okataina).
Residents of this area would have to be evacuated prior
to commencement of the eruption.
Residents on the north shore of Lake Tarawera
(along Spencer Road) would be the most vulnerable
to any eruption from Tarawera (Fig 3). This area is
likely to be completely devastated in a Kaharoa-type
eruption from Tarawera or Haroharo, particularly from
ballistic ejecta and thick near-vent ashfalls. Therefore,
it is imperative that this area evacuates pre-eruption.
An added problem is the single evacuation route into
and out of the area. Spencer Road ends at the edge of
20-100mm
100-
300mm
Slight inclines may be impassable to 2WD vehicles,
4WD vehicles need differential or hub locks to climb
moderate hills. Larger drifts (eg 300mm) may hinder
or stop 2WD vehicles on flat roads.
Uneven surfaces in the ash stop any 2WD vehicles,
compacted damp ash on flat surfaces is still able to
be driven on. 4WD utility type vehicles (not cars) may
be able to slowly progress on the flat. Drifts may need
to be cleared. Moderate inclines difficult, but may be
possible for experienced 4WD drivers. Steep inclines
generally impassable. Ruts easily formed on hills.
>300mm
Compacted ash may be driven on by 4WD vehicles,
softer patches may easily bog vehicles. Gradual
inclines possible on compacted ash, but after a few
vehicles, ruts in the ash will form, hindering uphill
progress for further vehicles.
Table 2. Ashfall effects on road transportation (from Barnard, 2003)
Fig 2. Evacuation Zones: Zone 1 = 10km radius (primary) zone;
Zone 2 = 40km radius (secondary) zone; Zone 3 = 100km radius
(potentially 'unsafe') zone.
A view looking northeast along the vents of the 1886 AD eruption, from Frying Pan Lake in the foreground
to Mt. Tarawera and in the background, the cone of Mt Edgecumbe. Photo: GNS Lloyd Homer.
47
TEPHRA
June 2004