The Fluvial and Volcanic Processes Field Trip
Janey Nolan and Lex Chalmers
We have taken the opportunity to combine these field trip
options because both trips planned to visit many of the same
venues, and because we could not fill a small bus with either
trip successfully. In addition, as the field trip will show, the
fluvial and volcanic histories of the upper Waikato Valley are
inextricably linked (nowhere better than the Taupo breakout flood of about 1800 years B.P.) and the sites we will visit
provide an on-the-day connection to the resources on the
Conference CD. Paper copies of key resources will be
available on the conference bus.
We assemble for a 7:45am departure from Gate 2a of the
University of Waikato on Knighton Road. (See the University of
Waikato map provided on the Conference website.)
7.45 am depart Hamilton, and travel over the depositional
Waikato River valley floodplain, noting the flat land and the
deeply incised valley of the Waikato River. The older (Jurassic,
145m years B.P) greywacke sediments are visible to the east
of the Hamilton basin. Note also the conspicuous (andesite)
volcanic peaks of Pirongia to the southwest of Hamilton,
Kakepuku to the south and Maungatautari to the southeast.
We see the mature and incised Waikato River briefly near
Cambridge, before seeing the dammed waters of Lake
Karapiro (the most northerly of the hydro stations on the
Waikato) for three or four km before the ‘Piarere bend’. Note
the river terraces reflecting the variable flows of the river. We
then leave the river until we reach Atiamuri.
At Piarere we see the distinctive ignimbrite deposits
associated with the intense volcanic activity of the Ruapehuto-White-Island volcanic zone eruptions almost a million years
B.P. The ignimbrite sheet at Piarere forms the east bank of the
ancestral Waikato River which used to flow into the Hauraki
Gulf from this point. When deposition blocked this channel,
the Waikato overflowed into the Karapiro Stream and joined
the Waipa River before cutting and depositing in its own
channel, joining the Waipa much further north at
Ngaruawahia. Our journey will not encounter the Waikato
River again until we reach Atiamuri. In our travel between
Piarere and Tirau we see close-up the first of the older
(Pleistocene, 2.6m years B.P) ignimbrite volcanic deposits.
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The Waikato River catchment was affected by the Oruanui
eruption (26,500 BP). An aggradation period followed with
transport of the volcanic material that fell during the eruption.
One of the main sedimentary responses was the diversion of
the Waikato River from the Firth of Thames into the Hamilton
Basin. Fluvial terrace sets formed (the “Hinuera Formation”).
These deposits occur in both the Hauraki Plains and the
Hamilton Basin. The Hinuera Formation was generated by a
series of fluvial deposits composed of pyroclastic minerals.
Fluvial terraces in Waikato River.
TERRACE
AGE
Hinuera D
17.6 – 14 ka
Hinuera C
22.5 – 17.6 ka
Hinuera B
26.5 – 22.5 ka
Hinuera A
220 – 26.5 ka
Climate change around 17,600 BP reduced sediment yields
from the hinterland, causing incision of the channel and
trapping of the Waikato River into the Hamilton Basin.
Aggradational processes along this river were a result of
volcanic activity, whereas incision was initiated by climate
conditions.
From Tirau to Tokoroa we see initially some riverine
depositional geology, and clear evidence of airborne volcanic
deposition before we climb onto the Whakamaru ignimbrites of
the Volcanic Plateau near Putaruru. If you Google Earth the
Ignimbrite outpourings that constitute this part of the field trip,
you will see that they are generally flatter that the rhyolites and
andesites that we see elsewhere on our field trip.
8:45 am Arrive at Tokoroa, allowing five minutes to collect preorder coffee at McDonalds. Quick comfort stop available if
required.
The Whakamaru ignimbrites extend over 35km of our route,
from north of Putaruru until we descend across the younger
extrusions of the Haparangi rhyolites of Tar Hill to Atiamuri.
Atiamuri marks a transition from the extensive ignimbrites
(Tirau to Tar Hill) to the complexities of the viscous rhyolites of
the heavily faulted volcanic zone. The stops from here on are
characterised by late Pleistocene rhyolites.
Note the sentinel Pohaturoa, a natural defensive site for Maori,
perched high (714 m) above the Waikato River near Atiamuri. If
it was undammed, the river at this point would be actively
eroding as the fall in metres per km is still significant compared
to that of the Waikato to the north of Hamilton.
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15 minutes before we arrive at Aratiatia, we pass through
Oruanui which gives its name to the caldera eruption over 2
weeks, 26,500 year B.P.
9.55 am First stop, the Aratiatia Rapids, where the Mighty River
Power scheme has transformed the flow regime of the river,
affecting both erosional and depositional processes.
10.30 am depart Aratiatia Rapids and drive to the viewing point
on top of the hill overlooking Lake Taupo and the Control Gates
Talk briefly about the headwaters of the Waikato River system,
and the Taupo reservoir that determines the downstream
fluvial processes. Comment on the intersection of volcanic and
fluvial processes with reference to the Taupo break-out flood
caused by eruption-induced failure of a natural dam; see
resources available on the bus and Conference CD.
10.45-11:15 am At Huka Falls to discuss fluvial processes.
11.25 am. Arrive at Craters of the Moon (Karapiti). The site is an
extensive area of geothermal activity where groundwater is
superheated and forced to the surface. The reduction in
pressure causes the hot water to emerge as steam. Karapiti is a
tourism venue within the 25 sq km of the Wairakei geothermal
area. View fumaroles and mudpools. Craters up to 20m deep
have been formed by hydrothermal eruptions.
12.25 pm. Depart Craters of the Moon for the Fairways
restaurant at the Wairakei Hotel. Here you may select and pay
for the light lunch of your choice. The Moroccan wrap filled
with lamb strips, grilled vegetables, tzatziki and harissa is $14, a
burger is $15, and the vegetarian quesadilla is $13. Gluten free
meals are available.
1.30 pm. Arrive at Wairakei Research Centre of GNS (the
Geologic and Nuclear Sciences CRI) to hear presentation by Dr
Gill Jolly. Part of the presentation will focus on the use of the
GNS website (see http://www.gns.cri.nz/) for teaching.
2.30 pm. Depart Wairakei Research Centre for geothermal visit
to the viewing platform in the Wairakei Valley. Explore the
management of geothermal steam generation – how humans
have affected processes associated with volcanism – through
the re-injection of water into the geothermal system.
2.50-4-10 pm. Travel for Lake Karapiro. A five minute comfort
stop before viewing the dam and discussing the impact of the
dam on the natural processes.
4.30 pm. Depart for Hamilton arriving 5:00 pm.
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