Geology of New Zealand Field Trip Guidebook - ResearchGate

Volcanic Center and examine some of the hazards that arise when humans live in proximity
to active volcanoes.
Tongariro Volcanic Center—Welcome to the heart of Mordor. This andesitic center is the
youngest on the North Island. It consists of the volcanoes Tongariro (1967 m), Ngauruhoe
(2287 m), and Ruapehu (2797 m, the highest point on the North Island. If the weather is fine,
you can see the Tasman Sea to the west and the Pacific to the east from the crater rim of
Ruapehu. It truly is a compact country!). Ngauruhoe is a satellite vent of Tongariro, but it
has the best developed volcanic cone shape and is easy to pick out. If you’ve seen Peter
Jackson’s film adaptation of “Lord of the Rings”, you might recognize it as the basis of Mt.
Doom. Tongariro and Ruapehu are larger volcanic features, but because of their longer and
more complex histories, they do not retain the typical stratocone volcano shape. Both have
been modified by lava flows, side eruptions, lahars, landslides, small vent collapse and rock
avalanches.
The vent complexes (the mountains themselves) are surrounded by gently sloping
“ring plains”. Eruptions seem to have begun in this area roughly a million years ago and
continue to the present. Ngauruhoe last erupted in 1975 but has active fumaroles along its
flanks. Ruapehu last erupted in 1995-1996.
Ruapehu Volcano—Ruapehu is located at the southern end of the Taupo Volcanic Zone, a
thermally expanding segment of the Earth’s crust and the source of spectacularly explosive
eruptions over the last 2 million years. Because the crust is heating up here, it is expanding,
creating tension, extension, normal faulting and subsidence in the central axis of the Taupo
Volcanic Zone. Prominent active faults have developed to the east and west of Ruapehu
volcano, which are downthrown towards the mountain. These faults mark the boundary of
the Taupo Volcanic Zone in this region, which terminates 20 km south of Ruapehu’s summit
[Neal, 2006 #12].
Accumulations of andesite lava flows interbedded with fragmental rubble radiate
from the summit region forming a stratovolcano that rises 2000m from the surrounding
lowlands. As stratovolcanoes build up they become steep and have a propensity to collapse,
spreading outwards onto the surrounding lowlands and forming a roughly circular apron of
fragmented rocks (volcaniclastics) termed the “ring plain”. Ring plain deposits are mainly
derived from debris avalanches and lahars, but may also include some river (fluvial) and
glacial deposits. Mantling the lowlands are various thicknesses of volcanic ash forming the
parent material of most of the soils in the region. Due to the dominantly westerly wind
direction, ash is always considerably thicker to the east than to the west. A crater lake
dominates the summit area. When full to overflowing, the lake contains 8-10 million m 3 of
warm to hot acid waters. Overflow of the crater lake can generate huge lahars or volcanic
mud flows. Lahars have been responsible for most of the loss of life on the New Zealand
volcanoes (Neal et al. 2006).
The oldest lava sequences preserved on Ruapehu are exposed on the northern flank and
seem to date from about 300,000 years ago. The distinct stratocone was built between about
250,000 and 130,000 years ago. During this period, voluminous lahars were also generated.
These inundated the nearby hill country of uplifted marine siltstones, sandstones and
limestones, creating the extensive ring plain, especially to the north and to the west of the
Whakapapa catchment. The resultant deposits total more than 12 km 3 in volume. A second
cone-building phase occurred between 60,000 and 15,000 years ago, during which time lavas
of variable composition were extruded from several vents between Tahurangi and the
northern Summit Plateau. Lahars were also generated during this time, leading to further
deposition on the western and southwestern ring plain. Considerable volcanic ash also
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