Geology of New Zealand Field Trip Guidebook - ResearchGate

in historical times (in 1945 and possibly 1861), but little remains of these today. Lava domes
are bulbous masses of viscous lava slowly extruded from a vent. The radius of a lava dome is
typically between a few hundred metres and 1-3 kilometres. Extrusion of a lava dome in the
currently active summit vent poses few direct problems, all of which can be managed with
use of an "exclusion zone" around the summit. The lava flows at Ruapehu have occurred
from summit vents during the Holocene. Lavas are channeled down existing valleys at rates
that seldom threaten human life. Hazard zones in which all structures would be destroyed are
elongate and would extend typically several kilometres from the active vent (Fig. 9).
Figure 9: The distribution of lava flows at Ruapehu both prior to and since 10,000 years ago.
Pyroclastic flows. Pyroclastic flows appear to be uncommon at Ruapehu. Pyroclastic flows
are dense, rapidly expanding clouds of hot gases that transport particles in fluidized masses
down valleys and across surfaces of low gradient until the flows lose mobility by dissipation
of the gases. If the speed, volume and momentum are sufficient, pyroclastic flows may travel
uphill or across areas of irregular relief, but usually they tend to be channeled into river
courses and into depressions. Pyroclastic flows travel at speeds of up to 200 km/hour.
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