Journal of the Royal Society
of New Zealand abstracts
Stratigraphy, age, and
correlation of voluminous debris-avalanche events from an ancestral
Egmont Volcano: implications for coastal plain construction and
regional hazard assessment
Brent Alloway1,
Peter McComb2,
Vince Neall3,
Colin Vucetich4,
Jeremy Gibb5,
Steve Sherburn6,
and Mark Stirling1
1Institute
of Geological and Nuclear Sciences (GNS), Gracefield Research Centre,
P.O. Box 30 368, Wellington, New Zealand.
2Marine
Consulting and Research, ASR Ltd, 3/17 Nobs Line, New Plymouth, New
Zealand.
3Institute
of Natural Resources, Massey University, Private Bag 11 222,
Palmerston North, New Zealand.
417
Ruru Street, Waikanae, New Zealand.
5Coastal
Management Consultancy Ltd, 555 Esdaile Road, RD 6, Tauranga, New
Zealand.
6GNS,
Wairakei Research Centre, Private Bag 2000, Taupo, New Zealand.
Abstract Two
previously unrecognised debris-avalanche deposits have been identified
on the eastern flanks of Egmont Volcano beneath a thick mantle of
tephric and andic soil material that has mostly subdued their
topographic expression. The Ngaere Formation is a
c. 23 14C
ka large volume (>5.85 km3)
debris-avalanche deposit that is widely distributed over
320–500 km2
of the north-east, south-east, and south portions of the Egmont ring
plain. The second deposit, Okawa Formation, is a c. 105 ka
large volume (>3.62 km3)
debris-avalanche deposit that has been mapped over a minimum area of
255 km2
in northern and north-eastern Taranaki. Both debris-avalanche
formations contain axial facies with hummocks composed mainly of
block-supported brecciated andesitic debris. A less conspicuous
marginal facies, texturally resembling a mudflow, is more extensive. A
third debris-avalanche deposit (Motunui Formation) is extensively
preserved along the north Taranaki coast where it is truncated by a
c. 127 ka wave cut surface (NT2) and closely overlies a
c. 210 ka wave cut surface (NT3). The source of this
debris-avalanche deposit is unknown.
Side-scan sonar and shallow
seismic profiling have been useful in accurately delineating the
distribution of combined Okawa and Motunui debris-avalanche deposits in
the offshore environment but cannot distinguish between the two
deposits or enable onshore spatial and volumetric estimates for each
unit to be revised. However, the widespread occurrence of
debris-avalanche rock material offshore does emphasise the importance
of this lag material altering the orientation of the coast influencing
both wave climate and rates of coastal erosion. Similarly, the
extensive onshore occurrence of debris-avalanche rock material appears
to be a significant factor in widening of the north Taranaki coastal
plain and preservation of the NT2 and NT3 uplifted marine terrace
surfaces.
Initiation of collapse by
magmatically-induced seismicity is apparently common at many
stratovolcanoes. Emplacement of Ngaere Formation was immediately
preceded by a magmatic fall unit and is directly overlain by a closely
spaced sequence of 13 fall units. In contrast, there is no evidence to
indicate that an eruptive event triggered or immediately followed the
Okawa debris-avalanche event, but seismically induced gravitational
sliding cannot be discounted.
Egmont Volcano has repetitively collapsed
over its c. 127 ka history and has generated at least five voluminous
landscape-forming debris-avalanche deposits. Probabilistically-based
return times are calculated at c. 196714C
yr for volumes
≥0.15km3
and c.
21 00014C
yr for volumes ≥7.5km3.
Despite lower
return times in
comparison to tephra emission, Egmont Volcano is an inherently unstable
cone because it comprises interbedded lavas and unconsolidated
volcaniclastic deposits with a high slope angle overlying a faulted
basement of Tertiary sediments. Should eruptive activity recommence and
coincide with significant upper cone dilation, then the likelihood of a
gravitational cone collapse is expected to increase although critical
thresholds remain to be modelled. Fortunately, the Taranaki Regional
Volcanic Contingency Plan is based on pre-emptive evacuation which is
intended to minimise loss of life in advance of an eruptive and/or cone
collapse event occurring.
Keywords Taranaki;
Egmont Volcano; debris-avalanche; gravitational collapse; andesitic
tephra; late Quaternary
R04013; Received 13 August
2004; accepted 5 April 2005; Online publication date 27 July 2005
Journal of the Royal Society
of New Zealand
Volume 35, Numbers 1 & 2, March/June, 2005, pp 229–267
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