VolcäiioIogy
Bull Volcanol (1989) 51:51-68
© Springer-Verlag 1989
A facies model for a Quaternary andesitic composite volcano: Ruapehu, New Zealand W R H a c k e t t ~* and BF H o u g h t o n 2 1 Department of Geology, Victoria University of Wellington, New Zealand 2 NZ Geological Survey, Rotorua, New Zealand
Ruapehu composite volcano is a dynamic volcanic-sedimentary system, characterised by high accumulation rates and by rapid lateral and vertical changes in facies. Four major conebuilding episodes have occurred over 250 Ka, from a variety of summit, flank and satellite vents. Eruptive styles include subplinian, strombolian, phreatomagmatic, vulcanian and dome-related explosive eruptions, and extrusion of lava flows and domes. The volcano can be divided into two parts: a composite cone of volume 110 km 3, surrounded by an equally voluminous ring plain. Complementary portions of Ruapehu's history are preserved in cone-forming and ring plain environments. Cone-forming sequences are dominated by sheet- and autobrecciated-lava flows, which seldom reach the ring plain. The ring plain is built predominantly from the products of explosive volcanism, both the distal primary pyroclastic deposits and the reworked material eroded from the cone. Much of the material entering the ring plain is transported by lahars either generated directly by eruptions or triggered by the high intensity rain storms which characterise the region. Ring plain detritus is reworked rapidly by concentrated and hyperconcentrated streams in pulses of rapid aggradation immediately following eruptions and more gradually in the longer intervals between eruptions. Abstract.
Centre, at the southern end of the Taupo Volcanic Zone (Fig. 1). Ruapehu is a very active composite volcano with a long record of historical activity including eruptions in 1969, 1971, 1975, 1977, 1978, 1979, 1980, 1981-82, and 1988. Destructive lahars accompanied the 1969, 1971, 1975 and 1977 eruptions. The volcano has been mapped in detail by Hackett (1985) and its historical eruptions studied by Gregg (1960), Healy et al. (1978), and Nairn et al. (1979). Ruapehu consists of a volcanic cone dominated by sheet lavas and autobreccias, and a surrounding ring plain of reworked detritus (Cole et
Introduction
Ruapehu is one of five large dominantly andesitic composite volcanoes within Tongariro Volcanic * Present address: Department of Geology, Idaho State University, Pocatello, Idaho, USA Offprint requests to: BF Houghton
Fig. 1. Map showing andesitic volcanoes and ring plains of Tongariro Volcanic Centre, after Houghton and Hackett (1984)
52
Hackett and Houghton: A facies model for a Quaternary andesitic composite volcano
al. 1986). Exposure on the cone is excellent, with continuous outcrop along many steep glaciated valleys. Good exposures are rare on the ring plain, and few of the lensoid volcaniclastic units
can be mapped beyond single outcrops. Intercalated fall deposits have greater continuity and serve as valuable marker horizons on the ring plain (Topping 1973). This paper presents the first
4
~....................
:»~:~::~»~:!~:~~~::;~:; ~ ! :i!~~~i!!i ~~ii!:iii!~iiii~~iii~~~:~:« ....
Mn
6-
"~i~~;:::~!~ ....~ ~
J
"-------
] X.f~~
/
~
~ /
_ z~.~
unmapped lavas & t e p h r a
U. Tama Fig. 2. Oblique aerial photograph of the northern slopes of Ruapehu showing major cone-building formations (see Table 1). Te Herenga Formation (TH), Wahianoa Formation (Wa), Mangawhero (Mn) and Whakapapa (Wh) formations form the modern main cone of Ruapehu. Note the youthful (ca. 5-10 ka) lava field and cone erupted from a flank vent of the Whakapapa Formation in the centre of the frame. Tama Lakes in the foreground occupy two 9.7 Ka old explosion craters between Ruapehu and Tongariro
Hackett and Houghton: A facies model for a Quaternary andesitic composite volcano
53
facies m o d e l for the volcano, based o n n u m e r o u s m e a s u r e d sections o n the cone, m o r e limited data f r o m the ring plain sediments, and the detailed t e p h r a stratigraphy o f T o p p i n g (1974).
Structure and history R u a p e h u is a c o m p l e x c o m p o s i t e v o l c a n o built in at least f o u r cone-building episodes involving b o t h central (summit) and flank vents. Absolute age data f r o m the v o l c a n o are sparse. A single rock s p e c i m e n f r o m n e a r the top o f the oldest c o n e - f o r m i n g s e q u e n c e has b e e n d a t e d at 0.23 _+0.06 M a and two specimens f r o m the third cone at 24 a n d 26 K a (Stipp 1968). The volcanic cone has a v o l u m e o f 110 k m 3 to which taust be a d d e d at least 100 k m 3 o f s u r r o u n d i n g laharic, pyroclastic a n d epiclastic ring plain deposits (Latter 1986). The v o l c a n o (Fig. 2) rises to an elevation o f 2797 m. T h e single active vent, Crater Lake, forms the s o u t h e r n p o r t i o n o f a b r o a d s u m m i t region s u r r o u n d e d by several small glaciers and p e r m a nent ice sheets. H o l o c e n e eruptions have also occurred f r o m at least five other flank and s u m m i t vents along a N N E - t r e n d i n g lineament.
1
m
Fig. 3. Geological sketch map of the Ruapehu area modified after Grindley (1960). t Lower Tama Lake, h Hauhangatahi, o Ohakune Craters, r Rangitau Lakes. Holocene vents (stars) have erupted the Tama (vents 1, 3), Pinnacle Ridge (2), Iwikau (4), Crater Lake (5) and Rangataua (6) members of the Whakapapa Formation
Stratigraphy H a c k e t t (1985) defines four f o r m a t i o n s o n Ruap e h u (Fig. 3), each c o r r e s p o n d i n g to a cone-build-
ing episode (Table 1). The oldest, Te H e r e n g a F o r m a t i o n , ( > c a . 120ka) is d e e p l y e r o d e d and only e x p o s e d on the n o r t h w e s t e r n flank o f the
Table 1. Lithostratigraphy of Ruapehu composite volcano and satellite vents Formation/ unit
Approximate age (Ka)
Summit and flank vents Whakapapa 0- 15
Estimated Original Volume (km3)
Composition
Andesite-dacite (57-66% SiOa) Basalt to dacite (52-64% SiOz) Andesite (54-61% SiOz) Andesite
Mangawhero
15- 60
35
Summit and flanks of modern cone Summit region of Ruapehu
Wahianoa
60-120
45
SE quadrant of Ruapehu
65
N and NW Ruapehu
Te Herenga
> 120
2.6
Location of vents
(54-59% SiO2) Satellite vents Ohakune
c.25
> 0.5
18 km SW of Ruapehu
Pukeonake
c.25
> 0.5
15 km N of Ruapehu
? 500
6.5
Hauhungatahi
12 km NW of Ruapehu
Note that the present (eroded) volcano has a volume of 110 k m 3 (Latter 1986)
Basic andesite (57% SiO2) Basic andesite (56-57% SiO2) Basic andesite (55-57% SiO2)
Approximate output rate [km3/ka] 0.17 0.78 0.75 <0.50
54
Hackett and Houghton: A facies model for a Quaternary andesitic composite volcano
volcano. The Wahianoa Formation (ca. 60120 ka) is also eroded and forms the southeastern side of Ruapehu. Mangawhëro Formation (ca. 15-60 ka) forms the present high peaks and main cone. The Whakapa Formation (0- ca. 15 ka) has been erupted from four flank and two summit vents at Ruapehu. The Ruapehu deposits range from basalt to dacite (52-66% SiO2) with a strong predominance of medium K andesite. The summit and flank vents have erupted more evolved compositions and a wider compositional range with time (Table 1). Satellite vents have consistently erupted relatively primitive olivine-bearing basic andesites.
Historical eruptions Activity at Ruapehu has been recorded for only the last 120 years (Cole and Nairn 1975). Some 53 small (104-106 m 3) eruptions have occurred since 1950 (Latter 1986), from the western crater now occupied by Crater Lake. The lake has an area of 0.16 km 2 and a volume of 107 m 3, and currently has the shape of a steep-walled inverted cone with a maximum depth exceeding 180 m. The geometry of the lake has changed rapidly in recent years (Hurst and Dibble 1981) with maximum depths varying from 70 to 300 m. Representative examples of the two styles of historical activity are described below.
the last 20 years. The explosions are the result of interaction between vesiculating andesite magma and external water, at or close to the base of the strongly temperature- and density-stratified lake. The explosions have the typical surtseyan form of a central dark slug of ejecta pierced by ballistic blocks and surrounded by a white clast-poor steam collar. The smaller eruptions generate columns less than 100 m high and leave no visible deposits. Such small events commonly occur with periodicities of 1-2 hours for periods of several weeks to several months. Larger discrete eruptions (e.g. 1969, 1971, 1975) eject up to 3 x 106 m 3 of lake water, juvenile bombs and accessory blocks (Healy et al. 1978, Nairn et al. 1979). These larger eruptions produce three types of deposit: 1) near-circular zones of wet, cold surge and blast deposits, and ballistic blocks, extending up to two km from vent, 2) downwind lobes of airfall material often rich in accretionary lapilli, 3) tongues of poorly sorted laharic deposits. The products of historical phreatomagmatic eruptions have been rapidly eroded and reworked, and have left no recognisable deposits on the volcano. Streams have immediately reoccupied the valleys removing or modifying the lahar deposits. Most of the surge and fall units were deposited on snow and rapidly subjected to runoff and slope wash. Within the basin of Crater Lake, much of the deposits were removed during the eruption by drainback of ejected watet into the lake.
Crater Lake 1945 In March-July 1945 lava rose under Crater Lake gradually displacing the lake waters into the Whangaehu Valley (Gregg 1960). A lava dome had filled the crater by July, and a central vent formed, which grew as a result of explosive eruptions. By late November the dome was largely destroyed and a 100 m wide, 400 m deep crater had formed about the central vent. Ash from the explosions fell up to 250 km away. A new lake started to form immediately and rose to a level eight m above the pre-1945 surface. No recognisable products of the 1945 eruption remain, suggesting that earlier dome building episodes may also be unrecorded in the stratigraphic succession at Ruapehu.
Crater Lake 1965-1985 Numerous small phreatic and phreatomagmatic explosions have occurred through Crater Lake in
Summary o f historical eruptions Ruapehu has had n u m e r o u s 105-106 m 3 (DRE) eruptions in historical times. There is however little record of this activity on the volcano, and no permanent deposits may ultimately result from the 47 episodes of this size recorded in the last 120 years, as much of the products of this volcanism has been, or is being, transported to and redeposited on the ring plain. Ruapehu, and other very active explosive andesitic volcanoes, contrast with very active basaltic volcanoes like Mt. Etna which have higher overall effusion rates and, being predominantly effusive, produce a permanent record on the volcano. This lack of preservation of the primary products of small eruptions, particularly those of the explosive phases, is almost certainly also a feature of prehistorical events at Ruapehu. Its significance is discussed after description of the products of the prehistorical activity.
Hackett and Houghton: A facies model for a Quaternary andesitic composite volcano
55
Table 2. Lithofacies associations for Ruapehu composite volcano
Association
Principal lithofacies
Minor lithofacies
Other notable features
Central and flank vent
Irregular lava dornes and plug like intrusions -welded fall deposits - - vent breccias Massive and autobrecciated lava flows - - lahar deposits
Thin lavas - - lahar deposits
Tectonically oversteepened dips -- alteration
Fall tephra-reworked sediments - - dikes - - block and ash flow deposits Debris avalanche deposits -lava flows - - loess
Complex gully-filling morphology - - numerous apparent stratigraphic inversions Fall deposits strongly windinfluenced
Reworked sediments -- distal fall deposits from composite volcano
Relatively primitive chemical composition
Proximal cone-building
Distal ring plain
Satellite vent
Hyperconcentrated and normal stream deposits - - lahar deposits - - fall deposits Strombolian bomb beds -phreatomagmatic surge and fall deposits - - aa lava flows
Lithofacies and lithofacies associations
The prehistorical Ruapehu deposits form four distinctive associations of lithofacies (Table 2). These are (1) the central and flank vent, (2) the proximal cone, (3) the distal ring plain, and (4) the satellite vent associations. Associations 2 and 3 are broadly concentric about the central vents of each cone.
The host rocks of the intrusives are thin lava flows and laharic and vent-filling tuff breccias, similar to those of the proximal association but
Whakapapa Formation
Central and flank vent association
The Holocene vent-filling deposits are poorly exposed but include welded spattered deposits, the remnants of the historical lava domes and thin lava flows. The floor of the Crater Lake vent consists of severely hydrothermally altered volcaniclastic sediments. Steep dips and local unconformities are typical of the vent association. Discrete unwelded pyroclastic strata are rare probably due to rapid erosion in the vent region. Several flank vents have formed small discrete scoria cones during intervals of predominantly effusive eruptions. This association is also exposed in the cores of the eroded cones of the Te Herenga (Figs. 3, 4) and Wahianoa formations, at levels perhaps several hundreds of metres below the original cone surfaces. The association consists of irregular plug- and dome-like intrusions, thin lava flows, welded airfall deposits and vent breccias. Halos of pervasive low-grade hydrothermal alteration characterise the vent regions, and radial oversteepened dips (45-90 degrees) are present. The intrusions are microdiorite, mineralogically and chemically similar to the cone lavas, and are irregular in form and up to several tens of metres in diameter.
]
Welded ruft
[]
Lavas and auto brecci
Te Herenga Formation Vent Association []
Hydrothermally alterec ruft breccias and lava.,
•
Intrusive plu9s
Te Herenga Formation Cone-forming Association ]
Lavas and auto brecc J
j
Dike /
Fault
• . • . . •
:
~.i~..!::
, ,
N
i
~.i(.
0 I
500 m [
Fig. 4. Map of the exposed portion of vent and proximal coneforming associations of Te Herenga Formation, at Pinnacle Ridge, northwest Ruapehu. The vent association is delinated by the extent of hydrothermal alteration (cross hatching). Note the over-steepened dips in this region and the irregular form of the intrusive mass. The Te Herenga Formation is unconformably overlain by the Pinnacle Ridge Member of W h a k a p a p a Formation
56
Hackett and Houghton: A facies model for a Quaternary andesitic composite volcano
[____.
LEGEND WAHIANOA FORMATION Lava flows [ ] Bornb-bearing lahar unit MI~B~EPEAK
Fig. 5. View of proximal and central Wahianoa formation exposed in Whangaehu Gorge, southeastern Ruapehu. Position of section shown in Fig. 6 is indicated (RS3). Note the tocation of the bomb-bearing unit described in the text and Fig. 8
faulted, oversteepened and altered. Inner zones of alteration extend up to 30 m from the inferred vent feeders and consist of orange-brown gossan, in which the rocks are intensely shattered, contain pervasive disseminated sulphides and are cut by networks of clay veins. The mineral assemblages are dominated by montmorillonite and mixed illite/montmorillonite. The inner zones are surrounded by a green outer zone of pervasive alteration to clay with only minor sulphide. The outermost margins of the hydrothermal aureole consist of veins of colloform amorphous silica and kaolinite.
Proximal (cone-forming) association The proximal cone-forming association at Ruapehu is dominated by block lava flows with relatively minor airfall and laharic deposits (Figs. 5, 6). Flows and autobreccias form 88% of one km of measured sections from this association (Table 3). The cone-forming sequences consist of numerous valley-confined flows on average between 5 and 10 m thick, with dips up to 25 degrees. The flows are typically lenticular, with numerous interflow erosional canyons and unconformities, creating apparent statigraphic reversals. Most flows are block lava flows, although basic andesitic lavas are commonly of aa type. The flows are enclosed in carapaces of autoclastic breccia, consisting of slabby or rubbly oxidised blocks with sparse interstitial matrix. Brec-
Fig. 6. Stratigraphic section through 223 m of Wahianoa Formation shown in Fig. 5. Position of Fig. 8 is indicated by a r r o w . a is autobrecciated lava, b fall deposits, f sheet lavas, 1 laharic deposits and s intercalated sediments
Hackett and Houghton: A facies model for a Quaternary andesitic composite volcano
57
Table 3. Percentages of individual lithofacies within measured sections through proximal cone forming sequences at Ruapehu Section
Formation
Distance (km) from vent
1 2
Te Herenga Te Herenga
1.6 2.0
3 5 6
Wahianoa Wahianoa Wahianoa
% sheet lava
% Autobreccia
Total Lava
% Massive tuff breccia (laharic)
% Sediment
145 80
33.6 72.2
29.8 27.8
63.4 100.0
35.4 .
--
2.4 2.6 2.6
223 50 192
42.3 47.7 49.6
42.3 52.3 32.3
84.6 100.0 81.9
13.4 . 15.9
Mangawhero Mangawhero Mangawhero Mangawhero
0.5 0.5 3.2 3.6
76 42 117 117
75.0 74.8 70.0 73.2
25.0 19.4 28.2 26.8
100.0 94.2 98.2 100.0
. -1.1 .
Total cone forming sequences
1042
57.4
30.5
87.9
10.9
9 10 11 13
Thickness (m)
cia blocks are unflattened, but are frequently welded adjacent to massive lava. A transition from irregular, slaggy or clinkery, weil jointed lava, through welded coarse breccia intruded by thin discordant lava tongues, into matrix-rich breccia is common at flow margins. The proportion of autobreccias to massive lavas increases and the character of the autobreccias changes with distance from vent (Fig. 7). The large blocks become progressively rounded, welding disappears and the proportion of matrix increases significantly. Only the pervasive red oxidation distinguishes some distal autobreccias from laharic and debris avalanche deposits at Ruapehu. Numerous dikes intrude the cone-forming sequences (Fig. 4). The dikes are thin (0.5-5 m wide), identical in petrography to adjacent lavas, and aligned radially to central or flank vents. The dikes are steeply dipping and in places have glassy chilled margins. Some dikes clearly fed flank vents. Heterolithological massive tuff breccias are an important but minor constituent of the association (Fig. 8), forming 11% of the measured sections. The deposits are broadly lensoid and valley-confined, and consist of dm- to -m-sized blocks set in tuffaceous matrix. Many units possess discrete inversely graded bases and thicker massive tops; others are massive throughout. The deposits range from almost clast-supported aggregates to widely dispersed blocks in abundant matrix. The clasts are petrographically variable. Some units are dominated by clasts of a single andesitic lithology, whereas others contain a range of lithologies including fresh grey or black andesite, red oxidised andesite and andesitic scoria or pumice, with no one lithology predominant.
.
.
.
.
.
.
.
1.2 . 1.2 . 2.3 0.7 .
. 0.7
% Airfall
% Dike
0.7
0.5
0.8
--
--
--
3.5 --
---
0.4
0.1
.
Fig. 7 A-C. Progressive changes in lava flows of the Whakapapa Formation, with distance from vent. A one km from source. Incipient brecciation of margins of sheet lava but no discrete autobreccia layers; B four km from vent. Sheet lava overlying matrix-poor autobreccia dominated by angular clasts; C six km from vent. Autobreccia contains abundant matrix and subrounded clasts
58
Hackett and Houghton: A facies model for a Quaternary andesitic compositevolcano
Fig. 8. Massive tuff breccia containing juvenile bombs with delicate cooling cracks and aligned thermoremnant magnetism, Wahianoa Formation, Whangaehu Gorge Some deposits contain clasts with delicate cooling cracks a n d / o r breadcrusted exteriors (Fig. 8). There is little other unequivocal evidence that the units as a whole were bot (e.g., carbon, thermal alteration) or fluidised (density grading, segregation pods and pipes) at the time of deposition. A wide spectrum of mechanisms from 'cold' (water-lubricated) lahars to weakly energetic block and ash flows probably deposited these and other similar tuff breccias (Hoblitt and Kellogg 1979). Breadcrusted clasts in one unit within the Wahianoa Formation show consistently aligned thermoremnant magnetism, whereas subordinate lithic clasts and the matrix show near random alignment (Hackett 1985). This and similar deposits appear to result from effusion of lava (Hildreth pers. comm. 1986) or eruption of hot pyroclastic ejecta onto snow or ice, triggering a lahar which incorporated some cold, older material. Most of the remaining deposits were probably emplaced in a wholly cold state and are lahars (s.s.). Primary airfall deposits, recognisable by mantle bedding, are rare (0.4% in Table 3) in the coneforming sequences. This appears to reflect susceptibility to slope processes and stream erosion rather than a predominance of effusive over explosive volcanism. Airfall material is rapidly eroded on the steep cone slopes and requires special conditions for preservation. An undated but youthful andesitic pumice unit, up to 0.5 m thick, drapes the northern slopes of Ruapehu but is already severely modified by erosion so that original thicknesses cannot be determined. It is unlikely that this unit will remain preserved in the
stratigraphic record. The conditions favourable to preservation of fall ejecta include welding, deposition in small closed basins and rapid burial by resistant lava flows. The most striking example is the welded Pinnacle Ridge tuff (Hackett and Houghton 1985), preserved only as three conspicuous lobes up to 25 m thick, of welded ejecta over 0.8 km 2 close to vent (Fig. 9). Elsewhere on the volcano its nonwelded equivalent has been completely eroded. Sequences of cross- or planar-bedded weil sorted sand and gravel occur in the proximal association but rarely exceed one m in thickness. Pockets of massive and cross-bedded sand commonly occur in the otherwise matrix-free interstices of autoclastic breccias and airfall deposits, suggesting sediment is washed into the pore spaces during flooding and heavy slope wash, in a fashion analogous to alluvial sieve deposits.
Distal (rin 9 plain) association Ruapehu, and its neighbouring composite volcano Tongariro, are surrounded by coalescing ring plains (Fig. 1) constructed from redeposited volcaniclastic material and distal airfall tephra. The reworked deposits consist of laharic and debris avalanche deposits and their fluvially reworked equivalents. The ring plain forms a 615 km wide girdle surrounding the volcanoes, at altitudes of 500-1100m in the west and 9001100 m in the east. Its form is a series of coalescing fans or aprons radiating outwards from the volcano (Fig. 10). The modern plain is cut by a series of steep-walled gullies up to 150 m deep, eroded by the major streams originating on the cone. The ring plain preserves a more complete record of the explosive volcanism than the parent cone. Andesitic fall deposits from Ruapehu interfinger with Tongariro tephras and distal plinian rhyolitic ashes from the Taupo volcano (Topping 1973). The ratio of primary fall tephra to redeposited material decreases markedly from east to west, due to a predominance of westerly to southwesterly winds. Topping (1974) has established a detailed stratigraphy for the Holocene tephras of the ring plain (Table 4) but insufficient exposure has precluded similar studies of the older fall deposits. Two types of bed occur in the Holocene record (Fig. 11): 1) dm to m thick massive or laminated fine to coarse ash, with proximal fine lapilli beds, and
Hackett and Houghton: A facies model for a Quaternary andesitic composite volcano
59
A
B Y
-0 -2 4m
(a) J
(c) (b)
basal~
\\~
i II
Mt RuapehuOB 0 I
( 5 km I
Fig. 9. a Proximal (A) and distal (B) sections, 1.3 km apart, through the Pinnacle Ridge Member of Whakapapa Formation. Open symbols indicate juvenile clasts, black accessory lithics; b Aerial view of northwestern Ruapehu with the extent of the welded deposit outlined; e Isopachs for the basal Okupata Tephra (after Topping 1973). R Ruapehu Crater Lake, T Tama Lakes, N Ngauruhoe, C Central Crater Tongariro. Welded Pinnacle Ridge tuff shown in black
2) dm thick, mantle and internally bedded lapilli beds. The ash beds, which are often deeply weathered, have near circular isopachs, and have accumulated over extended periods of thousands of years (Topping 1973). The near-circular isopachs reflect that no single wind direction strongly dom-
inated during the prolonged time interval when these units accumulated. Each unit thickens and coarsens towards one or more major vents on the cones, and Topping (1973) correlates each with the more explosive phases of one cone-forming episode, eg. ash accumulating between 1800 and 2500 years ago was principally erupted during
60
Hackett and Houghton: A facies model for a Quaternaryandesitic composite volcano
Fig. 10. Holocene fan of ring plain deposits, Whangaehu Valley, southeastern Ruapehu. Debris is channelised in the steep gorge of the Whangaehu River before merging to form a broad apron on the ring plain. Asterix indicates source area for the Whangaehu lahars
construction of Ngauruhoe cone at Tongariro, whereas that of 9800 to 13 800 years ago was associated with a northern vent at Ruapehu. The second category, the mantle-bedded lapilli units, have lobate wind-influenced distributions over 103-104 m 2. Four units, of volume 0.11.0 km 3, have been recognised in the Holocene sequence (Topping 1974). Some of these units can be tentatively correlated with welded, coarsegrained fall deposits close to vent (Topping 1973, Houghton and Hackett 1985). The best example of this is the ca. 10 ka Okupata Tephra (Topping 1973), the isopachs of which indicate a source in the vicinity of Pinnacle Ridge (Fig. 9). The Oku-
pata Tephra is tentatively correlated with welded Pinnacle Ridge tuff which has been eroded from all localities between 1 and 4 km from source. Extrapolation of the isopachs back to source suggests, that of a volume of 6 x 107 m 3 within the 20 cm isopach, 16% is preserved as the welded proximal deposit, 54% was deposited as primary distal tephra on the ring plain, and 30% was eroded from the flanks of the cone and redeposited on the ring plain. The lapilli beds are the products of brief plinian or subplinian eruptions. The primary fall deposits of the ring plan are interbedded with mixtures of rhyolitic and andesitic loess. Three recognisable units formed dur-
Hackett and Houghton: A facies model for a Quaternary andesitic composite volcano
61
Table 4. Summary of Holocene andesitic fall deposits present on the Ruapehu ring plain, after Topping (1973). Volumes are extrapolated to include all tephra within one cm isopachs Age or time interval (yrs B.P.)
Bed type
Description
Inferred source
Uncorrected Volume (km 3)
Area within 1 cm isopach (km z)
Nature
0-1800
1
Bedded fine-medium ash with proximal scoriaceous lapilli beds
Principally Ruapehu Crater Lake but also northern and southern (Ngauruhoe) vents of Tongariro
2.8
4 x 104
1800-2300
1
?
1
Principally Ngauruhoe vent of Tongariro Principally Ngauruhoe
?
2300-2500
Minor amounts of bedded ash Bedded ash with proximal lapilli deposits
?
?
2500-4800
1
Weathered bedded ash and fine lapilli
?
?
4800-7400
1
2.3
3x
7400-9700
1
Very minor amount of principally ash-sized tephra Weathered bedded ash-fine lapilli
Principally North & Red craters of Tongariro. Subordinate contribution from Ruapehu and Tongariro Tongariro and Ruapehu vents
Products of innumerable small explosive eruptions of magmatic, phreatomagmatic and dome-building character Relative hiatus in explosive volcanism Frequent small explosive eruptions during growth of Ngauruhoe cone Numerous small explosive eruptions accompanying conebuilding at 2 Tongariro vents Relative hiatus in explosive volcanism
?
?
9700
2
Principally northern and southern Tongariro vents, subordinate contribution from Ruapehu Blue Lake vent of Tongariro
1.3
1
9700-9800
1, 2
?
?
9800
2
Tama Lakes, between Tongariro and Ruapehu North Crater Vent of Tongariro
0.4
6 x 10 3
9800
2
Pinnacle Ridge area of Ruapehu
0.2
--
9800-13 800
1
At least 50 units from Ruapehu are recognisable mostly from a northern summit
?
?
13800
2
A northern vent of Tongariro
0.2
4 x 10 3
Trilobate lapilli deposit consisting of vesicular scoria. Lobes are dispersed to NNW, NE and SE At least 3 lapilli beds separated by ash deposits NNE-dispersed single lapilli lobe of andesite/dacite scoria NNW-dispersed single lapilli lobe of andesitic scoria Succession of several metres of bedded but undifferentiated ash and proximal lapilli
NE-dispersed broad lobe of bedded andesitic lapilli
ing the last glaciation (Topping 1973), and windreworking persists to the present day, but principally in areas of minimal cover by vegetation. The tephra/loess sequences are cut by several erosional unconformities. Some are related to glaciation but others have been interpreted as sedimen-
10 6
Frequent small explosive eruptions
x 10 4
Produced by a single discrete subplinian eruption
Products of several subplinian eruptions over a short interval Produced by a single short subplinian eruption Produced by single short subplinian eruption Products of innumerable small explosive eruptions, accompanying development of northem summit vent at Ruapehu Product of a single discrete subplinian eruption
tary pulses triggered by larger discrete eruptions (Topping 1973). The principal components of the Ruapehu ring plain are lensoid, coarse grained volcaniclastic deposits, comprising both matrix-rich laharic deposits and better sorted fluvial sediments (Fig.
62
Hackett and Houghton: A facies model for a Quaternary andesitic composite volcano
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12). Rapid vertical and lateral lithological changes are common. A wide range of sedimentary textures are present but there are three principal lithofacies. 1) muddy, matrix-supported gravel without stratification or imbrication (Gms of Miall 1978), 2) clast-supported gravel, usually with poorly defined subhorizontal bedding (Gm of Miall 1978), and 3) trough and planar cross-bedded sand (Sp and
st).
The Gms beds are generally poorly sorted and unstratified, and contain large rounded to subangular andesitic blocks in an abundant tuffaceous matrix. Texturally most are muddy sandy gravels. Most units show a weak basal inverse grading, defined by an absence of large clasts, overlain by more massive tops. Coarse blocks are often concentrated near the top of units, and less frequently define weak internal bedding. Most units contain some metre-sized clasts. The Gms beds show most features of debris flows (Pierson and Scott 1985) ie extreme grain size range, matrix support, very poor to extremely poor sorting, and a lack of stratification. They can be interpreted as the proximal products of lahars originating on Ruapehu, and to a lesser extent, at Tongariro. The Gm beds are mostly coarse framework-
';
|yo~
V' I I I
eN
Fig. 11. a and b Isopach diagrams for type 1 fall deposits (massive ash deposits) of the Ruapehu ring plain, after Topping (1973). Note the near symmetrical distribution, resulting from extended periods of volcanism and variable wind direction. Isopachs are in mm; e and d Isopach diagrams for type 2 fall deposits (bedded lapilli units) of the Ruapehu ring plain, after Topping (1973). Note the lobate wind-influenced distribution. Isopachs are in mm; e Plots of area enclosed by isopachs versus thickness for type 1 (solid lines) and type 2 (dashed lines) deposits: also shown, for comparison are the extreme examples of plinian deposits (dotred lines), cited by Walker (1981). Tis Lake Taupo, N Ngauruhoe, TL Tama Lakes, and C Crater Lake, Ruapehu
supported gravels, which are typically lensoid in cross section. Their sorting is poor but significantly better than the Gms beds. The characteristics of the Gm beds are those of concentrated to hyperconcentrated stream sediments found associated with volcanic debris flows (Smith 1986, 1987a, 1987b). By analogy with historical mudflows (Janda et al. 1981, Harrison and Fritz 1982, Pierson 1985, Pierson and Scott 1985), these beds were deposited by a variety of processes during and immediately following larger eruptions. Some were probably emplaced by more distal and diluted lahars directly related to eruptions, others during pulses of rapid erosion and sedimentation following the eruptions. The distinction between Gm(a) and Gm(b) beds defined by Smith (1987a) for Deschutes Formation sediments is equally applicable to the Ruapehu Gm beds (Fig. 12). The Gm(b) beds predominate and are characterised by intercalated lenses of weil sorted sand and gravel, crude internal bedding, relatively little matrix, rounded clasts and often imbrication. Like the Deschutes Gm(b) deposits, they appear to be traction deposits from sediment-choked but not hyperconcentrated streams during pulses of rapid aggradation probably following major eruptive episodes. Gm(a) beds are more massive, poorly sorted with appreciable matrix content, and often
Hackett and Houghton: A facies model for a Quaternary andesitic composite volcano
o -
" •
° o
×
= c
Fig. 12. Stratigraphic section through upper portion of ring plain deposits at Makatote Viaduct, west of Ruapehu. Inset shows lateral variability within unit x. Gms are muddy matrixsupported gravels of probable laharic origin, Gm are clast-supported fluvial gravels, and Sp, St, and Sh planar- and troughcross-bedded and horizontally bedded volcaniclastic sands
normally graded. Clasts are predominantly subrounded, but range from subangular to rounded. They show all features of hyperconcentrated flow deposits (Smith 1986). The Gms and Gm beds contain a variety of andesitic clasts. Some units contain predominantly a single lithology; others clasts derived from at least three cone-forming formations. Both scoriaceous and dense nonvesicular clasts occur. The Sp and St beds occur as 1 to 5 m thick intercalations in the coarser volcaniclastic units. Well-sorted, discontinuous pebble and cobble bands occur in the S beds. Thicker units often contain well developed paleosols and the root channels of plants, indicating prolonged intervals of relatively little sedimentation. The deposits are offen channelised with steep 'under-cut' margins. The Sp and St beds are normal fluvial sediments, produced both during the rapid sedimentation after larger eruptions, and during longer time intervals between eruptions when the primary and
63
reworked pyroclastic products were subjected to slope processes and normal stream erosion. The Murimotu Lahar Formation of Grindley (1960), a youthful example of a debris avalanche deposit (Fig. 13), is exposed on the Ruapehu ring plain. Topping (1974) obtained a 14C date of 9540_+ 100 years B.P. from wood in the deposits. The deposits, with distinctive mound topography, cover an area of ca. 25 km 2 on the northwestern flank of the volcano (Fig. 14). The mounds rarely exceed 10 m in height and are formed from deposits of up to three flows. Material at the core of the mounds is dominated by hornblende dacite, not exposed on northern Ruapehu. Presently, hornblende dacite is only found in a region of very low elevation, north of Ruapehu at Tama Lakes (Fig. 2). It seems more likely therefore that the patent body of dacite for this avalanche from Ruapehu was removed by the collapse and its remains buried by the products of subsequent eruptions. The two younger units are thickest on the upflow side of the mounds and contain only the lithologies presently found at Whakapapa gorge eight km to the southeast. A source in this region seems likely for these units. A plausible model based on events at Mount St Helens in May 1980 (Christiansen and Peterson 1981) is that a body of dacitic magma rose to form a shallow dome under the Whakapapa area, and was largely destroyed and removed in the first of three related collapses.
Ngaur~noe~ /SH 47
UpperTama~ ~ LowerTarna
lakapapanuiGorge 0
innacleRidge o~?EHu ~ CraterLake
Fig. 13. Map showing distribution and source region of debris avalanche deposits of the Murimotu Lahar Formation. The path of the flow is shown as the light stipple pattern and the final deposits by the random line pattern
64
Hackett and Houghton: A facies model for a Quaternary andesitic composite volcano
Fig. 14. Surface mounds of the Murimotu Lahar Formation. Source region of the debris avalanche is shown by the graph D. L. Homer
The two following failures involved mostly older wall rock, following stress release, by failure of the dome.
Satellite vent association
Deposits of four satellite vents protrude through the ring plain of Ruapehu and Tongariro (Figs. 1, 3). The vent locations are influenced both by the N N E trend of Taupo Volcanic Zone and by normal extensional faulting within the Miocene-Pliocene sedimentary basement. Magma composition is generally basic andesite (Table 1) and monogenetic scoria cones and tuff rings are the typical landforms. The Ohakune Craters, 18 km southwest of Crater Lake, consist of an inner scoria cone and
arrow.
Photo-
outer tuff ring with two small explosion pits (Houghton and Hackett 1984). The scoria cone consists of an alternation of welded and unwelded strombolian block and bomb beds, and the tuff ring of alternating strombolian fall and phreatomagmatic fall and surge deposits. Rangataua Lakes, 4.5 km south of Ohakune (Fig. 3), occupy two small maar craters with poor exposure. Hauhungatahi, 12km northwest of Crater Lake, forms the eroded remnant of a small cone consisting of lava flows overlying a ca. 50 m thick sequence of phreatomagmatic pyroclastic deposits. Pukeonake is a 143 m high scoria cone built from primary and slope-influenced strombolian deposits (Napp 1983). Two small hills north of Pukeonake consist of massive basic andesitic lava,
Hackett and Houghton: A facies model for a Quaternary andesitic composite volcano
and several aa lava flows extend up to eight km north and west of Pukeonake. The predominance of strombolian and associated phreatomagmatic volcanism clearly reflects the less silicic, fluid nature of the satellite magmas. Their localised distribution means that the satellite vents have very limited input to the adjacent ring plain, although quantities of ash and lapilli are deposited up to five km from vent, and both cones and lava flows appear to have deflected drainage channels on the plain.
Discussion and interpretation
Style and frequency of Ruapehu volcanism Ruapehu volcano has grown rapidly as a result of eruptions from both flank and summit vents. The styles of volcanism have included: 1) small phreatomagmatic eruptions from summit, flank and satellite vents, 2) strombolian eruptions from similar sites, 3) effusion of voluminous aa- and block-lava flows from summit and flank vents, 4) extrusion and disruption of summit and probably flank domes, 5) infrequent subplinian eruptions. Magma discharge rates averaged over the life of individual cones vary from 0.2 to 0.8 km3/ka (Table 1). These rates are an order of magnitude less than the very active rhyolite calderas to the north (Wilson et al. 1984), but the products of Ruapehu volcanism are localised over a much smaller area. On a smaller scale the historical record suggests frequencies of less than 10 years for eruptions of magnitude 106 m 3 (DRE) and less than 100 years for events of magnitude 107 m 3. The Holocene record, at least, can be subdivided into three types of activity: 1) cone-building intervals with numerous small explosive and extrusive eruptions (eg 0-1800 years BP, 2300-2500 years BP), and rapid sedimentation on the ring plain, 2) less frequent larger explosive eruptions followed by intense erosion and sedimentation on the ring plain (eg 9700-9800 years BP, 13 800 years BP), 3) intervals of hundreds or thousands of years with little or no eruptive activity (eg 1800-2300 years BP, 4800-7400 years BP), and erosion and resedimentation on the ring plain. The first category of activity results in a near-continual supply of reworked tephra to the ring plain, but at rates that are seldom sufficiently high to
65
cause major modification to drainage systems. The second category, the infrequent larger events, produce near-instantaneous deposition of 0.1 to 1.0 km 3 of tephra on, and adjacent to, the cone, initiating periods of intense erosion and sedimentation on the ring plain.
Processes on the volcanic cone(s) Both lava domes and flows have been extruded during growth of Ruapehu. The role of dome growth is probably underestimated as their preservation potential is low. Dome growth at Ruapehu is apparently commonly followed by explosive disruption (e.g., Crater Lake 1945) or sector collapse (e.g., Murimotu Lahars) and therefore, on at least some occasions, it contributes more to ring plain sedimentation than to cone-building. In contrast massive and autobrecciated lavas are the dominant constituent of the Ruapehu cones. During cone building episodes lasting hundreds to thousands of years, rapid successions of flows produce lava fields covering 5-20 km 2. Complex channelling and unconformities suggest that intervals of severe erosion and weathering separate short eruptive periods within each conebuilding episode. The Ruapehu flows are predominantly block lavas. Proximal exposures contain mostly sheet lava with only thin welded autobreccias which consist of large angular blocks and little matrix. Distal autobreccias are prone to erosion and are probably an important source of detritus for the ring plain. In rare cases, e.g. the Tama member of Whakapapa Formation, sheet lava flows extend on to the ring plain, but the most complete records of effusive volcanism are preserved in the proximal cone-building sequences. Explosive eruptions have occurred frequently at Ruapehu and probably matched the contribution of effusive volcanism to growth of the volcano. However, primary products of explosive phases are generally found only on the distal ring plain. Proximal deposits are rapidly reworked from the cone during periods of accelerated erosion after eruptions. Pyroclastic strata are only found on the cone where special conditions have prevailed. The conditions include rapid burial, primary welding, or deposition in local basins or depressions. The effects of explosive eruptions at Ruapehu have been to deposit thick ephemeral pyroclastic accumulations on steep, often snowcovered or water-saturated slopes. Much of this material probably was removed during the eruption by lahars and the remainder subjected to
66
Hackett and Houghton: A facies model for a Quaternary andesitic composite volcano
rapid stream erosion and slope wash. The lack of any mappable deposits on the cone from the 47 moderately sized explosive eruptions recorded in the last 120 years suggests little of the history of explosive activity has been preserved on the cone. The distal ring plain record suggests extended intervals of numerous small explosive eruptions, and lava extrusion alternating with discrete larger explosive events of volume 108-109 m 3, which probably lasted only hours or days. While glaciation has probably played a significant part in construction of the cones, its contribution is difficult to quantify at Ruapehu. The summit area is currently occupied by small, actively retreating alpine glaciers. More extensive older moraines occur on and beyond the flanks of Ruapehu (Fig. 3), correlated with at least three major advances in the last 100 Ka (Hackett 1985). The role of glaciation on the cones has been largely erosional, rather than constructional, and glaciation of the tones has carved deep gullies, which have often been infilled during subsequent eruptive episodes. At least one episode of cone collapse at Ruapehu has redistributed parts of three of the tones from the northwestern flank of the volcano. Earlier episodes are more difficult to recognise because erosion and subsequent volcanism have probably masked the distinctive debris avalanche morphology. The Murimotu debris avalanche suggests that such failures have had the capability at Ruapehu of removing entire dornes or lava flow fields from source, leaving no trace in the stratigraphic record (cf Voight et al. 1981). Smaller scale gravitational collapses have probably been widespread on the steep often metastable slopes of Ruapehu. These range from small rock falls and avalanches which merge into the talus and slope material on the volcano, through to lahar-producing events like the collapse of the southeastern margin of Crater Lake that was indirectly responsible for the deaths of 151 people in 1953 (Healy 1954). These small events leave no permanent trace on the volcano but contribute to the outward movement of detritus from cone to ring plain. Stream and slope processes have been very active on the modern cone, but fluvial and alluvial deposits are seldom preserved unless some favourable circumstances create sediment traps. These circumstances include ponding in valleys dammed by lava or pyroclastic deposits or in summit or flank craters, and deposition in the open framework of well-sorted breccias or agglomerates.
Rin 9 plain deposition The Ruapehu ring plain is a complex, unstable and rapidly evolving sedimentary environment. Volcanism and climate create steep slopes on the volcanic cones and a denuded landscape ensuring a continual supply of coarse detritus to the ring plain. Much of the ring plain formed during glacial maxima and has undergone only minor modification during the Holocene. These deposits are less well exposed than the Holocene strata and fewer inferences can be made about their deposition. The snowline was depressed by approximately 1000 m at the height of the last glaciation (Topping 1973), with an area of 56 km 2 above this level. The rates of supply and deposition of sediment on the ring plain were probably even higher during these times. Vessell and Davis (1982) propose a cycle of eruptive, posteruptive (fan-building) and interruptive phases for Guatemalan volcanoes. A similar concept applies to at least the Holocene portion of the Ruapehu ring plain, complicated by the occurrence of both large infrequent explosive eruptions, and prolonged cone-building phases, consisting of many smaller eruptions. There may be little distinction between the processes occurring on the ring plain during eruptive and posteruptive phases (sensu Vessel and Davis) during the prolonged intervals of small cone-building eruptions, although sedimentation rates probably fluctuated markedly in response to eruptive pulses. Volcanogenic detritus commonly undergoes three processes during a large eruption and the sedimentary cycle which follows. The processes are
1) transportation into the ring plain by lahars, pyroclastic falls and flows and debris avalanches,
2) rapid reworking into valleys and modification by hyperconcentrated streams seeking to reoccupy their former courses, 3) fluvial reworking over a longer time period. Much of the material entering the ring plain is transported by lahars, during and immediately following large eruptions. These include lahars which are direct products of volcanism and others resulting from the influence of torrential rainstorms on unstable pyroclastic deposits. Ruapehu is prone to voluminous lahars, because of the presence of the summit snow- and ice-fields and the crater lake. Some older lahars may have been produced by extrusion of lava flows or dornes onto snow-covered slopes (Hackett and Hough-
Hackett and Houghton: A facies model for a Quaternary andesiticcompositevolcano ton 1986). Debris avalanches have also transported large quantities of material infrequently on to the ring plain. Fan growth at Ruapehu is, and probably has always been, most rapid immediately after the large infrequent explosive eruptions, because of the increased availability of readily eroded material and enhanced surface runoff. The discrete subplinian lapilli beds produced during the eruptions were probably sufficiently widespread and thick to have caused changes in the patterns of sediment movement and dispersal, especially in the headwaters of valleys. Streams were probably dammed or choked by tephra, rates of snow ablation increased in the headwaters, and high sediment loads maintained for extended periods. There are therefore two components of such deposits which play roles in the development of the ring plain, the distal primary tephra deposited directly on the plain, and redeposited proximal deposits, which were rapidly eroded from the cone. The post-eruptive sediment load of streams draining Ruapehu and other composite volcanoes is exceptionally high. Once material reaches the ring plain it is rapidly subjected to erosion and reworking by sediment-laden streams struggling to occupy courses frequently blocked and modified by eruption. Laharic and primary deposits block and modify pre-existing drainage and so unstable channels form, widen and migrate to adjust to the new topographic and hydrologic conditions. Older fluvial sediments are also closely and complexly associated with lahars in the ring plain (Fig. 12). These deposits formed in rapidly migrating, unstable channels and show complex scour and fill relationships. The character of many lahars changes on leaving the cone, and they become floodlike concentrated stream flows. The resulting deposits form a spectrum of laharic to fluvial, coarse-grained, poorly sorted deposits, which alternate with primary fall tephra. The patterns of sediment movement following the 18 May 1980 eruption of Mt St Helens (Janda et al. 1981; Fink et al. 1981; Pierson 1985) are excellent models for the behaviour of Ruapehu and other ring plains. By analogy much of the "permanent" ring plain succession is established in short post-eruptive pulses. Wind erosion and loess deposition are also more significant immediately following larger explosive eruptions. There is also evidence that the plinian fall deposits from Taupo, the adjacent rhyolitic caldera, trigger enhanced erosion on at least the northern portion of the ring plain (Topping 1974). Less dramatic erosion and redeposition occurs
67
during the longer intervals, between the major explosive eruptions. Smaller explosive eruptions during prolonged cone-building episodes have contributed ash to the ring plain, but products of discrete events cannot be isolated. These smaller, strombolian, phreatomagmatic, and dome-related events have contributed much to the 'normal' sediment load of Ruapehu streams, without the disruptive effects on drainage of the subplinian eruptions. During the intervals of little or no volcanism, limited amounts of detritus enters the ring plain more gradually, transported by sheet wash and fluvial processes, during and following storms. The source terrain and the ring plain are both currently subject to high intensity rain storms unrelated to volcanism, The present New Zealand climate favours rapid erosion and reworking of the ring plain deposits in ephemeral river channels. Average rainfalls, for nine stations between 629 and 1100 m elevation on the plain, range from 1059-2851 mm. Much of this rain falls as short high intensity storms (Robertston 1964). Major flooding of the valleys of the ring plain has been associated with all storms with greater than 100 mm precipitation. Twenty four major floods occurred between 1850 and 1950, producing rise in river level of up to e i g h t m and up to one m of overbank sediments (Anon 1957). Rainfall is evenly distributed throughout the year, but the spring thaw in the headwaters of streams imposes a seasonal influence on flows. During glacial times, the rates of erosion and reworking of the ring plain deposits were probably even higher. Summary and conclusions Composite volcanoes like Ruapehu, with moderate discharge rates of magma and a wet temperate climate, are complex dynamic volcano-sedimentary systems. While discharge rates of magma are less than those of large rhyolitic calderas, the products have a more localised distribution and are subject to rapid erosion and reworking. Long intervals are characterised by small (104-107 m 3) but frequent eruptions, providing a continual supply of detritus to the surrounding ring plain. Larger explosive eruptions trigger major pulses of ring plain sedimentation. Proximal pyroclastic deposits form on steep, often water-saturated or snow-covered slopes. Much of this material is immediately transported to the ring plain by lahars triggered by syn-eruptive seismicity and deformation, and high-intensity rainstorms. The remain-
68
Hackett and Houghton: A facies model for a Quaternary andesitic composite volcano
der is removed by sheet wash and rapid stream erosion. Extruded lava domes are frequently disrupted by explosive eruptions or by gravitational collapse. Only aa and block lava flows during the protracted cone-building intervals are capable of a major contribution to cone growth. For these reasons, the cone and the ring plain preserve complementary portions of the volcano's history. The products of explosive eruptions, primary and reworked pyroclastic beds, are concentrated on the ring plain; the cone is built up during extrusive phases of activity. Acknowledgements. Parts of this study were completed while WRH held a scholarship from the Tongariro National Park Board, and the work would not have been possible without generous support and encouragement from staff of the park. The manuscript benefitted greatly from reviews by R Cas, BP Kokelaar, lA Nairn, DNB Skinner, DA Swanson, CJN Wilson and particularly GA Smith. Constructive editorial advice from JG Gregory has greatly improved the paper.
References Anonymous (1957) Floods in New Zealand 1920-53. The Soil Conservation and Rivers Control Council, Wellington, New Zealand, 239 p Christiansen RL, Peterson DW (1981) Chronology of the 1980 eruptive activity. US Geol Surv Prof Pap 1250:17-31 Cole JW, Graham IJ, Hackett WR, Houghton BF (1986) Volcanology and petrology of the Quaternary composite volcanoes of Tongaririo Volcanic Centre, Taupo Volcanic Zone. Roy Soc NZ Bull 23:224-250 Cole JW, Nairn lA (1975) Catalogue of the Active Volcanoes of the World 22: New Zealand. International Association of Volcanology and Chemistry of the Earth's Interior, Naples, 156 p Fink JH, Malin MC, D'Alli RE, Greeley R (1981) Rheological properties of mudflows associated with the Spring 1980 eruptions of Mount St Helens volcano, Washington. Geophys Res Lett 8:43-46 Gregg DR (1960) Volcanoes of Tongariro National Park. NZ DSIR Info Ser 28, 82 p Grindley GW (1960) Sheet 8 -- Taupo. Geological Map of New Zealand 1:250 000. NZ DSIR, Wellington Hackett WR (1985) Geology and petrology of Ruapehu volcano and related vents. Phd Dissertation, Victoria University, Wellington Hackett WR, Houghton BF (1985) Pinnacle Ridge member, Whakapapa Formation: A welded airfall deposit from Ruapehu Volcano, Taupo Volcanic Zone. NZ Geol Surv Rec 8: 24-29 Hackett WR, Houghton BF (1986) C4: Active composite volcanoes of Taupo Volcanic Zone. NZ Geol Surv Rec 11:61-114 Harrison S, Fritz WJ (1982) Depositional features of March 1982 Mount St Helens sediment flows. Nature 299: 720-722 Healy J (1954) Submission to the Tangiwai Board of Inquiry. Report of the Board of Inquiry, Tangiwai Railway Disaster. Government Printer Wellington, 31 p
Healy J, Lloyd EF, Rishworth DEH (1978) The eruption of Ruapehu, New Zealand on 22 June 1969. NZ DSIR Bull 224, 80 p Hoblitt RP, Kellogg KS (1979) Emplacement temperatures of unsorted and unstratified deposits of volcanic rock debris as determined by paleomagnetic techniques. Geol Soc Am Bull 90:633-642 Houghton BF, Hackett WR (1984) Strombolian and phreatomagmatic deposits of Ohakune Craters, Ruapehu, New Zealand: A complex interaction between external water and rising basaltic magma. J Volcanol Geotherm Res 21:207-231 Hurst A, Dibble RR (1981) Bathymetry, heat output and convection in Ruapehu Crater Lake, New Zealand. J Volcanol Geotherm Res 9:215-236 Janda RJ, Scott KM, Nolan KM, Martinson HA (1981) Lahar movement, effects and deposits. US Geol Surv Prof Pap 1250:461-478 Latter JH (1986) Volcanic risk and surveillance in New Zealand. NZ Geol Surv Rec 10:5-22 Miall AD (1978) Lithofacies types and vertical profile models in braided river deposits. Can Soc Petrol Geol Mem 5 :597-604 Nairn lA, Wood CP, Hewson CAY (1979) Phreatic eruptions of Ruapehu: April 1975. NZ J Geol Geophys 22:155-173 Napp JB (1983) Physical volcanology of Pukeonake scoria cone, Tongariro Volcanic Centre, New Zealand. BSc (Hons) Dissertation Victoria University, Wellington Pierson TC (1985) Initiation and flow behaviour of the 1980 Pine Creek and Muddy River lahars, Mount St Helens, Washington. Geol Soc Am Bull 96:1056-1069 Pierson TC, Scott KM (1985) Downstream dilution of a lahar: Transition from debris flow to hyperconcentrated streamflow. Waters Resources Res 21:1511-1524 Robertson NG (1964) The frequency of high intensity rainfalls in New Zealand. NZ Met Serv Misc Pub 18, 48 p Smith GA (1986) Coarse-grained nonmarine volcaniclastic sediment: terminology and depositional process. Geol Soc Am Bull 97:1-10 Smith GA (1987a) The influence of explosive volcanism on fluvial sedimentation: The Deschutes Formation (Neogene) in central Oregon. J Sed Pet 57:613-629 Smith GA (1987b) Sedimentology of volcanism-induced aggradation in fluvial basins: Examples from the Pacific Northwest. USA Soc Ecc Pal Min Spec Pub 39:217-228 Stipp JJ (1968) The geochronology and petrogenesis of the Cenozoic volcanics of the North Island, New Zealand. PhD Dissertation, Australian National University, Canberra Topping WW (1973) Tephrostratigraphy and chronology of late Quaternary eruptives from the Tongariro Volcanic Centre, New Zealand. NZ J Geol Geophys 16:397-423 Topping WW (1974) Some Aspects of Quaternary History of Tongariro Volcanic Centre. PhD Dissertation, Victoria University of Wellington Vessell RK, Davies DK (1981) Nonmarine sedimentation in an active fore arc basin. Soc Econ Geol Paleontol Mineral Spec Pub 31:31-45 Voight B, Glicken H, Janda RJ, Douglass PM (1981) Catastrophic debris avalanche of May 18. US Geol Surv Prof Pap 1250:347-378 Walker GPL (1981) Plinian eruptions and their products. Bull Volcanol 44:223-240 Wilson CJN, Rogan AM, Smith IEM, Northey D J, Nairn lA, Houghton BF (1984) Caldera volcanoes of Taupo Volcanic Zone, New Zealand. J Geophys Res 89:8463-8484 Received June 25, 1987/Accepted March 1, 1988
