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Late Quaternary glaciation of MtRuapehu, North Island, New ZealandJ. L. McArthur a & M. J. Shepherd aa Department of Geography , Massey University , Palmerston North ,New ZealandPublished online: 12 Jan 2012.

To cite this article: J. L. McArthur & M. J. Shepherd (1990) Late Quaternary glaciation of Mt Ruapehu,North Island, New Zealand, Journal of the Royal Society of New Zealand, 20:3, 287-296, DOI:10.1080/03036758.1990.10416823

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© Journal of the Royal Society of New Zealand, Volume 20, Number 3, September 1990, pp 287-296

Late Quaternary glaciation of Mt Ruapehu, North Island, New Zealand

J. L. McArthur* and M. J. Shepherd*

End moraines on Mt Ruapehu record extensive late Pleistocene glaciation and a much smaller late Holocene advance, The outermost moraines were fonned in association with an ice cap and outlet glaciers, which at the recorded maximum had a combined area of about 140 km2, The equilibrium line altitude of the ice body was 1,500-1,600 m, and the largest outlet glaciers extended downslope to approximately 1,200 m, The largest moraine systems are compound, suggesting two major glacial advances which are placed in the late Otira Glaciation,

Keywords.' Gtira Glaciation, Holocene glaciation, Quaternary geology, end moraines

INTRODUCTION Although a range of glacial features has been described from the central North Island

volcanic massif (Taylor, 1927; Grange and Williamson, 1930; Mathews, 1967; Hackett and Houghton, 1989), there has to date been no definitive account of the glaciation of Mt Ruapehu (Fig. 1), the North Island's highest peak (2,797 m). One important reason for this is the problem of identifying materials and landforms in a context of typically ambiguous deposits and landscape features. Thus, while many glacial features have gone unrecognised, some phenomena on Mt Ruapehu and in surrounding valleys that were at one time claimed to be of glacial origin (Park, 1910, 1916, 1926) have since been shown to be volcanic (Grange, 1931; Te Punga, 1952). There is no doubt, however, that during the late Pleistocene Mt Ruapehu was substantially covered on at least one occasion by an ice cap which both eroded the mountain sides and built moraines, and that glacial landforms created during the Last Glaciation remain as important components of the existing landscape. In addition, relatively minor glacial landforms record the limits of more recent glacier extension.

This paper presents data on end moraines recording the extent of late Quaternary glaciation on Mt Ruapehu and discusses possible correlations. The data comprise field observations made periodically over the last twelve years augmented with information derived from aerial photographs.

GEOLOGICAL AND GEOMORPHOLOGICAL BACKGROUND Mt Ruapehu, Mt Tongariro and Mt Ngauruhoe together comprise the Tongariro Volcanic

Centre (Fig. I), itself part of the Taupo Volcanic Zone. The volcanoes of the Tongariro Volcanic Centre are stratovolcanoes belonging to an andesite-dacite volcanic arc formed in association with the marginal basin between the Pacific and Indian lithospheric plates (Cole and Lewis, 1981; Healy, 1982). (For definitions of terms, see Glossary, p. 296). Although the volcanic history of the Taupo Volcanic Zone spans about a million years, the volcanic and erosive history of Mt Ruapehu is much briefer. Hackett (1985; see also Cole et al., 1986; Hackett and Houghton, 1989) has mapped four formations on Mt Ruapehu spanning approximately the last 250,000 years (Fig. 1). His interpretation of the history of Mt Ruapehu is that three main cone-building episodes alternated with periods of cone dissection. The highest peaks, together with a large proportion of the existing main cone, were built during

* Department of Geography, Massey University, Palmerston North, New Zealand.

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288 Journal of the Royal Society of New Zealand, Volume 20,1990

Late Quaternary andesites and pyroclastics

1 ~ <15 000 yr

2 [J 15000-60000 Y'

3 IT] 60000-120000 Y'

4 D >130 000 Y'

TVC Tonganro Volcanic Centre 5km

Fig. 1 - Location and geology. Geology from Cole et at. (1986). 1- Whakapapa Fonnation; 2- Mangawhero Fonnation; 3- Wahianoa Fonnation; 4- Te Heranga Fonnation.

the interval 60,000-15,000 years BP., and are therefore of last glacial (Orira Glaciation) age (McGlone, 1985; Suggate, 1985). This 'Mangawhero cone' (cf Mangawhero Formation, Fig. I) is at present being dissected. Mt Ruapehu has not been dormant during the last 15,000 years, however; lava and pyroclastics have been erupted from both summit and flank vents. The resulting Whakapapa Formation is shown in both Figs I and 2. The unconformity between the two youngest formations (Mangawhero and Whakapapa) is considered by Hackett to have resulted mainly from fluvial processes, in contrast to that between the two oldest formations (Wahianoa and Te Heranga) which he believes to be of mainly glacial origin, dating from the penultimate glaciation (Waimea Glaciation; Suggate, 1985).

A geomorphologically-important aspect of the more recent volcanic

activity within the Taupo Volcanic Zone has been the accumulation of a variably-thick surficial cover of ash and lapilli on the volcanoes of the Tongariro Volcanic Centre. On Mt Ruapehu, these tephras, where present, are thinnest on the higher slopes where they are considered on the basis of tephrochronology to be less than c. 10,000 years old (S. L. Donoghue, pers. comm. 1989). On the lower and middle slopes, the cover is thicker, and on tephrochronological grounds is principally in the age range C. 1O,000-c. 20,000 years BP. (S. L. Donoghue, pers. comm. 1989). These tephras, together with other less extensive, predominantly unconsolidated pyroclastic (including laharic) deposits, the weathering products of brecciated lava flows, and mixtures of these, provide a wide variety of unconsolidated material, much of which is similar in appearance to glacial deposits.

Small glaciers remain on the summit slopes of Mt Ruapehu above 2,100 m (Fig. 2). The cited evidence for a formerly much larger ice mass covering Mt Ruapehu includes abundant ice-moulded rock surfaces with conspicuous striations, and striated and polished boulders which have been found at altitudes down to 1,275 m (Taylor, 1927; Grange and Williamson, 1930; Topping, 1974; Hackett, 1985). It has been suggested by some investigators, however, that boulders may be scratched and grooved during transportation in lahars. The lowest altitude of striated and grooved bedrock encountered in this investigation was 1,400 m, in a rock basin. The presence of ice-moulded surfaces in rock basins on the flanks of Mt Ruapehu indicates that many if not most of the rock basins have been quarried by glacial erosion.

Several authors have noted that the morphology of Mt Ruapehu suggests a two-stage volcanic development. A generally more deeply dissected basal portion is separated from the upper and generally younger part by a pronounced break of slope at C. 1,500 m (Grindley, 1965). Healy's (1982) view is that the older, basal portion was deeply glaciated and modified by subaerial erosion before being partly covered by a younger cone, believed from lavas dated at 22,000-40,000 years BP. to have been built 'during the latter part of the glacial episode' (p.182). As the equivalent of Hackett's 'Mangawhero cone', this 'younger cone' was extensively developed below as well as above 1,500 m according to Hackett's mapping (Fig. I), and so it seems an open question as to whether the proposed glaciation of the 'basal portion' took place in the early or later part of the Otira Glaciation, or in both parts. The conclusions of the present investigation have a bearing on this question.

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~ ! f i o t; . ~ !, . " g ~ { ~ ~ ; ~ , 0

l l ~. ~ ..§.t: -0

III ..... :g!l .~ ~t E '& ~.~ ~ ~ "g = ~

!!,:, ;;:::

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~ ~~ ~

McArthur, Shepherd - Glaciation of Mt Ruapehu 289

~ "-in ~~.t,~

." ~'(-&~ •• '{}e

• N~l

"

Fig. 2 - Some aspects of the late Quaternary geology and geomorphology of Mt Ruapehu. Postglacial lava flows and pyroclastics from Hackett (1985). Significant sites locate: A- Fig. 4; 8- laminated silts referred to in the text.

RIDGE SYSTEMS: MORPHOLOGIES AND POSSIBLE ORIGINS Many of the main valleys on Mt Ruapehu head in rock basins and descend between

bounding ridge systems that are often looped in the manner of valley end moraines (sensu Flint, 1971) or appear to be remnants of such loops. Fig. 3 shows the best examples of such systems. Additionally, rock basins closed by blunter ridge loops are present on some planezes (e.g. Girdlestone moraine, Fig. 3). In non-volcanic terrain, such morphologies might reasonably

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290 Journal of the Royal Society of New Zealand, Volume 20,1990

Fig. 3 - The eastern slopes of Mt Ruapehu. Crater Lake (CL) is near the centre at left and is surrounded by the existing Mangatoetoenui (Mt), Whangaehu (Wh), Wahianoa (Wa) and Mangaehuehu (Mh) glaciers. Te Heu Heu (TH), Mangatoetoenui (Mt), Whangaehu (Wh), Wahianoa (Wa), Girdlestone (G), Mangaehuehu (Mh) and unnamed (M) end moraines are labelled. The location of site B of Fig. 2 is located with an 'x'. Note particularly the compound moraine system of the sinuous, broad-floored lower Mangatoetoenui valley with its ice-contact inner slopes; cf. the simple, straight and narrow lower Wahianoa valley. Photo published with the permission of the Department of Survey and Land Information.

be interpreted as end moraines, especially where the distal parts of the ridge systems are composed of diamictons containing boulders. In volcanic regions, however, similar morphologies can result from either the operation of volcanic processes per se or from the fluvial dissection of lava tongues. Ridge loops composed entirely of rock were observed in

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McArthur, Shepherd - Glaciation of Mt Ruapehu 291

the field, and it is clear that moraines cannot be identified with certainty on the basis of surface morphology alone. The difficulty of distinguishing till from diamictons of essentially volcanic origin compounds the problem of identifying ridge systems as moraines.

THE IDENTIFICATION OF END MORAINES

Intrinsic features observed in the field and used in identifying ridges as end moraines are: (a) exposures in the sides of the ridges of diamictons characterised by mainly sub-angular clasts of various sizes (including many large and some very large boulders) and varied lithologies, set in a fine-grained matrix; (b) a mix of mainly subangular clasts of various sizes (including many large and some very large boulders) and varied lithologies, strewn along the tops of the ridges; (c) the presence in ridge materials of ice-smoothed/striated boulders, rarely with chatter marks (Fig. 4); (d) features akin to glacifluvial overflow valleys cut into the ridges (Figs 2 and 3); and (e) inner slope morphology of apparently ice-contact origin (Fig. 3). Accessory features supporting such an interpretation are stratified deposits

Fig. 4 - Ice-polished and striated block with crescentic gouges, from the Mangaturuturu moraine (site A, Fig. 2).

abutting ridge systems, suggesting proglacial deposition, and laminated silts within ridge systems which could be the products of deposition in ice-marginal lakes. Additionally, the end moraines typically exhibit a distinctive signature on the air photos, possibly produced by responses to their greater permeability.

Extrinsic features supporting the interpretation of ridge loops as end moraines are: (a) moraine-like features in some valleys at different altitudes; (b) existing glaciers at the heads of the valleys with the ridge loops; and (c) striations and grooves on rock slopes in the catchments above the loops. The fact that the distal or terminal margins of many of the ridges identified as end moraines are within a narrow altitudinal range is also significant.

In combination, this evidence indicates that till is present on the flanks of Mt Ruapehu down to 1, I 00 m and that many of the ridge systems on the middle slopes are, at least in part, end moraines (Fig. 2). Many of the ridge crests are extensions downslope of rock ridges and at least some have rock cores in both their proximal and distal parts.

THE NATURE AND DISTRIBUTION OF END MORAINES End moraines are present on slopes of all aspects. The largest moraine systems are those

that enclose the main valleys, but smaller moraines are present within these valleys and at the outlets of smaller basins. In addition, some small end moraines extend down valley from the distal margins of existing glaciers. The best developed of these are in the Mangaehuehu and Mangatoetoenui valleys (Figs 2 and 3).

The main Mangatoetoenui and Mangaturuturu moraines are compound, comprising contiguous inner and outer ridges, mainly in their upslope parts. The inner ridges form

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292 Journal of the Royal Society of New Zealand. Volume 20. 1990

relatively entire loops whereas the outer ridges are incomplete. The outer ridges are around 40 m higher than the inner ridges at the proximal ends of the moraines (Fig. 2), but this height difference decreases downslope on account of the greater gradient of the outer ridge crests. Valley floors within the main moraines fall to between I, 100 m and 1,200 m, and within the smaller intervening loops to between 1,300 m and 1,550 m. The largest terminal or distal ridges rise about 100-150 m above their valley floors.

THE RUAPEHU ICE CAP Fig. 5 shows the limits of the

ice body defined by the moraines that seem to be contemporaneous on the basis of their completeness, their size as related to catchment area, and their relationship to a consistent equilibrium line altitude, assessed at between 1,500 m and 1,600 m from the altitudes of the proximal portions of the moraine systems. This ice body was a

I~

I ' (, I \ ...r-) , i,

/ ____ "'( j \1 ( ~ ,I (~ \ \, r I I \

LJE •• SI.ngglae.e,s

-- -: Ruapehu Ice cap al last mtlJor advance

Constructional ridge crests 01 Otlran - andmorames

~'-"'~ 'J \ I) \ /~ / '

'___.., "- b ~ \ --....,;: __ ~ I " " '\ C) '\ /~-

_' I' / " --.::- " ~"~ A \ ',/ ~~j c.::-I 0& ~~:(') / ,\ ~c.:..(J ' .. / 01 p~ 0 .r-" I

/7 ~ ---....... /~ " ( ,~o/-.-/~, 'd\ '\ '-~ _ \ if (~j

" I ~ /7 -. -.{ / ( 7' \ /~/ l 'I' J

l~j~~1 '\~ ! )~J ,,) \.~"o

/ /

I /

Fig. 5 - Moraine ridge crests and inferred ice limit of the Ruapehu ice cap and outlet glaciers at the maximum of the last major ice advance, which was probably during the late Otiran glacial age. The moraine ridge crests shown both outside and inside this limit are probably also of Otiran age. The circles are close approximations of the 1,200 m and 1,600 m generalised contours of the present surface. Question marks indicate uncertainty about moraine identification (outer ridge system) and correlation (inner ridges).

mountain ice cap with many outlet glaciers, and had an area of about 135 km2• The summit peaks and parts of what are now the main ridges would have risen above the general level of the ice cap; these are included in the ice body area. The Mangaturuturu glacier reached the lowest altitude (c. 1,100 m) but the Mangatoetoenui glacier extended farthest from the summit; its snout reached 10 km from what is now the crater rim. The portion of this glacier immediately downstream from the equilibrium line comprised three ice streams at this stage, the flow presumably following pre-existing valleys.

The compound moraines of the Mangatoetoenui and Mangaturuturu glaciers and moraines on the west of the mountain indicate that the ice limit shown in Fig. 5 is that of the smaller of two definable ice bodies of similar area and equilibrium line altitude. The area of the older, larger body was about 140 km2 •

DISCUSSION Age and correlation of moraines

The ages of the glacial deposits shown in Fig. 2 are not yet known from direct dating, but some inferences on age are possible from indirect lines of evidence. Almost all of the moraines on Mt Ruapehu have been deposited on lava flows that are younger than 120,000 years, and many on flows that are younger than 60,000 years (Fig. 1); they were therefore deposited either during the Otira Glaciation or in postglacial (Aranuian) time.

In the South Island, the established glacial history for the Otira Glaciation comprises three stadials, of which the last ended about 14,000 years ago (Suggate, 1965, 1985; Suggate and Moar, 1970; Burrows, 1983, 1988). In north Westland, where the glacial sequence is best

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McArthur, Shepherd - Glaciation of Mt Ruapehu 293

known, the penultimate (Larrikans) ice advance of the Otira Glaciation is thought to have begun before 22,300 years BP (Suggate, 1965) and to have persisted until at least 17,500 years BP (Burrows, 1988). The Moana advance, defining the final stadial, probably took place between about 16,000 years BP and 14,000 years BP (Burrows, 1988). There were many smaller glacial advances in the South Island during post-Otiran (Aranuian) time, which also built moraines (Porter, 1975; Burrows et aI., 1976; Burrows, 1977, 1988, 1989; McSaveney and Whitehouse, 1989; Basher and McSaveney, 1989). These moraines are much smaller than the Otiran moraines and are at higher altitudes.

The position of the laminated silts between the outer and inner moraine ridges of the Mangatoetoenui compound moraine on Mt Ruapehu is indicated in Figs 2 and 3. Data presented by Topping (1974) on the analysis of magnetite from the upper part of these silts suggest that sedimentation was concluding about 22,500 years ago (Oruanui Ash correlative; Wilson et ai, 1988). Thus, on this basis the silts were accumulating before and/or during the penultimate stadial (Larrikans) of the Otira Glaciation. Load structures in the silts are attributed by Topping (1974) to the final Otiran advance (the Moana advance of north Westland), which deformed the silts and deposited the inner moraine; he accordingly correlates the outer ridge with the penultimate glacial advance of the Otira Glaciation. Topping cautions that these correlations are tentative, given the indirect dating method. A sample from the upper part of the silts is awaiting thermoluminescence dating.

The absence of information on the stratigraphic relationship of the Mangatoetoenui silts and the moraine ridges (the silt/moraine contact is obscured) allows the alternative interpretation that the silts were deposited in association with the ice that built the inner moraine, rather than with the ice that built the outer moraine as proposed by Topping (1974). Thus, given that the date for the upper part of the silts is correct, both ridges could predate the Moana advance of north Westland. The proposition that the outer and inner ridges record, respectively, glacial advances within the cold climate intervals 40,000-32,000 years BP and 25,500-12,000 years BP is consistent with the established loess stratigraphy of the lower North Island (Milne and Smalley, 1979). In combination, therefore, the evidence suggests that the inner loop of the Mangatoetoenui moraine, and by extension all the moraines built by the ice body shown in Fig. 5, might have been built in the interval 25,500-22,500 years BP. On the other hand, evidence of equilibrium line altitude depression lends some support to the proposal that the moraines built during the last major advance (Fig. 5) are contemporaneous with the Moana moraines of north Westland.

The equilibrium line altitude (ELA) of the ice bodies on Mt Ruapehu for which there is a substantial moraine record has been assessed at 1,500-1,600 m (see above) on the basis of landform evidence. A similar ELA range is required to satisfy an accumulation/area ratio of 0.6 (Porter, 1975), assuming symmetrical area distribution about the median altitude of the outlet glaciers and their catchments. The ELA of existing glaciers is between about 2,300 and 2,400 m, which gives an ELA depression of 800 ± I 00 m for the event or events responsible for the main Ruapehu moraines. Porter (1975) has established ELA depressions of 875 m and 750 m respectively for the Mt John and Tekapo advances of the Otira Glaciation in the Tasman Valley of the Southern Alps, and has tentatively correlated the Tekapo Formation with the Moana Formation of north Westland. The ELA depression of the next-oldest (Balmoral) advance, dated at >36,400 years BP and assigned by Porter (1975) to pre-Otiran time (ef Gair, 1967), was assessed by Porter at 1,050 m. Thus, if ELA depressions on Mt Ruapehu were similar to those in the Tasman valley then the main moraine systems are the equivalents of the Mt John and/or Tekapo moraines in the Tasman Valley. (Conversely, if the main moraines on Ruapehu can be accepted to be the equivalents of the Mt John and/or Tekapo moraines on other grounds then the indication is that equilibrium line altitude depression was very similar on the central North Island volcanic massif to that in the Tasman Valley).

Historical records show that the moraine loops highest on Mt Ruapehu are of late Holocene age (= late Aranuian). The terminal moraine of the Mangahuehu glacier is about

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294 Journal of the Royal Society of New Zealand, Volume 20, 1990

200 m below the present snout, suggesting a correlation with Porter's (1975) 'Neoglacial' moraines in the Tasman Valley which he associates with an ELA depression of 140 m, Porter (1975) gives an ELA depression of 500 m for an early Aranuian event (the Birch Hill advance), which would place the proximal limits of moraines at around 1,800 -1,900 m on Mt Ruapehu, given a similar ELA depression, There is no convincing evidence for moraines at this elevation, although it is possible that the small till mounds between the late Holocene moraines and the main moraines record this event It is noteworthy, however, that in the Wahianoa and Mangaturuturu valleys these deposits are between 1,300 m and 1,600 m. They could therefore be recessional moraines formed during the final retreat of the Otiran ice, or the terminal moraines of a readvance during the closing stages of the last Otiran event, such as are present in parts of the South Island (Suggate, 1965). If, however, the main moraines were built before the Moana advance of north Westland, it is possible that the till between the late Holocene moraines and the main loops is the equivalent of the Moana Formation.

Relation to previous work Mathews (1967) and Hackett and Houghton (1989) have published maps of parts of the

central North Island volcanic massif showing, among other things, moraines. Some of the ridges identified as end moraines in this study, and a few more besides, are depicted on Hackett and Houghton's 'geological sketch map of the Ruapehu area' as 'glacial moraine'. The Mangatoetoenui system is one of those identified in this study that is not mapped by Hackett and Houghton, although small parts of it are indicated on Hackett's (1985) original map from which the later map is derived. The area of till shown on both Hackett's (1985) map and its derivative is much less extensive than given here in Fig. 2.

Neither Mathews nor Hackett and Houghton identified any compound moraines, although Hackett (1985: 123) concluded that 'there is good evidence for a minimum ofthree advances during Otiran time, as deduced from the nested triads of moraine ridges in two valleys of SW Ruapehu'. Only three of these six moraines have been recognised in this study and one of these is of late Holocene age.

Mathews' map shows moraines enclosing several valleys on Mt Tongariro which were interpreted by Mathews, from their lack of dissection, to be the correlatives of the youngest of the Otiran moraines in the South Island. The floors of the troughs within these moraines fall to between I, I 00 m and 1,200 m, which is the altitude of the floors behind the terminal portions of the primary ridge systems of the main valleys on Mt Ruapehu. A correlation between the Ruapehu moraines delineating the ice body shown in Fig. 5 and the Tongariro moraines seems justified on the grounds that the Tongariro moraines are similar in their completeness, size, position, and degree of preservation to the Ruapehu moraines. Such a correlation implies either that the ice fields feeding the Tongariro glaciers in late Otiran time were similar in size to those on Mt Ruapehu at that time or that, subsequently, there has been subsidence in the region of Mt Ngauruhoe.

CONCLUSION End moraines on Mt Ruapehu record several ice advances, the two most extensive of

which took place during the Otira Glaciation. During these events, Mt Ruapehu was covered by an ice cap which fed many outlet glaciers. The area of the total ice body at the maximum of these advances was about 140 km2 , and the largest outlet glaciers extended to around 1,200 m, suggesting that the unconformity between postglacial and synglaciallava flows is, like the oldest unconformity, of glacial origin. The equilibrium line altitude depression during these advances is assessed at 700-900 m, which suggests a correlation with the final two stadials of the Otira Glaciation, given that equilibrium line altitude depressions were similar on Mt Ruapehu to those in the Tasman Valley of the South Island. It is possible, however, that some of the moraines were built during an earlier Otiran event. The erosive action of the ice that built the main Ruapehu moraines was at least in part responsible for the excavation of many of the rock basins on the flanks of the mountain. Thus, the glacial dissection of Mt Ruapehu,

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which doubtless began in early Otiran time, was resumed, perhaps with most effect, in later Otiran time. Small moraine loops immediately downvalley from existing small glaciers record late Holocene glacier limits.

ACKNOWLEDGEMENTS We acknowledge the contributions of Dr C. J. Burrows and Dr R. P. Suggate who as referees of drafts of this paper made helpful suggestions, and thank Karen Puklowski for drawing the maps.

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Received 5 October 1989; accepted 21 March 1990

GLOSSARY

The following definitions follow Bates and Jackson (1980) and Schmid (1981). diamicton - a non-sorted or poorly sorted, non-calcareous, terrigenous sediment that contains a wide

variety of particle sizes. lahar - a mudflow composed chiefly of volcaniclastic materials on the flank of a volcano. laharic - pertaining to a lahar or lahars. lapilIi - pyroclasts between 2 mm and 64 mm in size. planeze - a wedge-shaped unit comprising the little-dissected sector of the constructional surface of a

volcano. pyroclast - an individual crystal, crystal fragment, glass fragment or rock fragment generated by

disruption as a direct result of volcanic action. pyroclastic - pertaining to pyroclasts. stratovolcano - a volcano that is constructed of alternating layers of lavas and pyroclastic deposits,

along with abundant dikes and sills. (Syn. composite volcano). tephra - a collective term for pyroclastic deposits that are predominantly unconsolidated.

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