The Volcanic Domes at Mayor Island, New Zealand
[Received by the Editor, May 1, 1956.]
Three distinct landforms, built during successive periods of volcanic activity, have been named thé main cone, the old dome, and the young dome. The main cone formed the outer slopes of the island, and its superstructural parts were removed during the formation of the main crater which now occupies the centre of the island. The main crater floor was covered by lavas of the old dome; this structure was later largely destroyed when a fault with north-east trend intersected the vent and initiated violent pumice eruptions. The greater part of the old dome collapsed into the vent and the caldera so formed was occupied by a series of lava flows belonging to the young dome.
The extinct volcano forming Mayor Island lies 16 miles offshore at the edge of the continental shelf in the northern part of Bay of Plenty, and 23 miles north of Tauranga Harbour. In plan the island is more or less circular, with a diameter about two and a half miles, but marine erosion along the north and east coasts has considerably modified the original volcanic form. Submarine contours indicate that the bulk of the volcano is submerged and that the base line at the 130 fathom contour is nine miles in diameter. The island is entirely of constructional volcanic origin, and three successive exogenous domes of rhyolite with minor pyroclastic content have been recognized in the present landforms.
The first general description of Mayor Island was presented by Goldsmith (1884) and later accounts were written by Bell (1914) and Sladden (1926). Volcanological observations have been recorded by Thomson (1926), Marshall (1936b) and Cotton (1941; 1944). Petrographic details of some of the acid lavas were given by von Wolff (1904), Bartrum (1926) and Marshall (1932; 1935; 1936a; 1936b).
The main object of the present paper is to present a morphological account of the volcano, based on landforms, and a chronology of eruptions. Similar descriptions have been given in outline by Marshall (1936b) and Cotton (1941), but oblique and vertical aerial photographs (only recently available) have allowed the addition of a large amount of detail and structure which could not otherwise be recognized easily on the ground. In November, 1955, the writer examined the island over a period of 11 days and made two circuits of the coastline by dinghy.
The highest peaks on the island, between 900 feet and 1,200 feet, are located on the outer slopes which are the remaining part of the “main cone,” and which are bounded on the seaward side by high cliffs and inland by the steep, roughly circular wall of the “main crater” (Figs. 1 and 2) Within the main crater, or old caldera, a crescentshaped relic of the “old dome” occupies the north and west parts and has for its inward margin a north-east-south-west fault scarp which crosses the island. Southeast of the fault lies the “new caldera,” partly filled by acid lavas of the “young dome”.
On the geological map (Fig. 1) prominence has been given to escarpments in the inland part of the island. These walls, whether of tectonic or volcanic origin, are dominant features of the landscape and are clearly defined in the aerial mosaic (Fig. 2).
The Main Cone
The main cone, or outer edifice, forms the main slopes of the island, and its seaward margin is bounded by steep cliffs which rise to as much as 350 feet in height. The inner margin is the roughly circular crater wall which is a conspicuous feature in aerial photographs (Fig. 2); the wall is steep and well preserved, and in places is over 1,000 feet high.
The greater part of the outer edifice is built of flows of stony and glassy rhyolite with perlitic obsidian selvages and occasionally strong columnar joints. Individual flows have no great vertical thickness and rarely exceed 100 feet, but laterally they sometimes extend for many hundreds of yards. Obsidian selvages on both upper and lower surfaces are a consistent feature of the flows. The upper surface invariably is extensively ruptured and penetrated by less glassy lava which has welled up from within the flow so that the top selvage rarely forms a continuous sheet. On the other hand, the lower selvage is usually of uniform thickness and is laterally continuous. Lower selvages vary from several feet to 9 feet in thickness; upper selvages are generally thicker, even where unbroken, and where flow brecciation of the upper selvage has caused mixing of obsidian blocks with stony lava this top zone may be up to 20 feet thick.
Building of the main cone was not a continuous process. Several soil horizons exposed on the northern and eastern coastlines are conspicuous in cliff sections by virtue of their vivid orange colour, caused by baking of the soil by younger overlying pumice deposits or lava flows. One such soil horizon underlying charred logs which range up to 8 inches in diameter is revealed in the crater wall in the south-eastern corner immediately behind Te Paritu lake. The downwards succession at this locality is as follows:
100 feet plus, pyroxene rhyolite.
5 feet, obsidian selvage (lower).
6 feet, interbedded pumice breccias and tuffs containing charred logs.
18 inches, baked orange coloured soil.
60 feet plus, massive rhyolite breccia.
The pumice breccia shows no internal bedding and no sorting, and in contrast with the associated tuffs contains no lithic fragments. This type of deposit is typical of Pelean eruptions, and the heat contained within such a pyroclastic layer is a satisfactory explanation of the charred condition of the included logs.
In general, breccias and tuffs interbedded with the lava flows are not common and rarely exceed 20 feet in thickness In most cases only a few feet of tuff separates the obsidian selvages of successive flows. However, there are several localities where thick pyroclastic layers are sandwiched between the lava sheets that make up the bulk of the main cone. The thickest deposit of these clastic rocks is found at the head of Crater Bay, where several hundred feet of bedded breccias and tuffs form a low and narrow wall separating the sea from the main crater. Near sea-level these beds are underlain by lava, and on north and south sides of the bay they are succeeded upwards by flow rocks. The overlying flows dip steeply seawards, so that penetration of this cover by marine erosion has allowed the sea to erode the softer pyroclastic beds that now form a narrow strip at the head of Crater Bay. A schematic profile across the island presented by Cotton (Fig. 3, p. 229, 1941) passes through the low saddle at the bayhead and gives a misleading impression that solid rocks are absent along the eastern coast; elsewhere along this coast lavas are consistently exposed in high positions in cliff sections. Pyroclastic debris in layers up to 60 feet thick and intercalated with flow rocks of the main cone appear also at the head of Oturu Bay, behind the lakes at the base of the main crater wall, and again on the south side of Opo Bay; along other parts of the coastline these beds are either thin or absent.
The only dyke seen on the island is located at the head of Oturu Bay, and it is intruded into the tuffs mentioned above. At sea-level the dyke is approximately 30 feet across, but this width narrows quickly upwards to a few feet where the intrusion disappears vertically into the cliff face with a trend of 070°. Along the dyke margins black obsidian up to 4 feet thick forms a distinct selvage against which adjacent tuffs have not been baked.
Successions of flows, with average seaward gradient about 25°, are best exposed on the northern coast where marine erosion has cut back the main cone extensively, and at one place (Hurihurihanga Bay) breached the main crater wall. On the south side of the island erosion has been less active and the original gradient of lava surfaces is preserved over larger areas. Between Waitangi Bay and Otiora Bay the upper surfaces of the youngest lava flows often disappear below sea-level. The abrupt rise in topography from the surface of the youngest flow at Omapu Bay to the flow rocks at the crater edge at 1,162 feet shows that, even allowing for some subaerial erosion, the primary slope on the flanks of the volcano must have been steep.
When viewed in silhouette the crater rim forms a sharply crenulate skyline due to the presence of incised ephemeral water courses on the flanks of the main cone. It appears that the major consequent valleys must have been eroded prior to the events that produced the crater. Similarly, the gradients of the valleys and the deeply scalloped notches which are made at their intersections with the crater rim indicate that the streams which flowed in them have been beheaded and deprived of a larger and more elevated watershed area. A considerable period of subaerial erosion must have intervened between completion of the building of the main cone and formation of the large caldera-type crater that is now marked out by the roughly circular wall.
Subaerially bedded pumice tuffs and breccias form a thick mantle on the outer slopes of the volcano. This material was not emitted during formation of the main cone, but was the product of eruptions which occurred at a late stage, during growth of the young dome. In these pumice beds there is a general lack of lithic ejecta and only occasionally do blocks or fragments of obsidian and rhyolite appear in quantities up to 10% of the total volume of material. Rounded fragments are absent and the rhyolite debris is not sorted. In addition, the bedding in the lower parts of these pyroclastic deposits is everywhere parallel to the upper surfaces of underlying lava flows and broadly follows the present land surface. Marshall (1936b) suggested that the pumice was derived by ocean drift from the mainland at a time when the volcano was depressed 600 feet below its present level, but the bedding and distribution of the material over the island do not support this hypothesis. The pumice tuffs and breccias occur on the outer slopes at heights of up to 900 feet and are particularly well exposed at the pass into the crater north-east of Otiora Bay.
These pumice deposits fall readily into two groups: the first group is finely bedded and fine-grained, and generally is found at the base of the pyroclastic succession overlying lava flows; the second group is coarser-grained, with blocks of pumice up to 1 foot across, and generally lacks bedding, sorting and fragments of country rock. The former group is regarded by the writer as resulting from paroxysmal explosions during an initial re-opening of the vent, and the latter group is considered to be a product of Pelean explosions during a later cataclysmic eruption when lava levels with higher gas content were reached, and the upper part of the lava column was violently evacuated.
The pumice tends to obliterate the form of major consequent valleys and in some cases to mask completely their lower reaches. Accordingly, these pyroclastic deposits are considered in age to belong to a late stage in the volcanic history of the island. The best developed bays on the island—Orongatea, Oturu, Waitangi, Opo, Omapu, Otiora, Oira and Mawai—have been eroded in the main cone where the uppermost lavas pass below sea-level and the covering sheet of softer pyroclastic material has been worn back. In the case of Orongatea and Otiora bays the pumice tuffs and breccias fill tectonic depressions which mark the opposite ends of a narrow rift zone extending across the island. The thickness of these deposits as exposed on the tops of headlands is not consistent and varies from a few feet to 60 feet.
On the northern part of the coastline between Opoupoto and Hurihurihanga bays erosion during a period of higher sea-level at 50 feet above the present level has removed entirely the outer slopes of the main cone and breached the circular crater
wall. However, the sea did not gain access to the crater through this breach, since within the wall it was blocked by a thick series of lavas belonging to a remnant of the old dome. These lavas were cliffed by marine erosion on their north side and now form the landward margin to a coastal strip of subaqueous sediments underlying Te Ananui Flat which stands at about 50 feet above sea-level. Evidence for the subaqueous origin of the flat is based mainly on lithology and bedding. The sediments are horizontally bedded breccias, sandstones and fine siltstones of rhyolitic origin; the larger fragments of lava in these rocks are mainly angular, but they show more marked rounding and are present in greater quantity than is the case in other sedimentary beds on the island. Furthermore, there is a clear graded bedding, from coarse to fine upwards, both in individual lensoid strata and on a larger scale throughout cliff sections, but the beds closing the succession are always composed of the finer grades. Four feet of light brown tuff forms the topmost layer underlying the surface of the flats. This layer of tuff follows the modern topography and lies unconformably across the older subaqueous beds where these have been gullied.
The age of Te Ananui Flat clearly post-dates the formation of the main cone and of the old dome within the crater. The evidence indicates that the flat was formed after the extensive pumice showers had mantled the outer slopes of the island, since deposits from that eruption are absent.
The Old Cumulo-dome
The roughly circular crater wall, forming the inner margin of lavas and pyroclastics of the main cone, is a dominant topographical feature of the interior of the island. The wall is steep and well defined, but it cannot be regarded as the true periphery of the original crater about which the main cone was built. For reasons given above, the original vent is presumed to have been located, smaller in dimensions, at a higher level in a superstructural part of the volcano that has been removed. The absence of any great quantities of fragmented lava on the slopes of the island points to removal of the superstructure and enlargement of the crater to approximately its present size by some process of collapse-caldera formation. According to this hypothesis, the upper part of the main cone collapsed into the central vent and the large caldera so formed in the centre of the island—the present main crater—was occupied by lavas of the old cumulo-dome.
Evidence given below indicates that this dome, in turn, was later largely destroyed by a repetition of the same process, so that at present only a remnant of the dome is preserved in the north-west part of the main crater. The materials of the old dome are massive rhyolite flows, with numerous glassy phases, bearing thin selvages of obsidian. Exposures of these rocks are not extensive, and outcrops are best seen along the south-eastern faulted margin. The crescentic relic of the old cumulo-dome is terminated on the seaward side by the cliffs behind Te Ananui Flat and by the valley that extends along the base of the main crater wall between Te Ohineiti and Te Kukuta. The eastern margin is clearly indicated by the scarp of a curved fracture line which crosses the island with a general trend of 045°. The trace of this fault shows clearly in aerial photographs of the south-west part of the island (Fig. 2), where it intersects the main crater wall and passes out to sea along the western side of Otiora Bay. At this locality there is an abrupt change in the type of rock at the shoreline, the lava cliffs to the west of the fault being replaced on the east side by pumice tuffs which extend for 8 chains along the back of the bay. Where the fracture line crosses the crater wall north-east of Otiora Bay there is a similar sharp break in the crestline of the crater rim which westwards rises rapidly an extra 300 feet or more to Te Kukuta at 985 feet. The fault line is not easily recognised in the north-east crater wall because of a dense cover of bush and a thick mantle of pumiceous pyroclastics, but on the sea coast at Orongatea Bay the exposure is excellent. As at Otiora Bay there is here a sharp break in the lava cliffs and eastwards for about 9
chains poorly bedded pumice tuffs and breccias occupy the head of the bay. Three large stacks of rhyolite lava, named Tawakewake Islands, are in line along the western, upthrow side of the fault. The tuffs terminate against a nearly vertical face of rhyolite lava at the western end of the bay and at the contact show well-developed concave bedding. Large blocks of lava up to 50 feet across, and sometimes bearing obsidian selvages, lie in the pyroclastic material adjacent to this contact and apparently have been derived locally, for similar lavas form the sea-cliffs to the west. On the east side of Orongatea Bay the tuffs overlie lava flows of the main cone group as a part of the pyroclastic mantle on the outer slopes of the volcano. The only hot springs found on the island—one group in the middle of Orongatea Bay and the other at the northern end of Oira Bay—are located at opposite ends of this fracture line.
In age the faulting post-dates the formation of the main cone and that of the old dome. It antedates the extensive pumice showers that cover the outer part of the island, for in aerial photographs the pumice deposits clearly bury the trace of the fault behind Otiora Bay, and in the field the same beds can be followed laterally into the pumice mantle of the main cone.
The Young Cumulo-dome
Considered to be part of this structure are a number of flows of different ages, but the youngest and most striking topographic feature is undoubtedly the central dome or tholoid which rises to 700 feet at Tarewakoura. The lavas associated with the young dome lie within the main crater and south-east of the fault, and they occupy a caldera which appears to have been formed by a process of explosion-collapse. The greater part of the old dome has been removed, but not by explosive disintegration, since significant deposits of fragmented dome lavas are not found on the island. It is believed by the writer that intersection of the fault with the vent of the volcano initiated explosions of magmatic material which was spread as the pumice mantle on the flanks of the main cone. The old dome roof over the vent then foundered and collapsed into the throat of the volcano and finally effusions of lava produced the flows described in this section.
The oldest lavas in the caldera form the area about Crossman Hill*, and the strip of low-lying land between Aroarotamahine Lake and the eastern crater wall. These two areas are regarded as upstanding portions of the foundered old dome. Crossman Hill acted as an obstruction to lava flow 1a (Fig. 1) which split and flowed westward around the hill on the north and south sides and rejoined on the other side. The hill stands out in the field as a steep-sided knoll, and the margins of the lava flow can easily be identified both from aerial photographs and a traverse on foot. Rocks forming the hill are various glassy and stony grey rhyolites which can be distinguished in hand specimen from those found in flow 1a and which are similar to lavas of the old dome. More importantly for determination of relative age, the hill is mantled by several feet of well-developed soil, and in this way alone is quite clearly differentiated from other areas of lava in this part of the crater.
East of Aroarotamahine Lake the low-lying land, bounded by the swamp on the south, a young lava flow on the north and the crater wall to the east, provides no clear outcrops. A large number of lava blocks are embedded in soil and these are similar in rock type to the material in Crossman Hill, but quite distinct from the darker glassy lavas of the younger flows.
Younger flows associated with the Tarewakoura dome have been numbered on the map in chronological order, but their rocks in hand specimens exhibit no distinguishing features from one flow to another. Flows numbered 1 and 1a are regarded as the earliest of the lava sheets that are visible in the caldera; it is assumed.
[Footnote] * No name for this topographic feature could be found in records or charts of the island; the name given here has been adopted purely for convenience of description.
that they were extruded soon after the collapse of the old dome. This older age for 1 and 1a is based on the visible form of the flows and the soils formed on them. The outline and form of flows of all groups can be identified in aerial photographs and verified on the ground. The margins of the youngest flows are remarkably steep and rise 60 to 90 feet in a short distance; such well-defined perimeters are present on lava sheets 2, 2a and 3 where they make contact with flow 1. The latter flow has the slightly convex surface that characterises the central parts of most flows in the caldera, but since steep-sided peripheral walls are absent it is concluded that the outer portions of flow 1 are buried under flows 2, 2a and 3. In a similar fashion flow 1a appears to be submerged along its northern margin beneath flow 3. The central part of flow 1a, about 8 chains east of Crossman Hill, has the form of a distinct small dome. From a high central point convex lava surfaces slope outwards, suggesting that this small flow may have been extruded from a temporary subsidiary vent formed by fracture and collapse of the old dome. In addition to morphological reasons, flows 1 and 1a are grouped together in the chronological scheme, since both carry soils which are less mature than that on Crossman Hill but which are better developed than soils on flow groups 2 and 3.
The lava flows numbered 2, 2a, 2b, 2c and 2d have many features in common. To the west and north-west they disappear beneath the flows of group 3 and along common boundaries they do not appear to have transgressed one over another. The boundaries of the group 2 flows indicate that there has been very little mutual interference between flow margins. A boundary common to two flows is always marked by a sharp, deep cleft, the walls of which are convex and steep, and formed of dark, glassy rhyolite. The five flows included in group 2 are perhaps best regarded as lobes of one extensive sheet of lava which was divided near to the vent into a number of individual flowing units.
A number of successive flows have been included in group 3, and these form the youngest dome at Tarewakoura. Each lava sheet is flat-lying and it is difficult in the field to identify margins of the individual flows. However, flow boundaries can be distinguished in vertical aerial photographs and at least eight sheets are seen to be present in the dome. These flows, like those of group 2, have spread east and southeast, and their composite margin rises so steeply above the earlier flows as to form a cliff. To the west the lavas of group 3 occupied an explosion embayment in the fracture line and almost overtopped the fault scarp. Banked-up flows have nearly obliterated the scarp in this locality and a view north-east along the fault from the south-west shows that the greatest bulk of lava is concentrated in the central area.
Where any of the flows in the caldera have reached the base of the main crater wall or the fault scarp there is almost always a flat-floored hollow between the wall and the lava margin. In only one place, immediately north-west of Tutaretare, does the lava appear to be hard up against the cliff face. In all other localities there is a gap of 20 to 40 yards, infilled by cliff talus, between crater wall and flow boundary. These flat marginal strips are the result of extreme viscosity in the outer shells of the flows; contact was made at depth between crater wall and lava, and the flow assumed a strongly convex peripheral form against which cliff talus has accumulated. Lake hollows for Aroarotamahine and Te Paritu were formed in a similar fashion. Te Paritu occupies a saucer shaped depression, 30 feet deep, on the eastern edge of the main swamp; Aroarotamahine is bounded on the east side by a straight shoreline against an arcuate foundered strip of the old dome. The deepest part of Aroarotamahine at 70 feet is excentric, and bottom soundings have provided underwater contours which reflect and extend the lobes of lava flows 2a, 2b, 2c and 2d that form its western shores. The lake surface lies between 5 and 10 feet above sea-level and a strongly flowing cold spring which merges a few feet below high tide mark in fissured lavas at the head of Crater Bay appears to be its outlet.
The surfaces of all lava flows in groups 1, 2 and 3 are extremely rough and broken into a chaotic assemblage of blocks of dark glassy rhyolite, vesiculated lava, and, in many places, of finely shredded pumice. True obsidian was not seen, for the glassy rocks invariably contained 30% or more of phenocrystic material. A thorough search was made for xenoliths from the collapsed old dome, but none was found. Elongated fissures are fairly common on the surfaces of flows. These have the form of long and sinuous steep-walled channels, usually located near the centres of flows, and aligned parallel to the direction of lava movement. Some are only 10 to 15 feet deep, but others, particularly in group 3, form miniature gorges up to 50 feet deep. Formation of these channels is probably due to outflow of the more fluid internal parts of flows from beneath an almost rigid glassy carapace, so that they are homologous with collapsed lava tunnels of basalt flows. The absence of autobrecciation or slickensides along the fissures does not allow their occurrence to be explained in terms of widespread tectonic fracturing of flows.
The sequence of main events in the volcanic history of Mayor Island is set out below and represented in diagrammatic form in the reconstructions of Fig. 3.
(a) The main cone was built up by a series of lava flows with gradients between 20° and 30° and chilled to obsidian on upper and lower surfaces. Cone building was a slow process, for minor quantities of tuff and breccia which are contained within the edifice in some places overlie soil horizons. Explosive eruption preceded some lava outpourings, since the section in the crater wall containing fossil logs indicates that the wood was charred by hot showers of Pelean type. During a succeeding period of dormancy the cone was ravined by subaerial erosion. Reconstruction of valley profiles and lava flow surfaces suggests that the volcano attained a maximum height of the order of 2,000 to 2,500 feet above present sea-level.
(b) The upper part of the main cone, about 1,000 feet in height, was removed probably by a process of caldera formation. Simple explosive disintegration cannot be invoked to explain destruction of the upper slopes, for lithic fragments are absent on the residual outer part of the volcano. A large sink, developed by collapse and measuring approximately one and a half miles across, occupied the centre of the island; the depression so formed is now bounded by the roughly circular main crater wall. This early caldera was filled by radially disposed convex sheets of lava which were extruded from the centrally situated vent and built up the old cumulo-dome. The lower flanks of the old dome slope at 13° to 18°; assuming a symmetrical invertedsaucer shape for the dome, reconstruction indicates a central height of about 1,200 feet and a circumference of eight miles.
(c) During a subsequent period of inactivity the old cumulo-dome acted as a roof to the throat of the volcano and as an impermeable vent cover beneath which magmatic gases accumulated at high pressure. The whole volcanic edifice was then broken by a curved tectonic rift with general trend 045° and the cumulo-dome blocking the vent was fractured. Gases confined at high pressure at the head of the magma column escaped upwards with the explosive violence of a Plinian eruption and foaming liquid rock was thrown outwards as highly vesiculated rhyolite pumice to form a mantle on the outer flanks of the main cone. Only a small quantity of country rock was included in the pyroclastic eruption, and this material was derived partly by explosive embayment of the fault scarp. At this stage the greater part of the fractured and weakened old dome east of the fault sagged and collapsed into the underlying cavity produced by rapid evacuation of the throat of the volcano. Remnants of the dome remained as marginal strips inside the main crater wall.
(d) Discharge from the magma chamber continued with upwelling and outpouring of gas-charged acid lava on to the broken floor of the newly formed caldera. A succession of viscous flows escaped from the vent and gradually covered most of.
the area of the new caldera. Chilled glassy phases of the rhyolite formed a skin on the lava sheets that made lateral movement slow and gave each flow a characteristic convex form with steep margins. Inner parts of the flows, with gases still mainly in solution, were more fluid than outer parts, and where pressure breaches occurred in the plastic glassy skin fresh outflows developed linear series of collapsed lava tunnels. A large amount of continued gas discharge from these lava flows is indicated by the numerous large blocks of pumiceous rhyolite scattered on their surfaces.
The early flows of groups 1 and 2, which moved outwards from a central vent situated near Tarewakoura, were banked steeply with convex margins against the main crater wall and upstanding remnants of the old dome. In this way deep marginal hollows were produced and basins developed for Te Paritu and Aroarotamahine lakes. As deeper parts of the magma reservoir were tapped, lava discharged from the vent became progressively more viscous. With this restriction on the ability to spread laterally, a number of flows were piled in succession in the immediate vicinity of the vent to form a new cumulo-dome which gradually assumed a generally convex steep-sided shape. These lavas, forming group 3, flowed mainly east and south-east for short distances and also occupied the explosion embayment in the fault scarp to the west. The flows contributing to the new exogenous dome gradually became smaller in bulk and a shallow dish-shaped depression at the summit of Tarewakoura probably marks the area where discharge ceased and was followed by a contraction at the head of the lava column, due perhaps to final escape of gases.
(e) At some stage after the formation of the old cumulo-dome, and during a temporary high sea-level at 50 feet above present level, the main crater wall was breached by marine erosion along the north-west part of the island. Lavas in the north-west part of the old cumulo-dome were cliffed and the sediments eroded were deposited subaqueously to form the beds underlying Te Ananui Flat. Like the remainder of the coastline, these beds have been cut back considerably by the sea after it attained its present level.
The main cone, old dome and young dome have been described as the three distinct volcanic forms on Mayor Island. No pyroclastic beds were seen in either of the younger domes, but the main cone is somewhat different in character for at least along the eastern side there are thick layers of breccia and tuff intercalated with lava flows. The present form of this outer edifice is broadly dome-shaped, but the exogenous method of growth with intervening periods of dormancy, or of pyroclastic ejection, was so distinct from the purely exogenous growth of the later domes that a different descriptive name has been used for it.
Perhaps the most outstanding feature in the history of the volcano is that caldera formation apparently has taken place on two distinct occasions. As pointed out by Williams (1941) the absence of fragmental material from the bedrock is a strong indication that large volumes of roof rock have collapsed into the volcanic hearth as part of the caldera forming process. However, when discussing the origin of the main crater Cotton (1941) has adopted the alternative explanation that “the northeastern part of the original Mayor Island dome (the main cone of this paper) has apparently been blown away long ago by an explosion which probably projected the obsidian and rhyolite fragments of it in a lateral direction out to sea.” In the case of the young caldera, formed by inward sliding of the fractured old cumulo-dome, the evidence seems conclusive. Space necessary for the collapse was made available by voluminous pumice eruptions which post-dated the tectonic fracture across the island, and remnants of the foundered old dome form the visible floor of the young caldera.
A broad magma reservoir situated at shallow depth could be invoked as an essential factor in allowing repeated collapse of the roof. However, it would seem
that the conduit on Mayor Island has been centrally situated at all times. The young dome at Tarewakoura marks the most recent position of the vent; but this position is also central with respect to the reconstructed form of the old dome, as well as to the main crater wall and the radially disposed lava flows of the main cone. It is difficult to visualise how this position was maintained during two separate phases of caldera formation by collapse. A general absence of xenolithic material in the later lavas refutes any suggestion that the relatively narrow pipe was kept open by internal solution of obstructing country rock. Indeed, the high proportion of intratelluric crystals in the lavas of both domes, as well as the high viscosity of individual lava sheets, indicates that at the time of effusion the magma had reached a low temperature state and was able to maintain a mobile condition only by containing a great quantity of gases in solution. The most likely explanation for the continuance of the orifice in its central position is to imagine that the conduit widens rapidly at depth so that insliding cores of disrupted country rock were easily accommodated.
Repeated collapse of the domical covering over the conduit does not support the general contention that dome formation marks the end of volcanic activity. According to Williams (1932) this generalization is true only if the dome manages to survive as a permanent seal over the vent. If, as with the old dome on Mayor Island, a line of weakness is developed in the rigid cover, and the pentup magmatic fluid has remained above the temperature of crystallization, there may be renewed activity from the same centre. Intersection of the north-east-south-west fault with the narrow conduit on Mayor Island can be regarded as a fortuitous convergence of the two physical factors necessary for reactivation of the volcano at the original vent.
With a general trend of 045° the fault is parallel to a number of recent scarps on the mainland that cross the volcanic area between Rotorua and Lake Taupo (Grange, 1937), and there is most likely a genetic connection between them. A survey by Wellman (1954) of fault lines in the northern part of North Island indicates a grouping of normal faults with strikes between 035° and 057° in a stress pattern which apparently has not changed orientation since the Upper Miocene. The trend of the Mayor Island fault is in accordance with this stress pattern. On Mayor Island there is no topographic evidence to suggest transcurrent movement. Where the northeastern end of the fault intersects the main crater there is a broad offset scallop in the crater wall on the downthrow side, but this is probably the result of rupture and insliding of a ring-fractured arcuate strip of the old dome.
Professor C. A. Cotton and Mr. E. J. Searle kindly read the manuscript and offered many helpful suggestions. Mr. W. S. Crossman, Mr. I. M. Paltridge and Mr. I. A. E. Atkinson gave the writer valuable assistance in the field, particularly during examination of the coastal sections. Mr. J. S. Edwards made available the bottom soundings of the crater lakes which had been taken by members of the Auckland University College Field Club.
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Dr. R. N. Brothers,Geology Department, Auckland University College, Auckland.