Pirongia’s first volcano

The Mahaukura Bluffs are best observed from Ruapane, across the Mangakara Valley. They are spectacularly sheer and pale grey, about 70 m in height. The bluffs are cross cut by a series of imposing dikes at least 20 m thick which connect to domes of andesite on the ridgeline. You can stand on one of these domes at Wharauroa, a great lunch spot. I have come to believe, through my research, that Mahaukura is the eroded remnant of Pirongia’s earliest volcano. This is supported firstly by its intense degree of erosion, as well the oldest age dated rock in the Alexandra Volcanic Group at 2.49 million years old.

After spending a month in Mexico, I decided that when I returned I needed to find a way down to the bluffs and observe their stratigraphy close up. I was encouraged, because a local farmer told me there is an easy way to the base of the cliffs just south of the bluff.

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Mahaukura Bluffs – a thick succession of basaltic-andesite lavas and dikes from Pirongia’s first phase of volcanism

Indeed, with careful GPS navigation and some bush bashing, I made it there after 4 hours. The area is far more overgrown than (evidently) it was in the 1970s when the farmer when there. That’s because the goat populations are significantly culled and the mountain is re-vegetating with native scrub.

When I reached a suitable viewing point, I quickly realised that the bluffs are composed of dozens of lava flows stacked upon one another. The lavas appear to be highly autobrecciated (rubbly) A’a’ flows with only thin layers of cohesive lava rock. Their compositions appear to be unanimously basaltic-andesitic with small phenocrysts of black clinopyroxene.

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Beautiful convex jointing patterns in a dike wall at Mahaukura. 

The lava sequence is cross cut by a spectacular array of andesitic dikes (vertical sheets of magma injected into the volcano). The dikes protrude from the bluff because they are far more erosion resistant than the lavas. This makes for incredible ‘wall’ like outcrops with well exposed joint patterns like the above photo shows.

I noted that the dikes project upwards to the small, randomly jointed domes of andesite on the ridgeline. It was a ‘smoking gun’ moment when I realised that all of the Mahaukura andesites found on previous trips were fed by the dikes I had long been aware of but never seen up close.

Where the dikes cross cut lava stratigraphy, there are small (<1 m) zones of breccia at the margins. These zones represent fragmentation caused when the dike cracked through the rocks.  One can imagination that this was a violent process associated with the ‘last gasp’ of volcanism at this vent area. Both the andesite and older basaltic lava sequence were probably stored within a magma chamber at crustal levels below Pirongia.

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Hornblende-andesite dike cross cuts a lava sequence. Note the brecciated contact zone.

The trip was symbolic because it is the last fieldwork I do before getting married in March (in eight days time). Perhaps accordingly, I also reached the very last page of my yellow field book on this day, and perhaps, closed a chapter of my young life.

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Done and dusted: The famous ‘Rite in the Rain 1’ is finally complete. It marks the end of my main Pirongia work.
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Top of Tahuanui

The 20th of September involved a steep climb from Corcoran Rd to the summit, for the purpose of mapping the uppermost section of Tahuanui track. This is the section I ran out of time to map on 24th of August. We had a small window of good weather between constant rain, so I made the most of it.

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“The Cone” – a basaltic-andesitic vent complex of Pirongia. It is essentially the second summit.

The main thing appreciated from this traverse was better recognition of volcanic dikes in the field. In the past, I had been puzzled by the co-occurrence of cohesive volcanic rocks and pyroclastic rocks. For instance, most of Ruapane bluff is composed of angular basaltic clasts in a fine matrix, but at several points, jointed cohesive units control the ridge structure. I now recognise these as dikes which have intruded through the pyroclastic sequence. A dike is a planar, or lensoidal sheet of magma which intrudes through a volcano in a subvertical orientation. All magmas, from what I’m aware, come to the surface as dikes. These dikes erupt and may form lavas, or they can stall and not reach the surface. In either case, we would expect to find dikes exposed on the top of Pirongia because of erosion of overlying lavas or other vent material.

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View NE towards Tirohanga bluff (bare rockface) and the Ruapane peaks. Photo taken from the Tahuanui track, at a lookout built on vitrophyric (glass-rich) hornblende andesite. 

I took the new understanding of dike occurrence and applied it as I continued the traverse. Expectedly, dikes occur at many intervals along the Tahuanui ridgeline. I measured the orientation of many and it will be interesting to see whether they form any kind of recognisable pattern (e.g. radial from vent).

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Tahuanui Peak (centre) is probably a flank vent of Pirongia. It is inaccessible by track.

Tahuanui is composed principally of two units: a basaltic succession, and a distinctive vitrophyric hornblende andesite cap sequence. The basalts occur as massive pyroclastic units that are intruded by dikes and sills, and possibly intercalated with small lava flows. All the basaltic deposits are intruded and overlain by andesite, which forms small resistant mounds on top of the ridges from about 800 m elevation. This andesite correlates to that observed nearby on my first expedition along the Tahuanui track.

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Tirohanga bluff – a remarkable monolith of hornblende andesite. I envisage that this formed by intrusion of a dike through soft pyroclastic sediments. Over time, the western face has collapsed under its own weight. 

An obvious question in the field was whether the Tahuanui andesite is related to the same phase of volcanism as the other andesites at Tirohanga, Wharauroa and Mahaukura. Petrographically, it is distinct because of its glass content, but otherwise contains the same mineral assemblage. A wider question I am yet to answer is whether andesites on opposites sides of Mangakara valley are part of the same unit, which has eroded in the middle to form a large valley, or they are separate intrusions. Given the ‘sticky’ flow rheology of andesites, I envisage that they formed discreet intrusions close to their vent source, and not extensive lava fields like the basalts did. Therefore, it may be wise to treat each of the andesite domains as separate deposits, produced by localised vents. The vitrophyric andesite at Tahuanui would therefore represent a small intrusion through basaltic-andesite flank lavas.

The Tahuanui Flank deposits

Ok, so mapping is underway on the northern side of the mountain. I was pleasantly surprised by the Tahuanui Track, which leads from farmland up towards the summit hut. The weather was warm and sunny, the bird were out, and the track gradient was gentle compared to the trip 2 days earlier to Hihikiwi. The walk is 12.5 km each way to the hut, but I managed to only get within 1 km of it. Winter daylight hours!

The main finding here was that there is a succession of distinct lavas that are exposed progressively upslope towards the summit. The lowermost flanks comprise of ‘mafic’ ankaramite, and the middle reaches contain basalts and basaltic andesites. The uppermost ‘cone’ exposes a domain of andesites (lighter coloured) that is intercalated with vent breccias and more vesicular basalts.

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These rocks represent a sequence of lavas that were laid down over time to produce a gently undulating flank of Pirongia. The earliest lava flows were basaltic, and these flows were overlain by more ‘evolved’ lavas of basaltic-andesitic composition. The last stage of eruptions produced the most viscous lavas – the andesites – which built up a steep cone and traveled no more than 2 km down slope. During this period, several pulses of basaltic lava erupted from the summit vent, producing fire fountains (like in Hawaii) and small lava flows.  The presence of abundant vesicles in these basaltic rocks indicates that their magma was gas rich, and their occurrence in breccias near the vent suggests that there were periods of fire fountaining.

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Fire fountaining of Bardarbunga, similar to that which produced vesicular basalts on Pirongia 2 million years ago.

 

 

Hihikiwi revisited

Although this field trip only happened 4 days ago, I’ve since been out in the field again to northern Pirongia and it’s further back in my mind now. The ‘Kawhia side’ of Pirongia (south and south-west) continues to be an expected source of interest in my mapping efforts. Careful examination of the digital terrain models (DEMs) and field observations clearly indicate that the area is built from numerous lava fields. These lavas originate from parasitic volcanic vents that have not previously been identified in geological surveys. Each of the lava fields seems to have erupted a distinct lava type that allows it to be differentiated from other nearby fields. The most obvious example is the lava plateau which rises above the intersection of Pirongia West Rd and Okupata Rd (see picture below). It comprises of basaltic-andesite lava flows with crystals of phlogopite, a golden coloured mica atypical of basaltic lavas. This vent forms a remarkably planar lava field that dips gently SW over a basement rock of limestone. It is dated at 2.3 million years old.

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Inferred vent location at the junction of Pirongia West and Okupata roads. While the rest of the landscape is dipping downhill (part of the Pirongia edifice), this vent cuts the landscape and rises steeply – signifying that it is a separate lava field that does not originate from Pirongia’s upper slopes.

The Hihikiwi track gets very little sun due to its southern aspect. It remains boggy throughout much of the year. I went a day after heavy rain, and also managed to forget my high-top tramping boots so had to battle it in my ‘rock-hopper’ hiking shoes. The mud pits are deepest in the pockets where old trees fell over. I estimated they get up to 1 m deep! I managed to keep mud-dry on the way up, but fell in a couple times going down.

The big discovery of the day was a ‘fresh’ landslide surface on a ridge that had exposed volcanic rocks. Rock exposures of this size are very rare on Pirongia. I deliberated with myself about attempting to climb down the cliff, as I was alone, and didn’t want to fall. Thankfully the inclination was shallow and I could clamber carefully over about 20 m of outcrop, avoiding the shear cliff below that. The slip surface exposes a basaltic block and ash unit that is overlain by fresh, andesitic rocks (lighter coloured).

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Landslide scarp with exposed volcanic rocks. Note shear cliff at the edge!

The basaltic unit comprises of angular basaltic blocks in a fine ashy-matrix that is now mostly converted to clays. This could represent an old eruptive deposit in which pyroclastic volcanic blocks, lapilli and ash cascaded down slope from a vent .

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Closer view of the basaltic deposits. The blocks are lodged within a fine red matrix.

At some point after the block and ash flow was emplaced, andesitic lavas erupted and covered the deposit. The andesite unit here is about 20 m thick and has bore the brunt of erosion. Only a ridgeline is left of what was probably a more extensive lava flow. The lava is essentially a protective ‘cap’ which inhibits erosion of the soft basaltic material underneath.

IMG_5672Contact zone between older basaltic deposits (bottom) and andesite cap lavas (top).

This outcrop confirmed to me that basaltic material underlies most of the Hihikiwi ridgeline. The basalts, which occur as pyroclastic material (i.e. the block and ash flows) as well as lavas, are punctuated by andesite lavas that flowed out towards the end of the volcano’s lifespan.

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To top it off, a clear view to Kawhia Harbour from Hihikiwi track.

The incredible Te Toto Gorge

Te Toto Gorge is a natural amphitheater on the west coast of Karioi, with spectactular exposures of the volcanic succession. Not too long ago, I did a 33 km walk from Raglan to the gorge to take samples and photos for my thesis. 

I began my walk to Te Toto from Raglan at 9 am. The weather was strongly overcast but relatively warm as I passed around the Raglan estuary from the sandbar. Mt Karioi was a reluctant feature of the skyline, partially obscured by mist.

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After several hours walking, I passed the main beaches of Ngarunui and Manu Bay. All of this coastline is composed of lavas and bedded ash deposits from Karioi. Eventually I reached Whale Bay, the spot where I proposed to Julieta just five months ago.

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Whale Bay – a romantic/volcanic coast.

Continuing uphill on a steady incline, I got to the edge of the paved road and began the long trek down the metal  to the gorge. On the way, I met a french tourist who was deaf and we discussed the landscape in sign language for a few minutes.

I reached Te Toto gorge later than expected, about 2 pm – a first leg trek of 5 hours. The path down to the gorge is located near the viewing platform and carpark, about 15 m from the Karioi track entrance on Whaanga Rd. I followed the slope of the ridge through tall grass and eventually got down to the forested part. Beautiful nikau trees, giant boulders and blue streams were abound.

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The last section of the walk to the beach involved navigating hummocky grass terraces. I found this challenging, as thousands of slump crevices were hidden by the grass and I managed to fall in to several of them.

Ah…now a breath and chance to examine the view.

Te Toto Gorge is truly a sight to behold. It is a horse shoe shape, surrounded by vertical cliffs greater than 70 m tall, and a pinnacle rock near the southern wall. The gorge is in-filled by landslide deposits. A true sense of quiet takes over down there, despite the crashing waves.

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Te Toto, view south from the beach. 

Geologically, Te Toto is a cross section through the Karioi volcanic edifice. It is almost a perfect succession, missing only the andesite dikes found near the summit. Work by Matheson (1981) and Briggs determined that the lowermost volcanics at Te Toto are ‘intraplate’ type basalts that relate to an older, underlying volcano called Pauaeke. Karioi itself began with fragmental explosive eruptions (Te Toto Member), followed by large scale effusion of basalts (Whaanga Member). At least 15 basaltic flows are preserved at Te Toto from this period.

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Volcanic stratigraphy of Te Toto Gorge, as presented by Goles, Briggs & Rosenberg (1996).

I searched through the boulders on the beach in the hope of finding green, olivine-rich nodules trapped within basaltic rocks (Te Toto is a well known locality of xenoliths). No olivine xenoliths were found but I did make general notes on the general rock types. Most of the boulders are augite-basalts, best described as ankaramites. Ankaramites are also the main rock type on Pirongia. In several ankaramite boulders I found fine-grained, cream-coloured xenoliths known to be quartzite (from similar xenoliths on Pirongia). Other species of basalt were present, including a finer grained version of ankaramite, and aphanitic olivine-basalts of the Pauaeke Member. Large blocks of bedded ash and lapilli lined the shore too. Pale blue, massive clay beds of the Ohuka Sandstone group outcrops near the central stream. Presumably, the uppermost country-rock of Karioi is this clay unit.

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Final thoughts

The Te Toto section of Karioi may be a perfect analog for the internal structure of Pirongia. The gorge deposits indicate that Karioi began with violent ash eruptions, and then proceeded to erupt thin sheet flows of ankaramite basalt. The upper flanks of Karioi were constructed from andesite feeder dikes and plugs. In a similar way, at Pirongia the base of the Mangakara Valley contains abundant ash and lapilli deposits, and these beds are overlain by ankaramite lavas of equivalent individual thickness and lithology to those of Karioi. The summit ridges are intruded by andesites, just like Karioi.

Given that Pirongia contains no spectacular outcrops like Te Toto Gorge, it seems important to draw as many conclusions as possible about the nature of volcanism from this locality. Karioi is the sister volcano to Pirongia, and their morphology and rock types are essentially identical. Therefore, Te Toto is probably a clear window into the early genesis of both Karioi and Pirongia, and more broadly into the eruption of strongly crystalline basaltic magmas. I hope to use this geologically significant locality as evidence to support my interpretations of the Pirongia Volcanic Succession over the next year of writing.

Winter traverse of Mangakara Stream

On the 8th of July, I was lucky enough to get a chance to try out the Mangakara Stream section of Pirongia with the help of two guys who know their way around the streams and thick bush of the mountain. The goal of our traverse was to cover a few kilometers of stream, walking uphill, and to map and sample the rocks along the way. The stream wasn’t too cold and mostly easy to walk up.

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Mangakara Stream traverse path. Looks small (we went about 1 km) but that’s 5 hours of walking.

The main finding from this first leg was that most of the basal sequence of the mountain is volcanic breccia. It appears light brown in outcrop, as a kind of clay with angular, surprisingly well preserved clasts throughout. Based on my knowledge of deposits it could represent a pyroclastic sequence, in which ash and lapilli were erupted from a proto-vent of Pirongia. The breccias are overlain, in parts, by lavas which are difficult to trace for any distance. Throughout the traverse, i couldn’t help but think of the similarities between the outcrops here and the basal sequence of Karioi, which is spectacularly exposed at Te Toto gorge (see coming post). Perhaps Pirongia, like Karioi, began eruptions with extensive tuff deposits (ash, small rocks etc) and then proceeded to erupt cohesive lavas…

In future trips, which will continue this year, we will drop down to the stream from the baiting tracks, which conveniently cross cut from ridges to streams across the Mangakara Valley. We will probably need at least 4 trips to adequately map the valley, which is Pirongia’s largest.

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Interesting rocks from Taranaki coast

On Monday this week, my colleague gave me several samples picked up from the coast of Taranaki. These rocks are all andesitic lavas with xenolithic (foreign) fragments contained within them. The xenoliths vary in size, lithology, shape, and mineralogy, which reflects their different source environments and emplacement processes. Xenoliths provide us with information on the source region(s) of magmas, such as the types of rocks found at mantle and crustal levels which are melting to form the magma itself. Much of our understanding of the composition of the earth’s interior comes from mantle xenoliths collected from volcanoes.

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Angular xenolith with green groundmass and deep red phenocrysts (or porphyroblasts?). This xenolith probably is one of two rock types: either it is a garnet gneiss (which explains the presence of red crystals) or it is some kind of olivine-rich ultramafic xenolith.  

Mt Taranaki is a high-K arc volcano with compositions ranging from basaltic andesite to andesite, and minor quantities of basalt and dacite. The basement rocks below the volcano are plutonic and metamorphic rocks of the Median Batholith, which is a a 375-110 million year old volcanic remnant of Gondwanaland.

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Green, olivine rich (ultramafic) mantle xenolith surrounded by a dark clinopyroxene reaction rim. During its ascent in the host andesite magma prior to eruption, the olivine began to convert to more stable clinopyroxene.

Six groups of xenoliths are recognised by Gruender (2010):

(1), mafic hornfels (2), garnet gneiss (3), granite and granodiorite (4), finely banded amphibolitic gneiss (5) and gabbros and ultramafic rocks (6).  Group 6 gabbros and ultramafic rocks are dominated by clinopyroxene, amphibole and plagioclase and are predominantly cumulate in origin.

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Diorite is the plutonic equivalent of andesite. It contains the same mineral assemblage of plagioclase + hornblende + clinopyroxene, but is much more coarse-grained. This xenolith could either represent a tabular xenolith of diorite entrained in andesite, or a thin dike of diorite that has intruded the andesite. The former seems more likely, given that the andesite is the late stage eruptive product.

Magma chambers of stratovolcanoes are complex, and operate across a range of depths, pressures and temperatures. Many igneous petrologists now believe that eruptions at stratovolcanoes are triggered by injection of fresh batches of basaltic magma into ‘simmering’ magma chambers. The injections ‘awaken’ the cooler magma body, a perturb it enough to reach the conduit. Our best evidence that this process exists is mafic inclusions in more evolved magmas. In the photo below, we see a dark basaltic intrusion within an andesite. This basalt is carrying angular brecciated material, which probably represent broken pieces of wall rock that it shattered during violent intrusion of the andesite magma chamber. The difference in temperature and physical properties between the two magmas meant that they did not mix (or have time to!) and erupted to the surface as a mingled magma, which is preserved in this beach cobble.

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Andesite with mafic ‘vein and breccia’ material. I am intrigued by this sample – it appears to be a mafic dike with angular felsic fragments (probably andesite). Mafic injections are now considered a fundamental process in stratovolcano eruptions. Perhaps we are seeing evidence of magma mingling here,  in which the basaltic magma has been injected into the andesite just prior to eruption.

A final photo here, which is not from Taranaki but from beloved Dunedin Volcano. I found this amazing dolerite in the teaching sample set. I never found a dolerite during my field trips around the peninsula, but saw a thin section once or twice. These rocks presumably represent tephritic magmas that did not quite reach the surface, and cooler more slowly in dikes. These dikes were eventually exposed by erosion, and provide a nice contrast to the usual finer grained variants.

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Dolerite from a dike of Dunedin Volcano. Oscillatory zoning and resorption textures in the plagioclase (peach coloured mineral) are visible to the naked eye. This sample is part of the teaching set here at Waikato.

Soon I’ll put a post up on my lab work during the last month. I am trying to be more spontaneous with the blog by using cell phone photos rather than those from the DSLR. I hope this increases the post output so I can better document all the great research projects going on at the moment (and my failures too).