Each has a perception of NZ and for me (Lord of the Rings aside) it was volcanic, mountainous and earthquake prone, few people and a lot of sheep jokes. Setting to one side Bilbo, the people, the sheep and starting with the North Island’s geology around the area of Auckland, where we were staying, the first surprise was it was generally sedimentary, heavily folded, graded beds with well rounded conglomerate, high energy rivers etc. There was some evidence of old volcanic activity around Piha’s black sand beach with an old volcanic plug and pyroclastic flows however that changed on arrival in the Taupo Volcanic Zone.

The narrowness of this zone can be seen from photograph 2a taken at Hells Gate just outside Rotorua showing the mud pools, fumaroles and geysers of Hell's Gate and the rolling countryside outside of the fault lines giving an idea of the limited area of the zone. The main rock type is low iron Ignimbrite/Rhyolite/Dacite. Many of the pools have high graphite concentrations and water temperature exceeds 115 o C (photograph 2b).

The Togariro Northern crossing trek (a theoretical 17 horizontal kms) goes across a complex of overlapping volcanic features which includes some 6 craters, 3 cones, thermal springs, glacial reworking and associated lava and pyroclastic fields.

The whole Taupo area is dated between 1.7 Ma and the present time and is extruded through series of lithologies. This includes upper Quaternary marine sediments and laharic sediments, Pliocene sandstones and siltstones, upper and lower Miocene limestones, calcareous sandstones and siltstones, Permian greywackes and lastly a covering of Andesitic tuff.

Lake Taupo was the scene of one of the biggest eruptions ever known when some 800 km3 of material blew out (~25000 years ago) in what is considered to be Phreatoplinian (steam driven) event. More recently around 181 some 60 to 100 km3 of material was erupted in an Ultra-Plinian event. Uniform ignimbrite deposits from these eruptions cover much of the area of North Island. As an idea of the size of the lake the river that flows from it provides some 15% of New Zealand’s hydro-electric power as well as cooling 2 geothermal power stations.
Great jet boating on the river and you should see the prawns from the geothermal station’s outflow.
Also for non-walkers Lake Taupo is reputed to be the best trout and salmon fishing in the world.

It is still a very active region and Mount Ruapehu last erupted in 1995/96. The magma feeding the complex is derived from the descending Pacific plate from a depth of 75 to 150 km.
Andesite and Basaltic Andesite are the principal magmas of the Tongariro complex but in the Taupo zone Rhyolite and Dacite are present. Much of the Basaltic Andesite is iron and magnesium rich (mafic) and oxidises into red scoria. Everything from ash (<2mm) to bombs (>64mm) and bigger can be seen or walked over.

This walk is a volcanologist's delight but you have limited time (the drop off bus will collect you in 8 hours time at the finish) so no rock bashing and it is a National Park (see figure 4). It is considered to be one of the best one day walks in the world.

You start at the car park (1100m) and then a "stroll" alongside/over the Mangatopopo stream until the Soda Springs (1380m). These are cold water; high gas content mineral springs with iron oxide action from the surrounding bogs. Prehistoric and more recent (1870, 1949, 1954 and 1975) lava and pyroclastic flows are seen on the flanks of Ngaurhoe and underfoot to the right as you walk around it.

Then the real test comes climbing the outer rim of the South Crater and this exceeds 1 in 1 in some places. The lava blocks (A-a) make climbing very difficult for those not built like mountain goats. If you make it without a coronary a treat awaits. Photograph 3 shows the climbing of the South Crater western rim from the Mangatopopo Stream (1380m). Roughly 320m vertical and 700m horizontal (~1 in 2) but old lava flow blocks (Aa) gave slopes of over 60o. Old lava flows are to the left. The Soda Springs are right of centre in the "grassy" section at the bottom of the slope in the photograph.

Photograph 4 is inside the South (1680m) Crater going towards the Eastern rim before a climb up to the Red Crater (top left). It is thought not to be a true crater but a glacial cirque but tephra, prehistoric lava flows and explosion pits are present. These are often infilled with melt water and mud from Ngaurhoe in winter/spring. Blocks, bombs and lapili can be seen from Ngaurhoe to the right. The local area of the Tongariro complex is about 275000 years old but the Ngaurhoe volcano is much younger at 2500 years. It is made up of Andesite/Basaltic Andesite pyroclastic ash and lava in a composite or strato-volcano. There are occasional nuée ardente events although the last eruption was in 1974/1975. The eruption type then was "Vulcanian" in that the vent was blocked by old lava and an explosive ash/pryoclastic eruption occurred when pressure is exceeded. It is estimated that the vent speed was supersonic around 400m s-1 or mach 2 .It is possible to walk to the top if fit and you start the walk very early (special trip/bus).

Photograph 5 gives a view looking back from the rim of the Red Crater across the South Crater. The Western Crater rim is to the right and the "pass" from the climb up from the Mangatopopo stream is in the centre and the track can be seen.

One of the spectacular features of the trip is the Red Crater (1886m). The red/russet colours are due to the iron in the mafic Basaltic and Andesitic scoria oxidising (photograph 6) to the left of the opening. In the centre is a view of an old lava tube (dyke) partly open. This crater is still active and the last eruption was some 100 years ago. One eruption (~10000 years ago) blew out the sidewall to the left and lava flowed down into the Oturere valley. Another flow (around 230) went into the Central Crater.

The highest point on the crossing (~1888m) is the scoria rim of the Red Crater before the steep descent to the Emerald lakes (photograph 7). That is volcanic steam issuing from the scoria, as the site is still active and the acidic Blue Crater lake can seen in left background. This lake is an infilled splatter cone eruption between 25000 and 10000 years old and is acidic at pH ~5.0.

Photograph 8 shows the steep descent of the scoria slope to the Emerald Lakes. The range of debris sizes can be seen although most of the descent is ash and gravel. However at this point I was glad of any down (guide book advises running down it!). Photograph 9 gives a close up of the texture and steepness of the descent.

Explosion pits on the breached rim of the Red Crater form the Emerald lakes. There are not deep about 4.5m but the pH is 3 to 5 so very acidic. Surprisingly given the nearby fumarole activity they are cold and the vivid green/blue colour is from polysulphide ions altering the leached mineralised melt water from the Red Crater. The track to the Blue Crater Lake is on the left diagonal again a bit of a detour of the main track.

Views of the A-a flow into the Central Crater (~1600m) is shown in photographs 10 and 11. This is about 1800 years old and flowed from the Red Crater. The blocks are clearly seen, but the tenuous hold some plants have established does not come out well in photograph 10. Ngaurhoe is in the background to the southwest in photograph 10. The North and West crater walls are shown in the background with photograph 11.

A good reverse view of the Red Crater, the scoria slope (the Red Crater colours are not so vivid in B/W) and the columnar jointing in the upper left corner can be seen in Photograph 12. The darker colours are old lava flows and the range of debris is from ash to blocks. The Emerald lakes are just off to the left. A good place for lunch and breathing but not a cup of tea!

Once down the slope a restful walk across Central crater follows. Photograph 13 gives a reverse view of the complex including the scoria slope (highest point), the Red Crater (to the left) and the Central Crater's lava flow (flowing from the centre to the right). The active fumaroles can be seen steaming in the left centre. Ngaurhoe is in the background

Photograph 14 is a view of the start of the descent route down to the pick up point near the lake. After a short climb over part of the North Crater rim from the Central Crater the track winds down the flank of the North Crater (on the left just discernable).

The Ketetahi springs, mud pools and fumaroles are on sacred Maori land and no access is allowed but the stream is often crossed and the steam can be seen. The descent to the Ketetahi refuge hut (only toilets en-route ladies) and car park often crosses many old lava flows and is subject to recent landslides giving a bit of excitement and careful steps. Eventually as you descend you enter the restored semi-tropical Ketetahi forest (podo-carp, beeches planted by the NZ conservation dept.) area arriving at the car park after an exhilarating, but exhausting, 8 hour trek and looking forward to a cold, cold beer, a new set of feet and a hot bath. As the owner of the Parklands Motel remarked when we limped/crawled in…“told you to go fishing”.
Why no photos of the descent? Find out for yourself!

Further Reading
If you do this trip is well worth reading up a bit first to get maximum benefit and there are many web sites.

NZ Dept. of Conservation one is good.
http://www.doc.govt.nz/Explore/002~Tracks-and-Walks/By-Region/007~Tongariro-Taupo/008~Tongariro-Crossing.asp

Also see http://www.alpinescenictours.co.nz/alpinescenictoursTC.htm

Volcanoes of the South Wind (2001) by Karen Williams Tongariro Natural History Society
(ISBN 0-473-07537-7)

Reed Field Guide to New Zealand Geology (2003) by Jocelyn Thornton Reed Publishing
(ISBN 0-7900-0856-4)

Volcanoes a Planetary Perspective (1996) by Peter Francis Clarendon Press
(ISBN 0-19-854033-7)