It is generally believed that the Mangatepopo Valley was glacially carved out during the last ice age and subsequently partially in-filled by lava flows from Ngauruhoe.
Note the different colours on the lava flows as you walk up the valley. The surface colour of younger lava is darker and absorbs much of the sun's heat – this is a harsh environment for plants to grow and the reason why the youngest flows only have a few plants, lichens and moss.
The older flows have progressively more species and large plants, which take advantage of the slow build up of precious soil. The vegetation has also been modified by fire and farming.
Near the head of the Mangatepopo Valley a short sidetrack leads to the cold water Soda Springs, which seep to the surface in a boggy area at the head of the Mangatepopo Stream. The springs are an oasis for the moisture-loving yellow buttercups (Ranunculus insignis).
The rocks at and below the springs are coloured golden by iron oxide, from the breakdown of volcanic ash in the bog. The water is slightly charged with dissolved gases and this effervescent quality inspired the name.
The water contains significant levels of dissolved minerals from the volcanic rock the water passes through on the way to the surface. It is not suitable for drinking.
The rim of the spectacular active Red Crater forms the highest point of the Tongariro Alpine Crossing. From the top you have extensive views across the Emerald and Blue lakes and beyond.
Its stunning red colour is from high temperature oxidation of iron in the rock. You can also see colours in the rock ranging from rich black, chocolate brown, rusty red and yellow.
You can see (and smell!) several active fumeroles (steam vents) in and around the crater. The gases coming out create bright yellow sulphur deposits. The top section of the crater rim is often free of snow in winter due to the thermal warmth of the ground there.
There is an interesting dike formation in the centre of the crater, a hollow lava tube formed when molten magma drained out from below and the outside solidified. You can see old lava flows from Red Crater extending into Oturere Valley, South crater and Central crater.
As you descend from the northern edge of Red Crater, three water-filled explosion craters called the Emerald Lakes come into view. Their brilliant colour is because of minerals such as sulphur leaching from the adjoining thermal area.
The Maori name for the lakes is Ngarotopounamu meaning greenstone-hued lakes.
Despite being surrounded by fumaroles, the lakes are cold and freeze over in winter. The water is acidic with a pH of around 3-5.
Formed in an old volcanic lava vent, this is a freshwater lake up to 16 metres deep. Like the Emerald lakes the water is acidic with a pH of around 5.
Once again, dissolved minerals are responsible for its distinctive colour, and its waters are cold and acidic. Its Maori name is Te Wai-whakaata-o-te-Rangihiroa, which translates as Rangihiroa’s mirror. Te Rangihiroa was the son of local chief Pakaurangi, and is said to have explored the Tongariro volcanoes about AD 1750.
This lake is tapu (sacred). Do not swim in or eat food around the lake.
Te Maari craters
The Te Maari craters on the northern side of Mount Tongariro burst into life on 6 August 2012 with a volcanic eruption. There was a second smaller eruption on 21 November 2012. Flying rocks from the August eruption caused damage to the track and the old Ketetahi hut.
This section of track passes through the Active Volcanic Hazard Zone - keep your stops to a minimum, and know the volcanic risks and what to do in the event of an eruption.
On the way down the track, you can see excellent views of the steaming vents, and evidence of the August eruption. There are impact craters from flying rocks, including a large one just below the Ketetahi shelter, and holes through the hut roof and floor. You can also see some of these rocks - any that look out of place and slightly yellow (from sulphur).
Prior to the 2012 eruptions, the Te Maari craters last erupted in 1896.
The Ketetahi springs is an area of thermal activity on the northern slopes of Mount Tongariro, at about 1400 m. The springs are located in a small area of private Maori land, and are not accessible from the Tongariro Alpine Crossing track. However, you can see wonderful views of the rising steam as you walk on the track.
There are a myriad of colours on the rocks around the springs area from the effects of the hot water and steam on the minerals in the andesitic rock. The largest fumerole in the springs area discharges superheated steam to about 138°C!
Lower down past the Ketetahi shelter, the track crosses a stream which originates at the Ketetahi springs. You can see here how the water and rock is coloured by the acidic and mineral-rich water.