Ngauruhoe Summit Crater

Ngauruhoe is a stratovolcano and forms part of the Tongariro Volcanic Centre. Ngauruhoe has been constructed over the top of older Tongariro volcanic rocks by eruptions of lava and ash going back approximately 7000 years. Since 1839 there have more than 70 eruption episodes, mostly small phases of explosive eruptions making eruption columns and ashfall. On three occasions the eruptions sent lava flows down the sides of the cone. The latest of these lava flow producing eruptions was in 1954-55. The explosive eruptions of 1974-75 were the most recent activity, making the period since then the longest phase of inactivity in the last 200 years. Before this quiescence Ngauruhoe was our most active volcano.
The summit of Ngauruhoe has changed significantly over time, with the small summit cone consisting of an outer and inner crater. The inner crater was established during the 1954-55 eruptions, and then further built up during the eruptions in the 1970’s.The crater is over 200 m across and the cliffs forming the crater wall are made of a sequence of unconsolidated or partially consolidated ash and spatter deposits, lava, blocks and bombs.

The naming of Ngauruhoe: Pre 1887 what we call Ngauruhoe today was referred to as Tongariro and the summit crater was called Ngauruhoe. After the first map was published in 1887 and revised ones in 1897 common usage of names changed following the labels on the maps. This is clarified by the explorer Kerry-Nicholls (1887) when he wrote:
“On the Government maps of the colony the active volcano of Tongariro is named Ngauruhoe and a smaller mountain adjacent to it as Tongariro. This is an error. The volcanic cone is known to the natives as Tongariro and its crater as Ngauruhoe (Aruhoe). The smaller mountain with its minor crater is Ketetahi”.

The crater rim is very narrow on the northern and eastern side and has a vertical inner face about 10m high. This was built up in 1974-75. Most of the material on the rim consists of fragments of red coloured scoria and bombs that are partly stuck together (agglutinated) as they were erupted in a hot, somewhat sticky condition. The rim is lower on the north-west side, where it partly collapsed back into the crater in March 1974.There are a number of large blocks (some over 2m across) that were thrown onto the rim during the last phase of the most recent eruptions in 1975.
The cliffs and slopes in the crater below the upper vertical rim wall are made up of material from the 1954-55 eruptions. These consist broadly of loose bombs and scoria at the bottom, a vertical cliff of lava and above that a slope of ash and scoria.