Mount Ngauruhoe
(In Maori ‘Nga Uru Hoe’ means ‘throwing heated stones’)
Mount Ngauruhoe is a large (2,291 metres tall), young stratovolcano that first erupted about 2,500 years ago- making it the youngest vent in the Tongariro volcanic complex of the Central Plateau of New Zealand’s North Island. Although to many it appears to be a separate volcano, it is in fact a secondary cone of Mount Tongariro to the north. It erupted 45 times in the 20th century, making it one of the most active volcanoes worldwide in this period. It is New Zealand’s most active volcano having had over 70 eruptive events since 1839. Its most recent claim to fame comes not from its last eruption in February, 1975, but from its debut as Mount Doom in Peter Jackson’s The Lord of the Rings movie trilogy. The crater’s floor has steadily cooled since 1979, which may suggest that the main vent is becoming blocked (Snelling, 1998). Present in this photograph are many rocks of andesite (fine-grained equivalent of diorite) composition, which exhibit a ‘salt and pepper-like’ texture of dark mafic minerals mixed into a lighter felsic mineral matrix. Andesitic lavas are more viscous than basaltic due to slightly higher silica contents, but there is still evidence of lava flows here. Flows are evidenced by large angular boulders that have broken off after cooling on the surface of other mobile liquid lavas that carried these boulders down the slope. The best evidence of flows are further down the slopes out of the view of this photograph. Many of these rocks also appear to have small crystal sizes, and a moderate degree of vesicles which may mean the lava had little time to crystallize or allow volatiles to escape while underground. This moderate lava composition may result from the subduction of the (basaltic) Pacific plate under the Australian continental plate, reaching a critical depth, dewatering, and then melting these plates, sending the less dense lavas toward the surface where they eventually build up sufficient pressure and erupt. Also present in the foreground of this photo is some type of old inactive crater, which could represent a shift in active volcanism to the present crater around 2,500 years ago. The layering present on the far ridge of the old crater probably represent distinct volcanic events. Finally, in the old crater one can make out small alluvial fans beginning to form inside the old crater from the erosion caused by melting of large amounts of snow off Nguaruhoe each year.