Sarah Holland- Postcard #2

There are many geological features present along the scenic Tongariro Crossing. The crossing passes through portions of both Tongariro and Ngauruhoe Mountain areas. The andesitic rocks that seem ubiquitous in this unearthly landscape show evidence of many lava flows over time. The volcanoes of the area are all stratovolcanoes. Eruptions are usually violent, and are followed by lava flow. The layering and jagged rocks in the area are both characteristic to lava flows. Cliff exposures include layers of very solid looking material sandwiched between layers of angular, unsorted substrate. The angular layer can be derived from two factors at play. Some of the material is colluvium, sediments (large and small) that gravity has pulled down the hill between eruptive events. Other parts of the angular layer is created during the cooling of lava flows. Due to exposure while traveling downhill, the outermost layer of lava congeals. As lava underneath continues to move downwards, it pulls the surface lava. This causes cracking, and piling of the recently hardened material. The valley that contains the first ascent of the Tongariro Crossing has a great deal of angular material which has been created by this process. The Tongariro Mountain area is still considered active.