Manganui Monster Gully

Taranaki Volcano has the classic conical profile of a stratovolcano, with andesitic eruptions that create a variety of volcanic rock types such as soft ash layers bound together by a framework of harder lava flows that radiate from the centre. Only 20-25% of the magma is extruded as lava flows, the remainder is thrown out as blocks and ash during eruptions that are generally gas rich and explosive, and may send dust and gas aerosols high into the atmosphere.
The mixture of soft and hard layers, along with the very steep angle of the slopes makes for a very unstable mountain. On occasions huge sector collapses send several cubic kilometres of rock debris across the ring plain to a radius of 40 or more kilometres in a single catastrophic event. In fact the steep profile of Mt Taranaki has more to do with these collapses taking out huge chunks of the mountain, than with the construction of lava and ash layers during eruptions. There is a much greater volume of volcanic material spread out in the ring plain than there is in the cone.

The mountain is close to a coastline on three sides and subject to extremes of temperature, wind, rain and snow. This leads to continuous rapid erosion of the slopes. Snow avalanches are common, particularly (but not exclusively) in winter, when the snow over wide areas across the upper slopes gets destabilised by rapid accumulation, changing temperatures, or rain, and breaks away to get funnelled into gullies, taking loose rock and boulders with it. These break down over time into smaller stones and particles, and get transported by rivers towards the coast.

As you walk along the track towards the skifield, keep your eyes open to the different rock types in the cliffs around you. You should be able to spot brownish layers of fine airborne ash, boulder conglomerates created by lahars and debris flows, and vertical walls of lava flows.
You will pass through a walking tunnel, protecting you from an eroding cliff, and into the heart of the Manganui Monster itself. Notice the size of some of the loose boulders, the confined dimensions of the gully, and imagine the power of snow and rock avalanches surging through it. Keep following the track as it sidles beyond the gully, beside another lava flow, until you arrive at the gentler grassy slopes of the ski area.