The
Alpine Fault runs across the lower North-western slopes of the mountain near the edge of the coastal outwash plain, and is the boundary between the
Pacific and
Indo-Australian tectonic plates. Mount Adams itself is composed primarily of
schist of
Permian–Triassic (depositional) age, which is increasingly metamorphosed closer to the Alpine Fault. On 6 October 1999, a large rock
landslide originating near the northern summit of Mount Adams deposited c.10–15 million cubic metres of rock in the
Poerua River below. This created a high
landslide dam, which formed a lake that extended 1.2 kilometres upstream. The dam failed six days later during heavy rain. Fears of major damage did not turn into reality when the dam was breached, though significant quantities of coarse
gravel were deposited downstream and the river's path was changed in places. == Access ==