 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.