Telephoto image of Solander Island taken on a clear day from McCrackens Rest (Nick Mortimer/GNS)
The geology is very varied. Pahia Hill, Rakiura/Stewart Island and Fiordland are mainly composed if granites and related plutonic rocks of the Median Batholith. Solander Island/Hautere is the extinct and eroded stump of a formerly much larger Quaternary volcano. Chemical studies of the lavas show that they were caused by subduction, like Mt Ruapehu and rocks in the Taupo Volcanic Zone. Subduction between the Pacific and Australian plates south of New Zealand is very slow and oblique compared to that in the North Island, which explains why there are not more volcanoes here.