The country owes its varied topography, and perhaps even its emergence above the waves, to the dynamic boundary it straddles between the Pacific and Indo-Australian Plates. New Zealand is part of Zealandia, a microcontinent nearly half the size of Australia that gradually submerged after breaking away from the Gondwanan supercontinent. About 25 million years ago, a shift in plate tectonic movements began to contort and crumple the region. This is now most evident in the Southern Alps, formed by compression of the crust beside the Alpine Fault. Elsewhere, the plate boundary involves the subduction of one plate under the other, producing the Puysegur Trench to the south, the Hikurangi Trench east of the North Island, and the Kermadec and Tonga Trenches further north.