About a fifth of the North Island and two-thirds of the South Island are mountains. Stretching from the north of the North Island to the bottom of the South, these mountains are caused by the collision of the Australian and Pacific Plates.
Over millions of years, alluvial deposits (eroded from the mountains by rivers) formed the vast Canterbury Plains in the South Island and a number of smaller plains in the North. These alluvial plains contain some of New Zealand’s most fertile and productive farmland.