New Zealand's oldest rocks are over 500 million years old, and were once part of Gondwanaland. This massive super-continent started to split up about 160 million years ago, and New Zealand separated from it about 85 million years ago.
New Zealand sits on two tectonic plates - the Pacific and the Australian. Fifteen of these gigantic moving chunks of crust make up the Earth's surface. The North Island and some parts of the South Island sit on the Australian Plate, while the rest of the South Island sits on the Pacific. Because these plates are constantly shifting and grinding into each other, New Zealand gets a lot of geological action.