Jordan Yee, of Merrin School, asks :-

Is it true that some geological plates in America are moving at a few metres per year? And what speed are the plates under New Zealand moving?

Jarg Pettinga, a geologist at the University of Canterbury, responded.

The major 'tectonic' plates which make up the outer skin or layer of the earth are all moving relative to each other. This movement forms three distinct types of boundaries. The first is where plates are separating and moving apart, such as occurs along mid-oceanic spreading ridges. The second type of boundary occurs where two plates collide. Here deep trenches, island arcs or mountain chains may form. Finally plates may slide past each other along great fault zones such as the San Andreas fault in California.

The fastest movements of one plate relative to another is about 10 centimetres per year and this occurs around some of the Pacific Plate margin. For the North American - Pacific Plate boundary - across the San Andreas Fault, the movement is about 5 cm per year. For the South American - Nazca Plate boundary along the Peru - Chile Trench the rate reaches nearly 10 cm per year, with the Nazca Plate being thrust (subducted) beneath the South African Plate.

In the New Zealand region, the Pacific and Australia Plates are colliding in a rather complex way, and so form much of the North and South Island landscape. Beneath the North Island the Pacific Plate is being subducted down under the Australian plate at about 5 cm per year. Deep earthquakes indicate that the Pacific Plate now reaches down to a depth of more than 500 kilometres, and this collision and subduction of the Pacific Plate has been going on for about 20 million years.

Across the northern and central South Island the Pacific and Australian Plates are colliding and sliding past each other, so forming the Southern Alps, but neither plate is subducted. Finally beneath Fiordland it is the Australian Plate that is subducted beneath the Pacific Plate, and the speed of collision is about 2.5cm per year.