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Rock Types

The three major rock classes are based on how they were formed.

Cape Kidnappers

Sedimentary Rocks, Cape Kidnappers, photo J.Thomson GNS Science

Sedimentary rocks

Sedimentary rocks form from deposition and joining together of particles, mostly eroded from the surrounding area. Most of New Zealand’s sedimentary rocks are mudstone and sandstone that were deposited beneath the sea. A sedimentary rock made from ejected volcanic fragments (e.g. ash) is called tuff. Some sedimentary rocks, such as chert and certain limestones, are formed from chemical precipitation – the formation of a solid from a solution. Sedimentary rocks are the best places to find fossils.

Find out more about New Zealand’s fossils.

Banks Peninsula

Banks Peninsula, photo Lloyd Homer GNS Science

Igneous rocks

Igneous rocks form when molten rock (magma) from deep within the Earth's crust moves towards or onto the surface and cools.

Plutonic igneous rocks are formed at depth. Because they don't get erupted onto the surface they cool slowly, and are coarse grained (i.e. they are made of of relatively large crystals) New Zealand examples include the granites of Fiordland, Stewart Island and the Abel Tasman National Park.

Volcanic igneous rocks are thrown out onto the land surface or beneath the sea, and cool quickly to form fine grained and sometimes glassy rocks e.g. rhyolite, ignimbrite, pumice, andesite and basalt. Active volcanism is occurring in the North Island, but New Zealand also has ancient volcanic rocks such as Banks Peninsula and around Dunedin.

Igneous rocks are also classified as acid, intermediate, or basic, according to the amount of silica they contain. Acid (silica-rich) igneous rocks are usually light in colour, whereas the basic (silica-poor) igneous rocks are darker. Silica rich magmas (e.g. rhyolitic magma) crystallise at higher temperatures, making stickier lavas and more explosive eruptions, whereas silica poor magmas (e.g. basaltic magma) crystallise at lower temperatures. They are therefore more fluid at a given temperature, and typically erupting with less violence.

Find out more about the different types of New Zealand’s volcanoes.

Haast Schist

Haast Schist, photo Lloyd Homer GNS Science

Metamorphic rocks

Metamorphic rocks are rocks that began as sedimentary or igneous rocks (or even previously metamorphosed rocks), and that have subsequently been recrystallised under conditions of high temperature and/or pressure.

Characteristic minerals form under certain conditions of temperature and pressure. For example, chlorite tends to crystallise in rocks metamorphosed at low to moderate temperatures and pressures, and biotite and garnet form at higher temperatures and pressures. Many of the rocks found in New Zealand have been metamorphosed by being buried deep in the crust, and then uplifted to the surface as the overlying rock has been eroded away. Examples are the schists that outcrop over much of the South Island.