What is a volcano?
A volcano is a landform (such as a mountain, island or crater) caused by the eruption of molten rock through a vent at the surface. This hot rock is known as magma when it is underground, but when it is erupted from a volcano it is called lava. When it explodes violently in fragments through the air, it is called tephra.
The Pacific Ring of Fire: earthquakes and volcanoes
The earth is like a giant jigsaw puzzle, made of slowly moving ‘plates’ of rock. Where one plate slides underneath another earthquakes are common, and the heating of rock forms magma. This molten rock rises to the surface and erupts, forming volcanoes.
New Zealand lies on the Pacific Ring of Fire, where movement of the Pacific plate causes many earthquakes and volcanoes. Some New Zealand volcanoes lie under the ocean on the Kermadec Ridge.
Active, dormant or extinct?
An active volcano shows signs of unrest (for example, earthquakes) or is currently erupting. For example, Whakaari (White Island) discharges steam almost continuously and ash every few years.
A dormant (sleeping) volcano is not currently erupting, but is expected to erupt again – such as Taranaki (Mt Egmont).
An extinct volcano has shut off and will not erupt again – for example, Dunedin volcano, which last erupted 11 million years ago.
Types of volcano
A familiar volcano is the cone-shaped mountain with ash coming from the top. But some of the most explosive volcanoes are very large craters, known as calderas, which are often filled with water.
New Zealand has three main types, each associated with different types of magma:
- Cone volcanoes such as Ruapehu and Taranaki (Mt Egmont). The shape has formed from many eruptions over thousands of years. The magma type is called andesite.
- Calderas such as Lake Taupō. A major eruption of ash and pumice can form a hole 10–25 kilometres in diameter. The magma type is called rhyolite.
- Volcanic fields as in Auckland and Northland. Over thousands of years, there are small eruptions in the area. Each time, a new volcano forms, but it normally does not erupt again. The magma type is called basalt.
Some New Zealand volcanoes
Lake Taupō was formed by a huge eruption about 26,500 years ago. Hot ash and pumice were blasted high into the atmosphere. It covered a large area of the central North Island with tephra up to 200 metres thick.
Ruapehu, Ngāuruhoe and Tongariro form a line of impressive landmarks in the Taupō Volcanic Zone, and have all erupted during the last 100 years.
Mayor Island in the Bay of Plenty has unusual black glass (obsidian), formed from rhyolite lava.