Inside the Earth there is a red-hot liquid rock, called magma. Volcanoes happen when magma rises to the surface of the earth. As the magma rises bubbles of gas form. This gas can cause pressure to build up in the mountain, until it erupts.
When the magma bursts out of the earth, it is called lava.
Why New Zealand has active volcanoes
Like earthquakes, volcanoes in New Zealand usually happen along the plate boundary. Under New Zealand, the Pacific plate is pushing into the Australian plate and is being pushed down into the Earth’s mantle, so the plate melts. Magma rising from this area called a “subduction” zone creates a line of active volcanoes called a volcanic arc. Most of New Zealand’s active volcanoes are caused by this process.
New Zealand’s volcanoes are part of a larger area of active volcanoes at the edge of the Pacific Plate.
How does the magma form at the edge of a plate?
When one tectonic plate sinks down under another one, it heats up and the rocks start to release water. This water then escapes from the sinking plate and rises into the plate above. Part of the plate above then starts to melt and pockets of magma form.
The magma may find its way upwards along cracks in the Earth's crust to reach the surface and burst out as a volcanic eruption. The opening at the earth's surface is called a vent.
Volcano types
There are three common volcano types:
- Caldera Volcanoes eg Taupō, Rotorua and Okataina
A caldera is a large, round-shaped basin formed by very large violent eruptions, when magma erupts from close to the surface. The land above then collapses. Lakes and domes can form in these basins (eg Tarawera).
- Cone Volcanoes eg Ruapehu and Ngāuruhoe
A cone volcano forms when eruptions of lava, ash and other rock build up layers over many years around the vent.
- Volcanic Fields eg Auckland
Volcanic fields do not form on a plate boundary. They form when a hot spot in the mantle melts its way to the surface. Volcanic fields have many small volcanoes that may only erupt once.
Taupō Volcanic Zone
The volcanoes of the central North Island make up the Taupō Volcanic Zone. This is a line of active volcanic cones from Ruapehu in the south to White Island in the north.
Auckland Volcanic Field
Unlike the Taupō Volcanic Zone, the volcanoes of Auckland are not on a plate boundary. They have formed in the middle of a plate. The magma comes from the mantle in an area called a hot spot.
Scientists don’t know why some areas of the mantle are hot spots. When enough mantle rock at the hotspot has melted, it rises to the surface to form a volcano. There are over 50 volcanoes in the Auckland Volcanic Field, most of these have erupted only once.
Types of eruptions
An eruption of magma at the surface of the Earth can be explosive or non-explosive (effusive).
The type of eruption from a volcano depends on a number of things:
- how sticky the magma is
- how much gas is present
- if water is close by then explosions can occur as the hot rock meets the cool water
Explosive eruptions happen when escaping gases tear the magma apart into pieces, and usually happen when the magma is sticky with lots of gas in it.
Sometimes, magma meets water. When this happens, the eruption can be very explosive.
Non-explosive eruptions happen when liquid magma flows out at the Earth’s surface (lava flows or domes). These eruptions are common when the magma is runny and has less gas in it.
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