Uncovering our explosive past

How do volcanoes work? Where do they form? And what does this mean for the people that live around them?

This resource provides explanations of the key concepts encountered when investigating volcanoes – the ‘basics’ that every student should understand.

Volcano types

  • A caldera is a type of giant volcanic crater that forms following a violent eruption that resulted in vast amounts of magma being emptied and leaving a void space that later collapsed. The subsequent depression or hole is often filled with water to form a lake. Examples are Lake Taupō and Lake Rotorua. Calderas are often associated with high-gas rhyolite magmas.
  • A cone volcano has steep sides and often a vent at the top and is also referred to as a stratovolcano. It is formed by many eruptions over a period of time, generally of andesite magma. Mt Taranaki and Mt Ruapehu are examples of cone volcanoes.
  • Shield volcanoes are often formed from a large diffuse area of low viscosity basaltic magma that forces its way through the crust at different points. This results in a large number of inactive volcanoes within a relatively small geographical region. Although each volcano is dormant, the field may remain active, and magma can push through and form another new volcano at another point.

Volcanic rock types

  • Rhyolite contains more than 70% silica and little iron so it is not heavy, and it is often white in colour (for example, pumice). Magma that contains rhyolite is generally cool (750–850ºC) in temperature as it emerges and quite thick. When rhyolitic magma erupts and contains only low amounts of gas, it forms viscous lava flows and domes (for example, Mt Maunganui). Usually it is associated with high amounts of gas, which cause highly explosive eruptions (Taupō eruptions). Rhyolite is often associated with caldera volcanoes.
  • Andesite contains moderate amounts of silica (between 55–70%) and some iron. Magma that contains andesite is generally around 800–1,000ºC and forms steep-sided cone volcanoes (stratovolcanoes). Mt Ngāuruhoe is an example of an andesite volcano.
  • Basalt contains not much silica (less than 50%) but a lot of iron so it is heavy and dark in colour. Magma that contains basalt is generally over 1,200ºC and runny. It forms shallow-sided volcanoes (shield volcanoes) or craters and is associated with volcanic fields. Rangitoto Island is an example of a basalt volcano.


The collective term for all airborne volcanic material is (commonly called ash). Ash is the smallest component of tephra (material less than 2 mm), lapilli is slightly larger (2–64 mm), and the largest fragments are called blocks and bombs (larger than 64 mm).

Volcanic field

An area of volcanoes that is not associated with a tectonic plate boundary, for example, the Auckland volcanic field. The area is associated with a ‘hot spot’ in the Earth’s mantle that causes the crust to melt slightly, forming a reservoir of magma. There is no single vent for the magma, which results in many volcanoes, and each eruption generally happens in a different place. Volcanic fields have a lot of inactive volcanoes spread over a particular area. They are associated with shield volcanoes like Mount Kilauea on Hawaii.

Subduction zone

An area where one tectonic plate is being forced under another. In a subduction zone, the plates melt and often form a pool of magma. This is often associated with volcanism. The ring of fire around the Pacific – a zone of pronounced volcanic activity – is also the edge of the Pacific plate. These areas are also often associated with frequent earthquakes.

Volcanic hazard

The chance of a volcanic eruption occurring. Scientists evaluate historical data to identify what sort of eruptions occurred and find out which part of the country might be expected to experience a volcanic eruption. They investigate what area would be affected, how much ash might be produced and how long an eruption would last to help define the volcanic hazard.

Risk of volcanic eruption

The combination of a volcanic hazard and the overall effect that the hazard has on a population of people. The risk posed by a volcano is determined by the probability of an eruption, how big that eruption would be and how people would respond. If people are well prepared, the overall risk posed by an eruption is decreased, but the risk increases if people are not prepared. The best way to reduce the risks associated with volcanoes is to be prepared for a possible eruption.

    Published 9 April 2010