Story: Volcanoes

Page 1. What is a volcano?

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A volcano is a landform that results from magma (molten rock within the earth) erupting at the surface. The size and shape of a volcano reflect how often it erupts, the size and type of eruptions, and the composition of the magma it produces.

Types of volcano

If asked to draw a volcano, most people will sketch a steep, cone-shaped mountain, usually with clouds billowing from the summit. This is one type, but some of the most explosive volcanoes are less obvious, and represented by large depressions that may be filled with water.

Although New Zealand’s active volcanoes look quite different from one another, they can be grouped into three main landform types:

  • classic cones or stratovolcanoes
  • volcanic fields
  • calderas and collapse craters.

Each of these has distinct landforms, and the violence and styles of eruptions are unique to each. These differences reflect the main type of magma erupted:

  • andesite at the cone volcanoes
  • basalt at the volcanic fields
  • rhyolite at the calderas.

Magma, lava or tephra?

Molten rock is called magma when it is beneath the earth’s surface. When it is erupted and flows through a volcanic vent it is called lava. And when it is erupted explosively as shattered fragments hurled into the air it is called tephra (which includes volcanic ash, pumice and scoria).

Types of volcanic rock

Magmas contain almost all of earth’s known chemical elements, but typically they consist of just nine: silicon, oxygen, aluminium, magnesium, iron, calcium, sodium, potassium and titanium. Oxygen and silicon together are the most abundant elements, making up 48–76% by weight of most magmas. The chemistry of magma, especially silicon content, is important for influencing the way it erupts. Three main magma types, and resulting volcanic rocks, are identified on the basis of their chemical composition.

  • Andesite magma is intermediate in composition and physical properties. Erupting at around 800–1,000°C it is more viscous than basalt, but much less viscous than rhyolite. Andesite magma cools to form dark grey lava if gas-poor, or scoria if gas-rich.
  • Basalt is rich in iron and magnesium, but has less silicon than other magmas. It erupts at very high temperatures (around 1100–1200°C) as a very fluid magma. Basalt magma with very little gas cools to form black, dense lava, but where magma erupts with lots of gas it cools to form ragged scoria.
  • Rhyolite magma is rich in silicon, potassium and sodium and erupts at temperatures between 700°C and 850°C as an extremely viscous (sticky) magma. Rhyolite magma containing lots of gas bubbles cools to form pumice. Because it is low in iron, rhyolite is normally light-coloured – it may vary from white to pink or brown. Obsidian is a type of rhyolite produced when lava is chilled to form glass.

Quiet or explosive eruptions

Whether or not an eruption is quiet or explosive depends on the gas content of the magma. When the gas content is low, magma extrudes at the surface as lava flows. Very fluid basalt lava can flow over long distances, whereas viscous rhyolite lava piles up around the vent, like toothpaste squeezed from a tube, to form steep-sided mounds called domes.

Ash, lapilli or bomb?

Volcanic ash is not ash in the sense of the remains of something that has been burnt. In volcanology it refers to the size of grains. Grains less than 2 millimetres across are called ash. Those from 2 to 64 millimetres are called lapilli (from the Latin word for little stones). Larger material is called a block if it is dense and angular, or a bomb if it is full of gas bubbles and partly rounded.

When magma is rich in gas, it can produce explosive eruptions. As magma rises to the surface, the drop in pressure causes its gases to expand violently like the foam that explodes out of a champagne bottle when first opened. Shattered magma and rock fragments (pyroclastic material or tephra) are carried violently into the air before settling back to the ground. The higher the material goes, the further from the volcano it will be carried by the wind, so the intensity of an eruption can be judged partly by the distance its eruption material is spread.

A second type of explosive eruption occurs when magma contacts water beneath the ground or at the surface (such as in a lake or the sea). The hot magma vaporises the water instantly, causing violent steam explosions. Such eruptions take place regularly when magma rises beneath Ruapehu’s crater lake.

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