The three main types of volcano are:
Cone (mainly andesite) – formed by many eruptions of lava flows and pyroclastic deposits. These build up a cone, over hundreds of thousands of years. Ruapehu is a typical cone volcano.
Caldera (mainly rhyolite) – a basin formed when a volcano collapses during an eruption. Pyroclastic deposits then build up around it. Magma often continues to come up through feeder dikes within the caldera or around the edges. Lake Taupō and Lake Rotorua are both calderas that are now filled by lakes.
Volcanic field (mainly basalt) – isolated volcanoes where magma erupts once at the surface. Normally there is no single vent that keeps erupting. Shown here are different types within a field: a crater, which is a hole in the ground formed by an explosion; a lava shield, which is a low cone; and a scoria cone, which is higher. Dikes are cracks filled with magma, and act as feeders for surface eruptions. Auckland city occupies a typical volcanic field.
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University of Waikato
Reference: R. C. Price and others, Volcanology of the Tongariro Crossing. CD-ROM. University of Waikato, 2003
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