Download Basalt has a high melting point and is very runny (like honey) – in

Types of Volcano
Basalt has a high melting point and is very runny (like honey) – in other words, it has a low
viscosity. This low viscosity means that the lava from a basalt volcano flows a long way,
resulting in a volcanic cone with a flat, shield-like shape. Basalt is very dark in colour and has a
silica content of only 50%. Basalt is also very dense and has a high specific gravity. Examples
of shield volcanoes include the Dunedin and Lyttleton volcanoes, and Rangitoto Island. The
‘Organ Pipes’ on Mt Cargill are an example of a basalt formation.
Andesite is an intermediate type of magma, and its qualities are between those of basalt and
ryholite. It has a silica content of 60% and is grey in colour. Andesite has a medium viscosity,
and it flows like cold treacle. Because if flows more slowly than basalt, it forms volcanic cones
with a much steeper shape, called cone volcanoes. Examples of cone volcanoes include Mt
Taranaki and Mt Ruapehu.
Rhyolite magma is the most viscous type of magma – it flows like tar. It is light in colour and
has a silica content of 70%. Rhyolite has a low density and a low melting point. Because of its
high viscosity, rhyolitic volcanoes tend to be very explosive. The viscous magma builds up until
there is enough pressure in the magma chamber to push it out. If sides of the volcano are left
behind, it is called a dome volcano, like Tarawera. Sometimes, if the explosion is very large,
nothing is left behind except a large hole in the ground, called a caldera. Examples of a
caldera include Lake Taupo and Lake Rotorua.
a.