Everyone knows what a volcano looks like – isn’t it a steep-sided cone with wisps of ash coming from the top, just like Rangitoto, White Island, Mt Ngāuruhoe or Mt Ruapehu? But what about small hills, peninsulas and lakes – are these volcanoes? Sometimes, yes they are – Banks Peninsula, Mt Eden and even Lake Rotorua are all volcanoes or the remnants of volcanoes.
If you’re asked to picture a volcano, the steep-sided, cone-shaped volcano that immediately comes to mind is called a cone volcano. (It’s also sometimes called a stratovolcano or composite volcano.) Mt Taranaki and Mt Ruapehu are examples of cone volcanoes.
A cone volcano is formed by magma forcing its way through the Earth’s surface (the crust) and, once erupted, the lava builds up near the vent. Over time, the lava and ash accumulate close to the vent area, building up until eventually a cone is formed.
This cone shape requires a specific type of lava, often made up of andesite. The original magma is relatively viscous and flows like treacle down the side of the mountain, solidifying as it goes. This process eventually forms the steep-sided cone-shaped volcanoes such as Mt Ruapehu or Mt Taranaki.
Shield volcanoes are formed from magma that is highly fluid and contains very little gas. Typically, because the lava is runny, it spreads over wide areas and forms gently sloped shield volcanoes.
Shield volcanoes are associated with basalt magma which is hot, runny and fast-flowing. The magma doesn’t erupt explosively, and the cone can form within a very small area (less than 5 square kilometres).
Rather than a pool of magma forcing through one vent to the surface, volcanic fields are often formed from a large diffuse area of 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.
The Auckland volcanic field is made up of nearly 50 small volcanoes spread over a large area. Most of them are scoria cones,formed by the explosion of basalt magmathrowing up ash and larger fragments. Some of the scoria cones also have lava flows around them.
Then, there are the really violent eruptions, throwing magma, ash and rock across large areas. These are the caldera volcanoes (sometimes called supervolcanoes). Following such a large eruption, the volcano collapses into the void left by the empty magma chamber and a large depression is formed.
This type of eruption formed Lake Taupō and Lake Rotorua. Each lake was formed by water filling the crater that was left after an eruption.
Caldera volcanoes are characterised by rhyolitic magma. This forms at relatively low temperatures (750–850ºC) and is thick. Gases like water vapour, sulphur dioxide and carbon dioxide are trapped until the magma reaches the vent and are suddenly released – a bit like shaking a can of fizzy drink then opening it. The resulting explosion sends ash (tephra) and rocks flying into the atmosphere
Caldera volcanoes can also generate pyroclastic flows – mixtures of hot dry rock fragments and gas that flow very rapidly and can travel for long distances over the land, sometimes at supersonic speeds.