Volcanoes: How often do they erupt and what happens when they do? Eruption of Mount Sakurajima
Lightning can form in volcanic ash clouds by ash fragments colliding to produce static electricity. (Getty Images: Mike Lyvers)
Planet Earth is covered in hundreds of volcanoes, many of which will be erupting at any one time.
Many of us only notice volcanoes when they are about to explode or disrupt our travel plans, but these spectacular forces of nature can have a significant impact on people living in the local area.
While volcanoes can be destructive, they are also responsible for creating rich agricultural soil, minerals like gold and silver, diamonds, hot springs and geothermal energy.
So how do these iconic wonders form, and what risks do they really pose?
What is a volcano?
A volcano is like a chimney that allows hot liquid rock, called magma, to flow from a layer within the Earth and erupt onto the surface. The magma can come from as far down as 200 kilometres in the mantle and once it erupts — at a piping hot 700 to 1,200 degrees Celsius — it is called lava.
As magma rises through many kilometres to the Earth's surface, dissolved gases contained within it form expanding bubbles.
These bubbles increase the pressure of the magma and, if this pressure is great enough, the volcano will erupt.
The amount, temperature and composition of magma, including the amount of trapped gas contained in it, determines the type of volcano formed.
The three most common large types of volcanoes are strato, shield and caldera.
Strato volcanoes
Illustration of a subduction volcano
Anatomy of a strato volcano such as Mt Agung in Bali. (ABC: Julie Ramsden)
Strato volcanoes are cone-shaped mountains that have been built up from layers of ash and lava. They are generally the tallest type of volcano and are known for their violent explosions.
Bubbles of gas build up in the magma — which has a high silica content — and explode creating volcanic ash, consisting of tiny gritty sharp fragments of glassy snap-frozen magma and rock from the sides of the volcano vent.
Examples of strato volcanoes include Agung in Bali, Yasur in Vanuatu, Etna in Italy and Fuji in Japan.
Shield volcanoes
This flatter type of volcano is named after the Roman Centurion shield shape of the volcano made by repeated gentle flows of lava down its slopes.
Shield volcanoes have magma with a relatively low silica content.
The magma is very hot and runny, so they are less likely to build up and create explosions — although they still can.
There are many shield volcanoes in Hawaii and Iceland, including Kilaeua and Eyjafjallajökull. Manaro volcano on Ambae Island on Vanuatu is also a shield volcano.
Aerial shot of Mt Kilauea
Mt Kilauea in Hawaii is a shield volcano (Supplied: USGS)
Caldera volcanoes
This type of volcano has the coolest and stickiest magma. It tends to erupt so violently its top collapses and leaves a large basin shape in its place.
Some caldera volcanoes are up to 90 kilometres across and are called supervolcanoes.
Examples of supervolcanoes are Yellowstone in the US and Lake Toba in Indonesia.
Where are volcanoes found?
Volcanoes are found all over the world but the most common location for active volcanoes is at the boundaries of tectonic plates where plates are converging.
One plate pushes under another (a process known as subduction) and as it sinks it melts and generates an explosive type of magma that is vented through volcanoes on the upper plate.
These kinds of volcanoes are common along the so-called Ring of Fire — a horseshoe-shaped area around the Pacific Ocean.
Volcanoes also occur in the middle of oceans where tectonic plates are pulling apart or diverging.
This mainly occurs underwater, where it can also lead to hydrothermal vents on the deep sea floor that harbour extreme forms of life.
Volcanic activity in Iceland also comes from diverging tectonic plates.
Hawaii's big island as seen from the International Space Station
Hawaii's Big Island is the largest active aboveground volcano on Earth (Supplied: NASA/ESA/Samantha Cristoforetti)
Some volcanoes occur in the middle of tectonic plates, and are created as the plate moves over a hot part of the Earth's interior.
As the plate continues to move across the "hot spot", a chain of volcanoes, like those seen in the islands of Hawaii, are created.
The Big Island is the biggest active aboveground volcano on Earth — around 180 kilometres wide by nine kilometres high.
How often do volcanoes erupt?
Some small volcanoes only erupt once in their lives, while other volcanoes erupt multiple times.
Kilaeua volcano in Hawaii, which has been erupting continuously since 1983, is the world's most active volcano.
While some volcanoes erupt at regular intervals, there are always exceptions to the rule.
And even volcanoes that haven't erupted for more than 10,000 years — traditionally thought to have been extinct — can start up again, says volcanologist Ray Cas, an emeritus professor at Monash University.
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