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Effusive and explosive eruptions

Watch Dr Wim Degruyter explain the difference between effusive and explosive volcanic eruptions.

In this video, Dr Wim Degruyter explains the two main types of volcanic eruptions, effusive and explosive.

One of the main things that distinguishes these two is the material that comes out of a volcano when it erupts

Explosive eruptions

When gas and broken fragments of magma are shot up into the atmosphere, this is called an explosive eruption. The broken fragments are called pyroclasts, or if they are smaller than 2 mm we call it volcanic ash.

Effusive eruptions

When lava flows out of a volcano, this is called an effusive eruption.

How do we measure explosive eruptions?

The most commonly used scale to determine the magnitude of an explosive eruption is called the Volcanic Explosivity Index or VEI.

It is based on the total volume of erupted material referred to as tephra. The VEI is a discrete scale with numbers going from 0 for non-explosive eruptions up to 8.

This means that VEI of 6.5 does not exist. It is either VEI 6 or VEI 7. It is a logarithmic scale, meaning a VEI 7 eruption is 10 times larger than a VEI 6 eruption.

In order to determine the VEI of an eruption, volcanologists go out in the field to study the material that has been deposited by ancient eruptions. Through careful mapping, they can determine the extent and thickness of the erupted deposits, which enables them to estimate the VEI.

An example of such a map is shown in the figure below.

Map showing Vesuvius with the area that was impacted by the pyroclastic flow highlighted in red. The thickness of the ash fall that spread many miles from the volcano is highlighted in blue Pyroclastic surge impact area and ash dispersal range by CUNY licensed under CC BY-NC-SA 4.0

From this field study, volcanologists estimated the 79 AD eruption of Vesuvius to have ejected about 4 km3 of tephra. This makes it an VEI 5 eruption.

How do we measure effusive eruptions?

Several effusive eruptions occur every year in locations such as Kilauea volcano in Hawaii, Piton de la fournaise on Reunion Island and Iceland. It is slightly more difficult to measure effusive eruptions.

The reason for this is that they tend to have long pauses and the slow outpouring of lava is not continuous in time. So it’s not easy to determine when one ends and a new one begins.

The most active effusive volcano in the world today is Kilauea in Hawaii. Its most recent eruption is called the Pu‘u ‘Ō‘ō eruption. It lasted 35 years, starting on the 3 January, 1983. It ended in spectacular fashion in June 2018 with a collapse of the crater floor and the lava lake contained within it.

The Pu'u 'O'o volcanic cone on Kilauea, Hawaii experiencing an explosive eruption and spewing lava 1983, Pu’u ‘O’o, a Volcanic cone on Kilauea, Hawaii by G.E. Ulrich, Public Domain.

After this there were some spectacular fissure eruptions elsewhere on the island up until September 2018. They are considered separate from the Pu‘u ‘Ō‘ō eruption, which occurred at the volcano’s summit.

The lava that erupted covered more than 144 km2, which would envelop a city like Cardiff entirely under lava. Its erupted volume is estimated to be more 4.4 km3.

This is similar to the erupted volume of the 79 AD eruption at Vesuvius. However, that one was much more destructive as the same amount of material erupted in only about 24 hours time. The rate with which material comes out of the ground is illustrative of the difference between effusive and explosive eruptions.

In order to compare the magnitude of effusive eruptions, 3 indicators are used (instead of 1 for explosive eruptions):

1. The area covered by lava (Pu‘u ‘Ō‘ō eruption: >144 km2)

2. The erupted volume (Pu‘u ‘Ō‘ō eruption: >4.4 km3)

3. The duration of the eruptions (Pu‘u ‘Ō‘ō eruption: 35 years)

Over to you

In 2.10 Experiencing volcanic eruptions you chose a volcano and added it to our map. What types of eruption has that volcano experienced? When did they occur?

Add and discuss your findings in the comments below.

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Extreme Geological Events

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