Article by Dr Ana Casas Ramos, first published on the ANU Reporter website.

Volcanic eruptions around the world have inspired fear, awe and curiosity among humans since the beginning of civilisation. They have played a key role in shaping our Earth since its formation.

More than 80 per cent of the land above and below the sea level was formed by volcanic activity and the Earth’s early atmosphere was formed through volcanic emissions, which provided favourable conditions for life to develop.

Flow or blow?

In simple terms, volcanoes are openings on Earth through which molten rocks, called magma, and gases from the Earth’s interior erupt to the surface.

How gases are released from magma during their ascent to the surface strongly determines the eruption style. If the gas can easily escape from the magma, the lava —which is erupted magma — will erupt in a ‘flowy’ manner, as rivers or even as ‘fountains’. When thick magma doesn’t allow for gas to escape easily, it causes a pressure build-up that leads to explosive eruptions.

Explosive eruptions break magma into fine particles called volcanic ash, injecting it along with hot gases into the atmosphere as kilometres-long columns or volcanic plumes. The January 2022 eruption of a volcano in Tonga is deemed the highest eruptive column recorded in modern history, reaching heights of 58 kilometres.

Destructive but essential for life

There are many ways that volcanic eruptions can impact life on Earth.

In the past, the most damaging consequences of volcanic eruptions have been the many casualties resulting from the large amount of material emitted from volcanoes. The 79 CE eruption of Mt Vesuvius in Italy produced a 30-kilometre-high column of hot ash and gases that buried nearby cities of Pompeii and Herculaneum, tragically suffocating their inhabitants.

In 1783, the Laki volcano eruption in Iceland released millions of tonnes of toxic gases such as hydrogen fluoride and sulphur dioxide, causing the death of around 60 per cent of Iceland’s livestock and severely affecting crops, which led to famines.

But at a larger timescale, volcanic eruptions are also beneficial for humanity. Volcanic soils, for instance, are extremely fertile thanks to the nutrients provided by erupted ash and rocks, which, upon weathering, release elements such as calcium, magnesium, potassium, sulphur and phosphorus that promote plant growth.

Volcanic activity is also an essential part of geochemical processes, allowing for the recycling of many elements. Carbon, sulphur, hydrogen and oxygen depend on volcanic activity to be transported from the Earth’s interior to the surface, where they undergo further transport and play key roles in biological systems.

Without volcanic activity, the supply and (re)distribution of these essential elements would be impossible.

Hot air, cooler climates

The influence of volcanic eruptions on climate has been of concern because it releases greenhouse gases such as carbon dioxide and water vapour into the atmosphere. But the impact of these gases on global warming is almost negligible when compared to emissions resulting from human activity.

Annual estimates of carbon dioxide emissions from volcanoes range from 0.28 to 0.36 gigatonnes (Gt). In comparison, human carbon dioxide emissions are estimated to be 38Gt. 

Similarly, water vapour emissions from volcanoes are unlikely to significantly increase global temperatures — at least, not in the long term. Water vapour from volcanoes tends to dissipate rapidly — within days or weeks — minimising its warming impact. 

Interestingly, the vast majority of large, stratosphere-reaching eruptions have caused global cooling, not warming. After the 1991 eruption of Mt Pinatubo in the Philippines, global temperatures dropped by around 0.5 degrees Celsius for the following two years. 

This cooling effect relates to the sulphur dioxide from eruptions that rapidly reacts with water vapour to form sulfuric acid aerosols. These aerosols can absorb and reflect sunlight, cooling the Earth’s surface for years or even decades.

Such cool periods are called ‘volcanic winters’, and their duration and reach are determined by how much sulphur is released and how high up into the atmosphere it goes.

From the field to the furnace

Because volcanic eruptions can have global impacts, it is important to study them — even in Australia, which is far from the ring of fire, an area of high volcanic activity along the Pacific Ocean.

At the ANU Research School of Earth Sciences, we try to understand what happens with sulphur during volcanic eruptions because of its potential to cool the Earth. We do so by making our own volcano in the lab! 

Our experiments consist of heating samples of rocks, glasses and minerals in a furnace at around 900 degrees while flushing sulphur dioxide gas through them, which is essentially what happens during eruptions. 

We ‘run’ the eruption various times and then analyse the samples to see how they’ve changed. 

What we have found is that chemical reactions occur between the samples and the sulphur dioxide gas that transform sulphur from a gas into a solid, forming sulphur products on the surfaces of our samples. 

This means that, during eruptions, not all sulphur gases will reach the atmosphere. Some will react with the rocks and ash to form solids, changing the amount of sulphur gases that can cause global cooling. 

With an increasing understanding of the fate of sulphur during volcanic eruptions, we can make better predictions of their past and future impacts on global climate