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Skip to 0 minutes and 7 seconds The Mount Etna eruption we discussed was neither rare nor large. It erupts frequently and has experienced much larger eruptions in the past. The main reason this eruption attracted so much attention was the fact that people were caught near the top of the volcano when it erupted, exposing them to direct danger. This raises an important point for us to consider– the difference between a natural hazard and a risk associated with that hazard. The hazard Mount Etna presents are its numerous eruptions, whereas the associated risk is the damage it can do to us humans and our infrastructure. Mount Etna has experienced larger eruptions over the last decade, but these eruptions haven’t caused much widespread damage.

Skip to 0 minutes and 55 seconds These eruptions were still relatively small, and because the volcano is continuously monitored, any potential risk is reduced. Normally, people aren’t allowed near the top of a volcano during increased activity. But in the case of this smaller eruption, people were caught near the top of the volcano by surprise, exposing them to a large risk. Mount Etna is one of many volcanoes on our planet. There are currently about 1,500 documented active volcanoes, and there have been many more that are now extinct. Approximately 800 million people live within 100 km of one of these volcanoes. They have produced eruptions of many different styles and sizes.

Skip to 1 minute and 37 seconds If we were to look at the historic record of the last thousands of years, we find that there were eruptions up to 10,000 times larger than our Mount Etna example. Looking at the geological record and further back in time, we can even find examples a million times larger, such as the super eruptions of Yellowstone volcano. So the most extreme eruptions didn’t even occur when humans were around to witness them. Obviously, extreme geological events are not just restricted to volcanic eruptions. They are just one of a string of geological events that occur on our habitable world. Flooding, tsunamis, drought, glaciations, earthquakes, landslides, meteor impacts have all had an important role in development of the planet.

Skip to 2 minutes and 26 seconds And just like volcanic eruptions, they range widely in scale and frequency. This causes us to pose the question, what is regular activity and what is an extreme event? Understanding whether an event is extreme or not becomes easier when we consider how we look at it. In this course, we will look at extreme events from different viewpoints. We will first look at the events at a cosmological scale that created our planet. We then zoom in to events at geological scales that produced the earth’s habitat. Finally, we will look at some events from our own perspective and investigate how they pose risks to our society.

Skip to 3 minutes and 7 seconds Our planet formed and was subjected to events on a cosmological scale much larger than any event we have discussed up to now. The origin of our atmosphere or oceans, and life itself, was shaped by slow, continuous processes that were punctuated by extreme changes to our planet. These things happened only once or a small number of times during the earth’s, or even the universe’s, lifetime. So how we look at these events is very important. What may appear extreme to human beings due to the risk presented to life isn’t necessarily an extreme event. Now let’s travel back in time, all the way to the origin of the universe.

Skip to 3 minutes and 47 seconds In the beginning, it all started with the ultimate extreme event, the very creation of the universe– the Big Bang.

Regular or extreme event?

In this video, Dr Wim Degruyter explains the differences between regular and extreme events.

The film crew caught up in the eruption on Mount Etna described this event as ‘extremely scary’. One volcanologist said it was the most dangerous incident he had experienced in 30 years.

This event was extreme to them because they were so close to a natural hazard and had never experienced something so dangerous in their lifetimes.

This helps us to consider one way we could define an extreme geological event:

An event driven by a natural process that presents a large risk to life [scale] and is very rare [frequency] for a human to experience.

The scale or size of an event, and how frequently it occurs, are certainly key. But, as many geological events happened well before humans were around to witness them, this discounts this definition.


One of the key ways we can begin to answer the question:

What is regular activity and what is an extreme event?

Is to understand the timescales over which we look at and compare events. We’ve referred to two timescales already, that we’ve called:


A timescale that begins from the very start of the universe, believed to be 14 billion years ago.


A timescale that begins from the formation of the Earth, believed to be 4.5 billion years ago.

If we look at the Mount Etna eruption over those timescales and compare it with volcanic eruptions that have occurred on our planet. We will see that this event was neither rare nor large enough to be truly extreme.

So at this stage, the key thing to remember is that we’re going to be comparing events over timescales of millions and billions of years, well before human history.

We’ll start to unpack this by looking at the the first and ultimate extreme event, the origin of the universe.

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Extreme Geological Events

Cardiff University

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