The fantastic thing about modelling volcanic activity is that we can take our current understanding of the physics surrounding a volcanic problem and apply it safely at a desk. It may not be as exciting as being 'out in the field' but is certainly as valuable as we always need real world data to validate and verify models! Modelling activity takes many forms from investigating the dispersal of ash in the atmosphere for aviation, certainly a very prominent and current problem, to analysing potential lava flow paths, vital for determining potential construction sites. The other arguably most valuable thing about modelling is that it often enables us to see things that we otherwise wouldn't be able to view. For example sub-surface processes such as the motion of magma and gas beneath the surface, which just happens to be my modelling area.

When involved with computational modelling, we try to constantly balance between levels of accuracy, an appropriate representation of the physics involved, computational power, solution time and storage space.

• Accuracy - when running a model you first have to define your area (or geometry), in my case its usually just a simple elongated rectangle. Great! I hear you say. Then we have to mesh the domain, that is define a grid which snaps onto our defined geometry. This is where it starts to get more complex, what kind of mesh do we use, do we make it adaptive (i.e. alters during model run), and the key thing what resolution do we use - too small and solution time may take an eon (and also produce unmanageable amounts of data), too large and we may not be able to represent the physics of the problem sufficiently!
• Appropriate representation of the physics - what equations are we going to use? Are they appropriate for the situation? How long do they take to solve?  
• Computational Power - I can guarantee that this is often the major problem for modellers. I am lucky, in that I have access to the University of Sheffield supercomputer Iceberg (don't ask me why its called that but it sounds cool!) to run applications off and hence a large amount of RAM to draw off. During my MSc I was on the other end of the spectrum, using a desktop computer with 2GB of RAM, waiting 3/4 days to perform 30 s of a model run (probably an under-exaggeration actually). From this you can certainly start to see the challenges mounting up!
• Solution Time - As mentioned above, solution time is certainly key, some climate models can take months to run, which if you are doing a 3 year PhD doesn't leave much room for error! It is therefore inherently important to plan model runs for available time and with available resources.
• Storage Space - In a world where data is easy to create but costly to store this is another key issue. Some of my recent model runs have produced >20 GB of data for just 10 s of model run (and took over 10 hours), when preforming multiple model runs it is obvious that cumulative data production will get larger and quickly run into the terabytes. Some ways of getting around this include only exporting/saving the data you actually need, if your only interested in temperature outputs only save this!

There are many excellent fluid dynamics software applications in existence, however, they are usually costly (running into the tens of thousands, notable exception here is OpenFoam). It is lucky therefore that the University of Sheffield has access to one of the leading applications Ansys and the dynamic Ansys Fluent package. Below is an example of Ansys Fluent model run, simulating the rise of a volcanic slug, generally believed to be the cause of strombolian eruptions at the such volcanoes as the archetypal Stromboli. Cheaper applications such as Matlab, certainly offer the ability to perform, less complex physics problems but lack the user-friendly interfaces such as Ansys and necessitate an in-depth knowledge of the maths and physics behind the problem!

I am sure I have missed many considerations off here, but these are just a few tasters!

It is vitally important that models are not just used on their own, as there is nothing to calibrate or validate the model against. The best way of modelling is by comparing observations in the field, in my case gas emissions, with laboratory proxies, and models. Anyhow, when weighing all of these points against the benefits, a clear and greater understanding of environmental processes is essential and is something which can only occur with significant modelling of processes.

Below is an example simulation of a taylor bubble (or slug) rising through a tube as a proxy for the root cause of a strombolian eruption (the type seen at stromboli).

Stromboli and its activity is some of the most charismatic and beautiful you will ever see, especially at night (my next post)! What visit to Stromboli would be without multiple attempts at trying to capture that perfect explosion (which many many tourists do during the day and night). During the day, explosions were ash-rich and the day-light prevents from seeing the majority of that amazing incandescence. The photo on the right shows a typical explosion from one of the summit vents, note how the ash and larger blocks are falling down the flanks and onto the Sciara del Fuoco. The Sciara del Fuoco is a large collapse feature on the island and presents one of the major hazards for tourists and locals alike. Inset (right) is another explosion from the same crater where you can see the hints of incandescence and the hot nature of the rock. Below is a real time video of an explosion from the same crater. In it you can see the initial thrust of the explosion and convective rise of ash into the atmosphere which eventually disperses as inertia is lost and mixing with cooler air occurs. The activity is fairly frequent, every 5-15 mins or so dependent on activity levels, so it is quite easy to get snap-happy if you have a lot of time on the summit!

I am now a few weeks into my PhD and am enoying it tremendously, it's certainly great enjoying what you do! As my PhD is based around gases and their importance in volcanic systems worldwide, I thought I would produce a series of posts on why they are important in specific volcanic eruption types and touch upon what they can tell us outside of eruptions. 

Firstly, a brief discussion on Strombolian eruptions.  

The classic example of the Strombolian eruption is from the Island of Stromboli, Italy and I am sure if you search google images you will find many wonderful images of the result of a Strombolian eruption, however, what causes these eruptions?

The cause of a Strombolian eruption is generally accepted to be the result of the movement of a large gas bubble (or slug as it is sometimes called) up a volcanic conduit and subsequently bursting at the surface, hence throwing the hot material out in a spectacular fashion. There is some debate about the origin of these bubbles, whether they are the result of the coalescence of bubbles (the merging of bubbles) as they rise up the conduit from depth or whether they are the result of a collection of gases which create a foam (high proportion of gas to magma) at a point at depth beneath where the surface where this could occur.

At depth, however these bubbles are formed, the size is constrained by the pressure they are under. As the bubble rises, faster than the magma which it is in, it begins to grow in size and accelerate as a result, allowing a high enough velocity to eject material at the surface.Evidently a gas bubble of insufficient size will not result in a Strombolian eruption but can still burst at the surface and contribute to a phenomenon known as passive degassing (which I will refer to in a later pos)t.

Strombolian eruptions can occur on timescales of seconds to minutes to hours or more and are generally associated with less viscous (more runny) basaltic magamas at high temperatures of around or more than 1100 degrees celsius. I hope you have enjoyed my very brief overview of Strombolian eruptions, any questions please post and I will try to answer!

On a recent trip to Etna, I was lucky enough to capture some Strombolian activity on the 27th July 2012 in action in Bocca Nuova at the summit of Mt. Etna (as per picture on the right). On a previous trip to the summit two days prior to this the magma column was low and was producing minor ash explosions (all pictures available here). On this particular day the magma column had risen high enough to produce regular strombolian explosions which occured from seconds to minutes apart. There was also evidence of a small outpouring of lava from the vent, although this was only visible using a heat camera.  

There are also a couple of videos of these events in the videos section. Apologies for the poor quality, down to a mixture of shaky camera work and strong wind!