Click for link to original site. Photo is Anak Krakatau
Vulcanian eruptions, generally considered as the next step up in explosiveness from Hawaiian eruptions, are very distinct from other forms of activity such as Strombolian as they are related to the emission of mostly ashy pyroclasts and blocks in more voluminous amounts. This is in direct contrast to the incandescent material ejected during Hawaiian and Strombolian events. These events can occur on time-scales of minutes to hours or more. They are generally thought to be caused by the build-up of gas below a cooler plug of material which builds up sufficient amounts of pressure to explosively force the plug of material from the conduit. This then creates a column between hundreds of metres and kilometres high filled with ash and can also lead to the ejection of large metre size blocks. Typically this type of eruption occurs with higher viscosity magmas which don't allow the rapid rise and expansion of bubbles (and hence allow large amounts of passive degassing). This then allows the upper conduit to cool. Of course there are always exceptions and different situations when considering volcanic eruptions!
Where do Vulcanian eruptions they get their name from? No surprises here when I tell you it is from the Volcano Vulcano, one of the Aeolian Island volcanoes which has most commonly exhibits this kind of activity. There are several great textbooks with more information on this type of activity - Volcanoes by Lockwood and Hazlett and of course Volcanoes by Francis and Oppenheimer. I am sure there are more!
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!