Stranded passengers. Grounded aircraft. Closed airports. The cause: an obscure volcano in Iceland with an unpronounceable name rocked aviation and transport in the Northern Hemisphere.
Now we’re eleven years on, and another volcano in Iceland is spewing hot lava and gasses: Fagradalsfjall of the Krýsuvík-Trölladyngja volcanic system on the Reykjanes Peninsula, not far at all from the capital Reykjavik. But… no flights cancelled, no passengers stranded. What’s the difference? Why could one volcano cause so much havoc, and another one, not too far away, be relatively harmless?
The way a volcano erupts, and how explosive it is, is controlled by different factors. The first lot of massive factors depend on the magma that is causing the eruption.
Magma is the mixture of hot, molten rock, gases and crystals that comes from deep below in the Earth making its way to the surface causing a volcanic eruption. Once its out and flowing, we call this mixture lava when still cohesive (although most of the gases are gone by then).
So what about the factors depending on the magma? How and what does this work?
The hotter, the better… and more
The main factors are the temperature of the magma, its composition (yes, there are different types of magma) and how much of it is gas or crystals.
Basically, the hotter the magma, the richer in iron and magnesium and the fewer crystals or gas bubbles, the more easily it flows. The opposite is also true, a magma flows less easily and becomes more sticky (the proper term is viscous) when it is cooler, richer in silica instead of iron and magnesium, and richer in gas bubbles and solid crystals.
Think of honey. When you heat up pure honey it becomes super runny. When you cool it, have it partially crystallize and maybe even trap some air bubbles in it, it is very hard to get it out of the jar.
The hotter the magma, the richer in iron and magnesium and the fewer crystals or gas bubbles, the more easily it flows.
So, how does this affect the type of eruption?
Well, when something flows easily, it also lets go easily. Think of water. When you blow bubbles in the water, they escape easily. Honey is less runny, so bubbles stay in their for longer
It is those gases that become important. When there is a lot of it, but the magma doesn’t flow easily, there is a pressure buildup. The magma acts like a lid to its own gas, holding it in. When the lid pops, the gas escapes. The stronger the lid (less runny the magma), the stronger the force released when the gases escape.
However, this is just half the story…
There’s something in the water
Water is also important. And, yes, water is present as one of those gases in the magma, but I’m talking here about water outside the magma.
When a volcanic vent, that part of a volcano through which the magma finds its way to the surface, is dry, all the explosivity of an eruption is controlled by the magma and its properties. But when it becomes wet, the story changes…
Liquid water and magma are a bit like oil and water, they don’t really mix. Think of your greasy pan, with just water you won’t get it nice and clean. Add dish washing liquid and all of a sudden the oil disappears in the water. Water and magma need something to help them mix, too.
The thing that keeps it all together…
The thing that helps liquid water and magma mix is steam. Steam is like a little in between layer, a film, between the rising magma and the water sitting or flowing through the volcanic vent.
It is also this steam that keeps the two at a safe distance, however small.
Think again of your greasy pan, but now when you were making it dirty while you were cooking. Your hot pan on the stove happily sizzling away, but then you put something wet on it. All of a sudden it spatters and sizzles even more. After a while, little droplets of water are dancing and floating over the surface of the hot pan till they disappear.
When the water droplets fall in the hot pan, they touch the hot surface immediately. Little explosions happen as water is flash-heated: the spattering and sizzling. However, little steam films develop between the water droplets and the hot pan, and they seem to be alright.
Steam does the same thing between hot magma and liquid water.
Everything’s alright then? Well… only up to a certain point.
…until it’s gone
Volcanic eruptions are very unstable. Lots of little and big earthquakes happen as the magma pushes its way to the surface, often cracking and fracturing rock along the way. Those earthquakes can be a lot of trouble.
When those tremors happen they can shake things up in the system. They can shake the liquid water and magma so much that the little steam layer collapses. Water and magma come in immediate contact, flash-heating the water, and more than a little sizzle or pop happens.
When the water and magma were well-mixed before that immediate contact, then we are all in for some trouble. Having lots of water touch hot magma instantly and flash-heating, causes a massive explosion. Sometimes, only part of the mixed system explodes first, but causing a second one close by, and on and on, until all the water and/or magma are used up.
This can leave big craters of kilometers in diameter that are called maars. Often times these craters are filled with lakes. The Blue Lake in Mount Gambier, South Australia, is such a maar.
Only if at the Reykjanes eruption the magma flow slows down and groundwater seeps into the volcanic vent, we may see a repeat of 2010. But only if…Tweet
The difference between 2010 and now
So how’s this important for the difference between the 2010 eruption in Iceland and the one happening now at Fagradalsfjall?
The 2010 eruption at Eyjafjallajökull (yes, that’s the one) started of dry. However, with a major glacier covering most of the mountain, the heat caused the ice to melt. The melt water seeped into the volcanic vent, especially when the magma flow slowed down.
The result was major explosive activity with a big plume rising up to 5 km high from the summit. Strong winds carried and spread the volcanic ash over most of north-western Europe. And, the rest was history.
The current eruption is happening in an area that is free of a glacier, so melt water won’t be a problem. There is still a chance, however, that things can turn. If the magma flow slows down and groundwater can seep into the volcanic vent, we may see a repeat of 2010. But only if…