A couple of days ago we received an email containing questions from a bright young student named Jennifer D in West Chester in the United States of America. It seems like she has developed an interest in one of our absolute favorite volcanoes, namely Bárdarbunga in Iceland (among other things).
So, I will attempt to answer her questions in this post. After all, Volcanocafé is about learning and teaching about new things concerning volcanoes. And in this case, I do not think we have actually attempted to answer these 3 questions.
How do you know when a volcano is no longer active?
Volcanoes are normally grouped into 3 categories of activity. First we have the erupting volcanoes, then we have the active ones and those that are judged to be dormant. Sounds like there should not be a problem to divide volcanoes into these categories, but as usual things are never that simple. Let us start at the beginning.
Volcanologists working with volcanic hazard mitigation normally color code volcanoes into green, yellow, orange and red.
One could define an erupting volcano as a volcano that is actively ejecting lava and ash from lava (tephra). This is a commonly used form by volcanologists. This definition though quickly run into all sorts of problems as all definitions do when they are faced with the hard reality.
First of all, there are volcanoes that have very short eruptions. One of those is Sakurajima in Japan, it literally has more than 1 000 eruptions every year. The Japanese authorities have decided that Sakurajima is only counted as an erupting volcano during those short eruptions, and otherwise it is downgraded into an active volcano. On the other hand, volcano agencies in some countries have different interpretations of the definition all together.
Then we run into problem with a class of volcanoes that suffer from something called a phreatic detonation. That is not a real eruption since no fresh lava is erupted. Instead it is hot magma (lava underground) heating up water causing steam explosions. Some agencies say that those volcanoes are erupting, others do not.
Erupting volcanoes are color coded red. I hope that you at least get a feeling for the problems with definitions now Jennifer, and that we have not scared you away.
An active volcano is a volcano that is showing signs of activity that could lead up to a future eruption. Those signs might be earthquakes, rapid motion of GPS-coordinates (signs of magma entering the system), Geysers, hot springs, fumaroles (steam vents), volcanic tremor recorded on seismographs, and so on. But, in the end this is not enough. For a volcano to be considered active it should be showing signs that it could erupt in a near future.
A splendid example of an active Volcano is Mount Sinabung in Indonesia. It shows earthquakes, volcanic tremor and active fumaroles. It also suffers from phreatic detonations hurling pulverized rock between 500 and 7000 meters up into the air. This is about as active as a volcano can get before she is erupting. These volcanoes are color coded orange.
Let us now take a look at Yellowstone and Bárdarbunga. Yellowstone shows all of these signs of activity, and Bárdarbunga only a few of them. Still most scientists are certain that Bárdarbunga is far more likely to erupt. The answer to the question why Bárdarbunga is more likely to erupt compared to Yellowstone is though very long, and if you are interested I have written an article about Yellowstone that I link to at the end. Anyhow, both of these volcanoes are considered dormant. The important part is that they are currently not showing signs of erupting in the near future, even though Bárdarbunga is currently closing towards a possible upgrade into being active.
From there it is a sliding scale of dormant volcanoes until you arrive at volcanoes like Lassen Volcanic Center in Northern California. It had a very small eruption in 1915, but what makes it interesting is not that. The researcher Erik Klemetti has shown that there was no signs of an eruption for 100 000 years. So, a volcano can be dormant for a very long time between eruptions, and that is highly interesting.
These volcanoes are either color coded green if they are at their normal state, if they show signs of unrest they can for a time be coded as yellow as a warning that this volcano is showing signs of unrest. Now we are ready for your answering your question number two.
I read that the Bárdarbunga volcano in Iceland is considered restless, what does this mean?
Well, it simply means that it is showing some signs that it might be moving towards a future eruption. In the case of Bárdarbunga we are talking about earthquakes that are deep enough to be of magmatic origin, we also see GPS motion indicating that magma could be moving into the system, and on occasion we see magmatic tremor. For these reasons it has been called “restless”. The reason why Bárdarbunga is not classified as Active is that we do not really know how she behaves before an eruption. This is because the last eruption started in December 1902 and ended in June 1903. Some sources list a lot of later eruptions, but those where both uncertain and located in a different volcano named Loki-Fögrufjöll. I do not have a good answer how those possible eruptions have been attributed to Bárdarbunga.
So, the phrase “restless” is used quite simply to point out that Bárdarbunga might be headed for an eruption in the near future, but that we are far from sure.
How are stratovolcanoes formed?
This is really a giant of a question that is fairly hard to answer since the answer is different from each volcano, and also varies in between the different types of processes that form volcanoes. So, instead I am going to use my favorite volcano as One example of how a stratovolcano can form and leave the rest for you to answer Jennifer since this is the type of question that you can spend your entire future career as a volcanologist on.
Hekla is a very young volcano, it might be as young as 8 000 years old. And that is a really short time for a volcano to evolve as far as Hekla has done. She is in many ways a unique volcano unlike other volcanoes in Iceland, and for that matter in the world.
Hekla started as fault in the crust of Iceland, a weakness that opens up as tectonic forces continues to pull apart the Mid Atlantic Rift that divides not only Iceland, but also continuously pull apart the North American plate and the Eurasian plates.
As the fault opened up an unusual type of acidic magma poured out of the ground instead of the usual Icelandic hotspot basalt. Normally Icelandic volcanoes first form basalt floods and then a shield volcano with gently sloping sides. The gentle slopes form due to the basalt being very fluid, not unlike the maple syrup on top of your pancakes. Instead Hekla’s lava (dachite/andesite) was much stickier so it skipped the stage of forming a shield volcano.
At first Hekla formed a serious of steep sided cones along the 7 kilometer long principal fissure. As the eruptions followed those cones merged until a single mountain had formed. Hekla is shaped more or less like a ship’s hull that is flipped over and not like a classical cone shaped stratovolcano. This is due to Hekla being so young, as time goes by it will most likely form a more classic shape.
Another sign that Hekla is very young is that she is still erupting along the entire 7 km fissure when a large eruption occurs. She has though by now formed a central crater at the top, and as time and more eruptions occur more and more of the action will take place there. And when that happens most of the lava will pour out over the sides in such a manner that the classical type of cone starts to form.
Another thing with Hekla is that she is unusually violent for such a young volcano. Normally volcanoes evolve and acquire larger magmatic systems as they grow older, but Hekla seems to have had massive eruptions the entire time.
What can be said about stratovolcanoes is that they normally evolve from either a crack in the ground or a gently sloping shield volcano. And some evolve directly into a stratovolcano if they are forming where a previous larger volcano has been. Like for example Monte Vesuvius in Italy that was born from the remnants when the Monte Somme volcano was destroyed in a large eruption.
Welcome Jennifer to the large and bewildering world of volcano formation, as you have perhaps noticed by now almost every stratovolcano on the planet have followed a unique path as it formed. I just used Hekla as one example of how it can happen.