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According to this person's surmises, volcanic activity appers to be increasing.
However, according to this report, volcanic activity is probably not increasing.

My question is:
Does Volcanic activity fluctuate?
If it does, what would cause this fluctuation in volcanic activity?

Image cited by both: Volcanic activity over time
Image: Smithsonian National Museum of Natural History, Global Volcanism Program

Is this fluctuation or recording increase?

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6  
Not my area of expertise, but I suspect it is partly because of the number of eruptions recorded, that could partially explain the WWI and WWII "dips", everyone was fighting in a war and nobody had time to watch a volcano. On geological timescales it is probably more dependent on tectonic activity, I'd definitely be interested to see a graph of more than just 200 years. –  hugovdberg May 7 at 18:55
    
Could you please give more information on the reports that you have read about this. –  seismo_steve May 8 at 9:32
2  
According to reports I've read, volcanic activity is increasing. The one report that you did cite, the source of those graphs, explicitly says no. This is a question of dubious quality. –  David Hammen May 8 at 14:19
    
I don't know where I originally saw it. I had citation (pointing to the only article I refound), but it was secondary citation and got fixed in an edit. The rising activity was just pointing at possible fluctuation. The real question was "Why would volcanic activity fluctuate?". –  Ben A. Noone May 8 at 14:43
    
Editied to clarify question. –  Ben A. Noone May 8 at 15:01

2 Answers 2

up vote 9 down vote accepted

This is probably an observational effect that is quite common in the Earth Sciences. In scientifically progressing societies there is a higher proportion of observations due to a number of effects:

  • awareness of science (not interpreting it as a wonder)
  • ability to record events
  • ability to observe (think 12 hour day in the factory vs. free weekend)
  • technology to do remote observations
  • ...

This is also why more and more meteorites are observed falling. In the middle ages you were either too busy to care, took it as a wonder, or found a meteorite, but had no ability to write it down. Similar scenario for volcanic eruptions. I could imagine that actually all those ships, airplanes, and oudoor-activities (no offense) lead to more observations of eruptions, but that people were to busy recording and writing them down. The geologists of their time were probably busy finding war-necessary resources.

Sources:

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Agreed. I looks like it shares a lot of the same problems with the observed tornado record. –  casey May 7 at 21:02
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Agreed. With the world's population growing and more people living in hazardous areas, more geohazards are being noticed and recorded. Overall, it is probably just a perception thing. –  seismo_steve May 8 at 9:32

Whether volcanic activity fluctuates depends rather on the timescales you are looking at. Crisp (1984) compiles available data on igneous activity lasting for longer than 300 years and concludes that over the past 180 million years the annual average magmatic output each year is around 30km3, of which ~75% is produced at mid-ocean ridges.

The most obvious variability in magmatic output arises from flood basalt eruptions, frequently correlated with continental rifting, and suggested to result from the first impact of a new hot upwelling mantle plume. These are enormous events - circa 106km3 of magma emplaced over timescales as short as one million years and always less than 10 million years - and even though they account for >95% of intracontinental volcanism, they account for only circa 5% of the global magmatic output due to their rarity.

So, yes, on short timescales volcanic activity can and does fluctuate - output rates are generally higher while Large Igneous Province emplacement is ongoing - and indeed on very long timescales (if the Earth was a magma ocean in its early history, it had more volcanic activity than the modern surface does!). However, on medium timescales - the ones geologists are typically interested in - the average output rates are fairly stable.

As has been mentioned with respect to earthquakes, earthquakes are not increasing in frequency. The key points, as with earthquakes, are improved technology and increased population density. Increased population density results in more events being observed, and individual events having greater negative impact; improvements in technology make it easier to perform remote observation, and to identify remote eruptions in the geological past. In particular, work with Antarctic ice cores (we have records going back 800,000 years at this point, and work is ongoing on a million-year core!) allow identification of discrete eruptions because they deposit layers of fine ash or, from further away, acid-rich layers. These eruptions can be correlated with one another between different cores - and even ice cores taken from different continents - through massive "marker" eruptions, such as Krakatoa, and to some extent through absolute dating via isotope chemistry in air bubbles trapped within the ice. However, even as our ability to interpret ice cores further and further back improves, our estimates of global magmatic activity stay pretty constant: despite being published 30 years ago, Joy Crisp's paper is still considered a decent piece of work worth citing.

ETA I note that the first report you cite, at "iceagenow.com", says about the author:

Felix is not affiliated with any university, scientific establishment, or corporation, and therein lies his strength. Untainted by institutional bias or conventional wisdom this architect turned author brings fresh insight to the study of the ice ages.

I suggest that he is probably less reliable than scientific consensus; peer review has its flaws but does exist for a reason! If you would like a close reading and critique of his report, can you make that clear? Cheers :-)

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