Mooning a volcano

By Phil Plait | August 9, 2006 12:24 pm

Can the gravity from the Moon trigger a volcano eruption?

Possibly. But will it trigger Mount Mayon in the Philippines tonight? Let me check the Magic 8-ball… signs say no. Here’s why.

A BBC report report this morning says that "experts" — no names, affiliations, or quotations were given — are concerned that the full Moon may trigger an eruption of the Philippine volcano Mount Mayon. It’s a very active volcano and ripe for an event. It could blow any time now. So is there cause for concern given the full Moon tonight?

I’m gona go with "no". The gravity of the Moon does affect the Earth, of course, mostly through tides. As it happens, tides are strongest when the Moon is full (and when it’s new as well, which the article doesn’t mention), so there is at least some reason to investigate this. And the BBC report says that the full Moon "coincided with at least three of Mayon’s 47 eruptions, including the two most recent ones in 2000 and 2001".

But let’s look at this critically, shall we? First of all, what does "coincide" mean?

First, Mayon is a very active volcano. It has quakes, minor explosions, lahars (mud flows) and such all the time. Certainly some will coincide with the full and new Moon. Let’s be generous and say that the time period around the full Moon is 2 days: a day before and a day after. The Moon goes through a complete cycle in roughly 29 days, so it’s full for 2/29 = 1/15th of the time. If you then look at 47 eruptions, then you expect to see 47/15 = 3 eruptions near the full Moon. And hey, that’s exactly what the report says!

So, statistically speaking, the Moon has nothing to do with eruptions. If it did, you’d expect to see a bump in the number of events near the full Moon. But the number of eruptions near the full Moon is what you’d expect from random chance. In other words, on average it doesn’t matter if the Moon is full, new, first quarter, or whatever. Now to be fair, the article doesn’t say how big a time period they used around the full Moon. Maybe they only used one day, not two. Even then, the correlation would be weak, because 47 eruptions isn’t a big enough sample to choose from. It’s small number statistics, like flipping a coin three times and having it come up heads each time. It’s rare, but it does happen on average one out of every eight times. You need bigger samples to get good statistics.

Now, there is some evidence that the Moon can cause earthquakes, and maybe even near volcanoes. But even then, if this were true in the case of Mount Mayon you’d expect more eruptions near the full Moon. It’s not seen, so again I think the correlation here is very weak.

So I am not totally discounting a connection between the Moon and this volcano, but I am saying that at best such a link is very weak, and probably not worth worrying about. The folks who live on the banks of Mayon have enough to worry about already!

I wonder: if it does erupt tonight, what will those "experts" say? But more interestingly, what if it waits three days?

Tip o’ the caldera to the half dozen people (so far!) who have sent me this story.

CATEGORIZED UNDER: Astronomy, Cool stuff, Science, Skepticism

Comments (34)

  1. Pete

    A full moon makes my hair grow quicker – Fact!

  2. George

    Perhaps it will happen during a new moon and they will finger point with their other hand. [I thought I’d see a wise crack, given the title. Glad I didn’t see one. :)]

  3. Nice straightforward analysis, Phil. I thought for a moment I was reading :-)

    – Jack

  4. Even I saw this news flash across the TV on BBC world – though I missed the details. And I remember thinking, “this should be interesting…”. How could BBC have been irresponsible enough to have broadcast this, especially since the usual suspects haven’t done so yet…?

  5. Stuart

    Amateur astronomer Stephen O’Meara has been studying the connection of the Moon and volcanoes. While it seems dubious, they apparentaly have good evidence that there is a link. There was a National Geographic program about it a few years ago:

    Perhaps it will be repeated sometime.

  6. Kaptain K

    Minor nitpick. The day before, the day after and the day of (the full Moon) equals three days, not two! ūüėČ

  7. BadGeology

    The article says, “It has quakes, minor explosions, lahars (mud flows)”. A geologist friend of mine was quick to react that lahars are not mudflows. This distinction was made clear during the Pinatubo episode in 1992, when lahar covered entire villages. I guess BadAstronomy has some BadGeology too. :)

  8. By day before and day after, I meant a 24 hour period before and 24 hours after. :)

  9. Laguna2

    On germans SPIEGEL-ONLINE the scientists have a name.
    Renato Solidum, Director of the philippine institute for vulcanology and seismology
    and his collegue Dr. Ernesto Corpuz are named as sources.

    From there:
    Renato U. Solidum, Jr. – Director
    Ernesto G. Corpuz – Chief, Volcano Monitoring and Eruption Prediction Division

    Nothing agains you Phil, but these guys know about volcanoes. That is their job.

  10. Pip

    Is it just me, or was that picture taken on June 37th?

  11. Mark Martin

    Your statistical reasoning reminds me of a gentleman who claimed to have figured out how to forecast earthquakes. As an example he forecast that, for a given date, there’d be an earthquake of magnitude 7 on the Richter Scale.

    Problem is, that’s all he predicted, that somewhere on Earth there would be a quake of that size. But there are quakes of that enormity somewhere on the planet… each day!

  12. Laguna, thanks for digging that up. I have no doubt that volcanologists know volanos. But if they are claiming a causal connection between phases of the Moon and Mayon eruptions, they’ll have to do better than 3/47! I’ll read that article a bit later.

  13. Roy Batty

    # Pip Says:
    August 9th, 2006 at 5:02 pm

    “Is it just me, or was that picture taken on June 37th?”

    No, that’s just the countdown clock ūüėÄ

  14. Mithandir

    Actually I think you’re slightly misinterpreting the article.
    First, under the little picture it says :
    * Some scientists say gravitational pull of sun and moon can influence eruptions
    * Influence is greatest at full and new moon
    * Others say more research is needed

    The first and last line there clearly show skepticism, they’re in no way presenting this as fact (tho yes, I agree that the “some scientists” and “others” is too vague). The middle line mentions the new moon issue.

    Those are just nitpicks tho, my main point is this:

    “A full moon coincided with *at least* three of Mayon’s 47 eruptions” (emphasis added).
    I interpret that as: They know three of them coincided with full moons. Presumably they also know of X that didn’t. However there’s an unspecified Y eruptions that they have no (accurate) moon data of, presumably mainly the older ones. This leaves us in a bit of a statistical mess, as they don’t state how many eruptions they have the data on. If they only know the moon phase for, say, the five most recent ones and three of those had a full moon, then it might be many more of the 47 that had full moons. Nowhere do they state it is just 3.
    So yeah, I think they’re basically basing their assumptions on insufficient data, but so, it seems, are you :)

  15. PK

    Phil, you got me intrigued. Particularly, I wanted to know what difference in the probability distributions (PDs) can be detected given those 47 samples. So we assume one PD for “no effect due to the moon”, where we have fifteen probabilities all 1/15 (I approximate 29=30), and the other PD with fourteen probabilities of (1-a)/15 and (1/15 + a) for the case of the full moon. Those PDs are distinguishable in N trials when

    N sum_k (delta p_k)^2 / p_k > 1, (known as the Cramer-Rao bound)

    where delta p_k is the difference between the probabilities of the distribution. In this case N=47. After some numerics, I found that a

  16. PK

    This is weird, somehow my conclusions got chopped off:

    I found that a

  17. PK

    D’oh! It’s thinking HTML tags! LOL! Please feel free to edit the comments, Phil:

    I found that a is smaller than 5%. So the effect of the moon results in a correction of less than 5% in the probability distribution.

  18. Beth

    Hi.. thanks for your amusing observations (I do understand lahar is ash and water –I was here for Pinatubo).. anyway.. I’m in the Philippines and read several legitimate reports about the likelihood of the eruption coinciding with the full moon. I’m not superstitious by any means… I think there are a lot of other factors that will go along with the volcano erupting than the full moon…. no one has said how the other planets affect the eruption either.. a detail over looked!? Also.. is there a significant occurrence that happened at the volcano around the last full moon? The more data or this theory could be proved perhaps if data was taken and correlated EVERY full moon etc… anyway.. thanks for the laugh my family thinks the same way!

  19. Roy Batty

    No real need to factor in the other planets, the BA has a nifty page explaining why:

    As you can see from the moon comparison table, tidal forces are pretty negligible :)

  20. George Greene

    Mark Martin speaks of someone who predicted that a M7.0 earthquake (or presumbly greater) would occur on a given date. That’s not exactly a good bet. Per the information I have, there are only about 20 quakes that large or larger per year. That gives a mean of 0.055 per day. By Poisson distribution statistics, the probability of exactly zero quakes on a day would then be exp(-0.055), or about 95%. He’d have only a 5% chance of being right.

    BTW, to forstall puns, there is nothing fishy about those statistics.

  21. Mark Martin

    Hi George,

    I just didn’t recall the magnitude correctly. The essence is that he forecast a quake for which there was a high likelihood of occurrance statistically. Had he specified a quake between 6 & 6.9, there’d be a chance of about 1/2 per day. If it was in the range 5 to 5.9, the chances were over 3/day.

  22. Irishman

    Mark, yes, a prediction has to be specific enough to be remarkable in order to be meaningful. Predicting there will be a major car accident today is meaningless (anywhere on Earth?). Predicting a major car accident in Houston today is somewhat more meaningful, but not incredibly unlikely. Predicting there will be a major car accident in Clear Lake is getting better. Predicting one on the corner of Bay Area Blvd and Space Center Drive would be very significant. Assuming the timeframe were constrained appropriately (define “today”).

    Mithandir, it isn’t valid to simultaneously list the 47 eruptions and then ignore them in the statistics. Either they are valid, in which case the 3 in 47 is accurate, or only list the ones with known dates (3 out of 5 where the phase to the moon is known occurred on Full Moons). In which case the correlation is much higher but the reliability is much lower, given the small sample size.

    And if the dates of the eruptions are known, it should be a simple matter to calculate the moon phase at those times, so we should know the status for all 47 eruptions.

  23. David_in_Australia

    Sorry if this is a dumb question, but what does the phase of the moon have to do with earthquakes or volcanoes?
    I can certainly understand how the proximity of the moon to one side of the Earth could affect our tides, geology and so on, but why would the amount of light reflected off the moon onto the Earth make any difference?

  24. The report on BBC was incorrect. The PHIVOLCS (Philippine Volcanology and Seismology) didnt exactly say that there was a connection between Full moon and Mayon Eruption. The Local News interviewed locals who live around Mayon Volcano and they believe that full moon was connected to Mayon’s eruption. Come on guys, give our geologists some credit. Why would our earth scientists believe in such superstitions in the era of computers, statistics etc.. when everything seems to be expalained using numbers?? It was pure superstition, nothing else. I suggest you be critical on what you’re watching. It’s not always the truth that comes out on news. Thanks.

  25. PK

    David in Australia: The point is that the sun and the moon line up at full moon and at new moon. Their combined gravitational pull then “stretches” the earth more (very loosely speaking, of course), and you get stronger tidal forces.

  26. BRigs

    I guess since magma is a lot more denser than water, the gravitational pull of the moon may not trigger erruption… and by the way, the title says “tonight” where in fact the moon is still there even if its daytime.


  27. I got into a discussion with a friend of mine about this after we read the BBC article. I said the article was junk-science, but he was not to be convinced, so in the end I did a quick analysis of the timings of eruptions worldwide since 1970 in relation to the phase of the Moon. Unlike the quoted 3/47 on the BBC, I have error bars! The results are here. I probably still won’t have convinced him, all because of that stupid USGS page.

    I also e-mailed the director of PhiVolcs. His response is at the link above.


  28. When I said “The results are here,” I was expecting it to obey the html tags. The link I mean is this:

    (posted on flickr ’cause my friend spends a lot of time on there, so he will get to see it)


  29. Lucy

    wow its interesting how some evidence find that the moon can cause earthquakes! =0

  30. Chris

    I find the criticism of this report to be a bit strange. The article is not about a scientific result describing the effect of terrestrial tides on volcanoes. It’s about current levels of activity at one particular volcano. Therefore there is no need for tide-related citations and quotes from named experts any more than an article about current tourism levels at the pyramids should be expected to provide citations if it were to include the phrase “Scientists believe that the pyramids were constructed 4500 years ago”.

    Are people forgetting that the BBC is a news organisation and not a scientific journal? Except for situations in which they are providing eye-witness reports, journalists are there to repackage and transmit information that’s provided to them by other people. Once the basic facts of an event have been established, if they then had to stop to double and triple check all the minor inconsequential details relating to what everyone told them (to avoid one or two readers throwing their toys out of the pram) then no news would ever get reported.
    I agree with this earlier comment, not only about the report being balanced by not presenting the tidal relationship as indisputable fact, but also about there being some vagueness about exactly what they mean by 3 out of 47 eruptions. In a more in depth feature extra information would be needed here, but the article does not invite people to draw any conclusions based on these figures and it in no way claims that this is a large statistical sample. The numbers are clearly being provided for information purposes only.

    There is no suggestion that some scientists saying that the gravitational pull of the sun and moon can influence eruptions is a conclusion that they came to by studying this volcano and these 47 erruptions alone. Therefore it makes little sense for any reader to assume that they are expected to come to the same conclusion based solely on the numbers quoted.

    The International Center for Earth Tides has a long list of papers investigating the relationship between tides and volcanoes, so the BBC did not pull this idea out of the ether or shoehorn it into the article after seeing some local villager do a war dance outside their hotel room to appease the angry moon god!
    In the era of computers, statistics etc. all these studies were no doubt carried out using numbers!

    I wonder if there would have been such an uproar if an article about a heavily pregnant elephant in Kenya happened to mention that some scientists believed the presence of a full moon might influence when elephants give birth and stated that in this particular herd 3 of the last 47 calves had been born on a full moon!

  31. Crazy Bob-Astronomy To Go


    As much as I usually am in agreement with you and your posts and comments on here, I must stray a little on this one. I was glad to see Stuart mention Steve O’Meara’s work and studies above. This is not to be brushed aside, when he and his colleagues were, at times, able to predict within minutes, the next eruption of an active volcano they were observing, based on the position of the Moon. Considering myself somewhat of an Astronomer as well, I know I am out of my field commenting on this topic, but geology had always been an interest of mine (not counting a passion/obsession with meteorites).

    I had hoped for more comments from real Geologists on here, but they seem to be lacking. I am definitely NOT one, but will give it a try. When you consider the tidal forces when the Sun, Earth, and Moon are closely aligned (Full/New Moon), there are measurably higher tides. Less well known is the fact that there is a land tide as well–a gentle but measurable swelling of the land surface of the Earth, not just the liquid part. I believe the thinking/reasoning/hypothesis being that, in the case of a volcano ready to erupt, the extra pull of gravity might be enough to weaken/crack the hardened skin of surface magma holding back the liquid form below, allowing the next eruption in an active volcano, or might add to or even be the final straw that might trigger a new one. If as you posted there is evidence of tides possibly effecting earthquakes, it seems these two go hand in hand with other possible geological connections.

    Before anyone discount the work of Steve O’Meara as a single source, besides being one of the founders of the respected Volcano Watch International, you might also recognize his name as the same author of several fantastic Astronomy books for Cambridge University Press (The Messier Objects, The Caldwell Objects), as a Contributing Editor to Sky and Telescope magazine, as well as being one of the finest visual observers in the world.

    I’m sorry for posting this so late, but I do think I might actually have something useful to add here. I am more than open to corrections if/as needed, but not flames.



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