Volcano Island in the caldera lake at Taal.
The Philippine government is upping its concern about an eruption at Taal - and cracking down on tourists and resorts trying to get near Volcano Island in the volcano's caldera lake. Tourists are only allowed to take boat tours in the lake that go halfway to the island - which doubles as the most recently active vent at Taal - and if resorts on the lake bring tourists any closer, they may face repercussions from the government. Of course, this still isn't stopping tourists and natives from going to the island anyway. Now, one thing interesting to ponder is Eyjafjallajökull has affected tourists in the Philippines mentality about visiting potentially active volcanoes. With all the footage, especially during the basaltic fissure stage, or people traipsing near the erupting volcano, some folks might have the mistaken idea that any volcano will behave in this fashion - and that could lead to disaster. Even foreign governments are warning "the capacity of the Philippine emergency and rescue services to deal with large natural disasters is limited." (UK Foreign and Commonwealth Office). The provincial government in Batangas has cancelled classes on Volcano Island and brought the students to the mainland - with 300 people voluntarily evacuating the island. An intense sulfur odor has been noted on the island - never a good sign when it comes to a potentially active volcano. There have also been more "folksy" reports of "strange animal behavior" since the Alert Status was raised to 2 - so take that as you will.
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Somehow, politicians's need to look good prevent them from doing their job properly. Instead of obfuscates, what needs to be said is something on the lines of:
"If you plan to visit Taal volcano, be advised: A typical eruption of Taal volcano WILL kill you if caught on Volcano Island. To venture onto Taal lake with an eruption imminent MAY kill you depending on the size of the eruption. The Phillipine governement and rescue services WILL NOT even try to rescue you if you are in breach of our Security Directives."
or
"If you plan to watch the next eruption of Taal, please do so via Webcam. Any closer than that and whether you live or die depends on your levels of curiosity and stupidity in observing Government Safety Instructions beginning with this one".
Another Nanny state to avoid it sounds like.
They will suffer as soon nobody will visit their country.
They should be showing the tourists and locals the Krafft's video on volcano hazards which was effective and persuasive at the time of Pinatubo. Maybe they already are?
BTW Taal shows that there's more to volcano danger than just the magma composition; Taal is more or less basaltic
Public safety seems to be more about psychology than geology...people recall the lava lakes at Hawaii and think all volcanoes are as well-behaved. Then they get caught in a pyroclastic flow and the next of kin blame the authorities. It is sad, but I see no simple remedy besides education.
Off-topic: As you recall, various nuts blame geological events on people annoying God. Just wait until they read this one: "Iceland Passes Gay Marriage. Unanimously."
http://scienceblogs.com/dispatches/2010/06/iceland_passes_gay_marriage_…
(There are some fun discussions at the link about just what will happen to Iceland now, and BTW Argentina passed a similar law some months ago so the earthquake in Chile was presumably a narrow miss)
Excellent point Erik, Eyjafjallajökull was really a very tame eruption. Another good video people should check out before thinking of visiting is the one on Unzen where the Kraffts and a number of others lost their lives.
Mike it's surprising that Taal is mostly basaltic. What's driving the explosiveness then, lake water?
@Lurking et al:)
Thank you for your lovely plots in post 154 (and 2 more) in the last thread.
They really confirmed my mental image of them being in-line and very centered in a straight tubelike way. Also it becomes clear that they started at the crust-boundary at depth, near the same depth that Weir gives for the A2 intersection. Then the quakes slowly moved upwards and ended (for now) at 6 kilometres depth.
Together with the february swarm I would seriously guess that something is brewing there. Now with your plot we see that it looks like a conduit leading upwards from depth, then disapearing and ending at 6km depth.
referense to Lurkings stunning plot, it is equally tubelike in all directions: http://i48.tinypic.com/jv69o7.png
Now I am going to give Erik and Boris and the rest who actually know what they are talking about an oportunity to laugh at me:) The rest of you can stomp me, I know this is out of my field and pure speculation:)
I interpret what is happening at Eldey as a conduit opening or re-opening from depth giving a possibility for magma to move up into a magma chamber that starts at 6km depth. I guess that this chamber is the same that gave the 1830 outbreak and perhap's those unconfirmed outbreaks that gave "fresh-looking" lavafields on the sea-bed that where noticed in the ninties.
I do not think there was an eruption in february, nor do I think one has started now, the reason for this being that even though it is 120 to 150 metres of depth there, I guess that one would notice something at the surface. But, I do think something will start there soon (like within a few years...)
Sounds fair enough to me Carl, so'll stand with you if anyone laughs.
Re lack of knowledge: is there any difference in the nature or character of dikes and intrusions on a MOR as compared to those in a subduction or intraplate setting?
Strange animnal behaviour around Taal? I thought this only happens shortly before an EQ? I would like to know more preciseley how these animal behaved exactly.
Just an idea from my former line of expertise...
Back in the days when everyone was still scared of Sovjet nuclear subs I worked at a company that constructed hydrophones and SONARs. So naturally the idea comes from this.
Back then the idea was to always know where all of the Red subs where. Since not even the US could deploy enough hunter-subs to keap track of the Red strategic subs they instead created a network of hydrophones that kind of closed the gate on the Sovjets, at least the idea was to always know when they left their own "turf". The name of the line is/was GIUK (Greenland, Iceland & UK).
The hydrophones used was/is really sensitive, and they where/are close enough together to actually achieving almost total coverege. The sensitivity was good enough to spot subs at up to 50 nautical miles, normal ships at 250 to 500 nautical miles, whales at up to 2000NM. So just imagine what a volcno did to the system.
Back then it was not uncommon for Sovjet subs to be drowned out, sometimes for weeks, by USNS (Ultra-Scale Nature Sounds). These could be anything from quakes, mantle-movement sounds, vent-holes (nobody knew what that keening kettle-like sound was untill the black smokers was found) and of course sub-surface volcanos going off now and then. The last ones where the most disturbing. Quakes are short, mantle-movement is easy to filter out and very low in frequency even though high order harmonics exist. The black steamers where a bit trickier, but was not really in the submarine frequency. But volcanos, phew... Under water they tend to make noice in every possible frequency, and at insane decibel-counts to boot. When there was an eruption the entire Sovjet sub-fleet could go by just metres from a hydrophone playing speaches by Stalin loudly on the speakers and nobody would notice.
So what is the point of my rambling? Well... Back then we knew when there was an under-water eruption going on, we knew exactly where, with the precision of the hydrophones and the vast number of them we could pin-point any eruption in the northern atlantic to within metres.
To the best of my knowledge the GIUK was still operational in the early 2000nds, it was updated during 91-94 and the beauty of it was that one of the access points is in Iceland.
If it is still active I think it might be possible to actually get a feed from the system since it has played out it's role. If it is closed down there should be leftover hydrophones in storage on Iceland, we delivered enough of them to last for more than 25 years, and you would just need five of them to give high enough sensitivity to presicily spot and monitor under-sea volcanos.
You can probably get them almost for free, or at least cheap, since they are today counted as junk.
With a feed of information like that the coverage would be almost as good under-sea as it is on-land.
Oh, and if somebody actually get something like that up and running I have some good software to analyze the sound-patterns;)
AAnd while I am at it and everyone else is slumbering over "the puddle" I have a small volcanic quizz for you when you wake up:)
Name the Volcono:) http://i.dailymail.co.uk/i/pix/2009/07/30/article-1203028-05E123E200000…
Slightly off topic but Sakurajima in Japan put on some impressive activity last night with intense crater glow and what looks like hours of Strombolian activity. I highly recommend anyone interested in watching volcano video subscribe to this channel on Youtube - http://www.youtube.com/user/cubhf137
It contains time lapse videos of activity at the volcano every day!
Carl, is that Anak Krakatau? Lovely photo!!
@11 nice recommendation!
back to Taal - this might be the strange animal behaviour:
"Fish that used to swim around the lakeâs deep areas were being seen on the coast, indicating high water temperature, a village official in San Nicolas said."
http://www.manilastandardtoday.com/insideNews.htm?f=2010/june/14/news8…
Does anyone know of any links to Taal relevant data sites?
carl, I bet its Anak krakatoa,.
Yepp James, it is my favourite Volcono, Anak Krakatoa:)
Tourist eruptions are not new - remember Surtsey and I am sure people went to Krakatau - after the event. More recently there was a lot of economic benefit (and disruption) from tourists to the wonderful Mt Mayon also in Luzon, Philippines to set partly against the misery and costs of evacuations.
As someone who knows the Philippines a little I think PhilVolcs should be given credit for superb work in forecasting Pinatubo and I have no doubt they have every reason to be super-cautious of Taal which has a history of being a real monster. No government could ever have the resources to deal with a "big one" just there.
(Taal distracting me between data input)
Perhaps the descriptions here
http://www.iml.rwth-aachen.de/Petrographie/taal-mas/ta-maso.htm
and accounts here
http://www.iml.rwth-aachen.de/Petrographie/taal-dt/ta-old-r.htm
would help to dissuade people from visiting the area!
Taal is part of a larger Caldera. Given the sundry descriptions, I'd be worried about a big-very big-eruption...
The Government should issue it's warnings then say: "Now it's up to you and Darwin...."
Mulakot and Ãórólfsfelli showing activity take a look at the far left side of the Ãórólfsfelli image looks like a ash plume but could be lighting and cloud confusing the issue
@19 The thoro cam is amazing those son't look like clouds they seem to be coming right from the crater and another area of activity off to he left. Jon's plots are on the uptick also.
Wind is low on the helicorders but there is an interesting little pulse showing.
what is on the left started as a black hump at the edge of the image and now is spreading to the right bottom first then rising into the mess above it now it is about 3-4 times wider than at the start
Good Lord what I wouldn't give for winshield wipers on the Thoro cam!
I wish the vodafone cam was operating dispite the nasty image i think there is water coming from the gigi glacier a lot of water
maybe the wind will blow some of the drops off the lens and we make out a better picture of what is happening.
I do hope so with both the flir and vodafone out of action and the mila image squirming it is frustrating
@Ems, 17. Thanks for the links. Taal is definitely capable of a big boom (the 1754 eruption reads like a description of Pinatubo) . As an older sort, I think that going out in a volcano might be really interesting. (compared to some ends--;-)
There have also been more "folksy" reports of "strange animal behavior" since the Alert Status was raised to 2 - so take that as you will.
Such as
Hello to all,
Renee & Gina,Mulakot(or should it be Murkalot)cam showing one ash plume coming from the usual location.
For those who are interested in an eCopy of The eruption of Taal volcano, January 30, 1911 , see link below.
http://www.archive.org/details/eruptionoftaalvo00philrich
The USGS uses SOSUS (and more advanced hydrophone technology) for marine geological monitoring
USGS Vents Program
http://www.pmel.noaa.gov/vents/acoustics.html
The program includes seismic monitoring on the MAR in the North Atlantic (since 1999).
http://www.pmel.noaa.gov/vents/acoustics/seismicity/mar/mar_seis.html
Mission Plan:
http://oceanexplorer.noaa.gov/explorations/sound01/background/plan/plan…
Map of hydrophone array positions
http://oceanexplorer.noaa.gov/explorations/sound01/background/plan/medi…
History of SOSUS (1950w-1990s), decommissioning (with a few notable exceptions), and eventual replacement with easier-to-maintain, high-end detection systems (2000s).
http://www.globalsecurity.org/intell/systems/sosus.htm#end
I suspect IES/IMO have hydrophone arrays in place as well.
Is that "Murkalot" as in murky-a-lot? ;-P
The USGS uses SOSUS (and more advanced hydrophone technology) for marine geological monitoring
USGS Vents Program
www.pmel.noaa.gov/vents/acoustics.html
The program includes seismic monitoring on the MAR in the North Atlantic (since 1999).
www.pmel.noaa.gov/vents/acoustics/seismicity/mar/mar_seis.html
Mission Plan:
oceanexplorer.noaa.gov/explorations/sound01/background/plan/plan.html
Map of hydrophone array positions
oceanexplorer.noaa.gov/explorations/sound01/background/plan/media/noaa_acoustics.html
History of SOSUS (1950w-1990s), decommissioning (with a few notable exceptions), and eventual replacement with easier-to-maintain, high-end detection systems (2000s).
www.globalsecurity.org/intell/systems/sosus.htm#end
I suspect IES/IMO have hydrophone arrays in place as well.
Diane yes, I thought it appropriate in the circumstances.Profuse apologies to all Icelanders !
To all Icelanders, Adrian and I were just kidding and we really didn't mean anything by it. Except for the murky part.
Seriously, I hope nothing more is going on than steam and not so great weather. May you get some good weather soon.
Perfectly put Diane,but as for the weather,we will just have to wait and see.Somethings never change....
Carl@9, SOSUS (now known as IUSS) has been used for scientific work since its declassification in the 1990s.
Heh.
Q: What is a high-pressure area over Iceland in the weather map?
A: An error.
I saw it once in RL, asked our Meteo Institute about it, and got that answer. A low over Iceland is a cliché.
So Greg are you going to go and camp on Volcano Island and wait until an eruption? Hopefully that would show you just how "nanny state"-like governments need to be round volcanoes.
Rarely have I seen a more silly comment at this blog than the second comment in this thread. Of such attitudes are Darwin Awards made.
@37: weak-to-neutral NAO index (weak Icelandic Low).
http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/nao_index_mrf.s…
Interestingly, the 10- and 14-day forecast patterns also match temperature patterns in the inland US PNW.
I believe this persistent pattern since Spring has sucked precip southward, out of the northern latitudes (Canada) and resulting in unusually dry conditions across the central Canadian prairies and upper midwest that may explain the unusual lightning-strike fires observed over Ontario and Quebec that sent smoke haze into New England, mid-May.
@38 Well it's currently only Level 2. At Mayon, foreign nationals were banned at Level 3, either Taal should be Level 3 when they say it's Level 2, or the government is just trying to show how tough it is?
http://www.gmanews.tv/story/179758/uk-canada-to-nationals-stay-away-fro…
We have a new Nanny state in Philippines...
I've family in the rescue services.
It is unfair for them to be expected to rescue candidates for Darwin Awards, but the sense of most people is just that-- I can go anywhere and someone (someone government) will bail me out.
I'm all for freedom of movement, but we need to have some sort of general understanding that if you go where you've been warned off, you are on your own. I think until government stops rescue for the unprepared, we'll be committing ourselves to the "nanny state".
While researching to try and back up my statement that Taal's (recent) products are essentially basaltic I found this link
http://www.abs-cbnnews.com/nation/regions/06/14/10/deped-postpones-open…
So much for PHIVOLCS attempts to declare Volcano Island off limits for everybody!
Bruce; I know I've read it somewhere, but haven't found where yet.
This has nothing to do with Nanny States. Asia is experiencing reasonably strong economic growth and a burst middle class affluence that affords increased opportunity for travel as never before. The airlines are anticipating (after whining about major losses in March-April 2010) major growth in ticket sales, much of it coming from the Eastern Pacific. Anything that smacks of major tourist-attraction potential will be targeted for drumbeat marketing - and that includes visiting volcanoes.
I think the Philippine Institute of Volcanology and Seismology and Federal government are wisely stressing risk control now, before the tourist numbers grow beyond control and add to the difficult evacuation situation near Manilla.
Because Taal has a benign visual profile and the last eruption - a mild one in 1977 that didn't cause death and property damage, perceived risk by the public tends to be under-estimated. If you read Ravings links to historic eruptions, you'll understand why it's a Decade volcano, beyond it's proximity to heavily populated areas. It can flare up and go quiet - and that is when it is most deadly, because it can return to full throttle very quickly.
The last thing the government wants is throngs of gawking tourists present during a major escalation in eruptive activity, with many foreign deaths on it's hands. Once the initial phase of the eruption - fast rise in seismicity, deadly gas cloud releases and a roiling steamy lake are in evidence, the public will state a beleated panic mass exit, complicating evacuation route management. Local emergency management officials already have ~4500 hundred locals to shuffle off the island in quick-time and many thousands more in the developed resorts and 5 cities that lies on the edge of the lake, under *very difficult* evacuation conditions.
The local topology and known toxic chemistry of Taals gas emissions is sufficient cause for elevated concern and warnings to avoid heavy ecotourism influx to the island.
All three major eruption precursors are present: major sulfurous gas clouds, jump in lake temperature and escalated seismic activity.
In my book, that's a level 3, not a 2.
Phivolic might consider stepping up their warnings and have evacuation routes and plans thoroughly pounded into the heads of the local municipal authorities.
When the volcanic shit hits the fan, everybody will have to act quickly and calmly to avoid additional death from vehicle and foot traffic stampede.
A link to RUV(icelanding) showing footage on Eyjaf's lake:
http://dagskra.ruv.is/sjonvarpid/4528075/2010/06/14/5/
There's also concern in Iceland of economic impacts the threat of flooding may pose to tourism.
See for timeline of Taal eruptions http://tinyurl.com/2ezyyez
As concerns the 1965 eruption, The (Montreal) Gazette reports 589 fatalities
See tinyurl.com/33s2z2c
This is almost 3 times greater than the 200 fatalities reported by the BoV.
Yes there are technicalities, this is the Philippines in 1965 and such. Nevertheless the BoV article is used as a reference, no? Perhaps The (Montreal) Gazette has an inaccurate count.
So, given there's a lake where the eruption was I see two possibilities:
1. The new vent got dammed (which is why we've not seen all that much from gigjokull) and the lake was created by the (stopped?) eruption and is kept going by melt caused by the cooling lava.
2. The lake is renewable, there is still hot magma close to the surface continuing to melt the glacier.
Either way, not so good for the valley below: dam breaks or is overtopped.
So, anyone else have ideas? This is just meant as the beginning of a discussion from a rank amateur. ;-)
QUESTION: Taal lake area certainly has a topography which resemples a huge caldera, and in a few places on the web it is called a caldera. However the references which I see never give any evidence that there was ever a caldera-like supervolcano eruption there. So what is the consensus of opinion on that: does Taal have supervolcano history and a present caldera, or not?
Obviously this is important, since if lake Taal is actally a supervolcano caldera, then it would increase the possibility of a super-eruption occuring there again.
Taal is, in fact, a caldera system - or at least had a caldera forming eruption as recently as 3580 BC according to the Smithsonian GVP, who go on to say The 15 x 20 km Talisay (Taal) caldera is largely filled by Lake Taal, whose 267 sq km surface lies only 3 m above sea level.
However, I wouldn't be too concerned about the "super-eruption". You'd be surprised about how many volcanic system have a caldera hidden somewhere in their history.
Erik, good point. Ever since the Yellowstone episode earlier in the year I've noticed a lot of confusion in the general public regarding calderas.
Calderas result from a collapse during a particularly large eruption*. There is either a collapse of a volcanic cone (Crater Lake, Krakatau) or a larger area of the crust (Yellowstone, Long Valley). Strictly speaking the caldera did not produce the eruption, it is the result of the eruption. The existence of a caldera tells us that a particular volcanic center has the potential for a very large eruption. Now some volcanoes may only have one caldera-forming event in the record (Mt. Mazama/Crater Lake). Some may no evidence of a caldera-forming event, but this doesn't mean they won't have one in the future. Others will build up a large stratovolcano, have a caldera-forming eruption, rebuild the stratovolcano and then have another caldera-forming eruption (example-Krakatau). Some centers have caldera complexes of overlapping calderas from different events.
The important thing to keep in mind is that activity within an existing caldera does not mean that there will be another caldera-forming eruption. If there is another very large eruption that causes a collapse, it does not need to be exactly within an existing caldera. For example, at everyoneâs favorite volcano Yellowstone, the last three very large eruptions each produced a caldera. They did not all form in exactly the same place with the same dimensions, but instead overlap.
What this all means is that you shouldnât assume that activity in a caldera means âthe caldera may eruptâ. If another caldera forming eruption does indeed occur, there would be plenty of precursors as the activity ramps up.
Erik posted on caldera formation earlier, he may be able to reference the post.
*Calderas in large basaltic shield volcanoes tend to form more quietly and slowly by subsidence.
"You'd be surprised about how many volcanic system have a caldera hidden somewhere in their history."
There are several Eruptions readers who will find that piece of information heartening, as evidenced by their comments on the Eyjafjallajökull eruption. I think the sequence (in mindspeak) might be something similar to this: "St Helens woW!" "MtMazama? That's Crater Lake? W-O-W! They can do that?!! Now that I'd LOVE to see!" Nothing perverted about it, just normal human curiosity, it's inbuilt into our genes. How close on the other hand depends on Darwinian factors... ;)
http://www.abs-cbnnews.com/nation/regions/06/15/10/text-message-sends-t…
Nanny state sends out fake txt messages to further panic the population
If it's really such a threat, level 3 it and evacuate.
Greg, there is no evidence that the Philippine government sent out the text. In fact, the article implies, if anything, it was somebody spoofing a government announcement. I, for one, think the Philippine government is doing exactly what it needs to do with a potentially hazardous situation.
Hiho, again the whole day our magnetosphere is not stable,
@google magnetosphere real-time@
it could be a that there is a connection.
We are facing the Coronal Hole today.
>What this all means is that you shouldnât assume that activity in a caldera means âthe caldera may eruptâ. If another caldera forming eruption does indeed occur, there would be plenty of precursors as the activity ramps up.
To elaborate on Ekohs comment: if you look at graphic representations of the historic (Holocene era) eruption record (by no means complete, but treated as approximations of temporal trends), you will see that there are periods of heightened global eruption activity, with many large and vigorous eruptions in geologically active regions.
Therefore, risk of large eruptions is relative to the baseline for the period of interest.
Taal may experience an eruption soon, but the risk is disproportionate due to the proximity to urban congestion of the more-than-doubled population of Metro Manilla since the last eruption, an aggressive tourism industry that sees opportunity in volcano curiosity and growing regional base of tourists with money to spend, and a local population (like all fast growing developing nations, it is top-heavy in the youngest age cohorts) that is inured to risk because they were born since the last and mild eruption.
To add a little tidbit to Ekoh's comments: the other really instructive caldera to look at is Katmai.
Not only was the vent, Novarupta, "relatively" small (relative to the diameter of the subsequent caldera that is, not in terms of what came out of it!!) but the vent wasn't even inside the perimeter of the later caldera but a full 10 km outside it. Pretty good illustration in my view that calderas are better understood as collapse pits and not craters.
But to confuse the issue: the name caldera originates (at least according to my touristy guidebook which is perhaps not quite so reliable) from Caldera Taburiente on the island of La Palma. This structure however is not strictly speaking a caldera but a scarp caused by sector collapse.
translator's note: any corrections of my abuse of geological terms gladly appreciated!!
Oh, dear. Impress the few of your friends who are willing to spend thousands of dollars and lots of their vacation time, and hope they get over the jet lag in time to attend the wedding.
What's wrong with city hall, or your local church/synagogue/meeting hall/etc.?
I would get married on a volcano, maybe Yasur or on Krakatoa. The church has probably killed more people over the ages then all the volcanoes combined :P
Greg, the church has saved more lives then any government entity ever has. Please refrain from your hateful and ignorant bigotry.
Churches are not perfect, and God knows the mistakes, but please try to think about what you say and use the brain that God gave you before you make hateful comments.
It's not the church - it's what men do to futher their self-interest and then justify their evil deeds as "ordered by Jhvh/God/Allah/(insert deity of your choice) and ordained by the church". In that sense Greg is correct. If you want a humdinger about religion, go to Martin Rundquist's Aardvarcheology Blog! There's some really nutty believers there (in the same mould as the infamous Bishop of Tours who used to beat people with a cudgel whilst roaring "Why should I forgive the wrongs done me just because I have been ordained!" and spent his free time doing some brigandage and highway robbery on the side), atheist as well as deist. xD
So Erik you are saying that this was a collapsed caldera with a cute little fire breathing dragon in it. The volcano supporting the original cone must have been huge with that kind of distance across. Then there is the now what?
Now what? Is this whole thing inflating or is just quakes. Anyone know what the estimated original size was?
I must admit I do not quite understand the term "caldera" (It. cauldron) and its usage in volcanology. It is very imprecise. What volcanologists themselves, advertently or inadvertently, call "caldera" ranges in size from the massive Toba to the puny, fissure-split-and-widened summit crater of Eyjafjallajökull. Some calderas are the result of a collapse of the roof an emptied magma chamber (Katmai, Monte Somma), some occur as a result of large bodies of water interacting explosively with a partially emptied magma chamber (Thera, Krakatoa) and some are caused by really large magma chambers first lifting the ground casing ring fracture caldera eruptions around the perimeter after which the roof collapses back in. So, please, what is the correct vulcanological definition of caldera and caldera eruption?
Henrik, I posed the same question a couple of months ago. When is a crater a crater and when is it a caldera? In the meantime I feel more comfortable with the explanation of the mechanism that forms it being the decisive feature.
So until the experts reply, here's my crack at a definition:
A crater gets blasted out by an explosion and is a surface feature.
A caldera is a surface expression of a deeper feature, namely the collapse of overlying strata into a cavity formed by the evacuation of a magma chamber.
That said, I have trouble distinguishing between the two. For instance Tambora and Pinatubo. Both mountains blasted their tops off, but are the subsequent craters formed primarily by explosion or collapse?
BTW, Lake Taal looked absolutely exquisite on the webcam today (it's sunset now). The thought of a monster roaring up out of the middle of the lake or on the island is horrific.
@ Randolf: I don't think the diameter of the caldera bears any relation to the diameter (or height) of any pre-eruption edifice. Rather it just roughly indicates how much was erupted from underneath.
I am studying volcanoes with the Open University and my textbook defines a caldera as a volcanic crater more than 1 km in diameter.
@ the caldera questions: As you might guess from this discussion, the definition of a caldera is far from hard-and-fast. In my mind (and classes), it does come down to mostly processes rather than size: a caldera is a collapse feature while a crater is an explosive feature. Now, it real terms, there is a size component as if there is a small collapse feature, like the collapsed top of Kilauea (Halema`uma`u), you tend to call it a crater rather than a caldera. As for Tambora and the like, their formation does have both an explosive and collapse component: explosions release the magma than then creates the open space for a collapse to form the caldera. This is what happened at Crater Lake in Oregon, where the explosion of Mt. Mazama released enough magma to no longer support the mountain, and it collapsed (incidentally, the lake is named after the crater on top of Wizard Island, not the collapse caldera, so the name isn't a misnomer).
So, does that help at all? Maybe not - and part of calling something a caldera or crater is interpretation and part is field observation of the geologic features. However, you likely wouldn't call the hole in the side of Mount St. Helens a "caldera" just like you wouldn't call the depressions at Yellowstone "craters" - and thats due to how they formed more than size.
Yup, science is fuzzy a lot of the time.
Thank you Eric, yes, but like all good answers it leads to further questions: According to this definition, neither Krakatoa nor Thera are calderas but craters as they resulted from seawater entering still partially filled magma chambers and the resulting phreatomagmatic blast was large enough to cause huge, caldera-sized craters?
Is the old joke about asking economists about what will happen to the economy in the next year like asking volcanologists what is a crater and what is a caldera? How many different answers per volcanologist will there be? Another area that is definitely true is asking astronomers about what is the exact definition of a planet.
@Greg (#52) Of course the messages were fake. They weren't sent out in 'jejemon' code. Use http://www.jejeschool.com/index.php for assistance.
Trying to define crater/caldera is almost as difficult as defining 'volcano' it seems. My personal (unscientific) classification might be:
Ignimbrite caldera: result of a very large volume (regional) pyroclastic eruption. Large size, generally low relief, may have a resurgent centre, probably NOT formed from collapse of an existing edifice. EG Yellowstone, Campi Flegrei
'Plinian' caldera: The 'textbook' caldera, large volume high eruption rate pyroclastic eruption from a specific locus. Generally when formed has strong relief, and is enclosed within the remains of an existing edifice EG Crater Lake, Tambora
Pit caldera: Formed (probably) by the rapid drainage of magma from lateral vents. Typically steep-to-near-vertical internal walls, and without the thick pyroclastic deposits associated with the above types. EG Mauna Loa
Avalanche caldera: Formed by sector collapse; that is the collapse is outward rather than inward. Typically asymmetric, forming an amphitheatre with a recognisable debris avalanche deposit in front of the breach. EG St Helens, Galunggung
Whether avalanche calderas should be so named is another matter..but they are, after all, COLLAPSE basins, as opposed to craters, which are essentially explosion features.
There have also been more "folksy" reports of "strange animal behavior" since the Alert Status was raised to 2 - so take that as you will.
Erik: just a thought that it might be that animals are sensitive to sounds/vibrations outside the human hearing range -perhaps harmonic tremor at certain frequencies is 'felt' as an unpleasant sensation?
Only halfway to the island? Haha; as if that would help if the volcano went 'pop'. Well, I guess it would help if it were a very small pop and the eruption was on the island rather than on the flanks of the caldera that the island is sitting in.
@Raving taal: Thanks for the link; I might download and read that book. The book was printed by the "Weather Bureau" which at that time was actually a private institution (the Manila Observatory, which was owned and operated by the Jesuit order). I wonder if the category of "no copyright" is correct, although I can't imagine the observatory objecting (if it still exists).
Pray for a variety ear and you'll ascertain you module lie about a protect entryway