Naked Science Forum
Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: DoctorBeaver on 21/12/2008 09:46:21
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I watched a program on Discovery Science last night called "Killer Lakes". There was a lake in Cameroon that released vast quantities of CO2 and killed 1,700 people in the valley. It was all connected with a volcanic vent under the lake and the CO2 from it being suspended in the depths due to stratification of the water.
They went on to talk about a crater on Mt Rainier that could send a lahar as far as Seattle, and also about Yellowstone Lake exploding. Apparently they could cause a lot of trouble. Just think of all those poor beavers!
I appreciate that a lot of programs go for the sensationalist angle and maybe overplay the dangers; so does anyone know anything about these sort of lakes and are they really as potentially dangerous as the program said?
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thats a good Question... I went a byear ago or so to crater lake in oregon... it was beautiful... hard to think it could be dangerous like that.. that is very interesting though and hope someone has some answers!
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Have a look at these two Wiki links:
http://en.wikipedia.org/wiki/Lake_Nyos (http://en.wikipedia.org/wiki/Lake_Nyos)
http://en.wikipedia.org/wiki/Limnic_eruption (http://en.wikipedia.org/wiki/Limnic_eruption)
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Crater lakes are formed in the craters of volcanoes, which obviously gives rise to the name. Having carbon dioxide in the water originating form the volcano is VERY RARE. Most often, the volcano is plugged by solidified magma that prevents any gas escape. In the case of the three known "killer Lakes" a noxious gas, here CO2, a rift system volcano just like the ones in Iceland and the one that formed Kilimanjaro (a stratavolcano) broke through to a large body of ground water and the resulting explosive nature of the two formed a crater - also a very rare occurrence.
There are two major types of volcanoes: Rift Systems exemplified by many Hawaiian eruptions and the Kilamanjaro and Oku type volcanoes and Subduction Zone volcanoes such as those around the Pacific rim or "Ring of Fire." The real difference is in the magma produced. Basaltic or
Volcanoes form when magma breaks through the earths crust. In a rift system the TENDENCY is for long fissures to form and eruptions to come from these fissures forming large lava flows. Shield Volcanoes (Hawaiian Islands) cinder cone fields are also associated with rift systems and extensional tectonics - places where the earth is moving apart.
In subduction zones the material of the plate being subducted, mostly basaltic material, starts to melt at points and when it melts, travels upward. As it travels upward this magma will also melt the rocks of the upper plate, incorporate this material into the mix and forming a different type or magma that tends to be more explosive than the rift or extensional tectonic type. The reason is the incorporation of a lot of water into the magma. Mount Rainer, as are all volcanoes of the Cascade Range, is one of the latter types. Cinder cone fields are also found with this type of magma. Mt Vesuvius and the Phlegrean Fields are a combination of a Stratovolcano and cinder cones in close proximity. Use Google Earth to view Napels and Mount Vesuvius. The area from including Naples all the way to the cost is an ACTIVE cinder cone area and Vesuvius is thee only active volcaano on the mainland of Europe. (there are two on islands.)
The Yellowstone Volcano is almost one of a kind. It is a super volcano caused by a hot spot. There are only about 8 super volcanoes know to have erupted in the last 30 million years or so.
The type of magma determines the eruptive type. Ultra-mafic and Mafic lavas usually are gentle in eruptions but are occasionally explosive. These are sometimes classed as tholeolitic magmas and usually are oceanic in nature but also occur where the crust is thin and oceanic magma underlies this crust. This is the case in Cameroon. It was the chance contact of the usually gentle erupting oceanic magma that caused the crater to be formed. It is also the source of the CO2 as andesitic magmas contain mostly water vapor as the eruptive (and more volatile) gas.
Magmas with higher silica content are called andecitic and are highly eruptive. For a detailed discussion of magma types see http://en.wikipedia.org/wiki/Magma
(I am almost there) A lahar is essentially a mud flow and land-slide incorporating volcanic material as well. When a volcano with snow on it erupts the snow is instantly melted starts to flow and picks up any loose dirt or debris and moves, along with the volcanic erupta and gases, down slope from the eruption - they are very destructive. Lahars destroyed Pompeii in the final act, Montserrat in the Leeward Islands and cause most of the deaths in the Mount St Hellens eruptions. In South American volcanic eruptions they can easily kill thousands. Seattle would suffer the same fate if Mount Ranier erupts - and it will. It's last eruption was in 1894. It is also a youngster in volcano time, only 500,000 years old. It is a highly active volcano and the hazard area is shown below.
(from Wikipedia)
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JimBob presented a great summary on this subject. Loathe that I am to add critical input to such a masterpiece- here goes:
Volcanic gases and minerals are present in virtually all crater lakes on active or dormant volcanoes. Gases are primarily CO2 and SO2; minerals include sulfur, arsenic, salts, chlorides, and metals. In Crater Lake, OR, carbon dioxide is added to the lake through springs and funaroles- and converted to carbonic acid. There are lakes in Indonesia with pH in the range of 1.6 to 1.8 (highly acidic) from the input of sulfur dioxide and carbon dioxide.
The size and depth of the lake, mixing, thermal layers, type of volcanism, relief and proximity to people determine how likely these gases are to cause catastrophe. Humans are lucky that the combination of these factors is extremely rare, making repetition of the Cameroon disaster unlikely.
Lahars, or volcaic mudflows, are far more dangerous, as JimBob pointed out. Stratovolcanoes are composed of mixtures of lava, ash, mud and debris. Heat a bit of water (snow, glaciers or rainwater), add some seismic or volcanic activity, mix in high relief and volcanic mudslides are almost guaranteed. These mudslides can be extensive, traveling 10's of miles and covering areas in several feet of debris.
However, it was pyroclastic eruptions, not lahars, that buried Pompeii, Montserrat and caused most of the deaths at Mount St Helens. Hot gasses keep ash and other volcanic particles suspended in the air, allowing them build up huge ash clouds before collapsing under their own weight (Pompeii), or flowing horizontally along the surface for miles before cooling enough to collapse- often still hot enough to weld the particles together.
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JimBob & Bass - thank you.
With regard Yellowstone Lake, it was said in the program that on average there is an explosion there every 800 or so years (according to some woman from the USGS). It is 3000 years since the last 1 so another is overdue. The danger would be a tidal wave caused by the underwater explosion. Apparently the pressure of the water acts as a bung, but any drop in that pressure could cause the magma underneath to erupt upwards into the water.
There was reference to SO2 in the section on the Mt Rainer lake; but the main danger there would be the lahar.
It was also said that the CO2 in the Cameroon lake was suspended in the lower strata of the lake and that possibly a large landslide cause de-stratification & released the gas. The exessive build-up was due to the lack of seasons in that part of the world so that the CO2-containing layer was never cycled to the upper parts of the lake to release the gas on a tearly basis. Instead, that particular layer stayed at the bottom absorbing more and more CO2 over the centuries. They said that the amount of CO2 released could have taken 300 years to accumulate.
The impression I got from the program was that most, if not all, crater lakes are dangerous for 1 of these 3 reasons. Can I take it that I got the wrong impression?
P.S. I thought also that Pompeii had a pyroclastic flow rather than a lahar.
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The eruptions they refer to in Yellowstone Lake are phreatic eruptions- or steam eruptions- not eruptions of magmatic material. Check Yellowstone Lake on Google Earth and you'll notice several semi-circular (or elliptical) bays on the north side of the lake, which are the craters left from steam explosions (Mary's bay for example). There are also phreatic craters in other parts of the park on land.
While catastrophic locally- along the shore of lake- these eruptions will not cause widespread problems. Undoubtedly they throw rocks around for several hundred meters and will set up a small seiche in the lake- some of the lodges and campgrounds on the north shore of the lake will be affected.
Several years ago, some misguided soul released lake trout into Yellowstone Lake. Lake trout, given the chance, will displace the native trout (they eat them). When the Park service went out to try to eliminate the lake trout, their nets kept getting fouled up on the bottom of the lake. This prompted investigation by the Park Service/USGS, who found numerous siliceous "chimneys" scattered about the bottom, especially at the north end of the lake. Hot springs, hydrothermal vents basically producing white smokers- if not for the lake, this would undoubtedly be another geyser basin.
The point of that story? Upon investigation, they also found a large dome- some 700 meters wide- that stands up above the lake bottom due to hydrothermal activity. Measurements just above the dome found superheated water (which quickly mixed with the cooler lake waters). Any event that causes the superheated water to flash to steam will result in a sizable explosion and crater. The question they haven't answered is how long this dome has been around- a few years, a few hundred years, a few thousand years? How much of a geologic hazard it presents will depend on it longevity and the amount of hydrothermal activity. Check out the Yellowstone Volcanic Observatory http://volcanoes.usgs.gov/yvo/ for more details.
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Bass - yes, you're right. I remember now. It was steam not magma. They did mention 1 particular crater at the north part of the lake, and that bulge.
The way things were portrayed in the program was that crater lakes are inherently dangerous and they only chose those 3 as examples. From what you & Jim have said, that's not strictly accurate.
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I hold no grudge!
I am not the expert on the subject - Bass is the geologist who knows more about volcanoes and stuff that comes out of the deep bowels of the earth. He makes his living working with rocks of this origin.
I make my living by looking at rocks formed at or near the surface of the earth.
Thank you Bass (you %*&%%&%%%##__&^$@#@@!@ !)
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Over the last several days, there has been a swarm of earthquakes occurring under Yellowstone lake in the vicinity of the "bulge". Good news is that if anything happens (a big "if"), there are very few people around the lake this time of year (a few snowmobilers and winter campers)
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It's gonna blow! [:o]
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It's gonna blow! [:o]
And it is good by Bass if it does.
Up for a prolonged Texas vacation, Bass?
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He'll be by the lake with a big net to catch flying rocks! [:D]
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Texans use nets. Montanans catch 'em with their teeth (which explains my lispthhhhhh).
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It's a pretty safe bet,
Texans use a net,
ANd Montana's the state,
For the edentate!
[;D]
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It's a pretty safe bet,
Texans use a net,
ANd Montana's the state,
For the edentate!
[;D]
GROAN !!! [xx(] [xx(] [xx(] [xx(] [xx(] [xx(]
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Over 900 recent earthquakes during the latest earthquake swarm located under Yellowstone Lake. While this sort of behavior is not unusual for Yellowstone, it certainly bears close watch. No indication of any kind of imminent eruption- I'm still sleeping soundly even though only a few hundred miles away.
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You'd sleep soundly if a rock landed on your head!
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It seems to have quietened down under the lake over the past week, but there's been a more recent little cluster about 20km NNW of the lake.