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I'm planning to climb a few volcanic mountains in Indonesia the coming year. Mostly Mt Kerinci and a few adjoining ones (I'm still making a list of those near by to Kerinci).

I believe that most volcanic mountains are dangerous due to the sulfurous gas emitted by the mountains. Are there any other specific threats from volcanic mountains?(apart for the obvious eruption threat). I'm talking of both active and dormant ones here. (Please exclude the threat due to altitude. I'm more concerned about specifics of a volcanic mountain climb)

I would also like to know if the lakes in the craters of some of these dormant volcanoes are safe sources of water.

PS: Kerinci is know to be active.

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Avoid stepping on the glowing gooy stuff. –  Olin Lathrop Dec 31 '13 at 14:46
@OlinLathrop man why didn't I get this before?! ;) –  Unsung Dec 31 '13 at 16:01

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One threat I've heard of is "heavy gases" (Carbon dioxide, hydrogen sulfide, etc.) collecting in valleys or craters.

Normally gases on a volcano will simply blow away, but some gases, being heavier, will sink into low lying areas of the volcano. These gases can replace the oxygen in these areas and can lead to HAPE like symptoms. This is made worse if you have respiratory problems (such as asthma) or at high altitude where the oxygen levels are low anyway.

Hydrogen sulphide was actually used as a gas in World War I, so you can imagine this isn't going to be pleasant if you encounter a pocket of this.

I don't know how big an issue this will realistically be but it is a threat that you may want to be aware of. I would imagine this would depend on the terrain and the type of volcano you are on.

Some info here

Most gases from a volcano quickly blow away. However, heavy gases such as carbon dioxide and hydrogen sulfide can collect in low-lying areas. The most common volcanic gas is water vapor, followed by carbon dioxide and sulfur dioxide. Sulfur dioxide can cause breathing problems in both healthy people and people with asthma and other respiratory problems. Other volcanic gases include hydrogen chloride, carbon monoxide, and hydrogen fluoride. Amounts of these gases vary widely from one volcanic eruption to the next.

Although gases usually blow away rapidly, it is possible that people who are close to the volcano or who are in the low-lying areas downwind may be exposed to levels that may affect health. At low levels, gases can irritate the eyes, nose, and throat. At higher levels, gases can cause rapid breathing, headache, dizziness, swelling and spasm of the throat, and suffocation.

To address your secondary question, Lakes, tending to be in low lying bowls or craters, could be particularly susceptible to this problem.

I also believe some of these gases when dissolved with water can produce some nasty chemicals. Hydrogen sulfide dissolved in water produces hydrosulfuric acid.

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