What causes an acid lake?
Do they take a long time to form or can a regular lake become one?
Les
Hi Les,
The easiest explanation for acid lakes is that they occur where water
collects in a place that volcanic gases are being emitted on a volcano.
This usually means a crater or caldera of some kind in a place where there
is sufficient rainfall. There are fumaroles on just about every volcano,
and these give off mainly H2O but also CO2 and SO2. The latter two
quickly react with atmospheric moisture to form carbonic and sulfuric
acid, respectively. That is why being around strong fumaroles is often so
unpleasant.
Of course if the fumarole happens to be in the bottom of a crater and the
crater fills up with rain, then there is a huge supply of water to react
with the gas and form acid. Depending on the amount of rain that comes
in, the lake can get very acid (with pH of often <1!). All that is
happening is that the gases are bubbling through the water and reacting to
form acid. Another process is that the gases can react with ground water
to form acid beneath the surface. This can then be injected into an
existing lake and make the lake acid as well. You can imagine all kinds
of variations on these themes as well as combinations of them going on at
the same time.
The amount of time required for a lake to become acid is really variable,
and depends on the amount of gas that is being emitted by the underwater
fumaroles, the size of the lake, the amount of rainfall (which works to
dilute the acid), and the amount of outflow of the lake.
There are some fascinating acid lakes on a volcano called Keli Mutu in
Indonesia. There is a recent article about them in the "Geochemical
Journal", volume 28, pages 243-262. Additionally, look here on
VolcanoWorld in the next month or so for some photos of an acide lake at
Anatahan Volcano in the Mariana Islands.
Sincerely,
Scott Rowland, University of Hawaii
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