The only major effect volcanoes have on the ozone layer is because of
the generation of sulfate aerosols. These are relatively short-lived, so
the effect is not as permanent as with carbon-halogen covalently bonded
compounds. The eruption of Mt Pinatubo (sp?) caused a decrease of ozone
layer thickness of about 2-4%, depending on location, but this was
resorbed at ~70%/year. I have no figures for the Mt St Helens 1980
eruption, would it was less important (geologically speaking).

The physical effect of the emissions will be largely stopped at the
tropopause, in the same manner as cumulonimbus thunder clouds forming
their famous anvil-shaped head, but it is probable that the level of the
tropopause would be pushed upwards 1,000 m or so, forming a "bubble".
Transtropopausal transport mechanisms are very slow, so relatively
little of the emissions gases or particles will reach the stratosphere
before thet are rained out or destroyed by photolysis or hydrolysis
(fortunately, otherwise there could be no ozone layer!).

OTOH, there are transient increases of various greenhouse gases but
their short-term effect is largely compensated by the transient increase
of dust particles, causing cooling. In addition, the earth's albedo is
locally increased during the time of the eruption by reflection of
radiation from the clouds. All these points average out, over a few
years, to an almost non-measurable effect on climate change, although
there may be short-term peaks in one sense or the other, possibly
causing some weather change (cf. the effects of the eruption of Krakatoa
in the late 19th c.).

We cannot predict the effects of a major caldera eruption, such as
happened at Yellowstone ~600,000 years ago and which could happen again
(at any time) within the next 50,000 years, but this could have a
profound effect on global weather for a few years.

Hope this helps.

Brian

Gary Bremer wrote:
> What effect does volcanoes such as Mt. St. Helen�s have on the atmosphere
> and environment? Dose not most of the plumes of ash, steam, and toxic gases
> go up into the stratosphere where it effect the ozone layer?
>
>
> Gary Bremer
>
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