I saw a program on TV that stated volcanic gas and/or ash is necessary for earth’s healthy atmosphere, and that volcanic erruptions cause a protective layer between us and the sun. Is this in fact true?

Our office deals with the atmospheric transport of volcanic ash, rather than the benefits of it. As one example, please seehttp://volcanoes.usgs.gov/Hazards/What/VolGas/SO2Aerosols.html.

Barbara Stunder