The most common gases released by volcanoes all over the earth

The most common gases released are water vapor, carbon dioxide, carbon monoxide, hydrogen sulfide compounds, sulphur dioxide, and a handful of others.

Climate skeptics often point to water vapor being the most important GHG, but this is normally used to suggest that carbon dioxide isn’t a problem. Water vapor does trap solar radiation into the atmosphere, but its concentration directly depends on the temperature. Warmer worlds have more water vapor in the atmosphere, and colder worlds have less. We cannot control this.

Carbon dioxide, however, we do control. Although not the most effective trapper of heat, it is still pretty good at it; its cumulative lifetime up there is around 20 to 200 years. This makes it the most important GHG.

Volcanoes release a fair amount of it. Sometimes they release so much that it causes mass extinctions. The Siberian Traps, the volcanic province that produced a continental-sized lava flow over 1 million years, 252 million years back. This released 85 trillion tonnes of carbon dioxide, triggering a roughly 6-8°C (10.8-14.4°F) rise in global temperatures in a very short space of time.

This likely triggered the Great Dying, a mass extinction that wiped out as much as 96 percent of all life on Earth. It was as close to the apocalypse as you’re going to get.

end partial quote from:

http://www.iflscience.com/environment/what-causes-more-climate-change-humans-volcanic-eruptions/all/