A volcano is a conical hill or mountain that ejects hot ash and lava in an eruption. In addition to hot ash a lava volcanoes erupt many gases. The most common gases erupted by a volcano are carbon dioxide, sulfur oxides and water vapor. Carbon dioxide is common in volcanic eruptions as it is formed mainly when subducting plates laden with the calcium carbonate skeletons of tiny marine organisms. Once the plate melts the carbon dioxide is incorporated into magma.