References:

http://www.gns.cri.nz/what/earthact/volcanoes/hazards/monitor.html

http://volcano.und.nodak.edu/vw.html

http://users.aber.ac.uk/rav/lis/theory.htm

http://vulcan.wr.usgs.gov/Hazards/NRC_Definitions/volcanic_gas.html

Major gases info http://volcanoes.usgs.gov/Hazards/What/VolGas/volgas.html

hydrogen info http://www.cdc.gov/niosh/ipcsneng/neng0001.html

carbon monoxide http://www.cdc.gov/niosh/npg/npgd0105.html  

http://www.geology.sdsu.edu/how_volcanoes_work/

Picture obtained from  http://volcano.und.edu/vwdocs/volc_images/img_st_helens_bw.html without permission

To see some video clips of volcanoes, follow this link: http://volcano.und.nodak.edu/vwdocs/movies/movie.html