Environmental effects of volcanoes

1.Volcanic eruptions can be extremely damaging to the environment, particularly because of a number of toxic gases possibly present in pyroclastic material. It typically consists mainly of water vapour, but it also contains carbon dioxide and sulphur dioxide gas. Other gases typically found in volcanic ashes are hydrogen sulphide, hydrogen chloride, hydrogen fluoride, carbon monoxide, and volatile metal chlorides.

Read more: http://www.lenntech.com/volcanic-eruptions-environment.htm#ixzz2t8I3OnuN

2.If the ash and mud from a volcanic eruption mix with rain water or melting snow, fast moving mudflows are created. These flows are called lahars.
http://www.bbc.co.uk/schools/gcsebitesize/geography/natural_hazards/volcanoes_rev5.shtml

3. Lava flows and lahars can destroy settlements and clear areas of woodland or agriculture.
http://www.bbc.co.uk/schools/gcsebitesize/geography/natural_hazards/volcanoes_rev5.shtml