Volcanoes cause a change in immediate weather resulting in cloudcover, rain and lightning. This is due to the property that the volcanic ash particles have; i.e. they promote the production of rain particles at a molecular level. There are also long term climatic effects of volcanic eruptions which are harder to understand. If the ash reaches the stratusphere it cannot be diminished by rain and therefore contributes to the greenhouse or warming sex.
Obviously there are also immediate physical damages caused by the lavaflows and acid seeping into surrounding water but this damage is relatively small in comparison to the climatic significance of Volcanoes
Electricity and even telephone supplies can be cut off because of the large explotion that has caused a strong vibration............................