There are many hazards associated with Volcanoes, but they are not the stereotypical ones that everyone thinks of. People are rarely incinerated by giant waves of lava but a pyroclastic flow, how gas and debris moving as fast as two hundred miles per hour, will definitely leave a mark. But when volcanoes do erupt, the major lasting consequences are not lava or waves of hot gas. Mudslides occur in areas where there is ice present during the volcanic eruption.
Although large chunks of debris, bombs, can cause major damage most particles released into the atmosphere from a volcano are ashes. This volcanic ash can cause major health problems from people breathing it and it can also cause many environmental problems due to the way ash can affect the climate.
The best example of this occurred many years ago when an Indonesian volcano called Tambora erupted and caused drastic climate change for a decade. 1810-1819 is the decade referred to as the cold decade because it was the coldest the earth has been for the past 500 years. Recent ice cores from Greenland and the Arctic hint that the cause for this was a major volcanic eruption that left so much ash suspended in the atmosphere sunlight was blocked for a decade.
The thing is the ash from Tambora would not have caused this big of a climate change so they have pinpointed that another eruption, an undocumented one, occurred in 1809 starting the decade off as cold and the eruption of Tambora in 1816 kept the coldness around for a few more years.
The fact that two volcanic eruptions caused so much of a change is alarming. You can go onto Google Earth and witness the fact that volcanic activity is an ongoing thing and that all it takes to send our world from one environment issue into another is two of hundreds of volcanoes to have a major eruption. The funny thing about volcanoes and earthquakes is the fact that we have no control over them. There is nothing we can do to stop these natural disasters from occurring, all we can do is learn as much as we can so that minimum human life is lost. As far as a cold decade from a major eruption, maybe we have found the cure for global warming.