Why are volcanoes dangerous?

Volcanoes produce a number of hazards depending on the type of volcano. Even a volcano that is not erupting can release toxic gasses and cause earthquakes, though major earthquakes are rare. Volcanoes with low-viscosity magma often produce lava flows that move downhill, burning what they come in contact with. Volcanoes with high-viscosity magma tend to erupt explosively. These eruptions produce volcanic ash, which consists of tiny shards of glass that can damage and clog the lungs, which can lead to a very painful death. The ash, which can fall like snow, is about as dense as concrete, which can cause roofs to collapse. The ash can also mix with water from rain or melting ice, forming dense mudflows called lahars. Finally, the greatest danger from these eruptions are pyroclastic flows. These are mixtures of hot ash, rock, and gas that race down the sides of a volcano at great speeds, destroying everything in their path.

Source article: Wiki.answers.com