Why are volcanic eruptions dangerous?
Not all eruptions are dangerous, but the sudden explosive variety can cause death and destruction by a number of mechanisms.
Volcanic eruptions associated with mafic flows (those rich in dark minerals) tend to flow easily and tend not to be explosive (e.g. volcanos in Hawaii). Flows from these kinds of volcanos don't move that fast, and people can generally outrun them if they have advanced warning. These flows can destroy homes, road and infrastructure. See 2nd video.
Those volcanos associated with felsic flows tend to not flow easily and often get stuck in magma chambers (e.g. Mt. St. Helens and most volcanos on the West coast of N. America). Consequently, when they do erupt, they are often explosive. This can result in a number of risks for humans including:
glowing ignimbrite are very fast flowing masses of ash and rock that can easily overtake a speeding car. The ancient city of Pompeii was destroyed in this fashion. See 1st video.
massive mudslides are often associated with ignimbrite.
ash the slowly settles out of the air and can become several feet thick - can choke out all animals and humans. Can also disrupt aviation for days or weeks.
large volcanic "bombs" and large pumice blocks that can kill
eruptions can also change the weather and tornados or thunderstorms have been known to form.
See Youtube for an amazing ignimbrite flow:
See the video for a much slower mafic flow in Hawaii: