There are lots of hazards surrounding Volcanoes, and they range in danger level. The below diagram shows these hazards.
A pyroclastic flow is a huge cloud made up of hot gas, rock and ash, reaching temperatures between 200 and 700 degrees. They travel at incredibly high speeds along the ground and pick up speed if crossing water, mostly travelling over 80 mph. They occur through an explosive eruption as well as the collapse of a volcano. Pyroclastic flows cause fires, the burning of farms, towns and villages and death. In Pompeii, the infamous statue people were burned alive and turned to stone as they inhaled the hot rock and ash.
Volcanic gases can leak out of a volcano and the surrounding areas into the atmosphere, soil and into lakes. They can be toxic and smell like rotten eggs due to the sulphur and carbon dioxide released. If the gas is stuck inside the magma chamber, it builds up overtime forcing the volcano to bulge until the walls give way. This creates a very violent eruption which can lead to pyroclastic flows.
Some volcanic gases leak into lakes, making the water toxic.
Lahars are rivers of hot or cold watery thick mud and rock that originates from a volcano. If an icecap volcano erupts and melts a glacier, then this has the potential to mix with the volcanic matter and become a river of liquid ‘concrete.’
Ground tremors or the collapse of a Volcano can cause landslides due to the loosening of the soil and rock on it’s cone. If in contact with water, the landslide can become a lahar.
Volcanic Tephra is the rock ejected out of a Volcano during an explosion. They range in size from ash to large boulders will be incredibly hot and dangerous.