Primary effects of a volcanic eruption
Tephra - solid material, ranging in size. E.g. volcanic bombs to ash particles, ejected into the atmosphere.
Pyroclastic flows - very hot (800 degrees Celsius), gas-charged, high-velocity flows made up of a mixture of gases and tephra.
Volcanic gases - carbon dioxide, carbon monoxide, hydrogen sulphide, sulpur dioxide and chlorine.
Secondary effects of a volcanic eruption
Lahars - Volcanic mud flows
Flooding - melting of glaciers and ice caps
Tsunamis - Giant sea waves generated after violent caldera-forming events such as that which occurred on Krakatoa in 1883 killing 36,000 people
Climate change - the ejection of vast amounts of volcanic debris into the atmosphere can reduce global temperatures and is believed to have been an agent in the past climate change.
Volcanic effects become a hazard when they impact upon the human and built environments.