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 tephra is the general term now used by volcanologists for airborne volcanic ejecta of any size. Historically, however, various terms have been used to describe ejecta of different sizes. Fragmental volcanic products between 0.1 to about 2.5 inches in diameter are called lapilli; material finer than 0.1 inch is called ash. Fragments larger than about 2.5 inches are called blocks if they were ejected in a solid state and volcanic bombs if ejected in semi-solid, or plastic, condition. [after USG, 1987]
TEPHRA FALLS Mode of action: By burial, filling, corrosion, accumulation, abrasion, conduction, absorption, dissolution and chemical reaction, magnetism. Distance: (average) 20-30 km - (max) > 800 km. Speed: (average) < 15m/s Dispersion often depends on the orientation of the wind, and the effects are variable, in particular according to the thickness of accumulated ashes. Environmental damage, disturbances of communication networks, water pollution, roof collapses... Effect on humans: Irritations of the mucous membranes...
BALISTIC EJECTA Mode of action: By impact, combustion, burial Distance: (average): 5 km Temperature: < 1000°C Speed: (average) 50-100m/s Fires (incandescent materials), collapse and perforation of the roofs, burial or obstruction of the infrastructures, broken windows. On humans: death or serious injuries
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Volcanic Risks