pyroclastic - refers to the fragmental materials formed by explosive processes of volcanoes.
aa - Lava of basaltic composition that has a very rough surface. (Contrast with pahoehoe lava)
ash - Fine granular pyroclastic material, with grains 0.04 inches or less in diameter. Hardened ash is called tuff.
basalt - Lava, usually black or very dark gray, that has 55 percent silica or less. Ocean floors of the world are covered by basalt lava flows.
base surge - A ring-shaped cloud of gas and suspended solid debris that moves (surges) radially outward from the base of a vertical explosion column at high velocity as a density flow.
blast surge - a blast or explosion directed outward from a volcano with great force.
block - a solid volcanic rock (pyroclast) ejected from a volcano and having a diameter greater than 2.6 inches.
bomb - a viscous particle (pyroclast) of a diameter greater than 2.6 inches ejected from a volcano before the particle is frozen solid. Shapes vary greatly - spindles, cow pies, ribbons, etc.
Bibliography
Out of the Crater: Chronicles of a Volcanologist, Richard V. Fisher, Princeton University Press, 1999
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