pyro-

in geology, "formed by volcanic agencies," especially in reference to fast-moving, dense, superheated surges of ash, gas and rock in a volcanic eruption; by 1862 in reference to the rocks that result; see pyro- "fire" + clastic, indicating "broken in pieces, fragments."

The word "ash" is not a very good one to include all the mechanical accompaniments of a subaerial or subaqueous eruption, since ash seems to be restricted to a fine powder, the residuum of combustion. A word is wanting to express all such accompaniments, no matter what their size and condition may be, when they are accumulated in such mass as to form beds of "rock." We might call them perhaps "pyroclastic materials," but I have endeavoured in vain to think of an English word which should express this meaning, and believe, therefore, that the only plan will be to retain the word "ash," giving it an enlarged technical meaning, so as to include all the fragments accumulated during an igneous eruption, no matter what size or what shape they may be. [J. Beete Jukes, "The Student's Manual of Geology," Edinburgh, 1862]

1858, as a proposed word for "electricity considered as a material substance possessing weight," from pyro- + -gen. Meaning "fever-producer, substance which, introduced into the blood, induces fever" is from 1896. Related: Pyrogenic. Greek pyrogenes meant "born in fire, wrought by fire" (compare pyrogenesis).