in reference to fast-moving, dense, superheated surges of ash, gas and rock in a volcanic eruption, by 1862, originally in geology, 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]

1840, from pyro- "fire" + mania "madness, frenzy." Used in German in the 1830s.

The propensity which leads an insane person to accomplish his purpose by burning, has been considered to merit particular notice, and to constitute a variety of monomania. Dr. Marc, of France, has published a memoir on the subject; he gives the name of pyromania to it, and considers that, like other insane propensities, it may be the result of instinct, or it may be the result of delusion—reasoning upon erroneous principles. [Alexander Morrison, M.D., "The Physiognomy of Mental Diseases," London, 1840]

An older word for it was incendiarism.

1704, "of or pertaining to fire;" 1825, "of or pertaining to fireworks," from pyro- + Greek tekhnikos "made by art," from tekhnē "art" (see techno-). Figurative use attested from 1847. Related: Pyrotechnical (1610s, from pyrotechny "use of gunpowder," 1570s).