Etymology
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Words related to pyro-

*paewr- 
*paəwr-, Proto-Indo-European root meaning "fire."

It forms all or part of: antipyretic; burro; empyreal; empyrean; fire; pyracanth; pyre; pyretic; pyrexia; pyrite; pyro-; pyrolusite; pyromania; pyrrhic; sbirro.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit pu, Hittite pahhur "fire;" Armenian hur "fire, torch;" Czech pyr "hot ashes;" Greek pyr, Umbrian pir "fire;" Old English fyr, German Feuer "fire."
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pyroclastic (adj.)

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]
pyrogen (n.)

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).

pyrogenesis (n.)

"the production of fire or heat," 1858; see pyro- + genesis. Pyrogenetic is attested from 1855 as "having the property of producing heat;" by 1838 in geology as "produced by heat."

pyrolatry (n.)
"fire-worship," 1660s, from pyro- + -latry "worship of." Related: Pyrolater.
pyrolysis (n.)

"decomposition by the action of heat," 1879, from pyro- + -lysis. Related: Pyrolytic; pyrolyse.

pyromancy (n.)

"divination by means of fire," late 14c., piromaunce, from Old French piromance and directly from Medieval Latin piromantia; see pyro- + -mancy. Related: Pyromantic.

pyromania (n.)

form of insanity marked by a mania for destroying things by fire, 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.

pyrophobia (n.)

"morbid fear of fire," 1871, from pyro- "fire" + -phobia "fear."

pyrophoric (adj.)

"having the property of taking fire upon exposure to air," 1779, from Modern Latin pyrophorus, literally "fire-bearing," from Greek pyrophoros, from pyro- (see pyro-) + phoros "bearer," from pherein "to carry" (from PIE root *bher- (1) "to carry"). Related: Pyrophorous.

Pyrophorus is by 1778 as the name of fine, powdery substances capable of catching fire spontaneously on exposure to air; with a capital P-, as the name given to the genus of the most brilliant of the American fireflies, from 1809.