ignimbrite
noun
ig·nim·brite ˈig-nəm-ˌbrīt 
: a hard rock formed by solidification of chiefly fine deposits of volcanic ash
Example Sentences
Recent Examples on the Web Its largest eruption put down a 200m thick layer of ignimbrite—a frozen foam of volcanic ash, rock, and gas—over much of the central north island. 
Chris Lee, Ars Technica, 16 Oct. 2017
Chris Lee, Ars Technica, 16 Oct. 2017 Word History
Etymology
German Ignimbrit, from Latin ignis + imbr- (from imber rain) + German -it -ite entry 1 — more at imbricate
First Known Use
1932, in the meaning defined above