The turning point when “hard rock” became “heavy metal”?

From Wikipedia:

Heavy metal (often referred to simply as metal) is a genre of rock music that developed in the late 1960s and early 1970s. With roots in blues-rock and psychedelic rock, the bands that created heavy metal developed a thick, heavy, guitar-and-drums-centered sound, characterized by highly amplified distortion and fast guitar solos. Allmusic states that “of all rock & roll’s myriad forms, heavy metal is the most extreme in terms of volume, machismo, and theatricality.