With its loud, distorted electric guitars (see entry under 1950s—Music in volume 3), powerful vocals, and often dark style, heavy-metal music became an important style of rock and roll (see entry under 1950s—Music in volume 3) starting in the 1970s. Amid the pop-rock and psychedelic rock of the late 1960s, musicians and groups such as Deep Purple and Led Zeppelin (see entry under 1970s—Music in volume 4) incorporated a harder, louder tone coupled with often mystical imagery that went far beyond anything else heard on the radio (see entry under 1920s—TV and Radio in volume 2) in the late 1960s. Heavy metal music is all about aggression, power, and pushing the boundaries of "respectable" music. Its critics are many, but its fans outnumber them because heavy-metal music speaks to raw human emotions.

Joe Elliot, lead singer of the heavy metal band Def Leppard. Photograph by Ken...