Heavy metal music emerged during the late 1960s and early 1970s as an offshoot of hard rock. Bands like Black Sabbath, Deep Purple and Led Zeppelin were forerunners in heavy metal, taking blues rock to new heights in technicality and volume. In the late 1970s, bands like Iron Maiden and Judas Priest stripped heavy metal of most of its blues influence and, merging it with punk sensibility, creating the New Wave of British Heavy Metal. During the 1980s, both hyper aggressive thrash metal and ultra poppy glam metal became the popular breeds of heavy metal. As metal continued to evolve, new genres like metalcore and nu metal popped up.
Heavy metal music is generally fast, powerful and loud. Heavy metal guitar is heavy distorted. Rhythm guitarists play very fast palm-muted style guitar, while lead guitarists play riffs. Heavy metal guitarists often play in modes once considered evil in Medieval times. Heavy metal bassists play root notes or follow the guitar, providing the extreme low end sound that gives the genre its "heavy" name. Heavy metal drummers play extremely fast and repetitious beats. Heavy metal music, on top of its roots in blues and rock and roll, often has roots in classical music.
Heavy metal's lyrics have explored darker and more philosophical themes than most of rock music's offshoots. Heavy metal lyrics often make references to death, violent imagery, blood, gore, drugs and sex, while commenting on larger themes like war and politics. Bands like Led Zeppelin and Black Sabbath incorporated fantasy elements in their lyrics, like Led Zeppelin's references to The Lord Of The Rings trilogy in "The Battle of Evermore," leading to its common practice by other heavy metal bands. Death metal bands bands like Cannibal Corpse and grindcore bands like Circle of Dead Children make extremely gory references in their lyrics.
Because of heavy metal's dark imagery and lyrical content, it has been the subject of controversy with parents and the media since the 1970s. Heavy metal bands have been accused of inspiring violence and suicide via their lyrics, as in the case of Black Sabbath's with its song "Suicide Solution" and Judas Priest's albums. In some parts of the world, such as in the Middle Eastern country of Jordan, heavy metal is actually forbidden and groups find it impossible to book gigs.
Different parts of the world are known for their different subgenres of metal. The Bay Area in Southern California is known for its thrash metal scene, which peaked in the 1980s with bands like Slayer and Megadeth. The death metal scene is particularly strong in Sweden with bands like At The Gates and Dismember. Louisiana is known for its sludge metal scene.