In the 1960s and 70’s, particularly in the United States and Great Britain, a new type of rock and roll music was evolving. Bands like Led Zeppelin and Black Sabbath were turning up the volume and thickening their sound to create a new genre called heavy metal music. From its earliest days, heavy metal music took elements from traditional rock, folk, jazz and blues, and combined them all to make a heavier sound that was both more vicious and aggressive. Widely panned by critics in its earliest days — and throughout its evolution as a genre — heavy metal music has continued to be a force in rock and roll and has since spawned several sub-genres.
Heavy metal music is comprised of any number of metal sub-genres: in the 1980s, glam metal became the craze as bands like Poison and Whitesnake donned long hair and make-up to accentuate their on-stage personas. Around that same time, thrash metal became extremely popular when bands like Metallica hit the scene and built off their predecessors by not only playing loud, but playing fast and with thick drum beats. Heavy metal music later spawned the genre of nu-metal in the mid 1990s and into the 2000s, which combined the elements of all the sub-genres before it but with the added element of modern instrumentation such as synthesizers. It also touched on genres outside of metal, such as hip hop and electronica.
But originally, heavy metal music was based off the same sound as blues, jazz, folk, and rock. Bands began to play louder and more aggressively, transforming the sound into a new genre that caught on with younger audiences but was panned by older critics for lacking structural roots and tried-and-true sounds. Experimentation became vital to heavy metal music, as guitarists like Jimmy Page dabbled into new effects as well as new methods of playing established instruments. Vocalists became a more powerful facet of the sound, and in later years, it became common for vocalists to scream with deep, raspy voices.
Eventually, heavy metal music became infused with punk rock sounds. Bands like Motorhead sped up the pace and simplified much of the song structure, and eventually the blues component of the genre began to disappear. Later on, a sub-genre called grunge hit the scene, with bands like Nirvana and Pearl Jam at the forefront. Their sound was aggressive and melodic and spoke to the angst of teens and young adults throughout the United States.
The characteristics of each sub-genre of heavy metal music are so varied that defining them all under the category of metal is sometimes a stretch. All the sub-genres share similar qualities — primarily aggressive sound coupled with excessive volume and experimental instrumentation — that classify them as distinctly heavy metal.