Who Are the 'Big 4' Bands of Heavy Metal?

Plenty of heavy metal acts have achieved astronomical success and elevated the medium over the past half-century, but only the biggest and boldest innovators can lay claim to the "Big 4" of heavy metal.

Rock bands started sowing the seeds of heavy metal in the' 60s. Some fans and scholars consider Blue Cheer's earth-shaking 1968 debut album Vincebus Eruptum to be the starting point for the genre. Later that year, the Beatles flaunted their screaming vocals and searing, distorted guitar squalls in the proto-metal anthem "Helter Skelter." And let's not forget Steppenwolf's "Born to Be Wild," whose "heavy metal thunder" lyric introduced the term into the cultural lexicon.

These artists — along with prototypical hard rock giants such as Led Zeppelin and Deep Purple — laid the foundation for heavy metal. But the genre wouldn't fully blossom until the beginning of the next decade.

READ MORE: Dave Mustaine Explains How He Influenced Every 'Big 4' Thrash Band

How Black Sabbath Invented — and Rejected — Heavy Metal

Heavy metal exploded in earnest in February 1970, when Black Sabbath announced their arrival with the tolling bells and devilish riffs that open their self-titled debut album. Perhaps unsurprisingly, Sabbath themselves balked at the "heavy metal" designation, but that hasn't stopped countless fans and fellow musicians from citing them as the genre's proper forebears.

Sabbath's thundering combination of seismic riffs and ghoulishly evocative lyrics set off a chain reaction that spawned several more heavy metal pioneers over the course of the decade. As the '70s progressed, metal bands ratcheted up the intensity, crafting increasingly heavy riffs and elaborate compositions to match their sky-high ambitions. This evolution continued into the early '80s as heavy metal reached heretofore unseen levels of speed and aggression.

More than half a century after its invention, heavy metal is still all about pushing the envelope. The brutal bands of today are worlds removed from Black Sabbath's electrified blues-rock, which sounds quaint by modern standards. But time cannot diminish the mighty roar of heavy metal's earliest movers and shakers, whom we're saluting now.

Read on to see the "Big 4" bands of heavy metal.

Below, see the Best Metal Song of Each Year Since 1970.