Heavy metal did not arrive with a committee-approved definition. It emerged from a crowded field of blues rock, psychedelia and hard rock, then announced itself through a new combination of sound and feeling: down-tuned weight, ominous space, blunt repetition and riffs that seemed to carry their own gravity. In 1970, that combination became impossible to dismiss. The year did not simply produce a few unusually heavy records; it gave the developing genre a centre of gravity.

Black Sabbath were the decisive force. Their self-titled debut, released in the United Kingdom in February, preserved the tension of the band’s live set with very little decoration. The title track’s rain, bell and tritone created an atmosphere closer to horror cinema than the peace-and-love residue still attached to psychedelic rock. Only seven months later, Paranoid sharpened the idea. “War Pigs,” “Iron Man,” “Hand of Doom” and “Paranoid” turned war, technology, addiction and mental distress into massive, memorable songs. Sabbath sounded frightening, but never stiff: Bill Ward and Geezer Butler brought jazz movement beneath Tony Iommi’s monolithic guitar.

They were not alone. Deep Purple in Rock introduced the Mark II lineup as a precision heavy-rock machine. Ritchie Blackmore and Jon Lord treated guitar and distorted Hammond organ like competing lead weapons, while Ian Gillan expanded the vocabulary of the metal singer. Led Zeppelin III moved in another direction, placing “Immigrant Song” and electric blues beside acoustic arrangements. That contrast mattered. Heavy music could be dynamic and exploratory without surrendering its power.

Beyond the giants, a wider foundation was being poured. Uriah Heep’s debut combined organ, progressive structures and theatrical vocals. Mountain carried a huge guitar tone into American radio with “Mississippi Queen.” Atomic Rooster found darkness through keyboards, while Sir Lord Baltimore and Lucifer’s Friend pushed fuzz, speed and high-register singing toward sounds that later generations would call proto-metal. Free’s spacious groove demonstrated that heaviness could come from what a band left out as much as what it played.

The future was also assembling off-record. Queen formed in London around Freddie Mercury, Brian May and Roger Taylor, with John Deacon joining the following year. Aerosmith formed in Boston and began the long club apprenticeship that would lead to its 1973 debut. In Birmingham, guitarist K. K. Downing and bassist Ian Hill connected with singer Al Atkins, creating the early core of the band that would become Judas Priest. None had yet released the albums that made them metal institutions, but the next chapters were already moving into place.

By the end of 1970, the boundary between hard rock and heavy metal remained porous—and it should. Deep Purple, Zeppelin, Mountain and Free supplied crucial colours even when they did not fully belong to the new genre. Black Sabbath supplied the centre: a sound, mood and worldview that later doom, thrash, death, black, grunge and alternative metal bands could all recognize somewhere in their ancestry. The vocabulary was not complete, but the first true chapter had been written.