Even though Deep Purple had been an active band since 1968 — and enjoyed quite a bit of success — it’s fair to say that they didn’t truly arrive until on June 3, 1970, when they collectively chiseled their claim to musical immortality on their fittingly titled fourth album, In Rock.

Taking a broader view of that period, Deep Purple’s new offering proved to be yet another seismic rumble signifying the rising tide of heavy metal bands (Black Sabbath, Uriah Heep, Trapeze, etc.) appearing in 1970, but this was hardly accidental.

Instead, it was a calculated change of direction undertaken by savvy veteran musicians based on the success of Led Zeppelin, who, a year earlier, had proven that heavy rock could sell millions of albums. It awakened Deep Purple’s leader, guitarist Ritchie Blackmore, into a sense of not so much inspiration as competition.

Like Zeppelin mastermind Jimmy Page, Blackmore had made his name as a six-string session ace, working with producer Joe Meek for much of the ‘60s while filling his live performing resume with artists like the Outlaws, Neil Christian’s Crusaders and Screaming Lord Sutch’s Savages before forming Deep Purple in 1968.

After enjoying some success with a cover of Joe South’s “Hush,” and then a relative period of sales decline and creative stagnation, Blackmore and his Deep Purple bandmates – organist Jon Lord and drummer Ian Paice – decided to jettison singer Rod Evans and bassist Nick Simper in exchange for Ian Gillan and Roger Glover, respectively, to establish the band's classic MK. II lineup.

The quintet devoted the second half of 1969 getting more familiar with their individual talents during rehearsals before indulging Lord’s classical ambitions on the historic Concerto for Group and Orchestra project. But this one-off concert, and its resulting LP, almost seems like a diversionary tactic designed to disarm listeners before the work that would define Deep Purple’s career.

Listen to Deep Purple Perform 'Child in Time'

In Rock’s opening number, “Speed King,” assaults unsuspecting fans with one of the heaviest and fastest metal numbers ever committed to wax up to that point. U.S. consumers were deprived of Blackmore’s blistering introductory barrage, but Gillan’s initial screams, intoning assorted Little Richard lyrics, suggested that the band saw this as a virtual reinvention of rock 'n' roll.

The same unrestrained aggression consistently permeates songs like “Flight of the Rat,” “Into the Fire” and “Hard Lovin’ Man” – all delivered by Blackmore’s riffs and solos, Gillan’s glass-shattering voice, Lord’s fleet-fingered keys and Paice’s thunderous percussion (all while Glover keeps the pulse pumping away).

Even comparatively calmer numbers like the groovy “Bloodsucker” and the unusually straightforward “Living Wreck” never allow the overall intensity to flag, and the album’s only throwback to Deep Purple’s psychedelic past, “Child in Time,” counters every gentle melody and Age of Aquarius musing with bombastic crescendos and apocalyptic visions of impending destruction.

Together, these songs make up one of metal’s most defining and oft-copied statements, and resulted in the band’s biggest sales yet. In Rock rose to No. 4 in the U.K. (and remained in the Top 10 for months), reached No. 1 in Germany and made the Top 10 across Europe, paving the way to a world tour that lasted more than a year. (The album didn't fare as well in the U.S., climbing to only No. 143.)

In Rock set the tone and template for Deep Purple for years to come. Over the next half-decade, they became one of the planet’s most successful bands in the studio and on the road. And as far as musical monuments go, In Rock ranks among the most imposing and career-redefining in classic rock.

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