Building on the innovations of psychedelic rock and blues rock in the 60s, hard rock developed in the late-60s into the 70s with bands such as Led Zeppelin and Deep Purple expanding the harder, riff-oriented sound. These bands continued to define the sound of hard rock with the records Led Zeppelin IV in 1971 and Machine Head in 1972. Around this time bands such as the Rolling Stones and The Who were fusing more hard rock elements into their writing with albums such as Exile on Main St. Hard rock became extremely popular and bands started to fuse hard rock elements with other genres, giving rise to a plethora of now iconic rock groups. AC/DC took blues roots and added extremely loud, distorted guitars playing thunderous power chord-based riffs, leading to hits such as Back in Black.