The 1970s was an incredibly diverse decade for recorded music: from hippie folk at the start to disco, punk, the rise of reggae and the very first stirrings of hip-hop. At the beginning of the decade, Frank Sinatra had a song on the charts for 122 weeks. There was soft rock, metal and country. Album sales and progressive radio were huge.
All this is true. That's why it is so fascinating to look at the songs that ended up at the very top of Billboard's pop chart for each year of the decade — they certainly don't always represent all the change that was going on.