<human>:  What general rock trends occurred
<bot>: Rock music experienced many waves of popularity during the second half of the 20th century. During the early 1970s, classic rock and heavy metal became popular genres. In the 1980s, the new wave of British heavy metal and hair metal emerged, followed by the alternative rock and grunge scene of the 1990s. Following this was the classic rock revival of the early 2000s, and by the 2010s, indie rock and other alternative rock sub-genres had become popular.