heavy metal vs rock music

heavy metal

noun
  • A genre descended from rock music, characterized by massive sound, highly amplified distortion, and overall loudness, often with extended guitar solos, and lyrics that involve aggressive or fantastic imagery. 

  • Any metal that has a specific gravity greater than about 5, especially one, such as lead, that is poisonous and may be a hazard in the environment. (There are many different definitions of what counts as a heavy metal; see Heavy metals for a discussion.) 

  • Great influence or power. 

  • guns or shot of large size. 

rock music

noun
  • A broad genre of popular music, including especially those styles ultimately derived from rock and roll, employing electrical amplification, heavy use of guitars, 4/4 time with strong percussive rhythm on beats two and four (backbeat), and usually themes of bold wildness, rebellion, and sexuality. 

How often have the words heavy metal and rock music occurred in a corpus of books? (source: Google Ngram Viewer )