Hard rock and Heavy metal

Heavy metal and hard rock are semantically related in form of rock and roll topic. In some cases you can use "Heavy metal" instead a noun phrase "Hard rock".

Heavy metal noun - A metal of relatively high density (specific gravity greater than about 5) or of high relative atomic weight (especially one that is poisonous like mercury or lead).

Hard rock and heavy metal are semantically related in harmonized topic. You can use "Hard rock" instead a noun phrase "Heavy metal".