open-pit mine vs strip mine

open-pit mine

noun
  • A type of mine where rock or minerals are extracted from the earth by their removal from an open pit or borrow. 

strip mine

noun
  • A mine where the material above the ore is removed (stripped) to expose it for removal and use. 

How often have the words open-pit mine and strip mine occurred in a corpus of books? (source: Google Ngram Viewer )