overburden vs overextend

overburden

verb
  • To overload or overtax. 

noun
  • A sterile stratum that lies above the stratum being investigated 

  • The rock and subsoil that lies above a mineral deposit such as a coal seam. 

overextend

verb
  • To apply (a term) to too many referents, by overextension. 

  • To push a pawn too far, so that it becomes vulnerable to the opponent's attacks. 

  • To expand or extend to an excessive degree, especially to do so beyond a safe limit. 

How often have the words overburden and overextend occurred in a corpus of books? (source: Google Ngram Viewer )