obstacle vs obstruction

obstacle

noun
  • Something that impedes, stands in the way of, or holds up progress, either physically or figuratively 

obstruction

noun
  • Something which obstructs or impedes, either intentionally or unintentionally 

  • The act of obstructing, or state of being obstructed. 

  • The condition of having the natural powers obstructed in their usual course; the arrest of the vital functions; death. 

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