He left a (modern) penny in his pocket as a "get out of jail free card" and accidentally triggered him returning to the (then) modern era.
I rewatched the movie after Shayne started posting this story. The modern penny was left in his vest pocket by accident.
The time-travel used in the movie was more a form of self-hypnosis. Shayne's time tunnel theory is much more believable. Anyway, there would be no way for Lois to show Clark her iPhone if any modern item sent her spiraling forward in time.
I was thinking that if Lois was pulled back to her time like time was a stretched rubber band....Maybe Clark's ship would have originally landed in the 1980's except for a little detour to the 1880's, so when he went into the tunnel he would have come out in 1980.
I love this theory. Brilliant!