Stock futures were plunging to start the week’s trading as U.S. oil prices topped $100 a barrel amid the U.S.-Iran conflict, raising fears higher energy prices could dramatically slow the U.S. economy. The Dow Jones Industrial Average

is coming off its biggest weekly slide in nearly a year.

Futures

tied to the Dow fell 806 points, or 1.7%. S&P 500 futures

and Nasdaq 100 futures