India, Oct. 12 -- A months-long calm on Wall Street shattered Friday, and U.S. stocks tumbled after President Donald Trump threatened to crank tariffs much higher on China.

The S&P 500 sank 2.7% in its worst day since April. The Dow Jones Industrial Average dropped 878 points, or 1.9%, and the Nasdaq composite fell 3.6%.

Stocks had been heading for a slight gain in the morning, until Trump took to his social media platform and said he's considering "a massive increase of tariffs" on Chinese imports. He's upset at restrictions China has placed on exports of its rare earths, which are materials critical for manufacturing everything from consumer electronics to jet engines.

"We have been contacted by other countries who are extremely angr...