U.S. stocks tumbled, erasing the week’s gain in the Standard & Poor’s 500 Index, as the Federal Reserve’s warnings over the global economic outlook rippled across financial markets.

The S&P 500 lost 1.6 percent to 1,958.13 at 4 p.m. in New York. Banks took a hit for a second day, wiping out a rally from earlier in the week, while energy shares plunged with the price of oil. The VIX, a measure of market volatility, climbed for the first time in four days.

Equity volatility may have increased as some futures and options on stocks and indexes expire today in a process known as quadruple witching. Trading in S&P 500 companies was 50 percent above the 30-day average.

The operator of the S&P 500 will also rebalance the index in a quarterly move to adjust member weightings. About $41 billion of shares was expected to be traded as investors buy and sell stocks to mimic the changes, according to a Sept. 8 estimate by Howard Silverblatt, an index analyst at the New York-based S&P Dow Jones Indices.

Equity markets have been turbulent amid concerns about a China slowdown and the Fed’s intentions. The Chicago Board Options Volatility Index endured its biggest weekly gain on record in August, and has closed above 20 for 20 straight sessions, the longest stretch since June 2012. The VIX jumped 11 percent to 23.45 today.

Source: Bloomberg