The Dow Jones industrial average plunged as many as 440 points in Thursday afternoon trading. It is now down more than 1,200 points since July 21.

The Standard & Poor's 500 index is down 3 percent, bringing it nearly 11 percent below its recent high of 1,363 reached on April 29.

All three major indexes are down 10 percent or more from their previous highs, a drop-off that is considered to be a market correction. A drop of 20 percent or more signifies the start of a bear market, an extended period of stock declines.

Investors are increasingly concerned about the possibility of another recession in the U.S. and a debt crisis in Europe.

"We are continuing to be bombarded by worries about the global economy," said Bill Stone, chief investment strategist at PNC Financial.

The Dow Jones industrial average was down 437 points, or 3.7 percent, to 11,455 in late afternoon trading. Thursday's losses turned the blue-chip stock index negative for the year. The S&P 500 lost 51, or 4 percent, to 1,208. The Nasdaq composite shed 116, or 4.3 percent, to 2,576.

Oil fell 6 percent to $87 a barrel on worries demand will fall because of the slowing economy. Oil had traded over $100 as recently as June 9. The yield on the two-year Treasury note hit a record low as investors sought out relatively stable investments.

The Vix, a measure of investor fear, shot up nearly 25 percent. It is up 77 percent for the quarter, which began July 1.