(CBS) — Stocks plunged on Wall Street Monday, with all three of the major indices closing down more than 3.5 percent, as overseas worries and valuation concerns shook equity markets around the globe.

The S&P 500 closed down 78; the Nasdaq dropped 180, and the Dow Jones Industrial Average shed 588 points, settling to 15,871.

“The sell-off on Friday was pretty dramatic and caught a lot of investors by surprise. Instead of gathering their senses over the weekend, their anxiety seemed to take over and many decided to sell this morning at the opening bell. That was exacerbated by the relative lack of liquidity due to a lot of traders on vacation,” says Jack Ablin, chief investment officer at BMO Private Bank.

He adds that he expects to see continued improvements in the overall U.S. economy and anticipates liquidity will pick up when traders come back from vacation.