<p>Pisani: Uptick in volatility</p> <p>CNBC&#039;s Bob Pisani reports on heavy volume in the markets, and which sectors are the weakest. </p>

U.S. stocks plunged nearly 2 percent or more on Monday with the Dow and S&P 500 wiping out gains for the year on escalating tensions between Greece and its creditors. ( Tweet This )

"I think the uncertainty around Greece is just making people skittish and we haven't had a meaningful pullback for some time," said Ben Pace, chief investment officer at HPM Partners. "It's hard for anybody to fight this uncertainty."

"It should be a reasonably isolated situation but we don't know that for sure," he said.

The Dow Jones industrial average closed 1.95 percent lower, falling below its 200-day moving average, with Visa and DuPont leading nearly all blue chips lower. The index is 1.27 percent lower year-to-date.

The Nasdaq Composite fell more than 120 points, or 2.4 percent, to end below 5,000 for the first time since May 13, with biotechs and Apple both more than 1 percent lower. The last time the index dipped below the psychologically key level was June 15.