BOSTON — Expect stock market volatility to stick around at least another month. Investors can say goodbye to the calm they recently began to enjoy.

That’s the message from the market’s fear gauge, formally known as the Chicago Board of Options Exchange’s Volatility Index.

The VIX surged again Friday, to post a two-day cumulative spike of 64 percent.

The index tends to rise when stocks fall. In an uncertain market investors are willing to pay hefty premiums for options that offer protection from price swings of stocks in the Standard & Poor’s 500 index. Options are contracts to buy or sell a stock at a specified price and time.

The VIX reflects what investors expect the market to do over the next 30 days.

“Two weeks ago, the VIX was extremely low, and investor and trader sentiment was very complacent,” says Scott Fullman, a strategist with WJB Capital Group. “Risks are greater, and people buying options are willing to pay more.”

On Friday, the VIX closed up nearly 25 percent at 40.95 points. It traded as high as 42.15 in the morning — its highest reading since April 7, 2009—as stocks fell sharply.

The rise in the VIX came as investors appeared to focus on the European debt crisis, rather than the morning’s better-than-expected report on the U. S. jobs market. The VIX calmed later as stocks recovered most of their morning losses, but spiked again just before the close.

Friday’s choppiness was still pretty tame compared with Thursday, when the VIX surged 63 percent in the afternoon before finishing the day up 32 percent, at 32.8. It came as the Dow Jones industrial average plunged nearly 1,000 points, before rebounding to finish down 348 points.

The volatility’s chief trigger has been Greece, where risk of a government bond default threatens to ripple throughout the European Union. The crisis also could slow the pace of a global economic recovery from a recession, and crimp U. S. exports to key trading partners in Europe.

The turmoil comes after relative calm had settled in markets. As recently as last week, the VIX had been trading around its historic average of 19 points. VIX’s historic peak came in October 2008, at 89.5, in the wake of Lehman Brothers’ collapse.

As for VIX’s next moves, they could prove harder to predict than the direction of stocks. The index’s movements late this week have been far sharper than those of stocks. For example, VIX’s decline on Thursday was 10 times greater in percentage terms than the Dow’s drop by the market close.

“The volatility in VIX itself is way up, so to try and give a market outlook on VIX is very difficult,” Fullman says.