NEW YORK (CNNMoney.com) -- Wall Street's key volatility measure hit a more than six-month high Thursday on fears of a European debt crisis and slowing global growth.
The CBOE Volatility (VIX) index, or the VIX, jumped as much as 13% Thursday to 28.50, before pulling back a bit to close at 24.91.
The high was the greatest peak since the close of November 3 and up 81% since April 12, when the VIX was trading at its lowest level in nearly three years.
Markets were rattled Wednesday after ratings agency Moody's placed Portugal's debt ratings on review for a potential downgrade, one week after Standard & Poor's cut the country's ratings. The fear accelerated Thursday after the Dow lost 200 points, the price of gold topped $1,200 and the euro fell to a 14-month low.
Investors also continued to worry that the $146 billion aid package for Greece won't be enough to stave off bigger debt problems in Europe.
"When we finally did address the Greece situation, the market doesn't believe it or just thinks that there's another shoe about to drop," said Joseph Saluzzi, co-head of equity trading at Themis Trading.
Greece is a member of the so-called PIIGS, a collection of five debt-laden European nations. Many investors fear that Spain and Portugal could be two of the next countries to need financial assistance. Italy and Ireland are the other two countries.
"This is not an isolated situation, because if it was just one country, the market would get over it," Saluzzi said. "It looks like we're now going to see the ramifications of this."
But it's not just concern over sovereign debt that's causing the VIX to surge, said Saluzzi.
"At this point there are just so many issues out there," he said. "You can pick anything, whether it's any country in Europe, the oil spill, volcanic ash or Goldman Sachs -- the market is having a hard time."
Given these lingering fears, Saluzzi said he wouldn't be surprised if the index continued to climb in the next few weeks, meaning stocks may be in for a rough ride. 
| Overnight Avg Rate | Latest | Change | Last Week |
|---|---|---|---|
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| 15 yr fixed | 2.80% | 2.76% | |
| 5/1 ARM | 2.60% | 2.58% | |
| 30 yr refi | 3.65% | 3.61% | |
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Today's featured rates:
| Index | Last | Change | % Change |
|---|---|---|---|
| Dow | 15,386.40 | 51.12 | 0.33% |
| Nasdaq | 3,497.92 | 1.49 | 0.04% |
| S&P 500 | 1,669.62 | 3.33 | 0.20% |
| Treasuries | 1.97 | 0.00 | 0.15% |
| Company | Price | Change | % Change |
|---|---|---|---|
| Bank of America Corp... | 13.51 | 0.08 | 0.60% |
| Microsoft Corp | 35.08 | 0.21 | 0.60% |
| Sprint Nextel Corp | 7.29 | -0.03 | -0.41% |
| Advanced Micro Devic... | 4.10 | 0.03 | 0.74% |
| Ford Motor Co | 15.05 | -0.03 | -0.20% |
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