A key measure of expected volatility in the stock market surged to its highest level in more than four years on Monday morning as global equities fell sharply.

The Cboe Volatility Index, or VIX, briefly broke above 65 on Monday morning, up from about 23 on Friday and roughly 17 a week ago. It had cooled to about 32 shortly after noon ET.

The Monday morning peak was the highest level the VIX has hit since March 2020, shortly after the Federal Reserve’s emergency actions during the Covid-19 pandemic, according to FactSet. The VIX rose as high as 85.47 in March 2020, according to FactSet.

The VIX is calculated based on market pricing for options on the S&P 500. It is designed to be a measure of expected volatility over the next 30 days, and is often referred to as Wall Street’s “fear gauge.”

Jim Carroll, senior wealth advisor with Ballast Rock Private Wealth, said that the size of the initial move on Monday could be a reflection of traders adding protection — or shifting out of short-term options into longer-term ones — during a period when the market struggled to meet that demand.

“It seems obvious, sitting where we sit now with VIX at 34-35 instead of 65, that we say an outside-of-regular-trading-hours squeeze. People were after those puts. They were chasing them. They couldn’t get them. They kept chasing them,” Carroll said late Monday morning. Put options are a type of derivative that traders use to add downside protection to their portfolios.

Since the Covid sell-off subsided, the VIX has been subdued, often trading below its longer-term average of 20. Some market experts have speculated that the proliferation of other types of derivatives, including increased trading in zero-day-to-expiration contracts, could be contributing to that.

While spikes in the VIX often coincide with deep market sell-offs, they can also be short-lived and precede a rebound for stocks.

“You have to watch the VIX. When the VIX peaks and starts to roll over and fall down, the recovery can be just as quick,” Fundstrat head of research Tom Lee said Monday on CNBC’s “Squawk Box.”

However, Carroll pointed out that the index also cooled during Friday’s session, only to spike over the weekend, and said Monday’s moves should not be viewed as an “all-clear” signal.

“If you just chopped off the long-tail at the top of the VIX candle right now and said, ‘Hey, we’re at 35.’ We’re at 35 relative to 23 on Friday’s close — that’s still a big move,” Carroll said.