Christina O'Haver, The Union Democrat

Gas prices throughout California have skyrocketed nearly 60 cents over the past month - one of the largest month-to-month spikes in the past decade, AAA said.

The average price of unleaded gasoline statewide was $4.22 per gallon Friday, which is a 12-cent increase from the week before.

AAA does not provide specific numbers for Tuolumne and Calaveras counties, but prices at Sonora gas stations ranged from $3.93 to $4.23 on Monday, according to

Prices in Angels Camp ranged from $4.09 to $4.28, the website showed.

California has the second highest average gas price in the country, trailing Hawaii, according to AAA.

However, California was not listed by AAA last week as one of the 10 states with the highest month-to-month increase.

The statewide surge reflects an uptick across the United States.

The national average was $3.80 on Sunday - a 20-cent jump over the past two weeks, according to the Lundberg Survey of fuel.

Experts attribute the escalating prices largely to changing trends at oil refineries.

Many refineries have already started converting from winter to summer blends, which emit fewer harmful vapors, AAA Northern California spokeswoman Cynthia Harris said.

Converting to the more expensive summer blends usually occurs in mid-March, she added.

Why the change?

"There's a lot of speculation, but there's no real concrete answer to that," Harris said.

She said one possible reason for switching to the summer blend earlier this year is that parts of the country have seen a mild winter season.

Refineries are also shutting down to conduct spring maintenance earlier than normal, temporarily limiting fuel supply and driving market prices higher, according to AAA.

The Associated Press contributed to this report.