The blaze in Mount Diablo State Park in Contra Costa County spread to 3,718 acres or nearly 6 square miles Monday afternoon, more than double the 1,500 acres reported in the morning, according to the California Department of Forestry and Fire Protection. It was 20 percent contained.
Officials said hot temperatures and wind gusts fueled the fire's spread.
Officials ordered the evacuation of 25 homes Monday morning, bringing the total to about 100.
At one point, the fire threatened electrical transmission lines, communications infrastructure, and a historic lookout and visitor center at the top of the 3,848-foot Mount Diablo. The Summit Museum was constructed in the 1930s of sandstone from the park.
The blaze broke out Sunday amid nearly triple-digit temperatures in the early afternoon. The cause was under investigation.
With his pickup truck already packed, Tom Fenner closely monitored the fire's movement from the roof of his house to decide exactly when to leave.
"I knew this was the big fire they're always talking about, that if a fire got onto the hill with all the fuel that was on the hill, that we were going to have a huge fire," Fenner, 53, said.
The fire spewed a plume of smoke visible for miles. It was burning in steep, rugged terrain near Clayton, a town of about 11,000 people northeast of San Francisco, alongside the park.
In addition to the difficult terrain, firefighters faced erratic winds and continued high temperatures. But they were confident they would get the blaze under control, Cal Fire spokesman David Shew said.
"We'll get it, but it will probably be a few days," he said.
Some 800 firefighters, aided by two air tankers and three water-dropping helicopters, were battling the blaze. Three firefighters suffered minor injuries.
The Bay Area Air Quality Management District issued a smoke advisory Monday for parts of Contra Costa, Alameda and Santa Clara counties. Residents were advised to take precautions, including setting air conditioning units and car vent systems to recirculate.
Elderly people, children and those with respiratory illnesses were told to be particularly careful.