CORRECTION (Published 9/11/2013)

A story incorrectly identified the management of the Mount Diablo Summit Visitor Center. The center is part of the Mt. Diablo State Park and thus part of the California State Parks system. Save Mount Diablo is a land acquisition agency unaffiliated with Mt. Diablo State Park and does not own, manage or staff the visitor center.

CLAYTON -- Fueled by thick brush on a mountain that hasn't burned significantly in years, a wildfire tore through Mount Diablo State Park for a second day Monday, more than tripling in size as hundreds of firefighters arrived to battle the increasingly stubborn blaze. But by Monday evening officials were more optimistic about the possibility of an earlier-than-expected containment.

Flames chewed through nearly 4,000 acres of mostly remote parkland, sending a plume of smoke that could be seen throughout the Bay Area, but officials said residents in communities near the mountain -- including Clayton and Danville -- had little to worry about.

About 100 homes were evacuated Sunday, with many rural residents scrambling to find places to house pets, horses and livestock. No evacuations were ordered Monday, though the Mount Diablo Summit Visitor Center and nearby wooden observation decks were threatened, along with telecommunication towers.

Fire officials said as of 8 p.m. Monday that the blaze had just "stopped," and that the total number of acres consumed had not grown since the afternoon.

"The total acreage has gone up dramatically today," said David Shew, staff chief for Cal Fire. "It's turning into a more difficult fight than we thought yesterday but we're throwing more resources at it."

For now, he said, "The fire is staying where we want." But he added: "Something could always go wrong."

The Morgan Fire, as it is being called, began as a grass fire Sunday at 2 p.m., grew to 1,500 acres by Monday morning, and exploded to 3,718 acres by midafternoon. Only 20 percent contained, it was expected to burn throughout the night, officials said. The cost of the fire had reached $580,000 by Monday night, said Steve Kaufmann, a spokesman for the fire's incident management team.

It's unclear what sparked the fire, which officials are investigating.

A combination of hot weather, occasional winds and thick brush that has seen only limited control burns in recent years is stoking the fire, Shew said. About 700 firefighters, some of whom were fresh off a days-long battle with the Rim Fire in Yosemite National Park, arrived from throughout the state as reinforcements Monday. Their goal for the day was to build a containment ring around the fire to either box it in or let it burn itself out, Shew said.

Firefighters in the area face temperatures Tuesday that could reach 95, with winds expected to be 10-20 mph.

A base camp has been set up for firefighters at Camp Parks, a U.S. Army mobilization and training center for reserve forces in Dublin.

Images of the fire from overnight show a seemingly volcanic mountain top, with flames and smoke shooting from the peak of Mount Diablo, as trucks pulling horses snaked up Morgan Territory Road and back carrying animals to safety zones at Heather Farm in Walnut Creek and the Contra Costa County Fairgrounds in Antioch.

Equestrians from around the area took in horses of residents who had to leave their ranch property behind.

"The horse community is really pretty tight," said Larry Sterling, as he fed hay and carrots to his new charges. "All over Clayton, you've got horse trailers hooked up and ready to go, ready to help people."

A few dozen residents showed up at the Red Cross evacuation center at the Clayton Community library Sunday night, but other residents slept overnight in trailers.

"We had a neighborhood sleepover," Valerie Sterling said with a smile.

Scott Arnott, who was evacuated from his five-wheel trailer on Curry Canyon Road, moved from Vacaville to unincorporated Clayton about three weeks ago.

"I might not have it now," he said of the trailer. "I moved up here for a fresh start and now this."

Displaced residents were told they could return home Monday to collect belongings and pets, though officials discouraged them from staying in the area.

Though there have been limited burns in recent years near Mount Diablo, officials said, this fire is the largest in recent memory. There have been some 100 wildfires on Mount Diablo since 1891, with the largest one burning some 25,000 acres in 1931, according to the conservation group Save Mount Diablo.

Many longtime locals remember the 6,000-acre fire on Aug. 2, 1977, the second-largest recorded fire. Former Mayor Pete Laurence said he was driving on Kirker Pass Road when he saw a lightning bolt strike the mountain and spark the fire. It burned for more than 82 hours, leaving the mountain "a battlefield with charred skeletons of trees," according to a report in the Oakland Tribune at the time.

"It was worrisome because Mother Nature was having her way with the mountain," Laurence said.

Richmond Fire Department's Merlin Turner arrived at the Morgan Fire on Monday, a week after returning from fighting the Rim Fire, and has had two days off in the past three weeks.

"When you work, you don't know how tired you are," Turner said. "At some point, I'll probably sleep for a couple days. I'll just go crash."

Added Mark Tait, 42, a father of two and a 15-year firefighter from Danville: "It's tough on them. You go from a two-parent house to suddenly it's a single mom; it changes things for everybody and it's hard." He then headed out to fight the fire.

David DeBolt covers breaking news. Contact him in Richmond at 510-262-2728. Follow him at Twitter.com/daviddebolt. Staff writers Eve Mitchell and Erin Ivie contributed to this report.