(Updates with 30 homes destroyed)
By Laila Kearney
SAN FRANCISCO, Sept 10 (Reuters) - A fast-moving wildfire ripped through rolling hills and ranch land in rural northern California on Tuesday, after destroying 30 homes overnight and prompting more than 500 area residents to evacuate, fire officials said.
The so-called Clover Fire, which broke out on Monday afternoon, has already scorched some 7,400 acres near the rural community of Happy Valley in Shasta County, some 200 miles north of San Francisco, according to the California Department of Forestry and Fire Protection.
Temperatures above 100 degrees have made for tough conditions on the fire lines, and four people, three of them firefighters, have been treated for smoke inhalation, CalFire spokeswoman Teresa Rea said.
In addition to the 30 homes considered total losses, at least another 50 outbuildings had been damaged or destroyed by the blaze, named after its place of origin, Cloverdale Road near Happy Valley. Officials had earlier said 80 structures were destroyed and 30 more damaged.
Some 300 homes remained under threat, prompting the evacuations and a number of road closures in the area. The fire was 40 percent contained as of Tuesday afternoon.
CalFire spokesman Mike Witesman said the blaze was moving quickly over hilly, rocky terrain and consuming dense vegetation, including grass, shrubs, oak and pine trees.
"Because of the very dry weather we've had, the conditions are very volatile," he said, adding that on Monday the flames had spread at a rate of about 500 acres per hour.
More than 1,120 firefighters were on the ground on Tuesday, digging containment lines around the blaze, while air tankers dropped water and fire retardant on the flames.
SPOT FIRES ERUPT
Crews have shoveled and bulldozed lines around the entire perimeter of the conflagration, one of dozens of wildfires that have spread across the drought-parched U.S. West in recent months, straining national firefighting resources.
Because of the dense vegetation and dry landscape, the fire can gain momentum and send out embers that create smaller fires outside of containment lines, and it was not clear if the lines will check its spread, Witesman said.
"We don't consider it contained until we're pretty sure that it's going to hold," Witesman said. "Yesterday, spot fires were a significant problem."
He said the fire had also created its own winds, which allow it to spread more quickly, adding to already windy weather in the area.
The Clover fire was burning about 200 miles north of a mountain blaze east of San Francisco that forced the evacuation of about 100 homes at the edge of the town of Clayton on Sunday.
The so-called Morgan Fire, which has blackened some 3,200 acres in and around Mt. Diablo State Park, was 45 percent contained as of Tuesday morning, according to a CalFire website, and cooler temperatures were helping crews make progress.
The state park, about 20 miles inland from the eastern edge of San Francisco Bay, has been closed to visitors.
Meanwhile crews were still at work on the massive Rim Fire, which has been burning in and around Yosemite National Park since mid-August.
That fire, believed to have been sparked by a hunter's campfire, has blackened nearly 400 square miles of timber and dry brush, making it the third-largest California wildfire on record. (Writing and additional reporting by Dan Whitcomb; Editing by Cynthia Johnston, Bernadette Baum and Steve Orlofsky)