The fire in the Altamont Pass area was 80 percent contained as of about 1 p.m., according to Cal Fire.
Cal Fire reported at about 12:30 p.m. that firefighters were battling two 25-acre fires. The two fires combined into one and spread.
Cal Fire spokeswoman Lynne Tolmachoff said the fire has spread because the area is grassy and windy. Tolmachoff said the area frequently burns this time of year.
"This year is no different," Tolmachoff said. .
Tolmachoff said the cause of the fire is still under investigation.
The two fires have been combined to one. The #ChristensenFire (Alameda County) is now 242 acres and 80% contained.— CAL FIRE (@CAL_FIRE) May 28, 2015