Fire dispatchers received a call shortly after 2 p.m. reporting a fire on Quimby Road and two others on Mt. Hamilton Road.
At 3:22 p.m., the blaze was reported at 28 acres but had grown to 40 acres sometime before 4:15 p.m.
The fires were 50 percent contained by around 3:55 p.m., said Pam Temmermand, fire prevention specialist for Cal Fire.
No structures have been threatened or lost and no injuries have been reported, Temmermand said. The cause and source of the fire is under investigation.