Officials say crews were conducting tests on the pipeline in the area when the incident happened around 4 p.m. No one was injured in the explosion, but the hillside was damaged.
California Highway Patrol Capt. Mike Maskarich says the vapor that was released from the explosion is not harmful.
"There was a lot of pressure that had come through that line," Maskarich told ABC7 News, adding that the side of the hill near the freeway had "a significant amount of erosion."
Authorities are advising motorists avoid the area and use US Highway 101 as an alternate route.
Three lanes on I-280 have reopened, but a fourth lane remains closed while crews continue the cleanup.