The crash happened around 10:40 a.m. on Highway 1 between Rio del Mar Boulevard and State Park Drive.
The big-rig was traveling north when it veered to the side of the highway for unknown reasons, possibly because of a blown tire, Aptos/La Selva Fire Protection District Division Chief Jeff Terpstra said.
The accident happened on an overpass, and when the big-rig went out of control its diesel tank clipped a concrete abutment, spilling fuel onto the roadway, Terpstra said.
"The diesel caught fire because of the sparks as he skidded across the bridge," Terpstra said.
The fire grew rapidly, engulfing the entire big-rig and spreading to some nearby vegetation, he said.
The driver of the big-rig was unharmed and briefly tried to put out the fire himself, unsuccessfully, Terpstra said.
Fire crews responded and extinguished the blaze, but water from the hoses carried the diesel fuel into the nearby Aptos Creek, he said.
The state Department of Fish and Game has been called in, as have personnel from the Santa Cruz County Environmental Health Department and Caltrans.
Cal Fire assisted Aptos/La Selva firefighters by extinguishing the vegetation fire, which burned less than an acre, Terpstra said.
The California Highway Patrol initially shut down traffic in both directions, but as of 1:45 p.m. all southbound lanes and one northbound lane had been reopened, Terpstra said.
He said it may be several hours before the other northbound lanes reopen.
Terpstra said the big-rig was destroyed. No other vehicles were involved in the crash.