SAN FRANCISCO A big-rig truck jackknifed Thursday morning on the lower deck of the Bay Bridge, blocking four lanes for more than four hours and causing massive traffic delays, authorities said.

The crash, which left the rig facing in the wrong direction but not overturned, happened about 5:10 a.m. on eastbound Interstate 80, just east of the First Street on-ramp. No other vehicles were involved.

The driver suffered minor injuries and was taken to a hospital as a precaution, the California Highway Patrol said.

The four lanes were blocked by the tractor and its trailer and a Sigalert was issued within minutes. Commuters were advised of massive delays and told to take alternative routes.

The truck was removed from the bridge about 8:24 a.m. and one eastbound lane was reopened while cleanup work was done.

The cause of the wreck remains under investigation. It was not immediately known if weather issues contributed to the crash, CHP Officer Mark Andrews said.

The Sigalert was lifted and all lanes were reopened at 9:22 a.m., the CHP said.