California safety inspectors are investigating what caused the Ninja roller coaster at
Park officials and witnesses described seeing a large branch fall across the tracks at about 6 p.m., causing the partial derailment, but a spokesman for the California Division of Occupational Safety and Health, Peter Melton, said Tuesday that an entire tree -- estimated at around 40 feet high -- had actually fallen.
Magic Mountain has shut down the ride and is investigating the incident along with Cal/OHSA.
"They're trying to find the problem and get the ride back up," Melton said.
The 22 people on the ride were safely evacuated around 8:30 p.m. after being stuck on the ride for nearly three hours. Four riders sustained minor injuries, Magic Mountain spokeswoman Sue Carpenter said.
One of the riders described being hit in the head by the branch after hearing a "loud noise."
"I ducked down just in time," Jeremy Ead told KCBS-TV Channel 2. "A hard branch hit me in the head. I was there bleeding from my head."
Introduced in 1988, the Ninja is one of Magic Mountain's oldest coasters. It is known as a suspended coaster, which means the train hangs beneath the track, but it is also a terrain coaster since the cars hug close to the ground as it swoops through the trees.
Of the park's 19 coasters, it is the only one from the 1980s still in operation.
Cal/OSHA inspects permanent amusement park rides annually, Melton said.
Times staff writer Brady MacDonald contributed to this report.