The Park Service stated that the rock was near the Weeping Rock Silo stop shortly before 6:00 PM, injuring three people and injuring several others.
One person was transported to the hospital with an ambulance at the park for further treatment, the release said.
The rock broke Mount Cable, about an hour east of St. George's in the southwest corner of the state, and fell about 3,000 feet.
The shuttles were stopped for about 90 minutes after the cliff, which showered visitors with rocks, branches and a stream of dust and sand.
Some visitors along the Weeping Rock Trail were temporarily stranded on the cliff, but managed to escape.
Rocks hit the top of a vehicle near the East Tunnel of the popular Mountain Road to the Sun, killing the teenager and injuring his parents and two other children in the car.  The park estimated that the rocks were fist-sized and 12 inches in diameter with enough debris to fill the pickup bed.