SALT LAKE CITY (AP) -- A tractor-trailer carrying 35,500 pounds of explosives overturned and exploded Wednesday, injuring four people and leaving a huge crater on Utah Highway 6 in Spanish Fork Canyon.
A motorist and a passenger in the truck's cab were among those transported to hospitals immediately after the 1:54 p.m. accident, Utah Highway Patrol Sgt. Todd Royce said.
The explosion left a crater in the two-lane highway estimated to be between 60 feet and 80 feet wide and between 20 feet and 35 feet deep, Utah Department of Transportation spokesman Tom Hudachko said.
''The entire road is gone, shoulder to shoulder, there's no asphalt left,'' he said, adding that much of the earth that supported the road's westbound lanes was also displaced in the explosion.
It is believed the truck accident initially started a fire, which triggered the explosion, Hudachko said.
LDS Hospital spokesman Jess Gomez said two people in critical condition were taken by medical helicopter to Utah Valley Regional Medical Center in Provo. A third was brought in by ambulance in satisfactory condition.
The truck's driver, a 30-year-old man, was transported by helicopter to University of Utah Hospital, spokesman Chris Nelson said. He was alert and talking to medical staff when he arrived at the hospital, but no official condition status was available, Nelson said.
The driver was able to get out of his tractor-trailer and warn other motorists away from the truck before the explosion.
J.D. Herbert of Denver said he also was trying to warn motorists about the truck accident when the explosion blew him off his motorcycle.
''It's truly just a powerful force that kicks you right in the butt,'' he told Salt Lake City television station KUTV.
After being knocked off the motorcycle, he said he looked up ''at a mushroom cloud of fire, and shrapnel just starts falling down.
''The shrapnel is hitting the forest, and crackling like bacon,'' said Herbert, the nephew of Utah Lt. Gov. Gary Herbert.
The canyon about 60 miles south of Salt Lake City was closed in both directions.
It was unclear how the accident occurred, Royce said. He said the truck was ''pretty much vaporized'' in the explosion and that that both lanes of the highway had been gutted by the blast.
The accident happened in an area about 20 miles east of the mouth of the canyon known as the Red Narrows. Nearby rail lines also were damaged.
Several small fires in the hills above the accident scene were believed to have been triggered by flying debris.
Firefighters from the Uinta National Forest were staged in the canyon waiting for clearance from the Utah Highway Patrol to enter the area and attack the fires, forest spokeswoman Loyal Clark said. She said the multiple fires were burning in an area of about 50 acres of sagebrush and oakbrush.
The highway patrol was trying to make sure there were no unexploded ordnance among the debris scattered in the explosion before allowing firefighters into the area, she said.
The rig from R&R Trucking of Duenweg, Mo., had just left commercial explosives maker Ensign-Bickford Co. at the mouth of Spanish Fork Canyon when the accident happened. The truck was headed to Oklahoma, company officials said. They wouldn't say what type of explosives the truck was carrying.
Hal Jaussi, an Ensign-Bickford manager, said the trucking company ''met federal regulations for transporting explosives.''
Trucking company controller Doug Greek said the company couldn't comment on the accident because they were ''still trying to investigate everything that's happened.''
The explosion forced the closure of the highway in both directions, something Hudachko said would remain in place until repairs could be made. More than 6,800 motorists use the highway daily, 19 percent of which are tractor-trailers.
''It's going to be a significant repair job, so it's going to take a while,'' Hudachko said.
Like firefighters, highway engineers were awaiting clearance from public safety officials to enter the area, he said.