Teenage US snowboarder Hannah Teter took gold in the women's half-pipe ahead of compatriot Gretchen Bleiler.
Teter's win was America's second half-pipe gold of the Games
The Americans looked set for a clean sweep, until Norway's Kjersti Buaas put in a great final run to push defending champion Kelly Clark into fourth.
British boarder Lesley McKenna had a nightmare day, finishing 33rd after falling on both qualifying runs.
Fellow Briton Kate Foster, 20, fared better - scoring 24.7 on her second run - but failed to qualify in 20th place.
Teter's win completed an American half-pipe double after Shaun White won gold a day earlier.
Teter, who at 19 is the same age as White, put in two scintillating performances in her final runs scoring 44.6 and then 46.4.
"I guess I was kind of distracted at the beginning," said Teter, who was only fifth in qualifying. But then I chilled out and made it. It's amazing."
Bleiler, 24, was delighted with her second place.
"The silver medal means everything to me, I've worked so hard to get here," she said.
Buaas said she had to throw caution to the wind to prevent a US one-two-three.
"I had to go big even if my tricks aren't as good as theirs," she said. "The Americans are really good and I'm happy to have a European on the podium."
McKenna - who has just returned to competing after a seven-month absence - said the Games had simply come too soon.
Despite having few chances to practice since her recovery from a shattered ankle, the 31-year-old was determined to go for broke.
"I did not come here to have a safe run and finish 16th or 17th," she told the BBC. "I wanted to get into the final, so I had to take risks and they did not pay off.
"In snowboarding you either go big, or you go home," she said. "This time I'm going home."
Foster, who enjoyed huge support from her family, was pleased with her first Olympic performance.
"It was shame I could not go for my better tricks, but I needed to get a safe run." she said.
Japanese competitor Melo Imai briefly lost consciousness after a nasty fall in the second round of qualifying.
She was flown to hospital in Turin where she was treated for a shoulder injury.