A head-on crash—the second within 24 hours—injured a Boone woman, tying up Friday mid-morning traffic on US 321 between Blowing Rock and Boone.  The crash occurred right at 10am near the Aho Road intersection with the small Saturn sedan of Crystal Walker and a large tractor -trailer colliding head-on, both ending up in the south-bound lane.  Melvin Terry of Morganton was pulled off of the road nearby when the wreck occurred less than 100 yards from his vehicle...Blowing Rock Fire/Rescue arrived to find Crystal Walker  pinned in the wreckage of her Saturn, and they had to cut the roof from the vehicle in order to safely remove her.  In spite of the damage, the woman suffered a serious cut to her leg but few other injuries. The trucker was not injured in the crash, the woman transported from the scene by Watauga Medics to Watauga Medical Center.  The State Patrol investigator, Trooper M.A. Grogan said that there were unsafe tires on the Saturn that caused Walker to hydroplane as she headed toward Blowing Rock, skidding into the path of the north-bound tractor trailer.  Crystal Walker was cited for unsafe tires and exceeding a safe speed.