There's an old article Mike Snyder in the Winter 2003 issue of Northern Woodlands that has a great look at the myriad ways trees can get scars like this. The article isn't available online, but UVM has it in their library. I've heard it on Camel's Hump before and it's incredible. It's hard to tell the difference between a frost crack erupting and a gunshot!
http://northernwoodlands.org/issues/issue/winter-2003
