only whit word is difficult ;if you send a pics is best, however if the crack is 2/3 mm in this period is normal the high temperature cause similar effect ;if the crack is more 2/3 mm is probabily or insects or (if after rain)thunder...
Have a look here: Attaching Images The important thing is to be patient and wait until the pic has loaded fully before clicking the 'post' button!
Got there at last! ;-) Looks to me most like a lightning strike; probably a fairly minor one (lightning bolt strength varies hugely), as the damage isn't severe. Not that rare a sight in forests, I've seen very similar cases many times, often old. The tree should be OK, it looks like it has already grown new bark across the split. Note that the new bark will always be black, that's normal for birches for injury response bark to be black (thus also e.g. when people carve names on birches, it stays visible for years). Lucky it wasn't a powerful bolt, or the tree might have ended up like this one did . . . http://www.pinetum.org/canon/sqgiga2.jpg
Thanks for all the help. I only recently discovered this site, and I will surely use it again - and refer friends.
Thanks for that photo of the tree - I only saw some Bluegums looking like this back in South Africa !