Blown head gasket, likely cause????

Are those "pits" on the quench domes? If yes , Id go with detonation.Detonation creates huge pressure spikes in the chambers.No 5 beside it looks to be failing too. Also,if those are detonation pits, you might consider checking the rod bearings, and the other head gasket as well.And finally, do something with the timing to prevent a subsequent failure.

But if those are not detonation pits on the quench-domes,then I guess the usual reasons; inferior gasket,lack of sufficient fastener torque/or retorque,a too-smooth surface machining,or, too much cylinder pressure.
That is a "spectacular" failure. lol. Did the hard surfaces survive?