Since it split, your expansion tank must have been over-pressurized when the car was running. I am curious to know whether you noticed if it were also pressurized after you stopped. I recently noticed that my expansion tank is pressurized for a while after stopping. I don't recall this being the case previously, so I'm not sure if this is a recent development or not. I have a new thermostat and gasket which I'm replacing this week, but I'm wondering if I shouldn't be looking for something else.

Ditto on the blown head gasket diagnosis.

/// Not if you install a thermostat with a air bleed hole in it. There is no bleeding necessary. In older times an aspirin tablet was put into the thermostat to allow air to escape the block. It would dissolve when the system filled. There should be a small hole (less than 1/16th inch) in your new thermostat. There may be a weight through it to keep the hole from plugging up.

A word of caution. The heater hoses are at the back of the engine and usually overlooked. If there is a hole in one of these it will allow the cooling system to slowly drain down and the coolant will finally boil. The hoses will eventually fail and drain the system quickly. Check the car by running it over a clean dry spot that will reveal any leak no matter where it is.

This sounds a lot like a blown head gasket. A compression test or analysis of dissolved gases in the coolant is in order. To some people this is the death knell for a car. To others it means a busy weekend with wrenches (not wenches) but only $50 for a gasket and maybe more for the motor mounts, o-rings, pipes and sundry things you find when you are in there. At worst $300 for a re-built head. No way would I retire a good Brick for a little thing like a blown head gasket. And the cars run like new with a rebuilt head, I think.