1998 Dodge Intrepid Coolant everywhere

Today my car started overheating. I pulled off the Interstate and hot coolant steam start billowing from under the hood. After the car cooled I was able to see that a cover between the valve covers right on the top front of the engine where the upper radiator hose meets the block had popped off allowing all of the coolant under pressure to blow out.

Could it have been the water pump went out or the thermostat stuck? Also I have no idea what that part's name is or its purpose. It's a small rectangular shaped cover with a small valve on it.

The car is sitting at a gas station off the exit and the garage mechanice says he has no idea what that part is either. I'm worried he will try to push an expensive repair on me and if it turns out to be the thermostat I kind of know what to expect.

Thanks.