There are lots of videos on YouTube showing sodium, potassium, etc. exploding when dropped into water (this, for example).
I understand that when an alkali metal is exposed to water, a violent exothermic reaction occurs where a hydroxide and hydrogen gas are produced, but why and how does the sample of metal end up detonating and fragmenting? Physically speaking, how can a block of metal seemingly blast apart from the inside when the reaction occurs on the surface of the sample?
The Wikipedia article on alkali metals explains this, but I still don't seem to understand how this would result in an explosion. Diagrams would be very helpful.
The key of the question is how does the release of hydrogen gas and energy result in an explosion? Is there thermal runaway, and is the metal physically destabilized in some way during the reaction?