Doesn't matter. A pound of aluminum in and F-4 weighs the same as a
pound of aluminum in a 757. Energy = 1/2mV^2 At over 400 knots,
there is a lot of energy in every pound of aluminum.

It is not necessary to melt the aluminum either to make the airplane
indistinguishable. Ablation is a process in which the aluminum is
pulverized by impact with a solid object. It is essentially ground up.
If you look at the video of the F-4, you will see that he aluminum is
literally being ground up by the impact with the concrete barrier. The
Pentagon is a very substantial stone building, and the 757 still
managed to punch through several of its rings. I am not surprised that
there are not many distinguishable pieces left. Also remember that
there was a post crash fire, and aluminum does burn when it gets hot
enough.

Same thing with flight 93. It went straight into the ground at high
speed. This is unlike the U.S. Air 737 that crashed on approach due to
a rudder hard over. The airspeed at impact was much higher for these
intentional crashes than for the unintentional crashes due to starting
the dive from a much higher alitude, and going straight in in a clean
vs. dirty drag configuration...

Am I just casting my pearls here, or does this make sense to you?

Dean