In such an impact, the pressure and temperature can exceed those found at the centre of the Earth, e.g. greater than 365 GPa and more than 6000 K.
The little divot on the backside of the plate is called spalling. Basically you have a thick plate of fairly brittle metal, and when it gets hit hard, the (compression) shockwave travels through the material without dissipating much. When it reaches the end of the plate, it can go no further and essentially 'bounces' back, turning into a pulling force, which pulls the material apart. If the impact had been a little stronger, that bottom part would have come loose entirely and could have ejected at lethal speeds.