Hyper-velocity impacts
Quote from ESA's site:
http://www.esa.int/SPECIALS/Space_Debris/SEMZL0P256E_1.html#subhead2
This image shows the results of a lab test impact between a small sphere of aluminum travelling at approximately 6.8 km per sec and a block of aluminum 18 cm thick. This test simulates what can happen when a small space debris object hits a spacecraft.
Al sphere diameter: 1.2-cm
Al sphere mass: about 1.7 g
Impact crater diameter: 9.0 cm
Impact crater depth: 5.3 cm
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 6,000 K.
Quote from NASA's site:
http://grin.hq.nasa.gov/ABSTRACTS/GPN-2000-001643.html
The photo shows the "energy flash" when a projectile launched at speeds up to 17,000 mph impacts a solid surface at the Hypervelocity Ballistic Range at NASA's Ames Research Center, Mountain View, California. This test is used to simulate what happens when a piece of orbital debris hits a spacecraft in orbit.
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The aluminum impact, with the inner surface spalling is a lot like what a really high energy impact will cause in even the best of armors. However tanks have spall guards of Kevlar as their innermost armor layer to catch fragments, then another layer of steel to form the inside of compartments.
The flash is caused by the materials compressing and thus heating. It's the same reason the rail gun round in the other shot causes an impact explosion.
It's strange that the projectile in the the top most picture is not deformed. My guess is an unfired projectile was placed in the impact crater.