Please explain the cause of the laser speckle pattern

[SF]

I was playing with a laser pointer and a concave mirror and I managed to get a 5mm beam to spread over a 30cm area.
http://img361.imageshack.us/img361/1884/mspaintfg0.png
http://img228.imageshack.us/img228/1866/lpuc9.jpg
http://img228.imageshack.us/img228/3581/lp0ii9.jpg

The irregular pattern remained stationary even when I moved the beam, so I guess it's due to (a) irregularities in the mirror or (b) impurities in the laser's lens?

 

[Billygoat]

I expect you are talking about a speckle pattern... Look up laser speckle in Wikipedia...