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Materials Science and Technology Dictionary from Chambers: "spalling: The flaking-off of material from a surface due to eg impact (as in flint knapping), expansion of sub-surface material (see rusting), ingress of water and subsequent freezing (common in stonework), etc."
In the context of cracking you've given so far, I'd say the spalling is being caused by a number of surface cracks growing together until small pieces of material at the surface become friable.