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White Extrusions on Black Oaks

I have noticed some white extrusions on Black Oaks (picture attached). What could be causing that?

A)    Your picture is very similar to bark beetle activity on conifers, which produces very visible pitch extrusions. Most bark beetles are host specific, meaning that individual species of beetles will only attack specific species of trees. Interestingly, each bark beetle makes a specific pattern beneath the bark between the phloem and the cambium layer that helps to identify the beetle. 

A Google search revealed a publication by the Pacific Southwest Research Station entitled “A Field Guide to Insects and Diseases of California Oaks” (tech report PSW-GTR-197, July, 2006).