Outgrowth on an ancient oak tree

Outgrowth on an ancient oak tree

Observation - Outgrowth on an ancient oak tree - UK and Ireland. Description: This very large, gnarled old oak tree had several of these warty outgrowths on its trunk. In some areas the bark had come off revealing a whitish layer underneath. What causes them?

This very large, gnarled old oak tree had several of these warty outgrowths on its trunk. In some areas the bark had come off revealing a whitish layer underneath. What causes them?