Mount Blackburn represents the heavily eroded core of a shield volcano. Because it is shrouded in permanent ice, its internal structure cannot be determined. It is believed to have a summit caldera, greatly modified by glaciation. The oldest rocks in the area are granites, about 4.2 million years old, representing an intrusive mass. The majority of the mountain is 3.4 million year old granite, intruding between andesite flows. From this it is inferred that a caldera collapse took place between 4.2 and 3.4 million years ago, after which activity ceased.