Phantom Ship (left) is an ancient volcanic plug from a volcano that preceeded Mt. Mazama, the volcano that exploded to create Crater Lake. Phantom Ship's volcanic composition is harder than Mt. Mazama's, which is why it erodes more slowly and remains a prominent outcrop. Pumice Castle, on the other hand, is pumice rock that erodes at a different rate from the surrounding rock, leaving a castle turret-like appearance.

Left, late evening along the Rim Drive. Above, a close up view of Pumice Castle.