How did Crater Lake, Oregon form? Thousands of years of erupting pyroclastic material from the volcano created a closed wall around the primary cone. Then that space was filled with water eventually, and the former cone is known as Wizard Island. About 1500 meters of summit of a composite volcano collapsed after an explosive eruption of about 50 cubic kilometers of pyroclastic material. Later the caldera was eventually filled with water. About 1500 meters of summit of a shield volcano collapsed after an explosive eruption of about 50 cubic kilometers of pyroclastic material. Later the caldera was eventually filled with water. About 1500 meters of summit of a composite volcano collapsed as a result of a gradual subterranean drainage of the magma chamber under the volcano leaving the summit unsupported. Later the caldera was eventually filled with water. About 3500 meters of summit of a composite volcano collapsed after an explosive eruption of about 500 cubic kilometers of pyroclastic material. Later the caldera was eventually filled with water.