The caldera complex at the peak of the volcano is made of at least six overlapping calderas and caldera segments (pictured). Calderas are formed by roof collapse following depletion and withdrawal of the subsurface magma chamber after an eruption. Each caldera thus represents a separate pulse of volcanic activity on the mountain. The largest and oldest caldera segment appears to have formed as a single, large lava lake. Using geometric relationships of caldera dimensions from laboratory models, scientists have estimated that the magma chamber associated with the largest caldera on Olympus Mons lies at a depth of about below the caldera floor. Crater size-frequency distributions on the caldera floors indicate the calderas range in age from 350 Mya to about 150 Mya. All probably formed within 100 million years of each other.