Pergousa is a volcanic islet that consists of weathered thick lava flows and domes of andesitic to dacitic composition, which are covered by pumice deposits of KPT from nearby Kos (they were deposited in a submarine environment, as indicated by bivalves and biogenic remains).
Its volcanic edifice with altitudes between 65 and 83m resembles in parts the pre-existence of a stratovolcano. Several monolithic blocks and pyroclastic deposits are interpreted as witnesses of a nearby collapse structure. Furthermore, the absence of pumices from Nisyros and Yali shows that the islet was completely underwater during the time of the young Plinian-type eruptions of these islands, and that it very recently uplifted to expose the KPT deposits. At the northwest coast of Pergousa, a gray, 1 m thick tuffite, rich in sponge needles, occurs.
