volcanism on graciosa was predominantly characterized by low explosivity, although there appear some deposits that show indications of elevated explosivity in its geological record. aerial volcanism had its beginning approximately 600,000 years ago with the formation of a shield volcano, which is still visible in the volcanic complex of serra branca (the oldest part of the island). this was followed by periods of explosivity, related to the faults that traverse the zone, forming numerous cones that constitute a major part of the serra dormida and serra das fontes region. the type of volcanism altered radically around 350,000 years ago with the formation of the central volcano (and later caldera) associated with a different magma chamber, resulting in greater explosivity and giving rise to thick sheets of pumice that layered serra branca. the formation of the santa cruz platform comes from the more recent geological volcanism associated with lower explosive basaltic eruptions resulting in numerous cones and sub-aerial deposits that cover the center and northern portions of the island. this phase evolved towards the southeast forming the caldera massif and many of the islets and submarine mounts along the coast. the last phase of the geological history of graciosa occurred 2,000 years ago, forming pico timão and the basaltic flows that are located in the surrounding areas.