Água de Pau is a stratovolcano

Stratovolcano

A stratovolcano, also known as a composite volcano, is a tall, conical volcano built up by many layers of hardened lava, tephra, pumice, and volcanic ash. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile and periodic, explosive eruptions...

 in central part of São Miguel Island

São Miguel Island

São Miguel Island , nicknamed "The Green Island", is the largest and most populous island in the Portuguese Azores archipelago. The island covers and has around 140,000 inhabitants, 45,000 of these people located in the largest city in the archipelago: Ponta Delgada.-History:In 1427, São Miguel...

. About 30,000 to 45,000 years ago the 4 km wide and 7 km long outer caldera

Caldera

A caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption, such as the one at Yellowstone National Park in the US. They are sometimes confused with volcanic craters...

 was formed. The inner caldera, which is 2.5 km wide and 4 km long was formed about 15,000 years ago. The younger inner caldera is partially filled by the crater lake Lagoa do Fogo

Lagoa do Fogo

Lagoa do Fogo is a crater lake within the Agua de Pau stratovolcano in the center of the island of São Miguel Island in the Azores. The governmental authorities do not allow any modern construction around this lake...

. Until 5000 years ago the activity in the caldera created several post-caldera lava dome

Lava dome

|250px|thumb|right|Image of the [[rhyolitic]] lava dome of [[Chaitén Volcano]] during its 2008–2009 eruption.In volcanology, a lava dome is a roughly circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano...

s on the northern and western flank.
Some of the cinder cones placed on the flank mark radial and concentric fissures. Some of them have been active during historical time. The latest known eruption took place in 1564. Some popular hot springs are located on the northwest flank.