Each major island, (with the sole exception of Isabela, which is formed from six volcanoes joined above sea level), consists of a single large shield volcano. The oldest in the islands is found on the southeastern island of Espanola. It is about three and one quarter million years old. The youngest are found in the western islands of Fernandina and Isabela, seven hundred thousand years old
Among all the volcanoes in the Galapagos the most active are Fernandina (1 volcano) and Isabela (6 volcanoes). Although Marchena, Santiago, and Pinta have erupted since settlers came to the islands. Eruptions have occurred on many of the Galapagos volcanoes, including Fernandina, Volcan Wolf, Alcedo, Sierra Megra, Cerro Azul, Santiago, Pinta, Floreana, and Marchena. Espanola and Santa Fe have been extinct for several million years. Pinzon and Rabida are both small extinct shield volcanoes that have not been active for about 1 million years. Though Santa Cruz and San Cristobal remain active volcanoes.
Two distinct types of volcanoes occur in the Galapagos. In the west, on the islands of Isabela and Fernandina, large volcanoes with an "inverted soup-bowl". In the east, smaller shield volcanoes with gentler slopes occur. Another peculiarity of the western Galapagos volcanoes is the large size of their calderas or bowls, particularly when compared to the size of the volcano. The presence of calderas is what gives a volcano its flat top, which can be clearly seen on Alcedo, it is the result of the collapse of an underlying magma chamber. Magma within a magma chamber helps to support the top of the volcano, when magma flows out, the surrounding rock may not be able to bear the load and results in a collapse. A partial collapse of the caldera on Fernandina happened in 1968, when the northern part of the caldera floor dropped 200 metres. The collapse occurred several weeks after a brief eruption. The floor of Genovesa’s caldra is below sea level and breached on the south side, forming Darwin Bay.