Sao Miguel is the largest of the Azores islands and is the second oldest after Santa Maria, gradually building from about 4 million years ago. It is formed from two separate islands which later became joined when a low central area erupted. Both shield and stratovolcanoes built the island providing a violent history that continued into the centuries following human settlement. The last eruption was in 1630. As a consequence of this long and diverse history there are many different geological structures for the interested visitor to enjoy.