The volcanic grabens are elongated depressions by sectors slaughtered between normal fissures, where volcanic and tectonic phenomena act together.
They are limited by scarps of normal fissures more or less parallel and extensive, which often occur in even number and are arranged laterally relative to the central sector more reduced.
The grabens of Pedro Miguel (Faial Island), of Praia da Vitória (Terceira Island) and of Mosteiros (São Miguel Island) are examples in the Azores.
These depressions are associated with drainage of magma from the magma chamber of polygenetic volcanoes, grabens neighbours, and strains affecting the volcanic edifices. In some cases, probably for most grabens, these volcanic and tectonic factors interact and influence the formation of these depressions.