Volcanism in the Madeira Islands
The Madeira Islands are an example of hotspot volcanism in an oceanic environment and arose from the action of two mantle plumes. One created Porto Santo during the Middle Miocene, around 14 million years ago (Mya), and later Madeira and the Desertas Islands during the Late Miocene, around 7 Mya. The other originated the Selvagens Islands during the Oligocene, around 28 Mya.
The island of Madeira rises up from a vast underwater plain, within the African Plate, forming a volcanic massif more than 5.5 km high, of which only 1/3 comes above the water.
The island evolved over successive stages of intense volcanic activity (early stage ≥ 7 Mya, middle stage 7 to 1.8 Mya and late stage < 1.8 Mya), separated by periods of very little activity or no activity at all. During these periods, erosion reduced the size of the volcanic structure, at times considerably. The erosion processes consisted mostly of fluvial erosion, marine abrasion (coastline retreat and the creation of cliffs), and gravity processes (landslips and landslides) that were triggered by the steep relief created by the previous processes. The periods of intense erosion carved valleys into the landscape and created uneven terrain that partially or totally destroyed the volcanic forms, while the products of the erosion were deposited on the underwater flanks of the great volcano or in the valleys. When activity started again, during the following volcanic period, it led to fossilisation of the sedimentary residue material and the eroded terrain. The most recent volcanic structure in the stratigraphy of the island of Madeira, which was formed until 6000-7000 years ago, represents the rejuvenation stage and is characterised by volcanism acting on an eroded topography, carved into the forms created during the previous stage. It is believed that Madeira is still in this stage of evolution, and volcanic activity is temporarily dormant. Although low, the risk of a volcanic eruption in Madeira is not non-existent.