The Azores are nine islands that occupy a triple junction between the North American, African and Eurasian Plate. It is a spreading center bound by the Mid-Atlantic Ridge on the west and the Terceira Rift on the NE and the East Azores Fracture Zone to the SE.
General motion of the North American Plate is WNW at some 19 mm/year. The Eurasian Plate moves generally NE at some 5 mm/yr while activity along the East Azores Fracture Zone and the boundary between the Eurasian Plate changes from a generally transform fault most of the way between the Azores to Spain at which point it becomes a convergent plate boundary as it is traced though the length of the Mediterranean Sea.
The conversion of this boundary from a transform fault takes place south of the Straits of Gibraltar. The epicenter of the disastrous 1755 Lisbon earthquake (8.5 – 9+ Richter) is located to the west of the fault’s arrival on the African continent.
The triangular area is variously referred to as the Azores Plateau, Azores Rise and the Azores microplate. Its formation is believed to have begun some 36 MA ago.
The area became more active some 5 – 10 MA ago. The oldest rocks are dated at just over 8.1 MA ago.
There is no small amount of discussion about precisely what the Azores Plateau and its islands actually are caused by. There is a good case to be made that it and the islands are simply products of a spreading oceanic ridge.
On the other hand, the Mantle Plumes guys point out the sheer volume of material making up the Rise and the apparent concentric aging of islands from the current most active island, Pico. The problem with this is that the island ages do not necessarily lay out in concentric age pattern. Some do, but some do not. https://notendur.hi.is/~ers12/skrar/Azores%20TJ%20and%20HS_presentation_Einar%20Ragnar.pdf
Just to thoroughly confuse the issue, Weiss et all noted in 2014 that the ridge jump and strong volcanism in the region are caused by the interaction of a hot spot with the Mid Atlantic Ridge. They believe that a moving hot spot initiated the northward migration of the triple junction. After the hot spot passed the ridge, the newly constructed plateau rifted and normal seafloor spreading reestablished between Flores / Corvo on the North American Plate and the rest of the islands. Involved mantle upwelling is partly responsible for the continued rifting processes along the Terceira Rift. The problem with this is that hot spots tend not to move around. Rather it is the plates floating across their locations over time.
Other authors referenced in the Weiss paper believe the islands are young and do not exhibit a hot spot track in terms of age progression from island to island. An alternate explanation is an enriched upper mantle domain with more volatiles than normal. Decompression of a wet mantle caused by rifting tectonics is a final explanation. http://ovga.centrosciencia.azores.gov.pt/ovga/art%C2%BA-tectonic-evolution-southeastern-terceira-rifts%C3%A3o-miguel-region-azores-2015-bj-wei%C3%9F-c-h%C3%BCbsc
At this point nobody knows, though my (thoroughly uneducated) guess would be the volcanism is driven by the migration of the triple junction between the three primary plates with little if any hot spot involvement. Behavior of this triple junction is very different from the subduction driven triple junctions previously discussed, as two of its boundaries are spreading centers and the third is a transform fault that is not moving very much at all.
Evolution of the Azores
The triple junction is now located some 150 – 250 km north of its location some 36 Ma ago. Its migration north is thought to have begun around the time the Iberian Peninsula became part of the Eurasian Plate. This changed the interaction between the Eurasian and African Plates. The migration took place in one to several steps.
The original location is thought to be the East Azores Fracture Zone (Gloria Fault) on the Nubian (Africa) Plate. The second location was the Pico Fracture Zone which was active some 33 – 20 Ma. At this point, some authors believe the tectonics formed a short-lived Azores Micro Plate. Around 20 Ma ago, a significant extension started in the Azores creating the Terceira Rift. That extension rate is still 4 mm / year.
Sometime between 10 – 20 Ma ago the extensional direction rotated 20 degrees clockwise. This rotation coincided with the deposition of large quantities of extrusives, intrusives and underplating material, building much of the Azores Plateau / Rise / Microplate. At this point the Monacco Bank (early Santa Maria Island) began. It stopped growing around 6 Ma ago.
Tectonics over the last 10 Ma have been driven by rifting of grabens, activation and deactivation of various rifts crossing the Plateau / Rise / Microplate. That rifting has allowed significant volcanic materials to reach the surface, building the ridges and the islands that sit upon them. The Monacco Graben and early evolution of the Santa Maria Island is thought to have begun some 5 – 6 Ma ago. Uniform sedimentation in S-Basin shows a period of relative quiet for a few million years, though the Povocao Basin reactivated. From 0.9 Ma to present, S-Basin went inactive and Sao Miguel grew from the ocean floor. The rise is a complex of grabens, basins, faults and rifts.
Another paper describes a block tectonic model for the Azores Rise that has multiple blocks. They move relative to one another in strike – slip motion or rotate clockwise depending which fault systems are active. This swapping back and forth of motions controls buildup of volcanoes and current seismic activity on the Plateau / Rise / Microplate. http://w3.ualg.pt/~jluis/artigos/mmiranda_mgr.pdf
Weiss, B.J., C. Hubscher, T. Ludmann, The tectonic evolution of the southeastern Terceira Rift/Sao Miguel region (Azores), Tectonophysics, 654, 75-95, 2015.