Are lava balloons -- and the formation of the Azores -- related to sea-floor spreading at the Mid-Atlantic Ridge or a hotspot below the Azores? The geology of the Azores is very complicated and lots of research is under way. To find an answer to this question, experts are studying ocean topography, the ages of volcanoes, and the chemistry of volcanic rocks in the Azores.
The Azores lie near the boundary of 3 tectonic plates: the North American, Eurasian, and African Plates. Geologists call this type of boundary a triple-junction. The Mid-Atlantic Ridge, a great undersea mountain range, separates the North American Plate from the other 2 plates. A fracture zone that marks the boundary between the Eurasian and African Plates extends from the Mid-Atlantic Ridge towards the Azores.
The Azores Islands occupy a region about 650km long and 200 km wide, alongside a valley called the Terceira Rift. At the Terceira Rift, which stretches southeast from the fracture zone at the Mid-Atlantic Ridge, the ocean floor is spreading. The geography of the ocean floor in this area is one clue to understanding volcanic activity in the Azores. What else do we know?
The 9 islands that form the Azores are the tips of active volcanoes. The age of these volcanoes tells us a great deal. If the Azores lie along a hotspot track, the youngest volcano should be closer to the Mid-Atlantic Ridge than the others.
Why? Because a hotspot is stationary, but the ocean plate and everything on it moves. After sea-floor spreading drags a volcano off a hotspot, the hotspot generates a new volcano. But the Azores volcanoes don't increase in age farther away from the ridge. Older volcanoes are mixed in with younger ones in no particular order. This seems to contradict the idea that a hotspot created this chain of islands.
One more clue helps us understand how the Azores were created.
The chemistry of volcanic rocks in the Azores has given us more clues to the origin of the islands. Many of the rocks look the same, but they have different chemical compositions. By studying the chemistry of the rocks, geologists can tell whether islands are created from sea-floor spreading or from hotspots.
So what's going on? How did the islands form? All the information gathered about the ocean floor, the ages of the islands, and the chemistry of the rocks suggests that the Azores resulted from volcanic activity that was a result of both a hotspot and sea-floor spreading at the mid-ocean ridge (which includes the Mid-Atlantic Ridge and its offshoot, the Terceira Rift) The magma that created the lava balloons was also probably generated from both the hotspot and the mid-ocean ridge.