The Mid-Atlantic Ridge is the divergent boundary that runs nearly the entire distance from the north pole to the south pole. As a slow spreading ridge, two plates move apart, causing the Atlantic Ocean to widen at a rate of about 2.5 cm per year. This has created the longest mountain range on Earth. Of course, the vast majority of this range is underwater. There are several islands near the ridge, but the only landform to exist directly on top of it is Iceland. Iceland is a hotspot of volcanic activity, with about 130 active and inactive volcanoes, and 30 active volcanic systems altogether.
According to the Guide to Iceland tourism website, this volcanism is due to its location along the Mid-Atlantic Ridge [1]. This certainly makes sense since “most volcanoes on earth are a result of plate tectonics” [2]. A map of all the volcanoes in the world reveals that the vast majority are along plate boundaries, with the rim of the Pacific plate so active to earn itself the nickname “Ring of Fire.” But the Mid-Atlantic is much quieter in comparison; Iceland is one of only two “hotspots” of volcanic activity along the ridge. So contrary to what many internet sources report, volcanism in Iceland cannot be attributed solely to its location on the Mid-Atlantic Ridge.
The other hotspot along the Mid-Atlantic Ridge is the archipelago of the Azores Islands. The Azores are a chain of nine volcanic islands that together constitute an autonomous region of Portugal, and are located about 850 miles off the coast of mainland Portugal. They lie around the “triple junction among the Eurasian, Nubian, and North American plates" [3]. This means that the Azores are along both the divergent Mid-Atlantic Ridge and the transform fault between the Eurasian and African (or Nubian) plates. This creates a different environment of tectonic activity than is seen in Iceland. However, the plate boundary may not be responsible for volcanic activity in the Azores either.
Hotspots, or regions with a high level of volcanic activity, “are not necessarily associated with a plate boundary,” according to Berkeley geochemistry professor, Donald DePaolo [2]. The hotspot of Hawaii for instance — one of the most volcanically active places in the world — is in the middle of the Pacific plate, thousands of miles from a fault line. The same is true of Yellowstone, which is near the center of the North American plate. Even the Iceland hotspot can’t seem to be explained by the presence of a plate boundary; if the Mid-Atlantic Ridge is responsible for Iceland’s volcanism, it would logically follow that the rest of the ridge be equally active. But the only active volcanoes along the Mid-Atlantic are in the isolated and concentrated hotspots of Iceland and the Azores.
In 1971, Princeton geology professor W. Jason Morgan proposed the existence of plumes caused by “convection in the lower mantle” as an explanation for hotspots around the world [4]. Mantle plumes are described as “hot, buoyant upwelling regions beneath the Earth’s lithosphere” [5]. Plumes are thought to act independently from plate tectonics since they originate so deep within the Earth, at the boundary between the core and the mantle, almost two thousand miles beneath the surface of the Earth. This means that plates and ridges — the crust — can move around without affecting the mostly stationary hotspots [2]. The clearest example of this in action is Hawaii. The plume underneath the Hawaiian hotspot has stayed in the same spot while the Pacific plate has migrated, creating a chain of islands 1,500 miles long.
It hasn’t been proven that mantle plumes exist, nor that they cause volcanic activity, but there’s strong evidence for a plume existing underneath Iceland. Morgan initially modeled that there were twenty deep mantle plumes around the world. One piece of evidence he used to justify this claim was a world gravity map made by William M. Kaula, a professor at the University of California, Los Angeles. Kaula’s map showed “gravity highs over Iceland, Hawaii, and most of the other hotspots” [4]. These gravity highs are thought to be evidence of a large swelling plume in the Earth’s mantle.
One indication of a gravity high for plumes under oceanic plates is the depth of the ocean. Hotspots often exist around “abnormally shallow parts of the ocean” [4]. “The Azores are located on a shallow plateau” toward the middle of the Atlantic Ocean, which points to the existence of a mantle plume [3]. The water around Iceland as well is significantly shallower than the usual depth of the North Atlantic [4].
There is also seismic evidence of a plume at the core-mantle boundary beneath Iceland. Donald V. Helmberger and Lianxing Wen of the Seismological Laboratory at the California Institute of Technology used seismic waves to model the Iceland plume in 1998. Seismic waves are caused by movement within the earth, whether that be earthquakes, explosions or volcanoes. This research was conducted to determine whether the plume under Iceland, at this point widely accepted to be true, exists at the “core-mantle boundary or at the base of the upper mantle” [6]. The researchers’ conclusion was that the plume originated very deep within the earth, at the core-mantle boundary.
Though not all hotspots appear to be caused by mantle plumes. The Yellowstone hotspot, which can’t be explained by a plate boundary, has yielded no seismological evidence for a lower mantle plume [2]. Furthermore, not all mantle plumes are created equally. Kaula’s gravity map reveals the differences in strength between hotspots through variation in “the magnitude of the gravity” [4]. In a similar way, Bernard Bourdon of the European Association of Geochemistry characterizes the strength of hotspots based on their “buoyancy flux and mantle temperature.” This essentially measures how much heat the mantle plume brings from the deep mantle to the upper mantle. Stronger plumes have higher temperatures and are high buoyancy flux. Since Iceland and the Azores have “lower excess temperatures” and are low buoyancy flux, they are considered weaker hotspots in comparison to the very strong hotspot of Hawaii [5].
According to Norman Sleep, professor of Geophysics at Stanford University, the Iceland plume has only “1/6 of the volume flux of Hawaii” and the Azores hotspot is even smaller and weaker. So, if these plumes weren’t positioned under the divergent Mid-Atlantic ridge, or another equally weak crustal spot, they would not be nearly as volcanic [7]. The strength of the Hawaii hotspot allows it to break through the thick and fast-moving Pacific plate, but a weaker plume would not produce as much volcanism.
Even though Iceland and the Azores have weaker plumes, the two are still highly volcanic. Iceland in particular is among the world’s most volcanically active places, considered one of the two most “prominent hotspots” along with Hawaii [7]. So the full explanation of Iceland’s volcanism is most likely due to a combination of the mantle plume and the Mid-Atlantic Ridge, and the interactions between the two. In Iceland, tectonic and plume activity come together to cause magmatism at the surface, along the rift. There are several fractures in and around Iceland. The Mid-Atlantic Ridge, of course, cuts right through the landform (called the Reykjanes Ridge where it’s above ground), and there is another main off-shoot to the south of the island. All the active volcanoes in Iceland lie right in and around the fault lines regardless of the size and scope of the underlying plume.
The volcanism in the Azores has a similar story, caused by a “mantle plume interacting with the Mid-Atlantic Ridge,” according to Daniele Trippanera, a Ph.D. student in the geology department at Roma Tre University [3]. The Azores differs slightly from Iceland, though, due to the presence of a transform fault between the Eurasian and African plates. Transform plate boundaries, where two plates move past each other, are usually not associated with volcanic activity.
A mantle plume can’t cause magmatism on its own. Cracks in the plate, whether at plate boundaries or in the middle of a plate, are “necessary for plume magma to ascend." Iceland and the Azores both have an obvious co-existence of plumes and faults, but “plate cracking is evident along the Hawaiian chain” as well [7].
In the case of Hawaii it makes sense that the strong upwelling mantle plume broke through the Pacific plate to cause these cracks in the crust. But looking at Iceland and the Azores, do plumes and plate boundaries have any causal relationship to each other? R.I Hill, a professor at Australian National University, writes that plumes and plate tectonics are caused by “two distinct modes of convection” in the mantle, and “operate largely independently” [8]. Craig Parkin, a professor at Cambridge University, takes a similar stance, referring to the conjunction of plate boundary and plume a “coincidence” [9].
Morgan, on the other hand, believes that plumes can lead to “continental break-up” especially in the case of the North Atlantic. He proposed that the Iceland and Azores plumes created “currents in the asthenosphere” that broke through the ocean crust and led to the spreading of the Atlantic Ocean. He points to the prevalence of mantle plumes at triple junctions, such as the Azores and the Galapagos, as evidence that plumes can create plate boundaries [4]. Kevin Burke, professor of geology at University College London, also believes that the relationship between triple junctions illustrate how plumes affect tectonic activity. In his 1973 article, “Plume-Generated Triple-Junctions,” Burke hypothesized that plate boundaries and rifts initially develop because of plumes, an idea that Morgan briefly mentioned in his article two years earlier [10]. Looking at how many plumes exist along plate boundaries, especially triple junctions, it does seem that these two forces are related.
The commonly found explanation for volcanic activity in Iceland centers on its location on top of the Mid-Atlantic Ridge. If it were as simple as that one would expect to find volcanic hotspots and continental growth all along the ridge, but in reality there is only volcanism in two isolated places in the Mid-Atlantic: Iceland and the Azores. During the past few decades there has been a lot of debate about the existence of mantle plumes and how they behave.
The full explanation of volcanism in Iceland is complicated, and combines the effect of both a relatively weak plume at the core-mantle boundary and the spreading center of the Mid-Atlantic Ridge. These two factors, however, may not have occurred in a mere coincidence. It’s likely that the plumes in both Iceland and the Azores played a part in creating the ridge in the North Atlantic, leading to the spreading of the Atlantic Ocean. In turn, the ridge in Iceland magnified the effect of the plume, making it one of the world’s most notable volcanic hotspots.
References:
1: Guide to Iceland website: "Volcanoes in Iceland"
2: "Deep Origin Of Hotspots -- The Mantle Plume Model." Donald J. DePaolo and Michael Manga.
3: "Relationships Between Tectonics And Magmatism In A Transtensive/ Transform Setting: An Example From Faial Island (Azores, Portugal)." Daniele Trippanera, et al.
4: "Convection Plumes in the Lower Mantle." W. Jason Morgan.
5: "Insights Into The Dynamics Of Mantle Plumes From Uranium-Series Geochemistry." Bernard Bourdon, et al.
6: "Seismic Evidence That The Source Of The Iceland Hotspot Lies At The Core-Mantle Boundary." D.V. Helmberger and L. Wen.
7: "Origins of the Plume Hypothesis and Some of Its Implications." Norman H. Sleep.
8: "Mantle plumes and continental tectonics." R.I. Hill, et al.
9: "Imaging The Pulsing Iceland Mantle Plume Through The Eocene." Craig Parkin, et al.
10: "Plume-Generated Triple Junctions: Key Indicators in Applying Plate Tectonics to Old Rocks." Kevin Burke and J.F. Dewey.
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