When the distribution of earthquake epicentres in Iceland during the period 1994–2007 is compared with the surface pattern predicted by the convection-roll model, a considerable degree of spatial correspondence becomes apparent.
The relationship takes somewhat different forms in different parts of the country. Across southern Iceland, much of the seismicity is concentrated within, along the margins of, or near the corners of the diamond-shaped sectors formed by the division lines of the model. North of Iceland, the pattern is different: within the Tjörnes Fracture Zone, earthquake epicentres tend to form elongated belts that can be compared directly with individual division lines.
Several examples illustrate this relationship particularly clearly.
1. The Reykjanes Ridge and the Njörður area
At this location, the Reykjanes Ridge, as an oceanic spreading ridge, meets an offshore structural system transverse to the ridge. A marked concentration of earthquake epicentres occurs close to this intersection.
The area is also associated with Njörður, a large submarine central volcano situated on the Reykjanes Ridge. Within the convection-roll framework, the concentration of tectonic and volcanic activity near this location is significant because it occurs close to an intersection between major division lines.
2. The curved seismic belt of the Reykjanes Peninsula
The earthquake belt extending across the Reykjanes Peninsula does not form a simple straight continuation of the Reykjanes Ridge. Instead, it bends progressively eastward toward the Hengill–Hveragerði region.
Within the convection-roll model, this geometry can be interpreted as a connection between the offshore Reykjanes Ridge and the Western Volcanic Zone. The two principal connection points are not located at the same latitude, and the complete seismic system consequently extends across two adjoining diamond-shaped sectors.
Of particular interest is that the curvature of the seismic belt from the Njörður area toward Hveragerði can be represented by the same mathematical arc used to construct the corresponding division line in the model.
3. Hveragerði and the Hengill region
The Hveragerði–Hengill region is one of the most seismically active areas of southwestern Iceland.
In the convection-roll model, this is not merely a minor intersection between division lines. It represents a principal intersection associated with the lower-mantle framework, where division systems derived from the polar and equatorial geometries converge.
The model therefore places an unusually important structural intersection close to Hveragerði and Reykjafell. The high concentration of earthquake epicentres in this area is consequently one of the more conspicuous spatial correspondences between the calculated geometry and observed seismicity.
4. The South Iceland Seismic Zone
The South Iceland Seismic Zone provides another particularly clear example. Earthquake epicentres are concentrated in a broadly east–west belt extending between the western and eastern corners of one of the model’s diamond-shaped sectors. The main trend extends approximately from the Hveragerði region toward Hekla, close to 64°N.
Thus, rather than following only a single division line, the seismicity occupies the interior of a geometrically defined sector and is particularly concentrated between its opposing corners.
5. Torfajökull
East of the main South Iceland Seismic Zone lies the Torfajökull volcanic system. This area can be regarded both as a distinct volcanic and geothermal region and, in geometrical terms, as an eastward continuation of the structural pattern associated with the South Iceland Seismic Zone. The concentration of earthquake epicentres around Torfajökull again occurs within a sector defined by the division-line geometry.
6. Eyjafjallajökull and Mýrdalsjökull
Farther south, earthquake clusters coincide with the volcanic systems of Eyjafjallajökull and Mýrdalsjökull, including the Katla caldera beneath Mýrdalsjökull. An additional feature is the approximately east–west arrangement of seismicity across the central part of the corresponding diamond-shaped sector. The relationship therefore involves both the location of the volcanic centres and the internal geometry of the sector itself.
7. Grímsvötn
The Grímsvötn area beneath Vatnajökull is one of Iceland’s most active volcanic regions. On the earthquake map, much of the seismicity in this region falls within the same diamond-shaped sector predicted by the convection-roll model. The concentration is therefore not restricted to a single line or intersection; instead, the active region occupies a substantial part of one geometrically defined cell.
8. Bárðarbunga
Bárðarbunga forms another major concentration of volcanic and seismic activity beneath northwestern Vatnajökull. Its caldera and associated fissure system lie within the same broader geometrical framework. The seismicity around Bárðarbunga therefore provides an additional test of whether the division lines and the diamond-shaped sectors have a systematic relationship with the distribution of tectonic and volcanic activity.
9. Askja and the northeast-trending fissure swarm
The Askja region forms a distinct seismic and volcanic centre farther north. From Askja, earthquake activity extends northeastward along the associated fissure swarm. In the convection-roll interpretation, both the location of Askja and the orientation of this northeast-trending seismic zone correspond with the underlying division-line pattern.
This is important because the comparison involves not only the position of an individual volcanic centre but also the direction in which tectonic activity extends away from it.
10. The Húsavík–Flatey Fault
North Iceland provides a different type of comparison. The Húsavík–Flatey Fault (HFF), one of the principal structures of the Tjörnes Fracture Zone, is marked by a pronounced belt of earthquake activity. Within the convection-roll model, this belt follows one of the calculated division-line trends remarkably closely.
Toward the western part of the Tjörnes Fracture Zone, the orientation of the seismicity changes as the system approaches the connection with the Kolbeinsey Ridge. This change occurs close to an area where division lines of the convection-roll framework also intersect and change their geometrical relationship.
The importance of this example is therefore twofold: the earthquake belt follows a predicted division-line direction over a considerable distance, while its change in geometry also occurs close to a significant intersection in the model.
11. The Grímsey Oblique Rift
The Grímsey Oblique Rift (GOR) forms the second major seismic belt of the Tjörnes Fracture Zone. Its orientation corresponds closely to the next division line to the north in the convection-roll framework. The earthquake epicentres do not form an infinitely narrow line, but rather a relatively broad zone extending mainly northward from the calculated boundary.
The width of this seismic zone corresponds approximately to two adjacent convection-roll sectors. A comparable relationship can be observed along the Húsavík–Flatey Fault, suggesting that the influence of the structural boundaries may extend across more than a single line.
Different Forms of Spatial Correspondence
The comparison between the 1994–2007 earthquake distribution and the convection-roll framework is therefore not based on one particular type of geometrical relationship. At least three recurring forms can be distinguished.
First, major concentrations of seismicity occur near intersections of division lines, as seen particularly in the Hveragerði–Hengill region and offshore along the Reykjanes Ridge.
Second, earthquake activity may occupy or follow the internal geometry of diamond-shaped sectors, as seen across southern Iceland from the South Iceland Seismic Zone through the central volcanic regions.
Third, elongated seismic zones may follow individual division lines, as is particularly evident in the Tjörnes Fracture Zone, including the Húsavík–Flatey Fault and the Grímsey Oblique Rift.
The significance of the comparison therefore lies not simply in individual earthquake clusters coinciding with individual calculated lines. Rather, similar geometrical relationships appear repeatedly in different tectonic settings: at volcanic centres, within seismic zones, at major intersections, and along long linear fault and rift systems.
This repeated spatial correspondence provides a basis for testing whether the surface geometry derived from the convection-roll model is related in a systematic way to the distribution of seismic and volcanic activity in Iceland.
A particularly striking feature of the proposed mantle-convection framework is a regular sequence of principal lower-mantle divisions along the equator. Taking 21.16°W as the theoretical reference longitude beneath the central Atlantic, equivalent divisions occur at intervals of exactly 30°. Moving eastward around the Earth, the relevant sequence considered here is:
111.16°W — East Pacific Rise
81.16°W — western margin of South America
51.16°W — Amazon mouth and northeastern South America
21.16°W — central Atlantic / Mid-Atlantic Ridge
8.84°E — western coast of equatorial Africa
38.84°E — East African Rift region
68.84°E — Central Indian Ridge
98.84°E — western margin of Indonesia
128.84°E — eastern Indonesia
158.84°E — possible additional boundary east of Papua New Guinea
These are theoretical longitudes. The model does not require the principal surface expression to occur precisely on each lower-mantle division. Upper-mantle upwelling lines occur approximately 1.5° to either side, while the effects of the adjacent convection roll may be expressed as much as 3° from the principal division. The appropriate comparison is therefore with a narrow longitudinal belt rather than with a geometrically infinitesimal line.
1. 111.16°W — East Pacific Rise
The first point lies in the eastern Pacific and corresponds closely to the East Pacific Rise, one of the Earth’s major oceanic spreading systems. Around the equatorial and southeastern Pacific, the East Pacific Rise occupies approximately the 110°W region, although its longitude varies with latitude. The theoretical longitude of 111.16°W therefore falls within about one degree of the broad ridge system. Studies of the eastern tropical Pacific commonly place the Rise near 110°W, while the EPR can be followed continuously southward from the equatorial region.
This is an important starting point because here the expected mantle division is expressed by an unmistakable oceanic spreading system. In the convection-roll interpretation, the surface ridge need not coincide exactly with the lower-mantle division; displacement toward an adjacent upper-mantle upwelling line would be expected.
2. 81.16°W — the western margin of South America
Thirty degrees farther east gives 81.16°W. At the equator this lies immediately west of the coast of Ecuador. Ecuador itself straddles the equator and its western boundary is the Pacific continental margin.
This is a fundamentally different tectonic environment from the East Pacific Rise. Instead of oceanic spreading, the eastern Pacific basin terminates against the South American margin, where the Nazca Plate converges with South America. Thus, within one 30° interval, the system passes from a major zone of crustal production to the continental margin toward which that oceanic plate moves.
The correspondence should not be interpreted as a claim that 81.16°W itself is the trench axis. Rather, it marks the equatorial longitude of the broader western South American tectonic boundary zone.
3. 51.16°W — the mouth of the Amazon
The next theoretical division, 51.16°W, reaches the opposite side of South America near the great outlet of the Amazon River.
This point is different from most of the others because the dominant surface expression is not an active plate boundary. Nevertheless, the Amazon mouth marks one of the major geological and physiographic transitions on Earth: the enormous Amazon drainage system reaches the Atlantic margin here. NASA describes the Amazon as carrying the largest freshwater discharge of any river, delivering roughly one fifth of global river discharge to the oceans.
Within the proposed framework, the significance would therefore not necessarily be present-day faulting. The question is whether the continental-scale drainage architecture, sedimentary basin and Atlantic continental margin preserve a longer-term structural influence associated with the same regular mantle division.
This point consequently needs to be treated more cautiously than the active ridges and rifts.
4. 21.16°W — the central Atlantic
Another 30° east brings the sequence to its reference point, 21.16°W.
This correspondence is particularly close. The Mid-Atlantic Ridge crosses the equatorial Atlantic in the vicinity of 21–22°W. For example, a large 2022 earthquake classified by the USGS as occurring on the central Mid-Atlantic Ridge was located at 0.934°S, 21.716°W. The theoretical value of 21.16°W is therefore within roughly half a degree of this active ridge position.
In the model, this longitude represents a principal lower-mantle division, while the actual surface spreading system may occupy the adjacent 1.5° roll positions. This distinction is important: the geometric reference is defined at depth, while the observable ridge records the response of the upper mantle and lithosphere.
5. 8.84°E — western equatorial Africa
The next 30° step gives 8.84°E, which reaches the Atlantic coast of equatorial Africa near Gabon. Independent geographical data place the equatorial portion of Gabon between approximately 9°E and 14°E, making 8.84°E essentially a coastal or immediately offshore position. Thus the 30° sequence produces another striking basin-scale relationship:
South American Atlantic margin → Mid-Atlantic Ridge → African Atlantic margin, at intervals of approximately 30°.
This does not mean that the Atlantic basin is geometrically symmetrical in conventional plate-tectonic terms. Rather, it suggests that the theoretical mantle sequence intersects three very different parts of the Atlantic system: one continental margin, the spreading axis, and the opposite continental margin.
6. 38.84°E — the East African Rift region
The next theoretical lower-mantle division is 38.84°E. Here the distinction between the lower-mantle division and the surface response becomes particularly important. The main Kenya Rift at the equator does not lie at 38.84°E. Its axial region is farther west. Lake Bogoria, for example, lies at approximately 36.05–36.12°E immediately north of the equator, within the central Kenya Rift. However, the model predicts:
38.84°E — principal lower-mantle division 37.34°E — upper-mantle upwelling line 35.84°E — outer influence/downwelling side of the adjacent 1.5° roll
The last value, 35.84°E, falls remarkably close to the active Kenya Rift. The central Kenya Rift itself is a roughly 100-km-wide asymmetric extensional structure in this region.
Seismic and gravity studies along the equator have additionally suggested that the elevated East African plateau requires a regional mantle contribution and is compatible with dynamic support by mantle convection. This makes the African point particularly useful for distinguishing the deep theoretical division from its displaced lithospheric expression.
7. 68.84°E — the Central Indian Ridge
Thirty degrees farther east lies 68.84°E, almost directly on the active ridge system of the central Indian Ocean. The Central Indian Ridge extends northward from the Rodrigues Triple Junction toward the Carlsberg Ridge. Near the equatorial region it lies broadly between 67°E and 69°E. Published ridge-axis locations include approximately 1.19°S, 67.52°E, and farther south several segments occur between about 68.1° and 68.6°E.
The theoretical 68.84°E position therefore falls comfortably within the ±1.5° influence interval and extremely close to the actual spreading system.
Together with the Mid-Atlantic Ridge and East Pacific Rise, this gives the sequence three major oceanic spreading systems separated by intervening continental and tectonic domains.
8. 98.84°E — western Indonesia
At 98.84°E, the sequence reaches the western Indonesian region, close to Sumatra and the Sunda convergent margin.
This is no longer a spreading environment. Southwest of Sumatra, the Indo-Australian plate converges with the Sunda plate along the Sunda–Java trench system. The USGS describes this as part of an immense collision zone extending from the Indonesian region toward the Himalaya and notes the intense seismicity and volcanism produced by the Sumatra–Andaman subduction system.
The theoretical longitude therefore lies near one of the Earth’s major transitions from Indian Ocean lithosphere into the Indonesian convergent system.
9. 128.84°E — eastern Indonesia
Another 30° east gives 128.84°E, in the exceptionally complex tectonic region of eastern Indonesia, close to the Molucca Sea–Halmahera system.
This is one of the most unusual convergent environments on Earth. The Molucca Sea plate has been consumed by oppositely directed subduction beneath the Sangihe and Halmahera arcs, producing an active arc–arc collision. Modern studies describe the Molucca Sea as one of Southeast Asia’s most complicated tectonic regions and document the interaction of the Philippine Sea, Eurasian/Sunda and Australian plate systems.
A USGS regional tectonic map of the central Molucca Islands uses 128°E as its central meridian and shows subduction zones, strike-slip faults, thrusts and volcanic arcs concentrated through this longitude range. Thus the 128.84°E point corresponds not merely to the eastern edge of Indonesia geographically, but to a major zone of tectonic reorganisation.
10. 158.84°E — a possible additional point east of Papua New Guinea
The next step, 158.84°E, is particularly interesting because it lies east of Papua New Guinea in the Woodlark–Solomon region. This point has not yet been incorporated into the principal sequence in the same way as the preceding nine because the geological relationships are much more complicated. Nevertheless, there is clearly major tectonic and magmatic activity within the predicted belt.
The USGS places the Woodlark Basin between approximately 157° and 159.5°E, directly encompassing the theoretical longitude of 158.84°E. The basin contains the boundary between the Solomon Sea and Australian/Woodlark plates and is an active extensional system. Immediately farther east, the Guadalcanal segment of the Solomon plate boundary occupies approximately 159–161°E.
The broader Solomon system combines active seafloor spreading, subduction of young Woodlark lithosphere, microplate motion, intense seismicity and interaction with the Ontong Java Plateau. The USGS tectonic synthesis shows spreading across the Woodlark Ridge and subduction along the Solomon system, while the Solomon arc has experienced very high levels of large-earthquake activity.
The importance of this tenth point is therefore not that a single simple plate boundary lies precisely at 158.84°E. Rather, the predicted 158.84°E ±3° belt coincides with an exceptionally active zone in which spreading, subduction, arc volcanism and microplate deformation interact. For this reason, 158.84°E should probably be retained as a candidate tenth equatorial node, even if its precise geological expression remains unresolved.
The larger pattern
The sequence is noteworthy because the theoretical coordinates are generated independently of the geological features:
Across these ten consecutive 30° positions one encounters, in order: East Pacific spreading → South American continental margin → Amazon/Atlantic continental margin → Mid-Atlantic spreading → African continental margin → East African rifting → Indian Ocean spreading → Sunda subduction → Molucca arc collision → Woodlark–Solomon spreading/subduction.
The geological processes are therefore not of one type. Some points correspond to spreading ridges, others to continental margins, rifts, subduction systems or complex collision zones. What they potentially have in common is their position relative to a regular deep-mantle framework.
This distinction is central to testing the model. The proposed lower-mantle divisions should not be expected to produce identical structures everywhere. The lithosphere above them differs greatly in age, thickness, composition and pre-existing tectonic architecture. The test is instead whether major changes in tectonic organization occur repeatedly within the predicted ±1.5° to ±3° belts around an independently defined 30° sequence.
The correspondence at 21.16°W, 68.84°E and approximately 111.16°W is particularly interesting because all three are associated with major oceanic spreading systems. The 38.84°E case provides a different and potentially important test: the lower-mantle division itself lies east of the Kenya Rift, while the predicted western influence line at 35.84°E approaches the actual rift axis closely. Eastern Indonesia at 128.84°E and the candidate point at 158.84°E show yet another manifestation, where the regular sequence enters some of the most structurally complicated convergent regions on Earth.
The Papua–Solomon point therefore deserves further investigation rather than premature classification. If 158.84°E ultimately proves to occupy a systematic position within the Woodlark–Solomon tectonic and magmatic system, it would extend an already remarkable 30° equatorial sequence by one further major tectonic node.
The 30° Equatorial Sequence and the ±3° Mantle-Roll Belts
Taking 21.16°W as a principal lower-mantle division, twelve equivalent divisions occur at exact 30° intervals around the equator. In the proposed convection-roll model, the surface expression does not necessarily coincide with the principal lower-mantle longitude. Upper-mantle upwelling occurs approximately 1.5° to either side, while geological effects of the adjacent roll may occur as far as 3° from the principal division.
The 30° Equatorial Sequence (yellow dots on the map)
30° stepstones along the equator (yellow dots).
No.
Principal division / belt
Main geological correspondence
Interpretation
1
111.16°W ±3°
East Pacific Rise
Major oceanic spreading system; ridge axis lies within the predicted mantle-roll belt.
2
81.16°W ±3°
Western margin of South America
Nazca–South America convergence and subduction along the Ecuador–Colombia margin.
3
51.16°W ±3°
Amazon mouth / NE South America
Major continental and sedimentary transition at the Atlantic margin; less directly tectonic than most other points.
4
21.16°W ±3°
Equatorial Mid-Atlantic Ridge
Major spreading system. The ridge/transform system approaches the eastern side of the predicted belt, near 18.16°W.
5
8.84°E ±3°
West coast of equatorial Africa
Atlantic continental margin and inherited structural zone of the Gabon Basin.
6
38.84°E ±3°
East African Rift
Kenya Rift lies toward the western side of the belt, close to the predicted 35.84°E outer influence line.
7
68.84°E ±3°
Central Indian Ridge
Major Indian Ocean spreading system; the ridge axis falls within the predicted belt.
8
98.84°E ±3°
Western Indonesia / Sunda margin
Major subduction system west of Sumatra, with strong seismic and volcanic activity.
9
128.84°E ±3°
Eastern Indonesia / Molucca–Halmahera
Exceptionally complex arc–arc collision, opposing subduction systems and intense magmatism.
10
158.84°E ±3°
Woodlark–Solomon region
Broad magmatic and tectonic zone involving spreading, subduction, microplates and the Solomon arc.
11
171.16°W ±3°
Phoenix / Nova–Canton region
Major old Pacific fracture-zone system; Nova–Canton Trough lies remarkably close to the 168.16°W outer line.
12
141.16°W ±3°
Central Pacific
Weakest equatorial case; no major active plate boundary directly at the principal longitude, although important fracture-zone and volcanic structures occur within the wider region.
What stands out
The most striking feature is that the sequence does not repeatedly select the same type of structure. Instead it encounters very different manifestations of lithospheric deformation:
That heterogeneity may actually be important for the model. If the 30° divisions represent a deep mantle framework, one should not necessarily expect identical surface structures. Oceanic and continental lithosphere respond differently, and the existing plate geometry determines whether a mantle-related influence is expressed as spreading, faulting, rifting, volcanism or enhanced deformation at a pre-existing boundary.
Three cases are particularly clean geometrically:
21.16°W: the Romanche/Mid-Atlantic ridge region approaches the predicted +3° position at 18.16°W.
38.84°E: the Kenya Rift near 36.1°E approaches the predicted −3° position at 35.84°E.
171.16°W: Nova–Canton Trough near 168.01°W approaches the predicted +3° position at 168.16°W.
There is also an important distinction at 148°E, which is not one of the 30° lower-mantle divisions. As we discussed, that longitude belongs instead to the strong tectonic line produced by the overlapping tectonic-ring geometry. Independent studies find major tectonic changes around 147–149°E: east of ~148°E the Papuan plate boundary becomes increasingly extensional, while around 147–148°E earthquake slip-vector orientations and microplate relationships change markedly.
So we now have two different geometrical systems that should not be mixed:
The 30° sequence describes the proposed regular lower-mantle/convection-roll framework.
The ~29°E → ~91°W → ~148°E set arises from the geometry of the tectonic rings and appears to identify unusually strong zones of tectonic reorganisation.
Occasionally, geographical coincidences appear that are interesting even though they have no particular scientific significance. One such example emerges when three well-known volcanic regions of Iceland, Hawaii and Java, are viewed in the context of the proposed convection-roll system.
Within this model, the volcanism of all three regions is related to the regular organization of mantle convection, although each occupies a different position within the system. Iceland lies near the northern end of a curved structure extending from the equatorial region, whereas Hawaii is situated closer to the middle of a comparable curve. Hawaii also lies approximately along the central axis of the Pacific tectonic ring, associated in this model with the Pacific Ring of Fire.
Farther west, Java provides the third element. In the proposed framework, one of the principal mantle divisions passes through this region and is associated with another major zone of mantle upwelling. The three principal mantle regions considered here are, in turn, associated with the central parts of the three large tectonic-ring systems: the Atlantic, Pacific and Indian Ocean rings.
There is also a purely cartographic curiosity. On a simple equirectangular world map, Iceland, Hawaii and Java can be placed approximately along a single straight line. The alignment is not exact and depends on the map projection and on which point within each island or island group is chosen. It is therefore not presented as scientific evidence or as part of the argument for the model. It is simply an intriguing geographical coincidence linking three of the world’s best-known volcanic regions.
One of the most striking features of the convection-roll model is the degree of spatial correspondence between the predicted roll boundaries and the observed volcanic zones of Iceland. This correspondence is not restricted to a single volcanic zone or to a few isolated localities. It is repeated in the positions of volcanic-zone margins, changes in direction, intersections, volcanic centres, and the polygonal areas enclosed by the modelled boundaries.
The principal volcanic zones used for comparison are the Western Volcanic Zone (WVZ), Eastern Volcanic Zone (EVZ), Northern Volcanic Zone (NVZ), the Reykjanes volcanic zone, and the volcanic belt crossing Central Iceland. These are well-established components of the Icelandic volcanic and plate-boundary system, although different terminology has been used for the Central Iceland connection between the main rift zones.
Specific examples of correspondence
Spatial Correspondence Between the Convection-Roll Model and the Volcanic Zones of Iceland.
A number of locations are particularly useful for illustrating the geometrical precision of the correspondence shown on the map:
The Reykjanes volcanic zone south of Hveragerði. Here, one of the modelled boundaries coincides closely with the transition towards the South Iceland Seismic Zone (SISZ). The relationship is particularly informative because the SISZ forms the tectonic connection between the western and eastern rift systems of South Iceland.
The Western Volcanic Zone and Langjökull. A modelled roll boundary closely follows the margin of the volcanic zone in the Langjökull area. The spatial relationship is sufficiently sharp to make this an important reference point when the model is compared with mapped volcanic systems.
The Eastern Volcanic Zone north of Mýrdalsjökull. Another model boundary corresponds to the margin of the EVZ immediately north of the Mýrdalsjökull–Katla region.
The Vatnajökull boundary and Grímsvötn. Within the Vatnajökull region, a major boundary in the convection-roll pattern passes through the area of Grímsvötn, one of the principal volcanic centres of the EVZ.
The boundary extending northeastwards from Kverkfjöll. From the Kverkfjöll region, the mapped volcanic structures and the modelled roll boundary follow the same general northeastward direction.
The outer boundary of the Kverkfjöll volcanic system. The extent of the Kverkfjöll system provides another example in which the margin of a volcanic system corresponds to a boundary in the convection-roll pattern.
Internal boundaries within the Northern Volcanic Zone. The correspondence is not restricted to the outer margins of the NVZ. Several internal divisions within the volcanic zone also coincide with modelled roll boundaries.
The northwestward bend of the Central Iceland volcanic belt. The Central Iceland Volcanic Zone, as defined here, bends towards the northwest in a manner that follows the geometry of the convection-roll pattern.
A comparable bend in the Western Volcanic Zone. The WVZ displays a related change in orientation, extending towards another boundary in the modelled system.
10–11. The polygon occupied by the Northern Volcanic Zone. A particularly clear geometrical relationship occurs in North Iceland, where the volcanic systems of the NVZ occupy a polygonal sector approximately 1.5° in longitudinal width. Both sides of this sector are defined by boundaries in the convection-roll system.
The northern intersection point. The northernmost point highlighted on the map coincides with an intersection of two boundaries in the convection-roll framework. This is significant because intersections represent geometrically more restrictive predictions than correspondence with a single line.
The Eastern Volcanic Zone
The most continuous correspondence on the map occurs along the Eastern Volcanic Zone (EVZ).
Four large polygonal sectors of the convection-roll framework are occupied by the volcanic systems shown in yellow on the map. Rather than simply following a single line, the volcanic zone repeatedly fills the areas enclosed by successive boundaries.
The relationship also places several major volcanic centres and systems—including Hekla, Tindfjöll, Eyjafjallajökull, Katla beneath Mýrdalsjökull, and the Vestmannaeyjar volcanic system—within the same geometrical arrangement.
This is important because the comparison therefore involves several different geometrical properties simultaneously: boundaries, intersections, enclosed areas, changes in orientation, and the positions of individual volcanic centres.
The Western Volcanic Zone
Immediately west of the EVZ, the eastern margin of the Western Volcanic Zone (WVZ) is particularly distinct.
In the convection-roll framework, this boundary occurs approximately 3° west of the corresponding eastern boundary of the EVZ. Thus the two volcanic zones are not treated as unrelated features. They occupy neighbouring parts of the same regularly spaced geometrical system.
The WVZ also changes orientation towards its northern end, and this curvature can be compared with the orientation of the modelled roll boundaries.
Central Iceland
The Central Iceland volcanic belt shows a somewhat different type of correspondence.
Here the most prominent geometrical control is not simply a volcanic zone centred on a modelled line. Instead, one of the sharpest boundaries of the volcanic area corresponds to a downwelling boundary extending northeastwards from the region.
This distinction may be important. If upwelling and downwelling boundaries have different mechanical effects on the lithosphere, volcanic zones should not necessarily occupy identical positions relative to each type of boundary.
The Northern Volcanic Zone
The Northern Volcanic Zone (NVZ) provides another particularly clear example.
Its principal volcanic systems are arranged along a predominantly north–south to NNE–SSW axis, while their outer limits correspond to polygonal boundaries in the convection-roll framework. Independent geological studies also show that the NVZ consists of several overlapping volcanic systems and fissure swarms, including Kverkfjöll, Askja, Fremrinámur, Krafla and Þeistareykir.
In the present model, both upwelling and downwelling lines appear to contribute to the geometry of the volcanic zone. The correspondence therefore concerns not merely the central axis of volcanism but also its width and internal subdivision.
Features not fully visible on the map
The map does not show all the relationships that can potentially be tested.
Among the additional examples are the position of Öræfajökull, the geometrical relationship between Snæfellsjökull and Snæfell, and the former Skagafjörður volcanic zone.
The Skagafjörður system is particularly interesting in this context because its position is approximately 3° west of the Northern Volcanic Zone in the geometrical framework used here. Older rift structures in the Skagi region are independently recognised in geological studies of Iceland.
The offshore continuation provides further constraints. The orientations and positions of both the Reykjanes Ridge and the Kolbeinsey Ridge can be compared with the same convection-roll geometry. The Icelandic volcanic zones therefore form only the subaerial part of a considerably larger geometrical pattern.
Plate motion and the orientation of the roll boundaries
The two-dimensional geometrical correspondence is only part of the problem. The system must ultimately be interpreted in three dimensions because the lines on the map represent boundaries associated with several superimposed levels of convection rolls.
Plate motion must also be incorporated.
In the interpretation proposed here, the motion of the North American Plate away from the spreading system produces a major westward to northwestward extensional component across Iceland. This regional movement interacts with the directions of rotation of the underlying convection rolls.
This provides a possible explanation for an important feature of the map: the eastern margins of several volcanic zones are commonly associated with the blue boundaries of the model.
In the convection-roll interpretation, the roll adjacent to such a boundary rotates in a direction opposite to the motion imposed from the North American side. The opposing motions would increase differential movement across the boundary and could therefore favour extension of the overlying lithosphere.
A comparable relationship can be seen along the eastern side of the Northern Volcanic Zone, where several of the principal extensional structures are again bounded on their eastern side by blue lines.
This proposed relationship between plate motion, roll rotation and surface extension is important because it introduces a dynamic component into what would otherwise be only a geometrical comparison.
From geometrical correspondence to a testable model
The significance of the pattern lies in its repetition.
No single coincidence between a volcanic centre and a modelled boundary would provide strong evidence for a relationship. The more relevant observation is that similar relationships recur independently at the margins of several volcanic zones, at changes in their direction, at volcanic centres, at intersections, and within polygonal sectors of the model.
The Eastern Volcanic Zone is especially informative because the correspondence is repeated through several consecutive polygons rather than occurring at only one locality.
The Icelandic pattern can therefore be tested at several levels:
the distance between mapped volcanic-zone margins and predicted roll boundaries;
the angular agreement between volcanic structures and the modelled lines;
the positions of volcanic centres relative to predicted intersections;
the width of volcanic zones relative to the predicted polygonal cells;
the relationship between active and extinct volcanic zones and successive boundaries in the model;
and the relationship between the direction of plate motion and the inferred rotation of individual convection rolls.
Such tests would allow the apparent correspondence to be expressed quantitatively rather than only visually.
The important point is therefore not simply that volcanic activity occurs near individual lines. It is that the geometry of the Icelandic volcanic zones repeatedly reproduces several different elements of the proposed convection-roll framework—lines, margins, bends, intersections and enclosed polygons—at a range of locations across Iceland.
When considering the spreading of Iceland and the boundary between the North American and Eurasian plates, three main types of tectonic boundaries or structures need to be taken into account. These are, first, mid-ocean ridges; second, volcanic zones; and third, seismic zones. Within the convection-roll model, all of these follow lines that can be constructed on a map on the basis of the geometrical analysis of the convection rolls.
Reykjanes Ridge, East Volcanic Zone and Tjörnes Fracture Zone.
The convection-roll system follows a simple mathematical pattern, with each depth layer having its own set of convection rolls. In Iceland, however, the situation is more complex than in many other regions because the convection rolls of the polar system and the equatorial system meet and interact there. The correspondence is clearly visible on the map. The Reykjanes Ridge, for example, follows convection-roll divisions associated with the 120 km level over a considerable distance.
Main Upper Layers.
In southern Iceland, the situation is further complicated by the fact that division lines belonging to convection rolls at different depths often lie very close to one another, so that they may appear almost as a single line. A precise description of the boundaries shown on the map is therefore considerably more complex than the simplified interpretation presented here. The depth levels are used primarily as convenient reference levels.
As can be seen, the comparison can be extended across the whole of Iceland, revealing correspondences between the calculated lines and geological structures. Blue downwelling lines reproduce the eastern boundaries of both the Eastern Volcanic Zone and the Western Volcanic Zone with considerable precision. The geometry also shows that the two volcanic zones are separated by approximately 3°.
A similar relationship occurs in the Tjörnes Fracture Zone in northern Iceland. There, two principal seismic source lines correspond to adjacent convection-roll division lines separated by 1.5°. As more geological information is incorporated into the comparison, an increasing number of features can be tested against, and found to correspond with, this convection-roll framework.
To compare these lines with the 120–670 km System Section, each mapped line can be related to the boundary of a particular depth interval. The section illustrated here applies to regions south of 60.7°N and also north of 67.3°N. Within the intervening latitude range, the geometry is considerably more complex: the number of layers effectively doubles, while their thicknesses vary with latitude. Nevertheless, the basic division is retained, and each layer can still be followed mathematically, allowing the boundaries of the convection rolls to be constructed with precision.
The polar system takes over progressively from the equatorial system and is superimposed upon it. At 64°N, however, the two systems act in combination, and the convection rolls of the two systems are aligned directly above one another. North and south of this latitude, the corresponding rolls gradually diverge from one another. Farther north, the polar system becomes increasingly dominant.
It is particularly noteworthy that the principal seismic zones—the Tjörnes Fracture Zone and the South Iceland Seismic Zone—are associated with different convection-roll division lines from those followed by the Reykjanes Ridge and the volcanic zones. These seismic-zone divisions are related to deeper parts of the system, specifically to the convection rolls immediately above the 410 km boundary. The same geometrical pattern is then repeated between the 410 km and 670 km depth levels.
