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.
Iceland provides an unusually detailed example of the relationship between surface tectonic structures and the proposed mantle convection-roll division system.
Convection Rolls System Division Lines of Iceland.
On the scale of the major oceanic plates, the convection-roll geometry can be compared with long plate boundaries, mid-ocean ridges and the elliptical tectonic rings. In Iceland, however, the same framework can be examined at much higher resolution. Volcanic zones, individual volcanic systems, fracture zones and even smaller polygonal structures can be compared directly with calculated mantle-division lines.
Four different types of correspondence are particularly useful.
1. Volcanic zones — the Eastern Volcanic Zone
One of the clearest examples is the Eastern Volcanic Zone of Iceland.
Its general orientation corresponds closely to the calculated direction of the convection-roll system beneath Iceland. Rather than treating the volcanic zone simply as an irregular surface continuation of plate spreading, the convection-roll model provides an underlying geometrical direction against which the zone can be compared.
This is important because the mantle divisions do not everywhere trend directly north–south. Their calculated orientation varies systematically with latitude. Iceland therefore provides a particularly useful test: the direction of the volcanic structures can be compared with a direction calculated independently from the convection-roll geometry.
The Eastern Volcanic Zone represents the first level of correspondence:
In other words, the model can be tested against the orientation of an entire volcanic belt.
2. Intersection points — Hekla
A second type of relationship occurs at intersections between calculated structural lines.
Hekla provides an important example.
Its position is associated not merely with the direction of one convection-roll division, but with a location where different geometrical elements of the system intersect. Such intersection points are potentially more important than individual lines because they represent places where different components of the underlying geometrical framework act together.
This provides a second level of correspondence:
Hekla is therefore useful not simply because it is a major volcano, but because its location can be examined in relation to a predicted geometrical crossing point. This may also help explain why some locations within a volcanic zone become much more prominent than others. A volcanic belt defines a broader structural corridor, while intersections may define particularly favourable locations within that corridor.
3. Polygonal structures, the South Iceland polygon
A third expression of the system appears when the calculated division lines are combined to form polygonal structures. South Iceland provides one of the clearest examples.
Here the relevant convection-roll divisions do not produce only isolated N–S or E–W lines. Their interaction defines a larger geometrical framework within which the tectonic structures of South Iceland can be analysed.
The South Iceland polygon is particularly important because it shows that the model is not restricted to matching the orientation of individual faults or volcanic zones. The intersections and connecting lines form a larger geometrical unit.
The relationship can therefore be written:
Within such a framework, individual fracture zones, volcanic systems and changes in structural orientation can occupy different sides, corners or internal divisions of the polygon.
This is also important for understanding the South Iceland Seismic Zone. The surface deformation need not reproduce the deeper geometrical framework as one continuous fault. Instead, the larger stress field can be accommodated through numerous smaller faults and fracture systems.
Thus, a relatively simple underlying geometry may produce a considerably more complex surface expression.
4. Linear boundaries — the Tjörnes Fracture Zone
The Tjörnes Fracture Zone provides a fourth type of comparison. Here the relationship is expressed through comparatively long tectonic lines and changes in direction that can be compared directly with the calculated convection-roll divisions. This differs from Hekla, where the emphasis is on an intersection, and from South Iceland, where the emphasis is on a polygonal framework.
The Tjörnes system illustrates the more direct relationship:
The fracture zone therefore provides another independent way of testing the model. A calculated mantle division can be compared not only with volcanic structures but also with a major transform and fracture-zone system.
Four expressions of the same underlying geometry
Taken together, these examples are particularly useful because they are not repetitions of the same type of observation. They represent four different geometrical relationships:
Volcanic-zone alignment — Eastern Volcanic Zone
Intersection point — Hekla
Polygonal framework — South Iceland
Linear tectonic structure — Tjörnes Fracture Zone
The significance of Iceland is therefore not based on one volcanic zone or one particularly favourable line. The same calculated framework appears to correspond to different types of geological structures at different scales.
The sequence can be summarized as:
This is one reason why Iceland has been so important in the development of the convection-roll model. The geological structures are sufficiently detailed to allow relationships to be examined on a much smaller scale than is possible around most major oceanic plates.
From Iceland to the global system
Iceland can therefore be regarded as a high-resolution calibration area for the larger geometrical framework. The same principles that can be examined locally in Iceland—directional divisions, intersections, polygonal structures and transitions between structural lines—can subsequently be investigated on the scale of entire tectonic plates.
This creates an important connection between the Icelandic examples and the analyses of the African, Somali and Nazca plates: In Iceland, the convection-roll framework can be examined at high resolution through volcanic zones, intersection points, polygons and fracture zones. On the scale of the major plates, the same geometrical principles appear as long plate boundaries and their interaction with the larger tectonic-ring system.
The tectonic structure surrounding Iceland can be viewed as the interaction of four related but distinct components. These are: (1) the mantle convection-roll system and its regularly spaced flow lines, (2) the overlap zone between the equatorial and polar parts of that system, expressed particularly through the Greenland–Iceland–Faroe Ridge Complex, (3) the large tectonic ellipse surrounding the Icelandic shelf, and (4) the active ridge–transform–volcanic-zone system that transfers plate separation through Iceland.
The continuity of the mid-ocean ridges – Reykjanes Ridge and Kolbeinsey Ridge.
These components should not be regarded as different descriptions of the same structure. Each appears to have a different geometrical and tectonic function. The convection-roll system defines large-scale mantle-flow trajectories; the overlap zone creates a special structural domain around Iceland; the tectonic ellipse appears to delimit a region in which ridge behaviour changes; and the active volcanic and transform zones accommodate the actual plate separation through Iceland.
Taken together, these four elements provide a coherent geometrical framework for the unusually complex tectonics of Iceland and its surrounding oceanic ridges.
The mid-ocean ridges and the tectonic ring framework, compared with the convection-roll system and the elliptical form of the Icelandic Plateau.
1. The Mantle Convection-Roll Framework
In the convection-roll model, tectonic structures are related to regularly spaced mantle-flow trajectories described by circular equations. Two sets of equations are important in the Iceland region.
The equatorial system is represented by:
[(x-C_n)^2+(y-32)^2=35.34^2]
whereas the polar system is represented by:
[(x-C_n)^2+(y-96)^2=35.34^2]
The values of (C_n) occur at intervals of approximately 1.5°. Consequently, the system consists not merely of individual lines but of repeated 1.5°-wide flow domains, each of which can be regarded as containing an upwelling side and a corresponding downstream or descending side.
This distinction is important when considering the oceanic ridges north and south of Iceland. A ridge axis does not necessarily have to coincide exactly with the calculated upwelling line. Instead, the ridge may occupy a particular position within the flow cell and may migrate laterally across it. The Reykjanes and Kolbeinsey ridges provide a striking example, as south of Iceland, the relevant upwelling trajectory is represented by approximately
[C_n=-7.66]
whereas the adjacent downstream line lies at
[C_n=-9.16]
North of Iceland the corresponding pair is
[C_n=-37.66]
and
[C_n=-39.16]
The difference is 1.5° in both cases. Furthermore, the northern and southern corresponding trajectories are separated by exactly 30°:
[-37.66-(-7.66)=-30^\circ]
and
[-39.16-(-9.16)=-30^\circ]
Thus, the two oceanic ridge systems occupy equivalent positions within two geometrically corresponding mantle-flow domains.
This suggests that the relevant tectonic unit is not simply the individual upwelling line but the entire convection-roll cell.
2. The Polar–Equatorial Overlap Zone and the Greenland–Iceland–Faroe Ridge
The second component is the overlap between the equatorial and polar parts of the convection-roll system.
The two equation families intersect systematically around 64°N. Their corresponding upwelling trajectories meet near approximately 64°N, 22.7°W, while the associated downstream trajectories meet approximately 1.5° farther west, near 64°N, 24.2°W. This creates a structurally distinctive overlap zone around Iceland.
The Greenland–Iceland–Faroe Ridge Complex appears to be particularly closely related to this geometry. The Greenland–Iceland Ridge to the west and the Iceland–Faroe Ridge to the east together define a broad WNW–ESE to E–W transverse structure across the North Atlantic. Their orientation differs fundamentally from the approximately N–S to NE–SW orientation of the active Mid-Atlantic spreading system.
This transverse ridge system can therefore be interpreted primarily in relation to the overlap of the equatorial and polar convection-roll systems. The Greenland–Iceland–Faroe Ridge should consequently not be treated simply as an extension of the active spreading ridges. It represents a different structural element. The overlap zone provides a possible explanation for why the Iceland region contains an anomalously broad and thickened crustal domain crossing the otherwise longitudinal Mid-Atlantic spreading system.
In this interpretation, the Greenland–Iceland–Faroe Ridge Complex is primarily associated with the interaction between two large-scale mantle-flow systems, whereas another geometrical feature—the Icelandic tectonic ellipse—controls the outer extent of a somewhat different tectonic regime.
3. The Tectonic Ellipse Surrounding Iceland
The third component is the large ellipse surrounding the Icelandic shelf. This ellipse should be distinguished from the mantle-flow trajectories themselves. Its centre is not located at the polar–equatorial intersection around 64°N and 22–24°W. Instead, the central tectonic point associated with the Icelandic shelf ellipse lies near approximately 65.6°N, 19.7°W.
The flow-line intersections then describe the geometry of the convection-roll system, whereas the shelf ellipse appears to define a tectonic domain surrounding Iceland. Iceland lies between an inner and an outer elliptical boundary. The Greenland–Iceland–Faroe Ridge crosses this domain approximately along its WNW–ESE structural orientation, but the most revealing evidence for the significance of the ellipse may be found where the active oceanic spreading ridges cross its northern and southern margins.
At both locations, the character of the ridge system changes, and this suggests that the ellipse may function as a tectonic regime boundary rather than simply describing the shape of the Icelandic shelf.
Outside the ellipse, the oceanic ridges can follow mathematically regular convection-roll trajectories for considerable distances. Inside it, the geometry becomes more complex. The spreading system divides into oblique volcanic zones, transform zones and multiple active rift axes. The ellipse therefore appears to mark the transition between two different modes of tectonic organisation.
4. Reykjanes Ridge: Entry into the Icelandic Tectonic Domain
Southwest of Iceland, the Reykjanes Ridge follows the mantle-flow geometry over a long oceanic distance.Its position is related to the flow domain between (C_n=-7.66) and (C_n=-9.16). The former represents the calculated upwelling trajectory, while mantle flow within the model is directed westward across the cell toward the latter.
An important feature of the southern Reykjanes Ridge is that the ridge axis itself shows a tendency to migrate westward within this flow domain. Instead of remaining strictly centred on the upwelling trajectory, it approaches the downstream side of the cell. Near the inner boundary of the tectonic ellipse, this behaviour becomes particularly important.
The ridge changes orientation. The more regular oceanic ridge system gives way to the oblique spreading structure that continues toward Iceland as the Reykjanes Oblique Rift and the volcanic systems of the Reykjanes Peninsula. The change is therefore not merely a bend in an otherwise uniform ridge.
It represents a transition from an oceanic ridge following a relatively regular mantle-flow trajectory into the more complicated tectonic regime surrounding Iceland.
Farther northeast, plate separation is transferred through the South Iceland Seismic Zone. This approximately E–W transform-related zone links the western volcanic systems to the active volcanic zones farther east. The sequence can therefore be expressed schematically as:
Reykjanes Ridge → elliptical boundary → Reykjanes Oblique Rift → South Iceland Seismic Zone → Eastern Volcanic Zone.
An important point is that the physical transition is not necessarily confined to a single sharp line. Changes in ridge morphology, crustal thickness, magmatic productivity and tectonic structure occur over a substantial distance. The ellipse may therefore identify the boundary of a broader transition zone rather than a single mechanical discontinuity.
5. Kolbeinsey Ridge: The Northern Equivalent
A remarkably similar relationship occurs north of Iceland. Much of the Kolbeinsey Ridge can be approximated by the polar-system trajectory with
[C_n=-39.16]
This is particularly significant because (-39.16) is not the calculated upwelling line. The corresponding upwelling trajectory lies one flow interval farther east, at approximately
[C_n=-37.66]
Thus, the Kolbeinsey Ridge occupies a position toward the downstream side of the same type of 1.5° mantle-flow cell seen south of Iceland. In this respect, the Kolbeinsey Ridge appears to display over much of its length the same tendency that becomes particularly clear on the southern Reykjanes Ridge: the spreading axis is displaced westward from the calculated upwelling trajectory toward the downstream part of the convection-roll cell.
As the Kolbeinsey Ridge approaches Iceland, however, its behaviour changes. Near the northern boundary of the Icelandic shelf ellipse, the ridge begins to assume a much more nearly N–S orientation. It subsequently enters the complex Tjörnes Fracture Zone, through which plate separation is transferred eastward into the Northern Volcanic Zone.
The northern sequence is therefore broadly comparable to the southern one:
The correspondence is particularly important because the observed change at the northern elliptical boundary is not restricted to geometry. Geochemical studies along the Kolbeinsey Ridge indicate changes in mantle-source and basaltic characteristics in this general region. The Iceland-related geochemical influence decreases northward, and changes occur between the southern Kolbeinsey Ridge and the more northerly ridge segments toward the Jan Mayen region.
Thus, whereas the southern elliptical boundary is expressed very clearly through a change in ridge orientation and tectonic style, the northern boundary may be expressed particularly strongly through petrological and geochemical changes. The manifestations are different, but both indicate a change in tectonic regime.
6. The Transform Zones as Lateral Transfer Systems
The South Iceland Seismic Zone and the Tjörnes Fracture Zone occupy corresponding positions within the overall system. Neither is simply a continuation of an oceanic ridge, instead both transfer plate separation laterally between offset spreading systems.
In the south, the Reykjanes system is connected through the South Iceland Seismic Zone to the volcanic zones farther east, but in the north, the Kolbeinsey system is connected through the Tjörnes Fracture Zone to the Northern Volcanic Zone. This creates an approximate large-scale symmetry:
South: Reykjanes Ridge → oblique volcanic belt → SISZ → Eastern Volcanic Zone
The geometry is quite symmetrical. Both transform zones perform the same fundamental function: they allow oceanic spreading axes outside the Icelandic shelf domain to connect with the displaced volcanic axes within Iceland.
7. The Northern and Eastern Volcanic Zones as the Internal Connection
The Northern Volcanic Zone and Eastern Volcanic Zone complete the system. Once the oceanic spreading axes enter the Icelandic tectonic domain, their plate-separation function is no longer carried by one continuous Mid-Atlantic ridge axis. Instead, spreading is distributed between volcanic zones and transform systems.
The Northern Volcanic Zone receives the plate-boundary displacement transferred through the Tjörnes Fracture Zone, and the Eastern Volcanic Zone receives a substantial part of the displacement transferred eastward across South Iceland. Together, these volcanic zones form the active internal connection between the northern and southern oceanic ridge systems.
The resulting geometry can be represented as a large tectonic pathway:
Kolbeinsey Ridge
Tjörnes Fracture Zone
Northern Volcanic Zone
Central Icelandic tectonic domain
Eastern Volcanic Zone
South Iceland Seismic Zone / Reykjanes system
Reykjanes Ridge
The Mid-Atlantic plate boundary therefore remains continuous in a kinematic sense, although its surface expression changes profoundly when it enters the Icelandic elliptical domain.
8. Four Components, Four Different Functions
The resulting model becomes clearer if the four components are assigned distinct functions.
1. The convection-roll system
This defines the large-scale mantle-flow geometry. Outside the Icelandic shelf domain, both the Reykjanes and Kolbeinsey ridges follow corresponding convection-roll cells for long distances and can therefore be approximated mathematically.
2. The polar–equatorial overlap zone
This produces a special mantle-structural domain around Iceland. The Greenland–Iceland–Faroe Ridge Complex appears to be particularly strongly related to this overlap and its WNW–ESE transverse geometry.
3. The Icelandic tectonic ellipse
This defines the region within which tectonic behaviour changes. At its margins, the relatively regular oceanic ridge system begins to reorganise. South of Iceland this is expressed especially clearly by changes in ridge orientation and physical structure. North of Iceland the transition is also reflected in petrology and mantle-source characteristics.
4. The volcanic and transform system within Iceland
This accommodates active plate separation inside the elliptical domain.
The Reykjanes Oblique Rift, South Iceland Seismic Zone, Eastern Volcanic Zone, Northern Volcanic Zone and Tjörnes Fracture Zone together replace the simpler ridge-axis geometry characteristic of the surrounding ocean basin.
9. A Coherent Tectonic Interpretation
Seen in this way, Iceland is not simply an enlarged section of the Mid-Atlantic Ridge, nor can its structure be explained by one geometrical feature alone. The oceanic ridges north and south of Iceland show a long-distance relationship with calculated mantle convection-roll trajectories. Their axes may be displaced within individual flow cells, particularly toward the downstream side, but their overall geometry remains mathematically recognisable.
When these ridges approach the Icelandic shelf ellipse, however, their behaviour changes. The Reykjanes Ridge changes orientation and enters the oblique Reykjanes volcanic system before plate motion is transferred through the South Iceland Seismic Zone.
The Kolbeinsey Ridge changes toward a nearly N–S orientation and enters the Tjörnes Fracture Zone before connecting with the Northern Volcanic Zone. Within Iceland, the Northern and Eastern volcanic zones maintain the active spreading connection. Crossing this approximately N–S active plate-boundary system is the Greenland–Iceland–Faroe Ridge Complex, which appears to belong primarily to another component of the model: the overlap between the polar and equatorial convection-roll systems.
The tectonic ellipse then provides the larger boundary within which these different structural systems interact. The resulting picture is therefore hierarchical rather than based on a single tectonic cause:
mantle convection-roll geometry provides the regional framework; the polar–equatorial overlap produces the transverse Greenland–Iceland–Faroe structural domain; the tectonic ellipse defines the Icelandic regime boundary; and volcanic zones and transform belts accommodate active plate separation within that boundary.
This arrangement also explains why the Reykjanes and Kolbeinsey ridges can be mathematically predictable over long oceanic distances while becoming structurally much more complicated as they enter the Icelandic region. The mathematical mantle-flow geometry remains present, but within the tectonic ellipse it interacts with a second-order regional structure that redistributes spreading into volcanic zones, oblique rifts and transform systems.
In this interpretation, the unusual tectonics of Iceland are not an exception to the larger mantle-flow geometry. Rather, Iceland represents a region where several geometrical and tectonic systems overlap, and where their interaction becomes visible at the surface.
The African Plate is characterized by several unusually long boundary segments with predominantly N–S or E–W orientations. To the north it is bounded by the Mediterranean region, to the west by the Mid-Atlantic Ridge system, to the east by the East African–Red Sea tectonic system, and to the south by the Antarctic Plate. Within the geometrical framework considered here, these boundaries can be analysed as a sequence of segments related either to the mantle-convection-roll division system, to the elliptical boundaries of the tectonic rings, or to transitions between the two.
The African Tectonic Plate.
1. The Mediterranean and the Gibraltar sector
The Mediterranean plate-boundary system has a pronounced E–W orientation, and this general direction continues westward through the Gibraltar region into the Atlantic, along the boundary between Africa and Eurasia. The endpoints of this broad E–W system are not regarded here as arbitrary. Its eastern end is located close to a major lower-mantle convection-roll division north of the northern end of the Red Sea, while the western end approaches an important N–S axis in the Atlantic. The Mediterranean system can therefore be viewed, in this model, as an E–W connection between two larger N–S structural elements.
2. Transition to the next N–S axis
The boundary then connects with the next major N–S structural axis farther west. Part of this section has a pronounced N–S component before reaching the plate-boundary system extending from the Caribbean region into the central Atlantic. The longitudinal separation between the principal structural lines is approximately 15°. This 15° spacing is important within the convection-roll model because it represents a secondary subdivision of the larger lower-mantle geometry.
3. Southeastward segment
The next section trends toward the southeast, approximately parallel to the calculated direction of the convection-roll system. Again, the longitudinal displacement is about 15°, but in this case the shift is from west to east. The bend occurs approximately on the same longitude as the transition between Sections 1 and 2, producing a repeated geometrical relationship between the major N–S axes and the oblique plate-boundary segments.
4. Equatorial E–W segment
At the equator, the boundary turns into an almost directly E–W segment. This part extends for approximately 15° of longitude. Its position directly along the equatorial reference line makes it particularly significant in the convection-roll framework, in which the equator provides the simplest geometrical expression of the major and subsidiary mantle divisions.
5. Long N–S oceanic boundary
A particularly long section of the African Plate boundary then trends approximately N–S. This segment follows a calculated central line within the convection-roll system, located midway between two principal lower-mantle division lines. It is therefore not associated directly with one of the major lower-mantle boundaries, but with the geometrically defined centre of the interval between them.
6. Southeastward turn toward the Antarctic triple-junction region
The boundary then turns toward the southeast, continuing to the region where the South American, Antarctic and African plates meet. Within the convection-roll model, this junction is also located close to an intersection of major lower-mantle division lines. The plate triple junction and the calculated mantle intersection therefore occupy approximately the same structural region.
7. E–W boundary toward the Atlantic ellipse
From this intersection, the plate boundary again assumes an approximately E–W orientation. It extends from the lower-mantle convection-roll intersection toward the inner elliptical boundary of the Atlantic tectonic ring. This section therefore forms a direct geometrical link between the convection-roll framework and the younger elliptical tectonic system.
8. Along the inner Atlantic ellipse
The boundary then turns toward the NNE and follows the inner ellipse of the Atlantic tectonic ring for a considerable distance. Here the elliptical geometry becomes the dominant structural relationship. This is an important contrast with the preceding sections, where the plate boundary was more closely associated with the convection-roll divisions.
9. Excursion into the African continent
Farther north, the boundary bends into the African continent before turning back toward the inner Atlantic ellipse. It returns to the region where the inner Atlantic ellipse and the inner ellipse of the Indian Ocean tectonic ring approach one another and become closely associated. This is the same broad geometrical interaction zone discussed in connection with the East African Rift and Lake Malawi.
10. African–Somali Plate boundary
Along this section, the boundary of the African Plate coincides with the boundary of the Somali Plate. The plate boundary occupies the narrow region associated with the two inner tectonic ellipses. This section is therefore especially important because the same geometry can be analysed from either side: as the eastern boundary of the African Plate or as the western boundary of the Somali Plate.
11. From the inner to the outer Atlantic ellipse
Farther north, the plate boundary leaves the inner Atlantic-ring boundary and crosses toward the outer Atlantic ellipse. Its orientation corresponds approximately to the direction of the underlying convection-roll system. The segment terminates at the Afar Triple Junction. This provides another clear example of a plate boundary transferring between two elements of the model: inner tectonic ellipse, convection roll alignment, outer tectonic ellipse.
12. The Red Sea
From Afar, the boundary continues northward along the Red Sea. In the present geometrical interpretation, the Red Sea follows the outer ellipse of the Atlantic tectonic ring. Near its northern end, the boundary develops a more pronounced N–S orientation, marking another transition between the elliptical tectonic geometry and the principal directional structure of the convection-roll system.
The African Plate as a large-scale geometrical structure
The African Plate provides a particularly useful continental-scale example because its boundary includes several fundamentally different types of tectonic environment: mid-ocean spreading ridges, continental rifts, convergent margins and complex transition zones. Despite these differences, the boundary can be divided into a sequence of relatively simple geometrical segments. Some are predominantly N–S or E–W, some follow calculated convection-roll divisions or intermediate lines, and others follow the inner or outer ellipses of the Atlantic and Indian Ocean tectonic rings.
A recurring feature is the approximately 15° longitudinal spacing between several major structural elements. The boundary repeatedly shifts from one such element to another before continuing along a new N–S, E–W or oblique direction.The African Plate is therefore not interpreted here as following one single geometrical structure. Rather, its outline appears to result from the interaction of an older mantle-convection framework with the younger elliptical tectonic-ring system.
The African Plate provides a particularly clear continental-scale example of how plate boundaries may alternate between mantle-convection divisions, equatorial and meridional structural axes, and the elliptical boundaries of the Atlantic and Indian Ocean tectonic rings.
The boundaries of the Nazca Plate can be analysed in relation to both the mantle-convection-roll division system and the elliptical tectonic boundaries associated with the Ring of Fire and the Atlantic tectonic ring. The principal segments of the plate boundary are numbered below.
The Nazca Plate compared with Convection Rolls Model.
1. The northwestern corner near the equator
At the equator, the northwestern corner of the Nazca Plate is characterized by plate boundaries with conspicuous N–S and E–W orientations. The N–S structures are approximately symmetrical with respect to the equator, while the complex geometry of the northwestern corner includes the Galápagos Microplate.
Within the convection-roll model, the N–S segment can be interpreted as a connection between two principal division lines of the lower-mantle convection system. To the north, the plate boundary then assumes a much more clearly E–W orientation. This makes the equatorial region particularly important, because two of the principal directional components of the proposed mantle geometry meet there.
2. A lower-mantle convection-roll division
The next section of the plate boundary follows very closely a calculated division line between lower-mantle convection rolls. Its western endpoint is also significant: it occurs close to the point where this division is intersected by another major division of the convection-roll system. The geometry of this segment can therefore be described largely in terms of the underlying mantle framework, without requiring an elliptical tectonic boundary to explain its orientation.
3. The major N–S boundary and the Antarctic axis
The boundary then turns into a pronounced N–S segment.
This section is particularly interesting because it lies approximately along the same longitude as the major axis of the Antarctic ellipse in the geometrical framework considered here. The Easter Microplate occurs along this general tectonic corridor, while farther south the Juan Fernández Microplate is also associated with the same broad N–S plate-boundary system.
The correspondence therefore involves not only the orientation of the Nazca Plate boundary but also two important microplates situated along it.
4. The eastward turn
The plate boundary then makes an abrupt turn of almost 90° toward the east. It continues eastward until it approaches the proposed outer elliptical boundary of the Atlantic tectonic ring, in a region where the Atlantic-ring geometry extends into and overlaps the broader geometry of the Pacific Ring of Fire. This represents a transition from a boundary segment primarily associated with the convection-roll system to one that can be examined in relation to the tectonic ellipses.
5. Along the outer Atlantic ellipse
From this point, the plate boundary turns toward the southeast and follows the outer ellipse of the Atlantic tectonic ring for a considerable distance. It continues in approximately this direction until it approaches the region where the outer Atlantic ellipse and the inner Pacific ellipse converge or intersect. This is therefore another example in which a plate boundary appears, within the present model, to occupy an interaction zone between two independently defined tectonic rings.
6. The return toward South America
Near the intersection of the two elliptical boundaries, the plate boundary changes direction again and turns eastward toward the South American continent. The location of this bend is important because it occurs close to the geometrically calculated transition between the Atlantic and Pacific elliptical systems. Thus, the change in direction is not treated here as an isolated feature of the Nazca Plate boundary, but as part of a larger geometrical pattern.
7. The southwestern margin of South America
Along the southwestern coast of South America, the plate boundary assumes a conspicuous N–S component. This direction can also be represented by a major N–S axis within the convection-roll system.The correspondence is especially interesting because this is a convergent boundary rather than a spreading ridge. The same underlying geometrical direction is therefore expressed through a very different type of plate interaction.
8. The northwestern turn along South America
Farther north, approximately in the central part of the western margin of South America, the plate boundary turns toward the northwest. Within the convection-roll model, there is a close correspondence between the calculated direction of the convection-roll divisions and the general orientation of the continental margin. The coastline and subduction system therefore appear to change orientation in a manner broadly consistent with the changing direction predicted by the underlying mantle geometry.
9. Return to the equator
As the plate boundary approaches the equator, its orientation again becomes predominantly N–S. This region coincides with one of the principal equatorial points in the convection-roll model. Such points recur at approximately 30° intervals along the equator and are interpreted in this framework as major divisions of the lower-mantle convection system. Several of these equatorial points correspond to important transitions between major crustal domains, including oceanic and continental plate systems. The northern termination of the Nazca Plate therefore brings the boundary system back to the same equatorial geometrical framework from which the analysis began.
The Nazca Plate as a combined geometrical system
Taken as a whole, the Nazca Plate provides a particularly useful example of how several geometrical systems may interact.
Some sections of its boundary closely follow major lower-mantle convection-roll divisions. Other segments correspond more closely to the inner or outer elliptical boundaries of the proposed Atlantic and Pacific tectonic rings. Still others coincide with major N–S axes, including the continuation of an axis defined by the geometry of the Antarctic Plate.
The most important observation is therefore not that the entire Nazca Plate follows a single geometrical structure. Rather, its boundary appears to transfer repeatedly from one structural element to another, with major bends occurring near intersections between these systems.
The Nazca Plate therefore illustrates how a single plate boundary may be organized by several overlapping geometrical systems: lower-mantle convection-roll divisions, equatorial reference points, Antarctic axes, and the elliptical boundaries of the Atlantic and Pacific tectonic rings.
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