Tag: geology

It has long been recognized that the Pacific Ring of Fire is linked, within the global plate-tectonic system, to the mid-ocean ridges of both the Atlantic and Indian Oceans. New oceanic crust is generated along the spreading ridges, while large amounts of old oceanic lithosphere are returned to the mantle through subduction around the margins of the Pacific. There are also major spreading ridges within the Pacific itself. Nevertheless, the Pacific differs fundamentally from the other two major oceanic domains: instead of being dominated by a central ridge system bounded largely by passive continental margins, much of its circumference is characterized by active plate boundaries, subduction zones, volcanic arcs and major fault systems. This is the basis for the familiar concept of the Pacific Ring of Fire.

By analysing the Ring of Fire more closely in relation to the proposed mantle convection-roll system, and drawing in particular on the detailed structural relationships that can be studied in Iceland and compared with other regions, it became possible to define two broad elliptical forms—an inner and an outer tectonic ellipse—within which most of the major active structures associated with the Ring of Fire can be placed. I presented this geometric interpretation at Stanford University in 2024.

The fit is not perfect, nor should it necessarily be expected to be. Some major tectonic structures initially appear to be exceptions. The Tonga–Kermadec system, for example, lies well inside the zone defined between the inner and outer boundaries of the Pacific Ring of Fire. This was difficult to explain if the Pacific system was considered in isolation.

A more important geometrical relationship became apparent when the inner ellipse was examined in greater detail. Its equatorial width is approximately 120° of longitude. Earlier work had concentrated more heavily on the outer ellipse, which theoretically spans about 150° from east to west at the equator. In practice, the outer boundary extends slightly beyond 150°, its geometric centre lies somewhat south of the geographic equator, and the ellipse itself is oblique: its general elongation is from northwest to southeast, while its minor axis therefore trends approximately southwest–northeast.

The 120° width of the inner ellipse raises a fundamental possibility. Since 120° is exactly one third of 360°, the Ring of Fire may not represent an isolated tectonic geometry. It becomes possible to test whether the Earth’s large-scale tectonic framework contains a threefold, approximately 120° organization.

When three comparable tectonic ellipses—or, more accurately, three pairs of inner and outer ellipses—are projected around the Earth, the resulting geometry shows a striking degree of correspondence with major geological structures. These may provisionally be called the Pacific Ring, the Indian Ocean Ring, and the Atlantic Ring.

This broader geometry changes the interpretation of several apparent anomalies.

The Tonga–Kermadec subduction system, rather than being an unexplained feature lying inside the Pacific Ring, falls close to the outer boundary of the Indian Ocean tectonic ellipse. In this interpretation it is located in an overlap zone between two large tectonic systems.

Similarly, the Red Sea no longer appears simply as an isolated, unusually linear rift system. It lies close to the outer boundary of the Atlantic ellipse. From there, the same broad tectonic boundary can be followed through Europe, where I described relationships between tectonic structures and the proposed convection-roll pattern in my 2023 Stanford presentation, and northward toward Iceland.

Iceland itself is situated on the outer boundary of the Atlantic tectonic ellipse. This provides a potentially important geometric context for its exceptional combination of a mid-ocean ridge, thickened crust, intense volcanism and major tectonic segmentation.

The geometry becomes particularly interesting where the large ellipses overlap. The continuation of the Atlantic ellipse toward North America approaches the regions of Yellowstone and the San Andreas fault system. At approximately the same locations, the boundaries of two tectonic ellipses intersect and the minor axes of the ellipses are expressed.

Thus, three independent geometrical relationships appear in approximately the same areas:

1. boundaries within the proposed lower-mantle—and, at smaller scales, upper-mantle—convection-roll system;
2. the minor axes of the large tectonic ellipses; and
3. intersections between the boundaries of neighbouring tectonic ellipses.

What makes this particularly interesting is that several exceptionally distinctive geological regions occur near such intersections. Yellowstone, the San Andreas system and New Zealand are obvious examples. They represent very different kinds of geological environments, yet all occupy unusually complex positions within the proposed global geometry.

This distinction is important. The model does not imply that Yellowstone, San Andreas and New Zealand were created by an identical local process. Rather, their positions may indicate places where several components of the global tectonic system interact. The geometry may therefore describe the framework within which different tectonic processes operate, rather than a single mechanism producing identical structures everywhere.

The three tectonic rings

The three proposed systems also have distinctly different geological expressions.

The Pacific Ring is exceptional because there is no major continent occupying the interior of its inner ellipse. The Pacific Plate and associated oceanic plates therefore dominate a huge part of the system. Subduction around its margins is correspondingly prominent, producing the classical Ring of Fire.

The Atlantic Ring has a different character. Its most obvious feature is the remarkable continuity of the Mid-Atlantic Ridge system from the Southern Ocean northward through the Atlantic and into the Arctic domain. Rather than being surrounded predominantly by subduction zones, as in the Pacific, the Atlantic contains a major spreading system through its central region. Its outer tectonic ellipse consequently interacts extensively with continental lithosphere in Africa, Europe and the Americas.

The Indian Ocean Ring is different again and may be regarded as fundamentally divided between north and south. Its southern part is dominated by oceanic lithosphere and the interconnected Indian Ocean ridge system, whereas its northern part is occupied by the large continental masses of Africa, Arabia, India and Eurasia and by the effects of major continental collision. Nevertheless, a comparable broad elliptical geometry can still be traced through the system.

The three rings therefore need not produce identical surface geology. Their importance lies instead in the possibility that a common large-scale geometry is being expressed through three very different lithospheric configurations.

One particularly striking example is the eastern margin of Australia, which lies close to the inner boundary of the proposed Indian Ocean ellipse. Likewise, the relationship between the Izu–Bonin system in the Northern Hemisphere and Tonga–Kermadec in the Southern Hemisphere becomes more meaningful when both are considered in relation not only to the Pacific Plate but also to the underlying boundaries proposed for the convection-roll system. Both appear to be associated with the same broad division in the western Pacific mantle system, even though their surface tectonic histories and plate geometries differ.

The Atlantic sector provides another useful example. When the proposed lower-mantle divisions are projected northward from Antarctica, one of the principal boundaries can be followed toward the North Atlantic and Iceland. The apparent continuity between Southern Ocean geometry, the Atlantic ridge system, Europe and Iceland is difficult to appreciate when these regions are examined individually, but becomes much clearer when they are placed within a single global geometric framework.

How could such rings form?

The origin of these large elliptical tectonic patterns is, of course, a much more difficult theoretical question than simply identifying their geometry.

The lithosphere cannot be treated as a perfectly rigid shell. Although its upper portion behaves rigidly over geological timescales, its deeper parts interact with the ductile asthenosphere and upper mantle. The large-scale geometry observed at the surface must therefore result from a combination of processes acting over very long periods.

At least three components may be involved:

• the rotation of the Earth and the directional constraints that rotation imposes on global-scale flow;
• persistent forcing associated with organized convection within the mantle;
• and horizontal displacement and deformation of the lithospheric plates themselves.

These processes would not operate independently. A plate moving horizontally across the Earth’s surface is simultaneously affected by its own internal stresses, by interactions with surrounding plates, by ridge generation and slab subduction, and by stresses transmitted from the underlying mantle. Continental and oceanic lithosphere would respond differently because of their very different thicknesses, densities, compositions and mechanical histories.

This may help explain why the same underlying geometrical system could produce such different surface expressions in the Pacific, Atlantic and Indian Ocean regions.

The concept should therefore not be interpreted as three rigid circles stamped onto the Earth’s crust. A better analogy is a set of overlapping, deformable tectonic domains, each responding to the same global physical system but modified by the heterogeneous lithosphere through which the forces are expressed. Their boundaries may consequently shift, broaden, bifurcate or become locally obscured.

This also provides a possible explanation for why the ellipses do not correspond perfectly to every geological structure. A geometrical model operating at mantle scale should define preferred zones and relationships rather than mathematically exact surface lines. Continental collision, inherited crustal structures, changing plate boundaries and the creation or destruction of oceanic lithosphere will inevitably modify their surface expression.

What is significant is therefore not that every tectonic boundary lies precisely on an ellipse, but that a recurring 120° geometry appears to organize a surprisingly large number of otherwise separate tectonic features.

If this threefold relationship is real, the Pacific Ring of Fire should no longer be viewed solely as a unique circum-Pacific phenomenon. It would instead represent the clearest surface expression of one member of a larger global system. The Atlantic and Indian Ocean systems would be less visually obvious because their lithospheric environments are different, but geometrically they could belong to the same underlying organization.

That possibility opens a much larger field for investigation. The exact positions of the inner and outer boundaries can be tested against ridge segments, transform faults, subduction zones, volcanic arcs, continental rifts, major fault systems and seismic structures. Their intersections can be examined independently, as can their relationship with predicted mantle-flow boundaries at different depths.

Particular attention should be given to locations where several elements coincide: ellipse intersections, minor-axis positions and independently predicted mantle boundaries. Such locations provide the strongest tests of the model because their positions can be predicted geometrically before the local geology is examined.

In this sense, the purpose of the model is not simply to draw ellipses around existing tectonic structures. Its scientific value depends on whether the geometry can be defined independently and then used to predict where unusual tectonic relationships should occur.

The Pacific Ring of Fire, the Atlantic system and the Indian Ocean system may therefore be three different surface expressions of a single, much larger geometrical organization of the Earth. The differences between them are substantial—but those differences may be exactly what should be expected when the same underlying mantle framework interacts with three fundamentally different arrangements of continents, oceans and lithospheric plates.

And this is where the analysis can now be taken considerably further.

Once one large-scale pattern in nature has been identified and described, it becomes easier to recognize analogous structures elsewhere. The Pacific Ring of Fire therefore does not necessarily have to be treated as a completely unique tectonic phenomenon.

After examining the geometry of the Ring of Fire in greater detail, its inner tectonic ring appears to span approximately 120° from east to west. This immediately raises a broader possibility: if one such 120° tectonic ring exists, comparable rings may be fitted around the rest of the Earth.

A complete circumference contains three such sectors: 3 × 120° = 360°.

This suggests a possible global system consisting of three major tectonic rings:

• the Pacific Ring of Fire,
• the Indian Ocean Ring,
• and the Atlantic Ring.

The comparison appears to work surprisingly well when these circles are placed against the known tectonic structure of the Earth.

The Indian Ocean Ring

The proposed Indian Ocean Ring provides one of the clearest examples. Its relationship with New Zealand is especially important because this is where it overlaps the Pacific Ring of Fire. The Kermadec–Tonga system, which appears to lie unusually far inside the broader Pacific Ring of Fire, can then be viewed differently.

Rather than being simply an inward deviation of the Pacific Ring, Kermadec–Tonga may belong primarily to the Indian Ocean Ring, within the region where the two tectonic rings overlap.This provides a possible geometric explanation for the complicated tectonic structure of the southwest Pacific. From Kermadec–Tonga, the same system can be followed northward through the Fiji region and towards the Philippine Sea Plate. The northern boundary of the Philippine Sea Plate lies close to the outer geometry of the proposed Indian Ocean Ring.

Farther inland, Lake Baikal also falls within the same large-scale ring. This is especially interesting because Baikal represents one of the world’s most prominent examples of active continental rifting. Its position may therefore be examined not only in relation to regional tectonics, but also within this larger geometric system.

The ring continues westward through regions of major tectonic activity. Turkey falls within it, as does Italy, close to the area where the proposed Indian Ocean Ring and Atlantic Ring overlap. The Cameroon Volcanic Line also lies within the geometry of the Indian Ocean Ring. This is particularly noteworthy because the Cameroon Line is an unusual intraplate volcanic structure that crosses both continental and oceanic crust.

None of these individual correspondences is sufficient by itself to establish the existence of such a ring. Their significance lies in whether the same geometric relationship repeatedly appears when independently mapped geological features are compared with the proposed system.

Erebus as an example of overlapping geometries

Mount Erebus in Antarctica provides a useful example of how several geometrical systems may interact. Erebus lies outside the main Pacific Ring of Fire, although it is not far from it. It is also situated slightly away from the minor axis of the elliptical Antarctic Plate geometry.However, when the proposed Indian Ocean Ring is added to the map, Erebus falls close to its outer boundary.

This illustrates an important principle of the model. A geological feature does not necessarily have to be explained by its relationship with only one geometric structure. Its position may instead result from the intersection or overlap of several systems: Antarctic plate geometry + mantle convection-roll divisions + global tectonic rings. A volcanic centre such as Erebus may therefore become particularly informative when its position is analysed simultaneously with respect to all three.

Relationship with the Mantle Convection Roll Model

The three tectonic rings also show a close relationship with the southern part of the Mantle Convection Roll system. From approximately 15°S to 60°S, considerable portions of the proposed rings follow the curvature of the convection-roll pattern and the corresponding division lines in the lower mantle. This relationship is important, but the tectonic rings and the mantle convection rolls should not be regarded as the same type of structure.

The convection rolls represent a three-dimensional mantle-flow system. The tectonic rings, by contrast, are essentially horizontal lithospheric structures, expressed through the tectonic interaction of plates at or near the Earth’s surface. In the interpretation proposed here, the tectonic rings develop as the lithosphere responds to the stresses generated by plate motion, while the larger-scale organization and direction of that motion are influenced by the underlying mantle-flow system.

The relationship can therefore be expressed schematically as:

Earth’s rotation
→ organization of mantle convection
→ systematic mantle-flow geometry
→ plate motion and lithospheric stress
→ large-scale tectonic ring structures.

The rings are therefore not themselves mantle convection rolls. They represent a surface-level tectonic response to a deeper global system.

The importance of the South Pole

When the Pacific, Indian Ocean and Atlantic rings are examined together with the Antarctic Plate, another important feature emerges: the South Polar region becomes a major geometrical reference point.

The elliptical geometry of the Antarctic Plate, its major and minor axes, the possible 60° sector division around Antarctica, and the geometry of the three 120° tectonic rings all appear to interact in this region.

This may help explain why Antarctica shows such pronounced large-scale symmetry around the pole.

The recently identified fan-shaped basin structures beneath East Antarctica add another independent observation to this picture. Their calculated Euler poles lie close to the geographic South Pole, indicating that large-scale deformation within Antarctica itself has developed around a nearly polar centre.

The tectonic rings then extend this geometry outward from Antarctica into the surrounding plate system.

A framework for further investigation

The most important value of this approach is not simply the possibility of drawing three large circles around the Earth.

The real research potential lies in examining the exact position of individual geological structures relative to several independent geometrical systems at the same time.

For any particular tectonic or volcanic region, it becomes possible to ask:

• Where does it lie relative to the inner and outer boundaries of a tectonic ring?
• Is it located where two rings overlap?
• How does it relate to the major or minor axis of the Antarctic Plate?
• Does it coincide with a mantle convection-roll division?
• Does it occur near a lower-mantle boundary or a predicted change in flow direction?
• Does its observed tectonic style agree with what would be expected from that position?

This opens a much wider field of comparison than studying each geological feature only in its local setting.

The Pacific Ring of Fire remains the clearest of the three systems because its tectonic boundaries are exceptionally well expressed. As its geometry becomes more precisely defined, however, the corresponding Indian Ocean Ring and Atlantic Ring also become easier to identify.

This may eventually allow the three rings to be analysed as parts of a single global tectonic framework rather than as unrelated regional structures.

One important question remains unresolved: how the Arctic Ocean fits into this geometry.

The southern hemisphere currently provides the clearest relationships because Antarctica offers a well-defined polar reference system and because the curvature of the proposed tectonic rings corresponds closely with the southern mantle-convection geometry. The northern polar region is considerably more complex, particularly because of the configuration of the Arctic Ocean, Eurasia and North America.

Nevertheless, the picture becomes progressively clearer as the Pacific Ring of Fire and the proposed Indian Ocean and Atlantic rings are defined more precisely.

The key point is therefore not that three tectonic rings have already been demonstrated as established geological entities. Rather, the same geometrical method that helps define the Pacific Ring of Fire can be applied globally. When this is done, major tectonic structures on different continents and ocean basins repeatedly fall into coherent relationships with three approximately 120° systems.

That correspondence is sufficiently systematic to justify much more detailed comparison with the independently derived geometry of the Mantle Convection Roll Model.

Starting from the possible sixfold division around Antarctica, it is possible to examine how this geometry relates to the tectonic plates and major tectonic systems outside the Antarctic region.

The Pacific Ring of Fire is particularly important in this respect because its geometry can be analysed in considerable detail. When examined in relation to the Mantle Convection Roll model, its overall form becomes even more clearly defined.

Several key locations show remarkably simple angular relationships when measured from the geographic South Pole. For example, the angle between the directions towards the Alpine Fault in New Zealand and Yellowstone is approximately 90°. This corresponds with another geometric relationship already identified: the Alpine Fault lies close to the continuation of the minor axis of the Antarctic ellipse, whereas Yellowstone lies close to the continuation of its major axis.

Thus, two major parts of the Ring of Fire can be related directly to the two perpendicular axes of the Antarctic geometry. The Ring of Fire also extends southward into the Antarctic tectonic system. This makes it possible to examine its geometry not only as a Pacific feature, but as part of a larger global arrangement.

From 90° and 45° to 60° and 120°

The two principal axes associated with the geometry of the Ring of Fire are themselves inclined by approximately 45° relative to one another in the larger construction used here. Once the possible 60° sector division around Antarctica is introduced, another question naturally follows:

How does the geometry of the Ring of Fire relate to 60° and 120° angular divisions? An interesting relationship appears at the equator, as the outer limits of the Pacific Ring of Fire, measured approximately from Indonesia to the western margin of South America, span about 150° of longitude.

However, the inner margins of the Ring of Fire lie approximately 15° inward from each outer margin.

Therefore we get; 150° − 15° − 15° = 120°, so the effective width of the inner Ring of Fire is consequently about 120°.

This is particularly significant because 120° is exactly one third of a complete circle: 360° / 3 = 120°.

It therefore becomes possible to construct three comparable great tectonic ring systems around the Earth, each displaced by approximately 120° from the next.

These may provisionally be called:

1. The Pacific Ring of Fire
2. The Indian Ocean Ring
3. The Atlantic Ring

The Pacific Ring is by far the most conspicuous of the three because the tectonic boundaries around the Pacific are exceptionally well developed: long subduction zones, major transform systems, volcanic arcs and associated seismic belts together produce the familiar Ring of Fire.

The other two proposed rings are less visually obvious. Their boundaries are interrupted by continents, ocean basins and tectonic systems of different ages, and they have not generally been considered as parts of equivalent circular structures. Nevertheless, when their positions are examined in the same geometric framework, substantial agreement with known geological features becomes apparent.

Three overlapping 120° systems

The important point is that these three rings should not necessarily be imagined as three completely separate circles placed side by side, and they overlap.

If three equivalent rings are arranged around the globe so that the inner margins of neighbouring rings meet or intersect at the principal division points, a repeating global pattern is produced. In this construction, each ring occupies approximately a 120° internal sector, while the outer tectonic expression may extend farther and overlap with the neighbouring ring.

This overlap may be geologically important. The Pacific Ring of Fire itself becomes easier to understand when viewed in this way.

Kermadec–Tonga and the overlap with the Indian Ocean Ring

One of the most interesting examples is the Kermadec–Tonga system. Kermadec–Tonga lies noticeably inside the broader geometrical outline of the Pacific Ring of Fire rather than simply following its outermost boundary. In a model based only on a single Pacific ring, this position appears somewhat anomalous.

However, if the Indian Ocean Ring overlaps the Pacific Ring, the position of Kermadec–Tonga becomes much more natural. It lies within the zone where the two systems interact.

Thus, what may initially appear to be an irregularity in the Pacific Ring may instead represent the superposition of two larger tectonic geometries:

Pacific Ring + Indian Ocean Ring.

This would also help explain why the southwest Pacific is tectonically much more complicated than a simple elliptical outline would suggest. The region around New Zealand, Kermadec, Tonga, Fiji and the neighbouring back-arc systems lies precisely where different large-scale tectonic systems would be expected to overlap.

In this interpretation, the complexity is therefore not necessarily evidence against the larger geometry. It may actually be one of its consequences.

Antarctica as the geometric reference

Antarctica provides an unusually useful reference system for examining these relationships because several different geometries appear to be centred on, or organized around, the South Polar region.

At least three levels can be distinguished:

A global rather than a regional geometry

The important point is not simply to draw three circles on a map and search for features that happen to lie upon them. The relevant question is whether independently mapped geological structures repeatedly coincide with the predicted boundaries, axes, intersections and overlap zones.

The Pacific Ring of Fire provides the clearest test because its tectonic structure is so well mapped. Its inner and outer margins can be distinguished, its major axes can be compared with Antarctic geometry, and key locations such as the Alpine Fault, Yellowstone and Kermadec–Tonga provide additional geometric constraints.

If the same construction can then be followed through the Indian Ocean and Atlantic regions, using independently mapped ridges, subduction zones, continental margins, transforms and other major tectonic boundaries, the comparison becomes considerably more significant.

The proposed relationship can therefore be summarized as:

Antarctic 60° sectors
→ pairs of sectors forming 120° divisions
→ three overlapping tectonic rings around the Earth.

The three proposed systems are:

Pacific Ring of Fire — Indian Ocean Ring — Atlantic Ring.

Of these, the Pacific Ring is presently the clearest geological expression. The Indian Ocean and Atlantic rings require more detailed examination, but their predicted positions already coincide with several major tectonic structures.

Perhaps most importantly, viewing the three rings together does not weaken the familiar geometry of the Pacific Ring of Fire. It may actually make it more complete, because features such as the inward position of the Kermadec–Tonga system can be interpreted as the result of overlap between neighbouring global tectonic systems rather than as departures from the general pattern.

A striking feature of the Icelandic plateau is its broadly elliptical outline. The ellipse discussed here has its centre at approximately 65.600°N, 19.625°W. Its major axis is oriented exactly east–west along latitude 65.600°N, extending from approximately 27.300°W in the west to 11.950°W in the east. Its minor axis extends from approximately 63.000°N in the south to 68.200°N in the north.

Map base: https://www.researchgate.net/publication/316330629_The_Greenland-Iceland-Faroe_Ridge_Complex

Using longitude λ and latitude ϕ, the ellipse is defined approximately by:

where λ denotes longitude, with western longitudes expressed as negative values, and ϕ denotes latitude.

The shape is particularly clear in the southeastern part of the Icelandic shelf, where relatively little later tectonic disturbance appears to have modified it. The most significant deviations occur in the northern sector, particularly around the Kolbeinsey Ridge, where younger spreading processes have substantially reorganized the crust. On geological and bathymetric maps, however, the elliptical geometry remains conspicuous. Of particular interest is that geological boundaries between the Icelandic part of the Greenland–Iceland–Faroe Ridge Complex and the adjoining ridge segments appear in places to follow the elliptical outline itself.

A stretched continental-crust interpretation

The geometry becomes especially interesting if the hypothesis proposed by Foulger and others is adopted as a working assumption: namely, that substantial amounts of stretched continental crust may remain beneath Iceland and parts of the Greenland–Iceland–Faroe Ridge Complex.

In such a model, Iceland would not consist exclusively of anomalously thick oceanic crust generated by excessive mantle melting. Instead, an older continental crustal component may have been progressively stretched, thinned, intruded by magma and covered by basalt during the opening of the North Atlantic.

If the remaining continental component beneath Iceland is approximately 20 km thick, whereas the original crust may have been 35–40 km thick, the crust must have undergone considerable areal extension. A simple volume-conservation calculation suggests that the original area could have been roughly one-half of the present affected area if the original crust was about 40 km thick.

The important point, however, is that the original block need not have been circular. It was probably elongated, because it represented part of the continental material being divided as Greenland separated from Europe. Its original structural connections would therefore naturally have extended toward Greenland on one side and toward the Faroe region on the other.

The Icelandic ellipse as a deformation envelope

The Icelandic ellipse should therefore not necessarily be interpreted as the original outline of a single continental fragment. A more plausible possibility is that it represents the finite deformation envelope of a wider zone of stretched continental crust.

The original continental material may have formed an irregular or elongated block, while long-term extension, lower-crustal flow, repeated rift relocation and magmatic intrusion produced a much smoother final deformation field. Over tens of millions of years, deformation distributed through a hot and mechanically weak crust can become considerably more regular than the original structural boundaries.

This possibility is compatible with several elements of Foulger’s model, particularly distributed extension, unstable and migrating spreading axes, broad transfer zones and ductile lower-crustal flow. Foulger does not propose the Icelandic ellipse itself, but the physical processes invoked in that model provide mechanisms through which a broad and relatively smooth deformation envelope could develop.

East–west extension

The major axis of the ellipse lies precisely east–west along 65.600°N. Within the mantle-convection-roll model, this is also an important structural latitude. The line passes through the central part of the northern volcanic-zone system when measured from north to south and through the geometric centre of the shelf ellipse.

The principal crustal extension can therefore be considered primarily east–west. This does not mean, however, that all deformation or material transport must have occurred east–west. The geometry suggests that the principal extension and the directions along which deformation was distributed may be different.

A useful distinction can therefore be made between the principal direction of extension, which was east–west, and the broader distribution of deformation, which appears to have taken place along northeast–southwest and northwest–southeast trends. This relationship may be fundamental to understanding the overall form.

Mirror-symmetric mantle boundaries

Within the mantle-convection-roll model, the major boundary associated with the Reykjanes Ridge continues through the central region beneath Iceland. It follows a curved SW–NE trajectory. A second underlying boundary forms a geometrical mirror image of this structure.

Both pass through the central region of the Icelandic ellipse. These two oblique systems can be considered mirror-symmetric components of a larger deformation field. This is mechanically important because two equally strong oblique deformation systems of opposite orientation can cancel their net shear while reinforcing their extensional components.

Schematically, if one system contributes a positive shear component, +γ, and its mirror-symmetric counterpart contributes an equal negative shear component, −γ, the two shear components cancel:

+γ+(−γ)=0

The net shear is therefore zero. The combined strain field can therefore retain principal axes oriented exactly east–west and north–south even though the underlying structural boundaries themselves are oblique. This offers a possible explanation for an otherwise curious feature: the Greenland–Iceland–Faroe system trends obliquely across the region, yet the Icelandic shelf ellipse itself is not similarly rotated. Its principal axes remain E–W and N–S.

Connections toward the Faroes and Greenland

The southeastern side of the ellipse connects with the Faroe segment of the ridge. The connection begins close to the eastern end of the major axis, near 65.6°N, 11.9°W, and continues southwestward toward approximately 63.3°N, 15.2°W.

This connection crosses the underlying convection-roll system obliquely rather than following a single roll boundary. According to the convection-roll model, it spans approximately five adjacent convection-roll elements, each separated by about 1.5° in longitude. Because the connecting zone crosses them obliquely, however, its real length and width cannot simply be calculated as (5\times1.5^\circ). The number five describes the number of underlying mantle units involved, rather than a direct geographical distance.

A corresponding connection occurs northwest of the major axis toward the Greenland segment of the ridge. It similarly extends across approximately five convection-roll elements but lies north of the 65.600°N major axis. The two connections therefore form complementary components of the same system:

• a southeastern connection toward the Faroes;
• a northwestern connection toward Greenland.

The Icelandic region between them is considerably wider than either connecting zone.

Why does the Icelandic region widen so much?

This may be one of the most important mechanical questions. If the connections toward Greenland and the Faroes are relatively narrow, why does the central Icelandic deformation zone expand into a broad ellipse roughly 700 km across? The answer may lie in the difference between a transfer zone and the deformation domain receiving the strain.

The Greenland and Faroe connections need not themselves be as wide as the Icelandic ellipse. Instead, they may transmit stress and deformation into a central region where the crust is hotter, weaker and more heavily intruded by magma. Once deformation enters such a mechanically weak domain, it can spread laterally.

A simplified sequence would therefore be: narrow oblique connection → broad, mechanically weak crustal domain → distributed extension and lower-crustal flow → development of an elliptical finite-strain envelope.

In this interpretation, the ellipse does not reflect the width of a single fault or rift. It represents the cumulative region over which deformation has been distributed.

Geological asymmetry superimposed on geometric symmetry

An important feature is that the ellipse is not preserved equally well on all sides. The southeastern sector appears especially regular. The geological boundary between the Icelandic region and the Faroe segment also seems to follow the elliptical geometry particularly clearly.

The Greenland-facing side appears sharper, whereas the Faroe-facing transition is broader and less abrupt. This difference does not necessarily contradict an originally symmetric underlying system. The subsequent tectonic histories of the two sides were different.

The largest disturbance occurs north of the 65.600°N axis around the Kolbeinsey Ridge. Spreading was reorganized in this region after the decline of the Aegir Ridge, and younger volcanic and spreading systems developed. These processes could have modified or overprinted an older elliptical deformation boundary.

Thus a distinction can be made between: primary geometric symmetry, controlled by the deeper system, and: secondary geological asymmetry, produced by later rift relocation, volcanism and crustal accretion.

Symmetry in degrees rather than kilometres

Another unusual aspect of the ellipse is that its symmetry is expressed in geographic angular coordinates. It is exactly symmetric about 65.600°N in latitude and 19.625°W in longitude. However, the Earth is spherical, and one degree of longitude becomes progressively shorter toward the north. Consequently, although the ellipse is geometrically symmetric in degrees, its northern half is physically smaller than its southern half when measured in kilometres.

At approximately 63°N, one degree of longitude corresponds to about 50 km, whereas near 68°N it is only about 41 km. The same east–west angular width therefore represents a greater physical distance in the south than in the north. This is important within the convection-roll hypothesis because the proposed mantle system itself is defined by angular divisions of the Earth. If the underlying mantle organization follows fixed longitudinal and latitudinal intervals, symmetry in degrees rather than kilometres would be expected within that model. This is quite different from an ordinary local strain ellipse constructed in a Cartesian coordinate system.

A possible combined mechanism

The emerging working hypothesis can therefore be summarized as follows. An elongated remnant of continental crust remained between Greenland and Europe during the opening of the North Atlantic. Beneath the Iceland region, the mantle-convection-roll system imposed a geometrically organized set of boundaries.

The principal crustal extension occurred in the east–west direction. At the same time, deformation was transmitted diagonally through two approximately mirror-symmetric systems extending toward Greenland in the northwest and the Faroes in the southeast. Because the two oblique systems are mirror-related, their shear components could largely cancel at the scale of the entire region while their extensional components reinforced one another. This allowed the principal axes of the cumulative strain field to remain east–west and north–south.

Within the Iceland region, hotter and weaker continental crust allowed deformation to spread laterally far beyond the widths of the narrower connections to Greenland and the Faroes. Ductile lower-crustal flow, repeated movement of spreading axes and extensive magmatic intrusion may have further smoothed the deformation field. The result could have been a broad, approximately elliptical finite-strain domain.

The Faroe and Greenland ridge segments would then represent narrower continuations of this larger crustal system rather than structures having the same width as the Icelandic central region.

Later tectonic reorganization, especially the development of the Kolbeinsey Ridge, subsequently disturbed the northern part of the original geometry. The most important observation is therefore not merely that the Icelandic shelf is approximately elliptical. It is that the geological boundaries, the central east–west axis, the connections toward Greenland and the Faroes, and the deeper mantle geometry proposed by the convection-roll model appear to form parts of the same symmetric system.

If the continental-crust interpretation is correct, the Icelandic ellipse may record the cumulative deformation of an old continental fragment whose extension was controlled not by a single rift axis, but by a wider and geometrically organized mantle deformation field.

The elliptical outline is particularly evident when compared with the geological and structural maps presented by Hjartarson et al. (2017). Their subdivision of the Greenland–Iceland–Faroe Ridge Complex shows distinct boundaries between the Icelandic shelf and the adjoining Greenland–Iceland and Iceland–Faroe ridge sectors. In particular, the southeastern geological boundary closely follows the elliptical outline described here. The elliptical interpretation itself is not proposed by Hjartarson et al.; it is introduced here as a geometric interpretation of the mapped structures.

The article: Hjartarson, Á., Erlendsson, Ö. & Blischke, A. (2017). The Greenland–Iceland–Faroe Ridge Complex. Geological Society, London, Special Publications, 447, 127–148. DOI: 10.1144/SP447.14.

https://www.researchgate.net/publication/316330629_The_Greenland-Iceland-Faroe_Ridge_Complex