Author: Steingrimur Thorbjarnarson

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.

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.

The boundaries of the Somali Plate can be traced around the plate in a remarkably systematic way when both the convection-roll division lines and the proposed elliptical tectonic-ring geometry are taken into account.

1. Afar
Afar occupies a key position at the southern end of the Red Sea. In the geometrical framework presented here, it also lies close to the outer boundary of the tectonic ring surrounding the Atlantic Ocean. It therefore forms an important transition between the Red Sea rift system and the plate-boundary system farther south.

2. Convection-roll section
From Afar, the Somali Plate boundary follows division lines associated with the mantle-convection-roll system. At the point indicated by the arrow, the boundary makes a distinct bend or step and shifts toward one of the major lower-mantle division lines.

3. Tectonic-ring section
This section is one of the main subjects of the present analysis. Here, the inner elliptical boundaries of the Atlantic and Indian Ocean tectonic rings approach and partly overlap one another. The boundary between the Somali Plate and the African, or Nubian, Plate follows this same zone remarkably closely.

Rather than being controlled by a single line, this region can therefore be viewed as an interaction zone between two large elliptical tectonic structures.

4. Lake Malawi
Lake Malawi occupies a particularly important position within this system. Near the lake and farther south toward the African coast, the two inner elliptical boundaries converge closely. This area also forms a key part of the plate-boundary system separating the Somali and Nubian plates.

The location of Lake Malawi is therefore significant not simply because it lies within the East African Rift, but because it occurs close to the junction of two independently defined tectonic-ring boundaries.

5. Return to the convection-roll system
Farther south and east, the plate boundary again tends toward the nearest major division of the lower-mantle convection-roll system. The geometry therefore appears to alternate between sections controlled primarily by the elliptical tectonic boundaries and sections more closely associated with the underlying convection-roll divisions.

6. Eastward step
A further eastward step then occurs. The boundary shifts from the vicinity of the inner Atlantic-ring ellipse toward the outer boundary of the Indian Ocean ring.

This change of position is important because it suggests that the Somali Plate boundary is not associated throughout with only one geometrical structure. Instead, different segments appear to follow different elements of the combined system.

7. Carlsberg Ridge
The Carlsberg Ridge has been discussed previously. Its orientation and position correspond closely to a major division in the lower-mantle convection-roll system—or, more precisely, to the next subsidiary division immediately east of the principal lower-mantle boundary.

This part of the Somali Plate boundary is therefore primarily associated with the convection-roll geometry rather than with an elliptical ring boundary.

8. North–south section
Another important segment of the boundary trends approximately north–south. In the geometrical model developed here, this segment lies in the direct continuation of the major axis of the Antarctic Plate.

This introduces a third geometrical relationship into the plate-boundary system: in addition to the convection-roll divisions and the tectonic-ring ellipses, some major boundaries appear to correspond to axes defined by the geometry of Antarctica.

9. Somali–Antarctic Plate boundary
The boundary between the Somali Plate and the Antarctic Plate extends from the region near the end of the major axis of the Antarctic Plate toward the outer elliptical boundary of the Indian Ocean tectonic ring.

This southern part of the system therefore provides a direct geometrical connection between the Antarctic Plate and the Indian Ocean ring.

A combined geometrical framework

Taken as a whole, the Somali Plate is particularly useful for examining the proposed global tectonic geometry. Its boundary cannot be described by only one type of structure. Different sections correspond to different components of the model:

• convection-roll division lines, including major and subsidiary lower-mantle divisions;
• inner and outer elliptical boundaries of the Atlantic and Indian Ocean tectonic rings;
• and major axes associated with the geometry of the Antarctic Plate.

The Somali Plate therefore provides a good example of how these different geometrical systems may interact. Rather than forming an arbitrary closed outline, its boundary appears, in this interpretation, to move from one structural element to another around the plate.

The geometry of the Somali Plate suggests that present-day plate boundaries may represent surface expressions of several interacting systems: the older mantle-convection-roll framework, the younger elliptical tectonic-ring geometry, and large-scale axes associated with Antarctica.

The Somali Plate — An Internal View of the Boundary System

The boundaries of the Somali Plate can also be described from the inside, beginning once again at the Afar Triple Junction. In the accompanying figure, a straight reference line has been drawn from Afar and marked as Line 1. From this starting point, the plate boundary can be followed through a series of distinct segments.

1. From the outer to the inner Atlantic-ring boundary

The first segment extends southwestward from the outer boundary of the Atlantic tectonic ring toward its inner boundary. The direction of this segment corresponds closely to the underlying orientation of the convection-roll system, which in this region trends approximately NE–SW.

This first section therefore represents a case where the surface plate boundary and the calculated mantle-convection geometry have approximately the same orientation.

2. Along the overlap of the two inner ellipses

The boundary then turns abruptly toward the SSE. From this point onward, it follows with considerable accuracy the narrow zone where the inner boundary of the Indian Ocean tectonic ring and the inner boundary of the Atlantic tectonic ring approach and overlap one another.

This is one of the most important sections of the Somali Plate boundary in the present analysis. The plate boundary is not simply following one ellipse; it occupies the interaction zone between two independently defined elliptical tectonic structures.

3. Return toward the African continent

Near the Indian Ocean coast of Africa, the plate boundary turns sharply toward the southwest, bringing it back onto the continent. This change occurs close to the point where the two inner elliptical boundaries—the Atlantic and Indian Ocean ellipses—begin to separate from one another.

The length and position of this section appear to be constrained by two different geometrical systems:

• to the east, by the tectonic-ring boundaries;
• to the west, by the nearest major lower-mantle division of the convection-roll system.

The boundary then turns southward and follows the convection-roll division for some distance. Farther south, another bend carries it toward the southeast, where the next segment continues offshore into the Indian Ocean, extending toward the inner boundary of the Atlantic tectonic ring.

4. A branch along the inner Atlantic ellipse

This branch develops where the plate boundary bends at the inner ellipse of the Atlantic tectonic ring. From there, it follows the elliptical boundary southward for a considerable distance. The segment ends approximately where it passes beyond the outer boundary of the Indian Ocean tectonic ring.

This provides another example of a plate boundary apparently changing from one controlling geometrical element to another as it crosses the larger tectonic-ring system.

5. Somali–Antarctic Plate boundary

The fifth section forms part of the boundary between the Somali Plate and the Antarctic Plate.This is a long oceanic boundary extending toward the major axis of the Antarctic Plate, where the Antarctic, Somali and Australian plates meet.

Within the geometry considered here, this is an important transition because the boundary links the Somali Plate directly with one of the principal axes associated with Antarctica.

6. From the triple junction to the equator

At the triple junction, the plate boundary turns northward. Remarkably, this northward segment terminates at the equator, at one of the principal equatorial division points identified in the convection-roll model. These principal points occur at approximately 30° intervals along the equator.

The position of the northern end of this segment is therefore not treated here as an isolated geographical coincidence, but as part of the larger equatorial organization of the convection-roll system.

7. The Carlsberg Ridge

From the equatorial point, the plate boundary turns toward the northwest and follows the Carlsberg Ridge. The Carlsberg Ridge has been discussed previously in relation to the mantle-convection-roll model. Its position and orientation correspond closely to the calculated division system and show how an oceanic spreading ridge can follow one of the major geometrical directions of the underlying mantle framework.

In this interpretation, the ridge is associated particularly closely with a subsidiary division immediately adjacent to one of the major lower-mantle divisions.

8. The return to Afar

At the southern end of the Red Sea lies the Afar Triple Junction, where one branch extends almost directly eastward. This final section is particularly useful for understanding the geometry of the Somali Plate. It lies between two different structural controls:

• the dominant tectonic boundary associated with the outer ellipse of the Atlantic tectonic ring;
• and the convection-roll division system associated with the Carlsberg Ridge.

The geometry therefore shows an identifiable gap or transition between the two systems. Afar occupies the point where these different structural elements are brought together.

A plate boundary controlled by more than one geometrical system

Viewed from inside the Somali Plate, its boundary appears to consist of a sequence of segments controlled by several different geometrical elements.

Some sections follow the convection-roll division lines. Others follow the inner or outer boundaries of the Atlantic and Indian Ocean tectonic rings. Still others connect with major axes associated with the Antarctic Plate or terminate at principal equatorial division points.

The Somali Plate is therefore particularly useful for studying how these geometrical systems may interact. Its boundaries do not follow one single calculated structure continuously. Instead, the plate boundary appears repeatedly to transfer from one structural framework to another, with sharp bends or step-like shifts occurring near the intersections between them.

The Somali Plate therefore provides a particularly clear example of how plate boundaries may be organized by the interaction of mantle-convection divisions, elliptical tectonic-ring boundaries, and the large-scale geometry associated with Antarctica and the equator.

The length and position of this section appear to be constrained by two different geometrical systems.

Three well-known volcanic and geothermal sites in the northwestern part of the Ring of Fire—Mount Fuji, Changbaishan, and the Valley of Geysers in Kamchatka—occupy very different positions within the geometrical framework considered here.

In the accompanying map, the inner and outer boundaries of the Ring of Fire are shown in red, while the calculated mantle-convection-roll divisions are shown as black lines.

Changbaishan is located close to the outer boundary of the Ring of Fire. At the same time, it lies within the region where the proposed Indian Ocean Ring overlaps the Pacific Ring. Its position can therefore be examined in relation to the interaction between two large-scale tectonic systems rather than to the Pacific Ring alone.

Mount Fuji, by contrast, is situated close to a calculated division between two large-scale lower-mantle convection rolls. It also marks the northern end of the Izu–Bonin subduction system, which follows this calculated mantle division southwards. Fuji therefore provides a particularly clear example of a volcanic centre associated directly with the geometry of the mantle convection roll system.

The Valley of Geysers in Kamchatka occupies yet another type of position. It lies close to the inner boundary of the Ring of Fire and also near a major N–S axis that can be regarded as an extension of the minor axis of the Antarctic ellipse.

This same meridional direction continues through the longitude of the Geysir geothermal area in Iceland. When viewed relative to the geographic poles, the Iceland–Kamchatka direction forms an approximately 90° relationship with Yellowstone. The three major geyser regions therefore occupy geometrically related positions within the broader global framework, although the immediate tectonic setting of each is different.

These three examples illustrate an important point. Exceptional volcanic or geothermal activity need not correspond to only one kind of geometrical relationship. In this case:

• Changbaishan is associated with the overlap of two proposed tectonic rings;
• Mount Fuji is associated with a major lower-mantle convection-roll division and the beginning of the Izu–Bonin subduction system;
• the Valley of Geysers is associated with the inner boundary of the Ring of Fire and a major meridional axis linked geometrically with Antarctica.

The locations can therefore be examined as different surface expressions of the same broader tectonic framework.

The Scotia Arc tectonic system, situated between southern South America and the Antarctic Peninsula, developed during the progressive disruption of the former geological connection between South America and Antarctica. Over tens of millions of years, extension, seafloor spreading, transform motion, subduction and back-arc spreading produced the exceptionally complex tectonic structure that now occupies the Scotia Sea. The system includes the North and South Scotia ridges, the extinct West Scotia Ridge, the active East Scotia Ridge, the South Sandwich volcanic arc and trench, as well as a number of continental fragments and smaller tectonic blocks.

The progressive opening of the region also created the Drake Passage, establishing an oceanic connection between the Pacific and Atlantic sectors of the Southern Ocean. A shallow gateway may already have existed by about 50 million years ago, while a deep-water connection developed later, particularly as seafloor spreading became established in the West Scotia Sea between about 34 and 30 Ma. This opening eventually allowed circumpolar ocean circulation to develop around Antarctica. The Antarctic Circumpolar Current profoundly influences the transfer of heat between Antarctica and lower latitudes and contributed to the increasing thermal isolation of the continent.

What is particularly interesting in the present study, however, is the geometry of the entire tectonic system. Although the Scotia Arc is conventionally described as an arcuate tectonic structure, its principal ridges, plate boundaries and subduction systems can, in the geometrical interpretation developed here, be contained within an approximately elliptical large-scale form. The minor-axis meridian of this ellipse lies at approximately 49.7°W.

This position is especially notable when compared with the elliptical form previously identified around the Icelandic shelf, whose corresponding minor-axis meridian lies at approximately 19.7°W.

The difference is almost exactly: 49.7°W − 19.7°W = 30.0°

Thus, the central axes of two geographically remote and tectonically very different regions fall on meridians separated by exactly 30° of longitude within the proposed mantle convection-roll framework.

This comparison does not imply that Iceland and the Scotia Arc formed through the same tectonic processes. Iceland lies on an active divergent plate boundary associated with the Mid-Atlantic Ridge, whereas the Scotia Arc contains a much more complicated combination of transform boundaries, extinct and active spreading centres, back-arc extension and subduction. What is being compared here is therefore not the detailed tectonic mechanism, but the large-scale geometry and its position within the proposed global convection-roll system.

This distinction makes the correspondence more interesting rather than less: two very different tectonic systems appear to be organised within similarly defined large-scale geometrical frameworks.

Iceland and the Antarctic Peninsula

There is another useful reason for making this comparison. Iceland lies between approximately 63° and 67°N, while the northern Antarctic Peninsula occupies broadly corresponding latitudes in the Southern Hemisphere. We therefore have exposed land at approximately equivalent absolute latitudes on opposite sides of the Earth, allowing some geometrical relationships to be compared directly.

An earlier comparison was presented here:

Iceland and Antarctic Peninsula compared

When the outline of the Antarctic Peninsula is transferred to the Iceland region according to corresponding positions within the proposed convection-roll framework, an intriguing relationship appears. The trend of the Antarctic Peninsula follows closely the principal division line of the North Atlantic tectonic system where that line crosses Iceland.

The same relationship can be shown on the geological map base of the Icelandic Institute of Natural History and the National Energy Authority:

And when the corresponding Antarctic line is added:

The geometrical agreement is striking. It should not, however, be interpreted simply as a similarity in coastline shape. The proposed explanation is that both regions occupy corresponding positions relative to the same large-scale system of mantle convection rolls. If the underlying geometrical controls are comparable, some similarity in the orientation of tectonic structures at the surface would be expected.

The new comparison with the entire Scotia Arc adds an important dimension to this earlier observation. The minor axis of the proposed Icelandic ellipse follows approximately 19.7°W, whereas the corresponding axis through the Scotia Arc system follows approximately 49.7°W. The two are therefore separated by 30° of longitude, one of the fundamental longitudinal intervals identified in the convection-roll model.

The Antarctic Peninsula extends away from the Scotia system in a position that can consequently be compared with Iceland after applying this 30° longitudinal relationship. When the two regions are superimposed according to that geometry, the Antarctic Peninsula follows a trend comparable to the principal tectonic and volcanic division across Iceland.

There is also a suggestive mirror-like aspect to the comparison. From its connection with the Scotia Arc, the Antarctic Peninsula extends predominantly southward, while the principal volcanic zones of Iceland extend through the Icelandic shelf in a broadly corresponding geometrical arrangement. The comparison is not exact at the level of individual faults or volcanic centres, nor should it be expected to be. The significant feature is the correspondence of the large-scale axes, orientations and longitudinal positions.

This gives the Scotia Arc–Iceland comparison particular importance within the model. It is not an isolated match between two selected geological features. Rather, the 30° separation between the two central meridians occurs in addition to numerous other cases in which major geological structures appear to coincide with the regular longitudinal framework predicted by the mantle convection-roll system.

The Scotia Arc therefore provides an unusually useful test area. Its tectonic history is independently well established, its internal structure is exceptionally complex, and yet the system as a whole can be examined geometrically. If such a complex assemblage of spreading ridges, transform boundaries, subduction zones and volcanic arcs is nevertheless organised within the predicted large-scale framework, the correspondence deserves closer investigation.

The geological features found at these corresponding latitudes in the Southern and Northern Hemispheres appear, to some extent, to mirror one another.

The broadly elliptical Scotia Arc tectonic system can be compared with the elliptical outline of the Icelandic shelf, while the volcanically active Antarctic Peninsula may likewise be compared with the volcanic regions of Iceland.

The two areas are by no means identical. Their geological settings are fundamentally different, not least because the Antarctic Peninsula is strongly influenced by the presence and tectonic history of the Antarctic continent, whereas Iceland is surrounded by oceanic crust and lies astride an active mid-ocean ridge system.

Nevertheless, the comparison is presented here because the large-scale mirroring effect is difficult to overlook. Despite the very different geological environments, the overall geometry, orientation and position of the principal tectonic and volcanic features show a striking degree of correspondence.