Tag: geology

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Eyjafjallajökull in a Global Context: Around the Earth

Steingrimur Thorbjarnarson

The volcanoes of Eyjafjallajökull and Katla are among the most famous volcanic systems in Iceland. They occupy a prominent position in the geology of South Iceland and have long attracted attention because of their apparent connection. Historically, eruptions of the two volcanoes have often occurred within relatively short intervals of one another, suggesting that some form of interaction may exist between the neighboring volcanic systems.

Eyjafjallajökull, an east-west aligned elliptical volcano,
with a glacial tongue flowing northward from the summit crater.
(https://www.visindavefur.is/svar.php?id=77035)

Geological maps reveal a remarkable east-west arrangement extending across the region. Katla lies beneath Mýrdalsjökull to the east, while Eyjafjallajökull forms an elongated volcanic edifice immediately to the west. The volcanic structures, fissure swarms, and associated tectonic features display a pronounced east-west alignment.

Eyjafjallajökull itself is an elliptical volcano. Its fissure swarm follows the long east-west axis of the edifice, indicating that the orientation is deeply rooted in the tectonic framework of the area. At the western end of this axis lies Seljalandsfoss, one of Iceland’s best-known waterfalls. At the eastern end, the glacier tongue of Kötlujökull marks the outlet area where meltwater and jökulhlaups from Katla most commonly emerge.

This eastern opening is particularly significant. When Katla erupts beneath its ice cover, large floods are generated. These floods frequently escape through the Kötlujökull outlet, effectively utilizing the same east-west corridor that connects Katla with Eyjafjallajökull. The repeated use of this pathway suggests that the axis represents a long-lived structural weakness within the crust.

Within the framework of the mantle convection rolls model, this east-west trend coincides with the central axis of a polygon formed by convection-roll division lines. In this interpretation, the geometry observed at the surface reflects deeper patterns within the mantle. The Eyjafjallajökull–Katla axis occupies the center of one such polygon, while other volcanic systems define its boundaries and corners.

Polygonal pattern formed by mantle convection roll division lines. According to the model, volcanic zones,
tectonic boundaries, and major geological features tend to develop along these persistent structural lines.

The north-south arrangement of volcanic centers is equally intriguing. Hekla occupies a prominent position to the north-east, while Vatnafjöll lies between Hekla and Eyjafjallajökull. Drawing a line through Hekla, Vatnafjöll, and the summit crater of Eyjafjallajökull reveals a clear north-south trend. According to the mantle convection rolls interpretation, this line represents another major axis of the same polygon.

The north-south axis and east-west axis through Eyjafjallajökull.

When viewed in a broader context, the north-south alignment does not end in South Iceland. Similar trends can be traced along the Mid-Atlantic Ridge, which itself forms one of the most prominent north-south geological structures on Earth. Extending the same geometric framework to a global scale, the trend can be followed toward the Southern Ocean and ultimately to the minor axis of the elliptical form of Antarctica identified elsewhere in this study, as seen here for example:

Location of Hekla (red circle) and the Elliptical Outline of the Continental Shelf of Iceland.

Whether examined locally or globally, Eyjafjallajökull and Katla occupy a unique position within Icelandic geology. Their historical relationship, their shared east-west structural alignment, the pathway of Katla’s jökulhlaups, and their position within larger tectonic patterns all point toward an underlying degree of geometric organization. The volcanoes are not merely neighboring volcanic systems; they appear to form part of a larger structural framework extending across Iceland and beyond.

Central Axis of Iceland Shelf and Eyjafjallajökull

The Eyjafjallajökull–Katla connection therefore provides an excellent example of how local volcanic features can be examined within a wider geodynamic context. The geological observations themselves are well established. The challenge is to understand whether these observations are isolated phenomena or components of a larger pattern linking Icelandic volcanism to the geometry of mantle processes on a planetary scale.

Geometric relationship between the elliptical Antarctic Plate, Iceland, and the Ring of Fire.
The minor axis of Antarctica aligns with Iceland to the north and, when extended southward,
coincides with the extrapolated minor axis of the Ring of Fire south of New Zealand.
The relationship becomes apparent on a South Pole-centered map
with the surrounding continents displayed on both sides of Antarctica.

The elliptical shape of the Antarctic Plate exhibits major and minor axes that are parallel to the corresponding axes identified in Iceland, particularly those associated with Eyjafjallajökull. Within the framework of the mantle convection rolls model, an apparent geometric connection can also be traced between Iceland and the Ring of Fire. The northern intersection of the Antarctic minor axis aligns with Iceland, while the opposite intersection point coincides with an extrapolation of the minor axis of the Ring of Fire south of New Zealand. This relationship can be examined by displaying world maps on both sides of a South Pole-centered projection, revealing the continuity of the large-scale geometric pattern.

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South Iceland Seismic Zone — A Geometric Link Between Iceland’s Rift Systems

Steingrimur Thorbjarnarson

 Iceland Seismic Zone — A Geometric Link Between Iceland’s Rift Systems

The South Iceland Seismic Zone is one of the most remarkable tectonic regions in Iceland. It is not only a zone of frequent earthquakes, but also a key to understanding how stress, volcanic systems, and crustal deformation interact across the island. While the volcanic zones of Iceland often receive most public attention, the seismic zone between them reveals an equally important part of the tectonic structure.

The mantle convection rolls division lines polygon
framing the South Iceland Seismic Zone.

The zone stretches across southern Iceland, roughly between the western volcanic systems near Hengill and the eastern systems connected with Hekla and the East Volcanic Zone.

The hypothetical sequence of north-south aligned earthquake faults
superimposed on a map of the South Iceland Seismic Zone.

Unlike the volcanic rift zones, where extension is expressed through volcanism and fissure swarms, the South Iceland Seismic Zone mainly releases tectonic stress through earthquakes.

Mapped surface faults of SISZ
Simplified map showing earthquake and volcanic zones of Iceland.
Hekla

A Transform Zone Across Iceland

In plate tectonics, Iceland is usually described as a place where the Mid-Atlantic Ridge rises above sea level. The North American and Eurasian tectonic plates move apart across the island. However, the spreading is not expressed as one single continuous rift. Instead, the volcanic zones are offset from each other.

The South Iceland Seismic Zone acts as a transfer structure between these volcanic segments. In standard tectonic terminology, it is often described as a transform zone, although it differs from classical oceanic transforms because deformation is distributed across a broad area rather than concentrated along one fault.

The earthquakes of the zone commonly occur on north-south oriented faults, even though the broader tectonic movement across Iceland is mainly east-west extension. This apparent contradiction is one of the most interesting aspects of the region.

The Diamond-Shaped Geometry

One of the clearest large-scale geometric features of the seismic zone is its tendency toward polygonal organization. The region can be interpreted as a broad diamond-shaped area between volcanic systems.

The eastern and western ends connect naturally with major volcanic centers, to the west: the Hveragerði geothermal region, and to the east at Hekla and the western margin of the East Volcanic Zone

Within this framework, stress appears to organize itself along lines that connect opposite corners of the polygonal area. The result is the repeated formation of north-south fracture structures inside an overall east-west tectonic setting.

This is one reason why the South Iceland Seismic Zone is so important geologically. It demonstrates that tectonic deformation is not simply linear. Instead, it becomes organized into geometric structures where local stress fields redirect movement into highly regular patterns.

Earthquakes and Historical Activity

The South Iceland Seismic Zone has produced many destructive earthquakes throughout Icelandic history. Some of the strongest historical earthquake sequences occurred in this region, affecting farms, churches, and settlements across the lowlands.

Notable earthquake episodes include:

  • The great earthquakes of 1784
  • The 1896 earthquake sequence
  • The June 2000 earthquakes
  • The May 2008 earthquakes

5

The earthquakes are usually shallow, which increases their impact at the surface. Even moderate earthquakes can therefore produce significant shaking.

Connection With Hekla

Hekla occupies a uniquely important tectonic position at the eastern end of the seismic zone. It lies where several tectonic tendencies intersect:

  • Connecting the South Iceland Seismic Zone and the East Volcanic Zone
  • A key function within the regional spreading across Iceland
  • Direct north-south structural alignments

Because of this, Hekla can be viewed as both a volcanic center and a tectonic focal point.

The geometry becomes especially interesting when Iceland is examined together with its continental shelf and offshore ridge systems. The South Iceland Seismic Zone appears not merely as a local fracture belt, but as part of a broader structural organization extending into the North Atlantic.

A Broader Geometric Interpretation

The South Iceland Seismic Zone also provides an opportunity to examine tectonics through geometric relationships.

The repeated north-south fracture orientation inside a broader east-west tectonic environment suggests that deformation is influenced by organized stress fields rather than random faulting alone. Similar geometric tendencies can be observed elsewhere in Iceland, particularly where polygonal crustal blocks form between volcanic systems and fracture zones.

In the mantle convection rolls interpretation, these polygonal structures emerge naturally from the division lines between adjacent convection cells. Pressure along the sides of such polygons can produce fracture systems that connect one corner to another, generating north-south alignments within larger east-west tectonic regions.

Whether examined through conventional tectonics or broader geometric models, the South Iceland Seismic Zone remains one of Iceland’s clearest examples of how crustal deformation organizes itself into remarkably regular patterns.

A Geological Laboratory

Few places on Earth allow such direct observation of active tectonics as Iceland. In the South Iceland Seismic Zone, earthquakes, volcanic systems, geothermal areas, and visible surface fractures all interact within a relatively compact area.

The landscape of southern Iceland preserves these processes in extraordinary clarity. Every earthquake sequence adds another chapter to the evolving tectonic story of Iceland.

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The Function of Indonesia’s Subduction Zones and the Caribbean Subduction and Transform Zones within the Ring of Fire

Steingrimur Thorbjarnarson

Comparing the geometry of Indonesia and the Caribbean reveals a remarkable degree of similarity. Although these two tectonic systems are located on opposite sides of the Pacific realm, many of their large-scale structural relationships appear comparable.

The Identical Shape of Indonesia and the Caribbean:
Comparable Subduction and Transform Geometry on Opposite Sides of the Ring of Fire

First, both systems are closely associated with the equatorial region. The Indonesian arc faces southward, with one important eastern endpoint located near the coast of Sumatra. Another major subduction-related crossing appears roughly 30° farther east along the equatorial zone, connected with the subduction framework surrounding the Philippine Plate.

A similar arrangement can be observed in the Caribbean region, although shifted slightly north of the equator. The Caribbean basin is associated with another pair of crossings of lower-mantle flow structures, where subduction-related zones occur at points approximately 30° apart, comparable to the Indonesian case. The Caribbean arc also extends outward in a manner resembling the Indonesian arc.

The Central American Trench stretches from the inner part of the Ring of Fire toward its outer margin, where the division line appears to turn toward the key region of the Peru–Chile Trench near the equatorial framework. This geometry resembles the relationship between the New Guinea Trench and the continuation of the Philippine Trench on the Indonesian side of the Pacific.

In this way, many aspects of the Indonesian and Caribbean plate-boundary systems appear strikingly similar, although effectively rotated by 180° relative to one another.

Considering that these two tectonic regions connect the Pacific Ocean with the Indian Ocean and the Atlantic Ocean respectively, the broader function of this arrangement becomes important to examine. The fundamental span of a 30° convection-roll unit, together with an additional extension east of the eastern crossing points, may help explain the tectonic importance of these paired regions on opposite sides of the Ring of Fire.

South of Indonesia, subduction is the dominant tectonic process, whereas in the Caribbean region transform motion and tectonic transition play a larger role, especially along the northern boundary of the Caribbean Plate. Even so, both systems appear to participate in a broader structural unity within the Ring of Fire.

The anticlockwise rotational tendency associated with the North American sector of the Ring of Fire is consistent with the northward-facing curvature of the Caribbean arc and the southward-facing curvature of the Indonesian arc. At the same time, the long-term expansion of the Atlantic Ocean on one side and the Indian Ocean on the other appears to be buffered near equatorial latitudes by these major subduction and transform systems situated between the large ocean basins.

The similarities are therefore sufficiently striking that they may partly reflect an underlying large-scale geometric organization. At the same time, this comparison leaves many important questions unanswered and suggests several directions worthy of further investigation. Further information: https://pangea.stanford.edu/ERE/db/GeoConf/papers/SGW/2024/Thorbjarnarson.pdf

All of these characteristics of the two archipelagic systems are, of course, well known. They can first be summarized as follows:

  1. Both archipelagos are associated with the Ring of Fire.
  2. Both archipelagos are located close to the equator.
  3. Both archipelagos have a predominantly east–west orientation.

However, when compared within the framework of the mantle convection rolls model, several additional observations can be made:

  1. When the tectonic features of Central America are included, the overall mathematical framework becomes directly comparable to that of Indonesia.
  2. The width of both archipelagic systems is comparable when measured in degrees of longitude along their respective latitudes.
  3. The relative position of both archipelagos appears comparable in the context of the large-scale convection-roll geometry proposed in the mantle convection rolls model.
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A Degree of Regularity Around Antarctica

Steingrimur Thorbjarnarson

Everyone notices that Antarctica is centered around the South Pole. It has not always occupied this position, and over geological time it will eventually drift toward other latitudes. So why should its present location matter? Many people may consider it coincidental, yet the degree of regularity surrounding Antarctica is nevertheless worth examining.

The mainland of Antarctica is not perfectly circular, but a circle drawn around the coastline of East Antarctica, centered on the South Pole, also broadly encompasses the Antarctic Peninsula. This gives the continent as a whole a remarkably regular appearance https://www.sciencedirect.com/science/article/pii/S1674984722000775.

Antarctica is also almost completely encircled by mid-ocean ridges. The Antarctic Plate can therefore be regarded as a growing tectonic plate, since mid-ocean ridges expand outward on both sides according to the standard plate-tectonic model accepted by most geologists.

When the oceanic part of the Antarctic Plate is examined, it roughly fits an elliptical form centered on the South Pole, with one side extending from East Antarctica and the other from West Antarctica, along the geometrical major axis. Tracing the East Pacific Rise, and the Mid-Indian Ridge, they connect closely with the endpoints of the ellipse’s major axis.

Looking at the minor axis, the Mid-Atlantic Ridge approaches one endpoint rather closely, while the Puysegur Trench subduction zone, extending from New Zealand, connects near the opposite endpoint. Another major subduction system, along the western margin of South America where the Nazca Plate meets the South American Plate, touches the elliptical form at a position approximately equidistant from the Pacific and Atlantic ridge systems.

This degree of regularity can be observed directly on a simple world map. In addition, it fits well with the convection-roll framework examined here, since mid-ocean ridges outside this elliptical form around Antarctica, mainly tend to follow north–south alignments inherent in the mantle convection rolls model.

Within this interpretation, Antarctica is confined within the polar portion of the convection-roll system, bordering the transition zone of the model between 60.7°S and 67.3°S. From this perspective, the present position of Antarctica becomes relevant not only because of the regularity visible on the world map itself, but also because of the way these large-scale features appear to correspond with the mantle convection-roll model being explored here.

In addition, comparing with the Icelandic elliptical form, the minor axis is found along the same longitude.

Location of Hekla (red circle) and the Elliptical Outline of the Continental Shelf of Iceland.

Those two elliptical forms of the Antarctic and Icelandic continental shelfs, which can be identified with reasonable accuracy, both have a minor axis along 19°40′W. The Mid-Atlantic Ridge is obviously north-south oriented, but this consistency between Iceland and Antarctica clearly adds to that concept.

Major and minor axes of the Antarctic continental shelf, centered around the South Pole and reflecting the large-scale elliptical geometry of the Antarctic Plate margin.

The Icelandic continental shelf is indicated on the map. This interpretation is based on the mantle convection rolls model and the related large-scale structural pattern. The minor axis coincides with the transition zone between polar- and equatorial-related convection rolls, while the major axis extends toward the central part of the equatorial convection rolls. Map base: https://oceanobservatories.org/2022/11/nature-review-paper-reveals-new-understandings-of-mid-ocean-ridge-systems/

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The Geometry of the Pacific Ocean

Steingrimur Thorbjarnarson

Comparing large-scale convection rolls with the shape of the Pacific Ocean reveals several intriguing geometrical structures. The first step is to consider the equator itself. It forms a simple reference line, but one with important physical implications, because the effects of Earth’s rotation differ there from those at other latitudes. At the equator, the clockwise deflection characteristic of the Northern Hemisphere, and the tendency of horizontally moving particles to turn anticlockwise, do not apply in the same way.

The Geometry of the Pacific Ocean.

The presence of major subduction zones on either side of the Pacific, Indonesia in the west and South America in the east, approximately 150° apart, provides two stable reference points.

As the Ring of Fire is a well-established concept, the volcanic regions can be outlined in a relatively simple way, beginning from these two equatorial reference points. The San Andreas Fault provides particularly strong support for this geometry, as it forms the sliding boundary between the Pacific Plate and the North American Plate. https://www.youtube.com/watch?v=YQtMBmS49Ew It thereby marks the approximate position of the minor axis of the elliptical form that appears when the Ring of Fire is sketched onto a map. This essay contains a lot of details:

Stanford 2024Download

Several principal lower-mantle division lines from the mantle convection rolls model are also drawn on the map. It should perhaps have been mentioned earlier that the major intersections between the main lower-mantle convection rolls coincide with the subduction regions on both the Indonesian and South American sides of the Pacific (shown with red dots). This observation alone deserves attention when comparing the model with the map.

Additional features emerge when examining the northeastern section of the Ring of Fire. When the outer limits of the Ring of Fire are outlined, the resulting elliptical form crosses Yellowstone National Park, a volcanic region unlike any other. Within the model, the pattern created by the division lines between convection rolls of different mantle layers produces a north–south axis between the predominantly north–south-trending rolls. Yellowstone is located directly on such a north–south axis.

With this in mind, the position of Hawaii also becomes significant. Hawaii is located not only on the north–south axis emerging from the convection-roll division pattern, but also on the central north–south axis of the Ring of Fire as represented on this map.

Looking at the western side of the Pacific, one of the first indications of a broader geometrical structure is the position of New Zealand along the minor axis of the elliptical form, opposite the San Andreas region and Yellowstone on the same axis. The Taupō Volcanic Zone is located where this minor axis intersects a principal lower-mantle division line. On the map, that mantle division line is drawn somewhat exaggerated in order to emphasize its significance.

The western Pacific is subject to intense geophysical and tectonic stress, and this particular division line — extending from Japan to New Zealand — appears to accommodate much of that stress. Comparing the Northern and Southern Hemispheres, New Zealand and Japan, together with the subduction systems extending southward and northward from them respectively, display a mirrored relationship. Because stress is concentrated along this division line, and because rotational effects differ symmetrically between the hemispheres, New Zealand appears within the inner elliptical form of the Ring of Fire.

Further similarities emerge when examining the convection-roll division lines. Not only do Japan and New Zealand occupy corresponding positions within the model, but major volcanic centres such as Mount Fuji and Taupō are also found in analogous hemispherical settings on opposite sides of the globe. Likewise, the Kermadec Trench and the Izu–Bonin Trench display mirrored alignments.

The circular — or more precisely elliptical — geometry of the Pacific can therefore be compared with many details of the mantle convection rolls model. At the same time, the dimensions and shape of the Pacific have continually changed through tectonic drift. Over geological time, plate motion gradually alters the geometry of the basin itself.

The geological development of the region is equally revealing. Research has shown that much of the crustal material now forming southern Alaska originated at far more southern latitudes and has since been transported more than a thousand kilometres toward the northwest. The Denali Fault and Tintina Fault systems appear to have played a major role in this transport after island arcs had been sutured onto the North American continent. This partly illustrates how the Ring of Fire functions and why it extends across a geometrically well-defined region composed primarily of two concentric elliptical forms and the area enclosed within them.

This perspective also suggests that subduction zones may remain relatively fixed once established within the mantle convection rolls system. As the geometry evolves through tectonic drift, slabs descending into the mantle may become detached from their original surface plates, while new subduction zones eventually develop elsewhere.

It should be added here, tht there is a possible explanation for the formation of north–south axes within the framework of convection rolls. This becomes apparent when examining the detailed geometry of the division lines and polygonal regions formed between them. These polygons often tend toward a diamond-shaped geometry, with corners aligned approximately east–west and north–south.

Pressure exerted along the sides of such polygons may then create preferential lines of weakness or activity extending from one corner to the opposite corner. When this process is repeated across many adjacent small polygons, the resulting alignment can be extrapolated into the large-scale north–south patterns observed across the Pacific and elsewhere.

A particularly clear example of this type of geometry can be examined in Iceland. There, the North Volcanic Zone forms a pronounced north–south volcanic alignment, while the South Iceland Seismic Zone follows an east–west-oriented structural trend. Together, these systems illustrate how polygonal stress geometry within the mantle convection rolls framework may influence both volcanic and seismic alignments on the surface.