Tag: science

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The Great Rift Valley as a Great Rift Zone

Steingrimur Thorbjarnarson

The African Rift System has undergone continuous change over millions of years. In reality, it is a complex system that extends across most of eastern Africa south of the Sahara.

Inserted map from: https://www.geologyin.com/2024/11/african-rift-valley-africa-splitting.html

The system connects to the Afar Triangle, where a major tectonic junction occurs, and continues northward along the entire length of the Red Sea. There, a mid-ocean ridge follows a NNW-SSE orientation from the continental rift system in Africa. This shift can be explained by referring to different division lines within the mantle.

When this structure is compared with mantle flow patterns, a striking relationship emerges: the width of the continental rift valleys corresponds closely to the width of a single convection roll in the lower mantle. In contrast, the Red Sea spreading ridge appears to align primarily with a boundary between convection cells. The northern continuation of this boundary lies at the eastern most part of the Mediterranean Sea, trending roughly north–south.

The rift system is broadly symmetrical about the equator, although the NNE–SSW-trending block extends farther south than north. The Red Sea itself reflects a division line and related rifting process from Afar north to boundaries near 32°N, where the structural trend shifts into a more direct north–south orientation.

At the equator, the inferred boundaries in the lower mantle pass directly through the central rift zone. This is the only location on Earth where such deep mantle boundaries appear to coincide with continental surface structures. From this point, the angular distance is approximately 60° to the western coast of Indonesia and about 30° to the eastern coast of Africa.

When mapped, these regions display an unusual rectangular geometry, a pattern that has also emerged from surface geological observations. When compared with the mantle convection system, there is a notable correspondence between deep mantle boundaries and the corners of the rectangular rift domain in East Africa.

This relationship helps explain both the large-scale splitting of the African continent and the formation of the rift valleys themselves. The rifts can be understood as the result of interaction between mantle convection rolls and plate tectonic forces. Only through this combined framework can we explain why extension occurs across such an exceptionally wide region.

To understand the Great Rift Valley better: https://www.youtube.com/watch?v=D9vajUjaEYM

The symmetry around the equator is particularly striking. It is not only observable in mapped data but also consistent with mathematical formulations. When mantle convection rolls are modeled as uniform in both height and width within Earth’s layered structure, the positions of the continents align closely with the boundaries of these rolls. At the equator, or as for the Afar region 11°N, a section based on that method looks like this:

The broad nature of the East African Rift helps explain why Africa differs from regions such as South America and Indonesia. In those regions, continental margins tend to align with intersections of lower mantle convection rolls, whereas in Africa, the rifting occurs within the interior of the plate and does not coincide with such boundaries.

The bifurcation of Africa, and the way the rift system aligns with calculated mantle flow divisions, provides further evidence that can be used to reconstruct the internal structural framework of the Earth. This section of the equatorial plane of the Earth is made according to information about the Earth’s layers. The convection rolls 2D sections fit into those layers, and simultanously explain the existence of relevant transition layers. This also fits with the distribution of land mass along equator.

The pattern along equator is so amazingly clear, that it is strange no one mentions it, at no one therefore tries to explain it. The lack of discussion is a topic to be dealt with according to social science though, or other disciplines.

The East African Rift as a Distributed Zone of Extension

Mantle Flow, Plate Motion, and the Geometry of a Splitting Continent

https://www.researchgate.net/publication/327821244/figure/fig1/AS%3A686045574209537%401540577419901/Map-showing-major-tectonic-provinces-and-features-of-the-East-African-Rift-System-EARS.png
https://upload.wikimedia.org/wikipedia/commons/7/79/Tectonic_African_Arabian_Rift_System.jpg

The East African Rift is often presented as a simple example of a continent breaking apart. While this description is not incorrect, it does not capture the true scale and nature of the system. This is not a single line or a narrow plate boundary, but a vast zone of extension covering a large portion of eastern Africa south of the Sahara.

At certain latitudes, this zone spans up to half the width of the continent and measures approximately 15° in latitude, corresponding to roughly 1000–1500 km. This is highly unusual compared to most rift systems on Earth, which tend to be much narrower and more localized.

The question is therefore not only why Africa is rifting, but:

why the extension is distributed across such a broad region.

Not a Single Rift, but a System

https://upload.wikimedia.org/wikipedia/commons/c/c0/Tectonical_map_of_East_Africa.png

4

The term “rift valley” can be misleading. It suggests a single structure, but in reality the East African Rift consists of:

  • multiple parallel rift branches
  • fault systems and fractures
  • broad zones of crustal thinning
  • large basins occupied by lakes

This is therefore not a single fracture, but a case of:

distributed extension

Deformation is spread across a wide region rather than localized along a single boundary. This immediately suggests that the causes are not confined to shallow levels, but are linked to deeper processes.

Lithosphere: Brittle Above, Ductile Below

To understand how such distributed extension occurs, we must consider the mechanical behavior of the lithosphere.

Above the Moho, the material is brittle and responds to stress by:

  • fracturing
  • forming faults
  • generating earthquakes

Below the Moho, the uppermost mantle behaves ductilely:

  • it deforms gradually
  • it flows over long timescales
  • it does not fracture in the same way

Under horizontal plate motion:

the ductile portion yields and accommodates deformation, while the brittle portion fractures.

This is why the system does not form a single rift, but many.

Below the lithosphere lies the asthenosphere, a lower-viscosity layer where flow is more easily sustained. However, the coupling between this layer and the overlying plate is not constant.

In some cases, a no-slip condition applies:

  • mantle flow is directly transmitted to the plate
  • motion in the mantle influences plate movement

In other cases, slip occurs:

  • the plate moves independently
  • mantle flow has limited direct influence

This variability is crucial:

mantle flow does not always directly control plate motion.

Mantle Rolls and the Scale of the System

To explain the width of the rift system, the mantle must be considered as a layered system of flow.

In the upper mantle, from roughly 120 km to 670 km depth, one can identify a system of smaller convection rolls. In this framework, these may be considered as approximately ten rolls, dividing the region into smaller dynamic units and influencing localized deformation.

Below this level, in the lower mantle, the structure appears different. The convection rolls are larger and extend over broader areas. Their surface expression corresponds to a width of 15°.

This is a key observation:

The width of the East African Rift corresponds to the width of these larger lower mantle rolls.

If so, the rift is not a local feature, but:

a surface expression of deeper, large-scale mantle structure.

Resistance to Plate Motion and the Origin of Extension

To understand why such a broad rift develops, it is necessary to consider the balance of forces.

If plate motion were unopposed, the lithosphere would move as a coherent unit. There would be no reason for it to break apart over such a large region.

The key factor is that:

some mantle rolls rotate against the direction of plate motion.

This creates resistance. We therefore have two interacting components:

  • the overall plate motion driving the system
  • opposing mantle flow providing resistance

The net plate motion is stronger and continues to drive movement. However, where resistance occurs:

  • tensile stress builds within the lithosphere
  • stress is distributed across a wide region
  • the system begins to deform

The result is:

  • ductile deformation at depth
  • brittle fracturing above
  • and the onset of rifting

Difference from Mid-Ocean Ridges

This mechanism differs fundamentally from what occurs at mid-ocean ridges.

At ridges:

  • convection rolls exist on either side
  • they move away from the ridge axis
  • and directly drive symmetrical extension

Thus:

extension is a direct result of divergent mantle flow.

In East Africa, however:

  • there is no single central axis
  • extension results from interaction between plate motion and opposing mantle flow

This produces:

  • a wide deformation zone
  • distributed extension
  • multiple rift branches

Why Africa Differs from Other Regions

https://www.researchgate.net/publication/357018373/figure/fig1/AS%3A1101017022234624%401639514322237/Trench-fill-sediments-and-tectonics-of-South-American-subduction-a-Sediment-thickness-in.png
https://images.openai.com/static-rsc-3/20LpD3CGEVulF8jClQX7mQRwgS46KSWpUp-E3yvOA0W_MAVGkFdg7pXieB5SNvbYA2I5OUXn7L-_HDnE4ktStbBl9EsdZu9Cb4v4l9tsiGU?purpose=fullsize&v=1
https://www.researchgate.net/publication/342008815/figure/fig3/AS%3A959165941948417%401605694389536/Tectonic-setting-of-the-East-African-Rift-System-a-Plate-kinematic-configuration-of-the.png

This framework also explains why Africa differs from regions such as South America and Indonesia.

In those regions:

  • plate boundaries often lie along continental margins
  • they coincide with boundaries between mantle flow systems
  • coastlines reflect deep structural divisions

In Africa:

  • rifting occurs within the interior of the plate
  • it does not coincide with a coastline
  • and it is distributed across a broad region

This indicates that the relationship between surface tectonics and mantle structure varies between regions.

A Distributed Response of the Lithosphere

The East African Rift can therefore be understood as a distributed response of the lithosphere to underlying mantle flow.

The lithosphere stretches across a wide region:

  • the ductile portion deforms and distributes stress
  • the brittle portion fractures into multiple rifts

The result is a coherent but distributed system that reflects both material behavior and deeper forces.

Evolution Toward an Ocean Basin

If this process continues, the system may evolve over time.

Distributed extension may gradually localize:

  • forming a more defined plate boundary
  • eventually developing into a mid-ocean ridge

The East African Rift may therefore represent:

an early stage in the formation of a new ocean basin.

Conclusion

The East African Rift is not a simple valley, but a large and structured zone of extension. Its width — approximately 15° — suggests a connection to large-scale mantle flow systems.

By analyzing the system in relation to:

  • smaller convection rolls between 120–670 km depth
  • larger rolls below, spanning ~15°
  • and the resistance generated where mantle flow opposes plate motion
  • surface tectonics
  • lithospheric behavior
  • and the internal structure of the Earth

In this way, the rift system is not merely a local feature, but part of a larger and organized dynamic system within the Earth.

it becomes possible to link:

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The Large Eruptions of Eldgjá 939 and Laki 1783

Steingrimur Thorbjarnarson

The two major fissure eruptions of Eldgjá (939 CE) and Laki (1783 CE) display several striking similarities. The eruptive fissures are broadly parallel and occur within the same general region of southern Iceland. Lava from the Laki eruption partly overlies lava produced during the earlier Eldgjá event..

Despite these similarities, the eruptions are associated with different volcanic systems. The Eldgjá eruption is linked to the Katla volcanic system, whereas the Laki eruption belongs to the Grímsvötn system. These central volcanoes are separated by approximately 120 km. Remarkably, the Laki crater row lies almost exactly midway between the two calderas.

Both calderas also coincide with intersections of the structural division lines shown on the map. The dykes responsible for the eruptions propagated along the structural line situated between the two calderas, forming fissures that follow the same general orientation. This geometry shows that the regional stress field and crustal structure guide magma transport over large distances.

In both cases, the propagating dyke appears to cross a perpendicular structural division located between the two volcanic systems. The onset of eruption occurs shortly after this crossing. This recurring pattern suggests that the intersection between these structural elements plays an important role in controlling where magma reaches the surface.

One interpretation is that the perpendicular division represents a zone of enhanced crustal permeability, where fractures or weaknesses allow magma within the dyke to ascend more easily. Another possibility is that this structural boundary marks a deeper source region where additional hot magma is supplied from below. In this case, the propagating dyke may intersect a region of increased magma pressure or temperature, destabilizing the system and triggering eruption.

Along the Ring Road, informational displays describe the severe consequences of the 1783 Laki eruption. Images depict the suffering experienced by the Icelandic population during the event. Another image refers to the French Revolution, illustrating the wider climatic and societal effects of the eruption.

Images representing Laki eruption.
Image related to the French revolution.

The volcanic haze produced by Laki spread across large parts of Europe, contributing to crop failures and famine. These environmental stresses intensified social tensions in France and are considered one of the contributing factors to the unrest that culminated in the French Revolution.

These two eruptions are did affect the history of Iceland more than any other volcanic events.

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Reykjanes, West Volcanic Zone, Húsavík Fault, and Grímsey Oblique Rift as Surface Expressions of a Single Mantle Convection Roll

Steingrimur Thorbjarnarson

The convection roll bridging the gap between Reykjanes Ridge and Kolbeinsey Ridge

The West Volcanic Zone (WVZ) and the Húsavík–Flatey Fault (HFF) can be understood as two fundamentally different surface responses to the same mantle convection roll, whose flow direction is opposed to the absolute motion of the overlying plate. Where this roll is effectively coupled to the lithosphere, its opposition to plate motion promotes either extension or shear, depending on local boundary conditions.

Mantle Convection Roll at 120 km depth.

Map base: https://www.vedur.is/skjalftar-og-eldgos/frodleikur/greinar/nr/450

The WVZ represents the extensional response to this roll–plate opposition. South of approximately 65° N, the uppermost convection roll maintains sufficient mechanical coupling to the lithosphere to drive sustained rifting and volcanism. Northward of this latitude, however, the polygonal pattern of roll-division lines becomes increasingly complex, and the uppermost roll progressively loses its grip on the plate above. As a result, the rift architecture of the WVZ terminates, and extension is no longer the dominant mode of deformation.

These conditions prevail until the system reaches the HFF. At this location, the same convection roll appears to regain effective coupling to the lithosphere, but now in a geometrical setting defined by a lower division line of the asthenosphere. The result is not extension, but efficient dextral strike-slip motion, localized into a narrow, long-lived fault zone. The HFF thus represents a shear-dominated expression of the same roll–plate opposition that elsewhere produces rift volcanism (RPR, WVZ and GOR). The area south of HHF freely drifts with the North American Tectonic Plate (a smooth displacement of 60 km has been measured). North of the HHF, things are still a bit more complicated.

North of the HFF, a small number of polygons form a region with less seismic activity in which deformation is limited and distributed neither as rifting nor as focused volcanism. Farther north, beyond the next division line parallel to the HFF, the system enters the domain of the Grímsey Oblique Rift (GOR). This boundary lies east of the division associated with the Kolbeinsey Ridge (KR), and therefore clearly within the Eurasian Plate.

Within the GOR domain, the same convection roll continues to influence deformation, but in a setting where tension is created as the North Volcanic Zone (NVZ) fissure swarms extend to the east of the GOR. The GOR is therefore trapped between the Kolbeinsey Ridge and the northern most part of NVZ. This leads to the formation of a volcanic area, linking the NVZ with the Kolbeinsey Ridge. Here, shear and extension are combined, and as a consequence, both seismic activity and volcanism become widespread. Volcanic systems within the GOR represent the volcanic manifestation of this distributed deformation, while seismicity reflects ongoing strain accommodation across the polygonal framework.

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The Mid-Iceland Belt

Steingrimur Thorbjarnarson

The Mid-Iceland Belt (MIB), also referred to as the Central Iceland Volcanic Zone, forms the volcanic connection between the West Volcanic Zone (WVZ) and the East Volcanic Zone (EVZ). Although the precise outlines of individual volcanic systems—particularly those associated with Hofsjökull and Kerlingarfjöll—vary somewhat between published maps, the overall geometry of the belt is consistent.

Within the Mantle Convection Rolls Model, the MIB occupies a central polygon bounded by division lines separating adjacent convection rolls. In addition, many tectonic maps depict a plate-boundary trace aligned approximately east–west through the MIB, reinforcing its interpretation as a zone of plate-scale interaction rather than a simple rift segment.

The Mid-Iceland Belt Polygon

Why the MIB differs fundamentally from WVZ and EVZ

The adjacent volcanic zones, WVZ and EVZ, can be interpreted as rifting zones located above convection rolls whose flow direction opposes the absolute motion of the overlying plate. In those zones, mantle flow and plate motion combine to promote sustained extension and focused rift volcanism.

The MIB does not fit this configuration. Located between the opposing roll-controlled rift systems, it occupies a region where this rifting mechanism does not apply directly. Its existence therefore cannot be explained as a primary spreading axis driven by roll-opposed plate motion. Instead, the MIB must be understood as a structural connection zone accommodating the transfer of deformation between the WVZ and EVZ.

Analogy with the South Iceland Seismic Zone

It has often been noted that the MIB performs a role analogous to that of the South Iceland Seismic Zone (SISZ). The SISZ is characterized by tectonically driven strike-slip and oblique faulting, accommodating lateral plate motion rather than sustained rift volcanism. The MIB is broadly parallel to the SISZ and occupies a comparable central position within its convection-roll polygon.

Between the polygons associated with the SISZ and the MIB lies an intermediate polygon, here referred to as the Hreppar Polygon (HP). Some interpretations treat the SISZ and HP—together with adjacent areas, often including the western margin identified as the Þingvellir rift—as a distinct tectonic microplate. In this framework, the northern boundary of that microplate coincides with the transition across the MIB.

Internal complexity of the MIB polygon

The MIB polygon itself displays a dual structural character. In its southern half, the bounding division lines of the convection rolls converge closely, producing a relatively unified structural pattern. Farther north, these division lines diverge, and an additional micro-polygon appears in the north corner. In this area, fissure swarms extending from the Hofsjökull central volcano exhibit an orientation that differs from that of the southern MIB, indicating localized reorganization of stress.

Comparison with the Tjörnes Fracture Zone

A useful comparison can be made with the Tjörnes Fracture Zone. Within the TFZ, the Húsavík–Flatey Fault accommodates almost exclusively horizontal dextral motion and has done so continuously for millions of years, producing a cumulative offset on the order of 60 km on each side. In contrast, the Grímsey Oblique Rift exhibits both seismic and volcanic activity. Although it is broadly parallel to the Húsavík Fault, it serves a different function: accommodating the combined tectonic and magmatic processes required to link the North Volcanic Zone with the Kolbeinsey Ridge.

In an analogous manner, the SISZ and the MIB together form a paired system: one zone primarily accommodating horizontal tectonic motion, the other incorporating significant volcanic processes.

Latitudinal variation in polygon patterns

The geometric arrangement of convection-roll polygons differs markedly between southern and northern Iceland. In the south, large and relatively regular polygons give rise to east–west-oriented structures, with one zone dominated by horizontal shear (the SISZ) and another incorporating volcanism (the MIB). In the north, the polygon pattern instead permits the development of a single, long-lived, continuous transform fault, represented by the Húsavík–Flatey Fault. Within the adjacent row of polygons, the Grímsey Oblique Rift fulfills the volcanic role associated with plate-boundary connection.

Closing synthesis

Taken together, these observations suggest that the Mid-Iceland Belt is neither a simple rift nor a conventional transform zone. Instead, it represents a mantle-controlled connection zone, complementary to the South Iceland Seismic Zone in the south. The pair of MIB and SISZ is analogous in function to the Grímsey Oblique Rift and Húsavík Fault in the north. The geometry of the MIB, internal complexity, and relationship to adjacent volcanic zones are best understood in terms of the spatial organization of mantle convection rolls rather than solely through plate-boundary kinematics.

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Seeing the Invisible: Visualizing Mantle Convection Rolls with AI

Steingrimur Thorbjarnarson

Visualizing mantle convection rolls becomes significantly easier with the use of AI-based three-dimensional models. What were once abstract concepts—difficult to imagine and even harder to communicate—can now be rendered as coherent flow structures extending through the mantle. These visualizations provide an important bridge between mathematical models of mantle dynamics and the geological features observed at Earth’s surface. Here is a beginning:

AI-made simplified version of convection rolls under Iceland.

Understanding mantle convection is essential if we are to understand nature correctly. When long-lived convection rolls are taken into account, the spatial distribution of volcanism, rift zones, and seismic belts becomes more intelligible. Location of volcanoes and earthquake zones can be explained, and mid-ocean ridges are not randomly distributed features; instead, they align along sections that can be calculated, and notably, the same mathematical framework can be used to trace subduction zones.

Iceland is exceptionally well suited for testing this type of model. Few regions on Earth display such a concentration of geological features within such a limited area. This makes it possible to compare predicted mantle-flow patterns directly with mapped surface expressions. AI-generated 3D visualizations allow these comparisons to be made more intuitively, helping to explain how the geometry of convection rolls corresponds to volcanic zones and rift systems.

The functioning of mantle convection rolls is not immediately intuitive and requires time to grasp. The mantle behaves neither as a simple liquid nor as a rigid solid. Instead, temperatures are close to the solidus, allowing slow but organized flow to take place over geological timescales. Molten magma—which may eventually erupt as lava at the surface—is originally supplied along the division lines between adjacent convection rolls, where hot mantle undergoes partial melting and ascends through the tectonic plate.

A model showing correct proportions of the convection rolls along 64°N.

These rolls exert a direct influence on the tectonic plates above them. Through basal traction, the organized mantle flow causes tectonic drift. In most cases, the direction of mantle flow reinforces the dominant tectonic drift. However, in certain regions, a smaller convection roll may locally oppose the main trend of plate motion. When this occurs, extensional stresses can develop in the overlying crust, leading to rifting.

Such rifting is not merely conceptual but measurable. Over time, an active rift zone does typically span 1.5 degrees from east to west. The East Volcanic Zone in Iceland provides a clear example. Its width, orientation, and volcanic productivity are consistent with localized interaction between mantle convection rolls.

By combining mathematical descriptions of mantle flow with AI-based visualization and geological observation, mantle convection rolls can be treated as physically coherent structures linking Earth’s deep interior to its surface expression. Rather than being abstract or speculative, they offer a unifying framework for understanding why geological features appear where they do.