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The division line of the West Volcanic Zone of Iceland

May 4, 2026May 4, 2026 Steingrimur Thorbjarnarson

By examining a relief map of Iceland, key geological features become far more apparent than they would otherwise. One such map is displayed in the lobby of the visitor center at Þingvellir.

The main division line of West Volcanic Zone and its surroundings

A broken red line has been drawn to delineate the boundary along the western margin of the Western Volcanic Zone. This boundary can be traced continuously from north to south across the landscape. At its northernmost extent lie Hveravellir, a significant high-temperature geothermal field situated in the central highlands. Immediately to the west is Langjökull, beneath which two volcanic systems are located. Proceeding southward, one encounters Hvítárvatn, a proglacial lake that serves as a major outlet for meltwater from Langjökull and lies directly on this geological boundary.

Further south, the Jarlhettur form a row of hyaloclastite ridges aligned along the same divisional trend. This boundary corresponds to the interface between two mantle convection rolls beneath the approximately 120 km thick lithospheric plate. In addition to this division, deeper convection rolls boundaries are present, above which lies the Geysir geothermal area.

South of this part, volcanic formations of a different character emerge, including Laugarvatnsfjall, located above the lake Laugarvatn, pointed out on the map. These formations extend westward to Kálfstindar at the side of the Þingvellir graben, which follows the same orientation as the main boundary shown here. The geothermal activity at Laugarvatn is well documented. Immediately south of Laugarvatn lies the shield volcano Lyngdalsheiði. The boundary intersects the summit crater Þrasaborgir.

Notably, the hydroelectric power stations along the Sog river are situated on this boundary, marking a transition from highland terrain to the west to lower elevations in the east. The former waterfall Ljósafoss was located precisely along this division. Further south lies the mountainous region associated with Hrómundartindur, of which Reykjafell, near Hveragerði, forms a part. Here, deeper boundaries between mantle convection rolls extend westward beneath the Hengill volcanic system. South of this area, sharply defined structural boundaries known as Hlíð appear. These correspond in practice to the eastern flank of the shield volcano Skálafell and associated volcanic formations along the eastern edge of the Western Volcanic Zone.

In this region, the mantle convection rolls rotate in opposite directions. The western side coincides with the West Volcanic Zone, where one convection roll resists the northwestward motion of the North American Plate. In contrast, the area to the east is transported along with the underlying mantle flow. Consequently, the western region is subjected to continuous extensional forces, resulting in crustal rifting. This process is most clearly expressed in the rift valley at Þingvellir.

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Mapping Volcanoes to Mantle Flow in the Aegean Sea

May 2, 2026May 2, 2026 Steingrimur Thorbjarnarson

The South Aegean Volcanic Arc, including Santorini, is one of the most active and famous volcanic regions of the Mediterranean. It formed mainly through subduction of the African Plate beneath the Aegean/Eurasian region along the Hellenic subduction zone. The arc includes, from west to east, the volcanic fields of Sousaki–Aegina–Methana–Poros; Milos; Christiana–Santorini–Kolumbo; and Kos–Nisyros–Yali/Gyali.

In the convection-roll model, the arc is highly symmetrical around the central axis of the Aegean Sea, and the model as well, while still being affected by the westward tectonic drift of the Anatolian region. The volcanoes appear to fall along specific modeled convection-roll lines. This close fit between a mathematical model and observed surface volcanism adds statistical support to the relevance of the model.

The volcanic alignment can be summarized as follows:

Sousaki, Aegina, Methana, and Poros are situated above the westernmost set 1.
Milos lies above the second set of downwelling lines.
Central Axis: Santorini and Kolumbo are located near the central axis of the Aegean Sea.
Nisyros, Gyali (Yali), and Kos are positioned above the easternmost downwelling line of set 4.

Within the South Aegean volcanic arc, four sets of lines are identified in the convection-roll model. However, volcanic centers are concentrated above only three of these. Notably, no volcanoes are observed directly above the line of the pair marked as number 3. Instead, volcanic activity, most prominently at Santorini and Kolumbo, is located between lines marked as 2 and 3, aligning closely with the central axis of the Aegean Sea.

This pattern suggests that the central axis may act as a preferential path for magma ascent. Similar axial focusing has been observed in other tectonic settings, such as Iceland (particularly the North Volcanic Zone), where mantle upwelling and crustal weaknesses combine to localize volcanism and high temperature areas along a central N-S aligned axis.

Within the framework of this model, it can be hypothesized that mantle material associated with line 3 does not produce surface volcanism directly above it. Instead, the flow may be laterally redistributed, potentially westward, before rising along zones of reduced lithospheric strength at the central axis. This would imply that the interaction between convection patterns and lithospheric structure plays a key role in determining the final location of volcanic activity.

This suggests that the division lines of the convection rolls play a decisive role in determining the exact locations of volcanic activity. The activity itself is, of course, initiated by the subduction of the African Plate and the associated processes. As in many other regions of the world, subduction creates a volcanic arc that is clearly detectable at the surface and is dotted with volcanoes. It is precisely at the points where the convection-roll division lines intersect this arc that volcanic centers appear.

In this context, the blue downwelling lines may also be interpreted as marking conduits for ascending magma. The term “downwelling” refers to the slow convection of plastic mantle material; however, along the boundaries between convection rolls, at the division lines, partial melting can occur. This process may create zones of weakness within the lithosphere, providing favorable conditions for magma ascent and, ultimately, volcanic activity at the surface.

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How Basaltic Magma Ascends within Subduction Zones

May 1, 2026May 1, 2026 Steingrimur Thorbjarnarson

I did discuss how magma must ascend from below the descending slab. If not, there would not be enough mantle material available for partial melting, not enough heat because of the cooling effect of the slab, etc. Simplifying the picture, emphasizing on the ascending magma only, the idea can be drawn AI-style:

In this way the production of basalt for Mt. Fuji, for instance, can be explained.

This can be tested by comparing sections of subduction zones with the model.

There are many examples of how the model shows consistency with actual circumstances, position, alignment, and length. Trenches and volcanic zones also tend to coincide with convection rolls, the trench following the scope of a roll, then one roll is found in between the trench and the volcanic roll, and finally the bulk of volcanic activity is found in context with one roll. These three rolls are parallel to each other. The volcanic activity also tends to terminate at the border of a polygon. Here, two examples of this type of subduction are provided, from different corners of the Pacific Ocean:

The geology of these areas should then be analyzed for each of those diamond-shaped areas marked on the maps.

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The Distribution of High-Temperature Geothermal Areas in Relation to Plate Boundaries and Mantle Convection in Iceland

April 30, 2026May 1, 2026 Steingrimur Thorbjarnarson

There are two main rules governing the distribution of high-temperature geothermal areas:
(1) their location along the boundaries between tectonic plates, and
(2) their alignment with the boundaries of mantle convection rolls.

High temperature areas in Iceland pointed out.

Not all the high-tempereature areas are directly “pointed at” here, because the purpose is not to show a list, only that the location of those areas can be linked to an understandable process within the whole tectonic plate. Intrusions of magma were responsible for all those formations.

In practice, almost all high-temperature areas are located either directly on, or immediately adjacent to, these lines. The main exceptions are Eyjafjallajökull, which is known to be connected to its neighbouring system beneath Mýrdalsjökull, and Hofsjökull, where the presence of a high-temperature area is somewhat uncertain. Map base: https://vatnsidnadur.net/wp-content/uploads/2023/12/NI-03016.pdf

The Torfajökull area lies somewhat distant from the plate boundary, yet it is the largest and most powerful of all high-temperature systems in Iceland. However, the area is highly fractured, suggesting that its roots may extend toward the plate boundary at depth.

Taken together, all 24 areas appear to occur within a similar structural context. Not every area is pointed out with an arrow, but in this way the consistency becomes more obvious.

This map is more accurate, and the consistency becomes more obvious. On the other hand, the locations are less clearly marked. Comparing those two maps is therefore ideal.

The fact that two different factors are relevant here, division between tectonic plate and convection rolls division lines is of course intriguing. It must be kept in mind that the origin of these lines is to be traced 120 km below the surface. This AI drawing tells a story:

AI-version of the ascending path of magma – not to scale.

The remarkable aspect is how precisely the vertical flow from asthenospheric roots appears to be aligned along the boundaries of convection rolls. In contrast, along plate boundaries, according to this model, magma travels laterally before ascending at its final location. The divergent tectonic process creates pathways that allow magma to accumulate in these zones, leading to the formation of high-temperature geothermal areas.

List of Main High-Temperature Geothermal Areas in Iceland:

Hveravellir
Reykjadalur
Prestahnúkur
Geysir
Hengill
Reykjanes
Kerlingarfjöll
Mýrdalsjökull
Hágöngur
Vonarskarð
Þeistareykir
Öxarfjörður
Gjástykki
Krafla
Námafjall
Fremrinámar
Hrúthálsar
Askja
Kverkfjöll
Grímsvötn
Jökulskálar
Hofsjökull
Eyjafjallajökull
Torfajökull

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What We Learn from Ophiolites

April 27, 2026April 30, 2026 Steingrimur Thorbjarnarson

Ophiolites are slices of oceanic crust that have been uplifted and exposed, making it possible to examine a cross-section of a tectonic plate. This provides a valuable opportunity to study the different layers of the Earth. There are many ophiolites, the most famous probably being those found in Oman and Cyprus. These were emplaced onto continental crust through obduction and are in some cases tilted or rotated.

After: http://www.gulfpetrolink.com/geoarabia/index.php/ga/article/view/31997

The uppermost ~5 kilometers of the brittle oceanic crust can thus be examined in cross-section. At the top lies a layer of pillow lavas, beneath which are sheeted dikes, followed by a gabbro layer extending down to the Moho discontinuity. In favorable cases, ophiolites also expose portions of the underlying mantle.

At the Moho, there is typically a transition zone composed mainly of two rock types: wehrlite just below the gabbro, followed by dunite. Beneath this lies the lithospheric mantle, which is composed mainly of harzburgite. This is a type of peridotite that remains after the original lherzolite of the asthenosphere has undergone partial melting to produce basalt.

Basalt originates from the asthenosphere at depths of around 120 km. The temperature of basalt at eruption at the Earth’s surface is surprisingly close to the temperature at which it originally formed. In Iceland, basalt may represent up to about 20% partial melting of the original lherzolite.

One particularly important feature revealed by the exposure of mantle below the Moho is the presence of conduits through which partial melt (basaltic magma) has traveled. These conduits consist of dunite, composed almost entirely of olivine. They form vertical channels leading up toward the Moho. This indicates that partially molten material can flow upward relatively rapidly, entering the gabbroic section immediately above the Moho transition zone.

The gabbro zone is commonly divided into two parts, with the lower portion showing layered structures. Within this zone, basaltic magma can accumulate in sills and magma chambers, where it may partially crystallize before continuing its ascent. When conditions allow, the magma rises again, typically vertically, through the sheeted dike complex, forming successive dikes.

As frequently observed in Iceland, such processes can eventually lead to volcanic eruptions at the surface. On the ocean floor, these eruptions typically produce pillow lavas. The vertical continuity and rather fast flow of basalt up through the tectonic plate can explain how it maintains temperature (almost) found at the depth of 120 km at the surface. This AI picture expresses the process:

The vertical movement of partially melted material through the tectonic plates can partly explain why division lines in the asthenosphere can be detected at the surface. This process continues over long distances, forming dikes of considerable length. Even more importantly, this ongoing process creates divisions around which different parts, often described here as polygons, can adjust to the tectonic drift that constantly alters the positions of continents and oceanic crust. Another contributing factor is the local horizontal movement, combined with the global tectonic drift trend, which can lead to localized rifting or pressure at the surface.

For mainland crust, the section looks slightly different for the upper most part. Again, this AI image can be made: