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Should the Volcanic Zones of Iceland be Redefined?

Steingrimur Thorbjarnarson

Each of Iceland’s volcanic systems shows a double character, meaning that the volcanic zone or belt tends to appear as a pair of structurally or spatially related units — in most cases corresponding to two adjacent polygons within the framework of the convection roll system. Since mantle convection rolls occur as paired structures, the volcanic systems that form above them naturally reflect this geometry. In this way, nearly every volcanic zone or belt in Iceland can be divided into two complementary parts.

West Volcanic Zone (WVZ) – The WVZ displays a continuous rifted region along its southwest portion, including the famous Þingvellir rift valley, and a secondary, wider area toward the northeast. Although the polygon northeast of the WVZ, here marked with a question mark, is commonly considered part of the same zone, its characteristics suggest it may instead be linked more closely with the Mid Iceland Belt.

East Volcanic Zone (EVZ) – The EVZ is sometimes defined as covering five polygons, but here only two, where active spreading takes place.

South Iceland Volcanic Belt (SIVB) – The SIVB also follows a paired pattern, the two polygons being quite different though, with Katla and Eyjafjallajökull in the southern one, and long fissures covering the northern one. Hekla is found between them, marking the division between spreading and non-spreading along the 64th latitude.

Westman Islands (WI) – The WI system is an exception, as it currently appears confined to a single polygon. However, if the general two-part pattern holds true, there should be a counterpart polygon located southwest of the islands. This hypothetical extension, designated as WI(2), would complement the known island polygon WI(1), completing the expected double structure. That is one part of science, right? Let’s go there and check it out!

Reykjanes Oblique Rift Zone (RORZ) – The RORZ demonstrates its double character both geographically and structurally. The Reykjanes Peninsula forms the onshore part of the system, while the offshore segment of the rift continues as a second polygon on the seafloor. These two parts together define the complete oblique rift zone.

Mid Iceland Belt (MIB) – The MIB is another exception to the general rule, as it occupies only one polygon in present definitions. However, given its central location and the symmetry of the convection roll system, it is logical to expect a complementary polygon in close proximity. The missing half of the MIB is therefore indicated by one of the question marks on the map. It is mainly up to petrologists to answer that question.

Skagafjörður Volcanic Belt (SKVB) – It volcanically extinct, and neglected, and even here I do not define the two parts 🙂

North Volcanic Zone (NVZ) – The NVZ can be divided into two principal parts: the northern portion associated with the Theistareykir–Krafla systems, and the southern portion extending toward Askja. These two subdivisions mark distinct volcanic corridors following separate polygons but functioning as a single rifting system.

Grímsey Oblique Belt (GOB) – The GOB likely mirrors the Reykjanes Oblique Rift Zone in both form and behavior. Its paired geometry supports its classification as an oblique twin to the RORZ.

Öræfajökull Volcanic Belt (ÖVB) – The double character of the ÖVB is particularly pronounced, as it clearly spans two distinct polygons: one centered on Öræfajökull and the other on Snæfell to the north. The two volcanic centers form a strongly defined pair, consistent with the structural control of the convection rolls beneath.

Snæfellsnes Volcanic Belt (SVB) – The SVB, is very similar to the ÖVB, and is divided into two parts here, although it actually extends over parts of 3 different polygons, it should not break the main rule of duality of each volcanic zone or volcanic belt.

In summary, the double character of Iceland’s volcanic systems reflects the paired arrangement of mantle convection rolls beneath the crust. Each volcanic zone or belt typically spans two adjacent polygons, forming a conjugate structure that mirrors the flow patterns of the underlying mantle. The only exceptions — MIB and WI — highlight potential locations where the complementary polygon remains unrecognized, indicated on the map by the two question marks.

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The Skagafjörður Volcanic Zone: A Relic of Iceland’s Shifting Rift System

Steingrimur Thorbjarnarson

Around 3 million years ago, a volcanic zone developed in the Skagafjörður region, extending across what is now the Skagi Peninsula in northern Iceland. This area was part of the Neovolcanic Zone at the time — the active rift that carried most of Iceland’s volcanic and tectonic activity.

For roughly 2–2.5 million years, the Skagafjörður volcanic system produced extensive basaltic lava flows, which now blanket the Skagi Peninsula. These lava layers form a thick sequence of Pleistocene basalt plateaus, showing clear evidence of successive fissure eruptions and long-lived rift activity.

The Skagafjörður volcanic zone formed approximately 3° farther west, but at the same latitudes as the present-day Northern Volcanic Zone. This spatial relationship is not coincidental: it reflects the underlying mantle convection pattern. In Iceland’s mantle, long convection rolls extend roughly 1.5° in width from east to west. These rolls guide upwelling zones and determine where rifting and volcanism are concentrated at the surface.

Thus, both the Skagafjörður and Northern Volcanic Zones are expressions of the same large-scale convection pattern — successive manifestations of upwelling between the same pair of convection rolls, but active at different times as the spreading axis gradually shifted eastward.

Volcanic activity in Skagafjörður ceased less than 700,000 years ago, marking the end of its active phase. By then, the rift axis had shifted eastward and thereby replaced by the current Northern Volcanic Zone. During the active period of the Skagafjörður system, tectonic drift continued, resulting in approximately 10 km of crustal extension. This stretching contributed to the widening of Skagafjörður, later sculpted by glaciers into the broad fjord we see today.

The Skagi Peninsula, now far from any active volcanic centers, remains a silent geological record of this earlier rift episode — a remnant of the same convection-driven dynamics that continue to shape Iceland’s landscape today.

The two NS-axis of Iceland – old and new

Here you see the light blue colored area of Skagi, isolated from other volcanic areas.

https://www.langdale-associates.com/iceland_2017/prologue/geology_map.htm

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About Mary Tharp and the Mid-Ocean Ridges

Steingrimur Thorbjarnarson

Mary Tharp was a pioneering geologist and oceanographic cartographer who, together with Bruce Heezen, created the first comprehensive maps of the ocean floor. Her work revealed the global mid-ocean ridge system, a continuous chain of underwater mountains stretching around the planet, and provided some of the first convincing visual evidence for plate tectonics.

This painting is still the clearest depiction I know of the ridge system. When zooming in on the ridges around Iceland, we can clearly see that a continuous structure lies beneath the island: the Reykjanes Ridge to the south and the Kolbeinsey Ridge to the north, separated only by Iceland and the Icelandic shelf.

It is also fascinating to look at the globe Tharp and Heezen created, where the ridges are marked all around the world. Seeing the system on a sphere makes it much easier to grasp how continuous these features really are, something that is very difficult to comprehend on a flat map. Creating a globe like that must have been a crucial part of their work, helping them visualize Earth’s dynamic structure as a truly interconnected system.

https://en.wikipedia.org/wiki/Marie_Tharp

1977: The culmination of Tharp’s decades of work came with the publication of “The World Ocean Floor”, a full world map of the ocean ridges, created in collaboration with artist Heinrich Berann. This was the first global visualization of the continuous mid-ocean ridge system encircling the planet.

Of course it is tempting to add some aspects of the convection rolls model here:

t’s hard not to be impressed by the remarkable regularity of the pattern — at once striking, convincing, and precise. The main structural divisions appear at consistent 30° intervals, outlining a symmetry that is anything but coincidental. Even with the thinnest possible lines, the geometry stands out clearly: the 30° spacing traced along the equator, the continuous arc of the Ring of Fire encircling the Pacific Ocean, and the 90° separations linking the major mid-ocean ridges across the Southern Hemisphere. Together, these alignments suggest a coherent global framework — a kind of planetary rhythm — that underlies both surface geology and deeper mantle dynamics.

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Volcanic and Geothermal Activity along 64° N, Iceland

Steingrimur Thorbjarnarson

The 64th parallel intersects some of Iceland’s most significant volcanic and geothermal centers, including Hekla, Landmannalaugar, and Öræfajökull, which line up remarkably well along that latitude, about 3° of longitude apart. Let’s now trace the 64th parallel north (64°00′ N) carefully from west to east.

Tracing 64° N, Iceland

1. Western Start — Reykjanes Peninsula (22°–21° W)

The 64th parallel enters Iceland at the southern Reykjanes Peninsula, intersecting the Reykjanes Volcanic Belt, the onshore continuation of the Mid-Atlantic Ridge.

  • Key systems: Reykjanes, Svartsengi, Krýsuvík–Trölladyngja, Brennisteinsfjöll.
  • Geothermal activity:
    • Svartsengi (Blue Lagoon) — high-temperature field exploited for power and bathing.
    • Krýsuvík — intense fumaroles and sulfur deposits along faulted rhyolite ridges near Lake Kleifarvatn.
  • Volcanism: Basaltic fissure eruptions, most recently the Fagradalsfjall events (2021–2024), lie within a few kilometers north of 64° N.

2. Hengill and Hveragerði Region — Western End of SISZ (21° W)

At 64° N, the parallel runs just north of Hveragerði and crosses the Hengill volcanic system, a major geothermal center and the western terminus of the South Iceland Seismic Zone (SISZ).

  • Geothermal:
    • Hellisheiði and Nesjavellir plants utilize the Hengill field; production wells reach >300 °C.
    • Surface features include fumaroles, silica terraces, and mud pots in Hveragerði valley.
  • Tectonics: The SISZ runs eastward from here to Hekla (~19.5° W), spanning roughly 1.5° of longitude, as you noted.
    • It’s a transform zone, accommodating lateral spreading between the Western and Eastern Volcanic Zones.

3. South Iceland Seismic Zone (SISZ) — Between Hveragerði and Hekla (21°–19.5° W)

At 64° N, the line traverses the seismically complex Hreppar microplate.

  • Numerous NNE–SSW strike-slip faults and en-échelon fissures characterize this belt.
  • Though volcanic activity is minimal, earthquakes (M 6–7 historically) are frequent.
  • Geothermal springs occur near Flúðir and Laugarvatn, used locally for heating and bathing.

4. Hekla Volcano (19.7° W)

At precisely 64.00° N, 19.7° W, lies Hekla, Iceland’s most active stratovolcano.

  • Type: Elongate ridge volcano, 1491 m high.
  • Eruptive behavior: Mixed basaltic-andesitic, with both explosive and effusive events; last eruption in 2000.
  • Structure: A central fissure system about 40 km long trending SW–NE.
  • Geothermal: Weak surface manifestation; heat flux mainly magmatic.
  • Relation to SISZ: Marks the eastern terminus of the seismic zone and transition into the Eastern Volcanic Zone (EVZ).

5. Landmannalaugar–Torfajökull Region (18.8°–18.5° W)

Still right at 64° N, this area sits within the Torfajökull volcanic system, a vast rhyolitic caldera overlapping the Fjallabak fissure swarm.

  • Geothermal:
    • Active hot springs, mud pots, and steam vents along the Laugahraun lava and Brennisteinsalda area.
    • Surface temperatures reach >100 °C; deep hydrothermal systems exceed 250 °C.
  • Volcanic features:
    • Rhyolitic domes (Brennisteinsalda, Bláhnjúkur).
    • Mixed eruptions between Torfajökull and Veiðivötn fissure systems (e.g., 1477 AD event).
  • Significance: It’s one of Iceland’s largest silicic geothermal regions, directly intersected by the 64th parallel.

6. Veiðivötn–Bárðarbunga Fissure Swarms (18°–16° W)

Crossing the central highlands, 64° N passes just south of Veiðivötn, part of the Bárðarbunga–Grímsvötn volcanic system.

  • Recent activity: 1477 AD Veiðivötn eruption produced >5 km³ of basaltic tephra; the fissure extends ~100 km.
  • Geothermal: Subsurface high-temperature zones exist beneath Holocene lava fields; limited surface expression due to remoteness.
  • Topography: Alternating lava plains and crater rows — a direct result of fissure rifting at the EVZ’s central axis.

7. Northwest Vatnajökull Margin – Hamarinn & Kverkfjöll (16°–15° W)

Approaching the Vatnajökull ice cap, 64° N crosses areas where subglacial volcanism dominates.

  • Hamarinn (Loki): Central volcano beneath ice, source of jökulhlaups into the Tungnaá and Skaftá rivers.
  • Kverkfjöll (64.7° N, 16.7° W): Slightly north of the parallel but significant. Features:
    • One of Iceland’s most vigorous geothermal fields, extending under the ice margin.
    • Fumaroles, hot ice caves, and sulfur deposits at the ice edge.
    • Heat flow >1 W/m², indicating magmatic heat directly below.
  • Interaction: These subglacial systems release jökulhlaups, linking geothermal processes to glacial hydrology.

8. Öræfajökull Volcano (64.00° N, 16.65° W)

Exactly on the 64th parallel, Öræfajökull forms the southeastern corner of Vatnajökull.

  • Type: Stratovolcano; Iceland’s highest peak, Hvannadalshnúkur (2110 m).
  • Eruptive history: Catastrophic 1362 and 1727 eruptions, both explosive (VEI 5+), with ash fallout across Europe.
  • Geothermal activity:
    • Weak at the surface due to heavy glaciation, but meltwater and hydrothermal alteration indicate subglacial heat flow.
  • Structure: A large caldera, with geothermal vents beneath the ice—likely connected to a shallow intrusive complex.

9. Eastern Termination – Breiðamerkurjökull to Höfn (15°–14° W)

The 64th parallel exits Iceland across the southeast coastal plain and the Vatnajökull outlet glaciers.

  • Geothermal: Minimal visible activity; only low-temperature springs.
  • Volcanic: Ancient subglacial ridges and pillow lavas beneath the sands mark earlier Holocene eruptions under ice.

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Northward Drift and Symmetry of Plate Motion Across Iceland: Insights from ISNET Data

Steingrimur Thorbjarnarson

This figure presents the results of ISNET geodetic measurements from the ISN93 and ISN2004 campaigns.

https://www-gamli.lmi.is/wp-content/uploads/2011/09/isnlet2004-skyrsla.pdf

The data clearly indicate that the overall horizontal plate motion across Iceland is directed predominantly towards the north. Furthermore, the velocity field demonstrates that the plate motion in northern Iceland is approximately symmetric with respect to the north–south axis: motion vectors on the Westfjords are oriented toward the northwest, whereas those on the Eastfjords are oriented toward the northeast. The vectors on both sides of the island form nearly identical angles relative to geographic north.

The figure is based on the original measurement data and therefore represents the true directions of crustal motion. The displacement rates were derived using the SOPAC velocity field at epoch 2007.6, under the assumption that the motion at station REYK is equivalent to that observed at LM0082. Consequently, the figure provides a realistic depiction of the current kinematic framework of Iceland.

The magnitude of the plate motion is notably greater than commonly assumed. On the western part of Iceland, the motion typically reaches close to 2.5 cm/yr towards the northwest and the northeast, as shown (see arrow at lower left corner on the map, showing the length of 25 cm in 10 years). On the other hand, the spreading rate between the North American and Eurasian plates—i.e., the rate of divergence across the rift zones—is approximately 2 cm/yr. It is important to distinguish between absolute plate motion and spreading rate: while the total drift relative to a stable reference frame is on the order of 2.5 cm/yr on each side, the divergence between the two plates amounts to roughly half that value. Many descriptions of Icelandic tectonics cite about 1 cm/yr of motion to the west and 1 cm/yr to the east; however, these represent the half-spreading rates rather than the absolute plate velocities observed in geodetic measurements.

https://en.wikipedia.org/wiki/Plate_tectonics

The two different drift trends at the eastern and western sides of Iceland can also be seen on this map from Wikipedia.

Geodetic networks such as ISNET have been instrumental in quantifying this relative motion and in delineating the internal deformation of the island. The observed symmetry in motion across northern Iceland reflects the geometry of the ridge system, with extension distributed among several rift zones and connected by major transform fault systems, including the South Iceland Seismic Zone and the Tjörnes Fracture Zone. These results are consistent with the regional plate-tectonic model and provide refined constraints on the present-day strain field within the Icelandic plate boundary zone.