Author: Steingrimur Thorbjarnarson

Here is a good way to take a first look at how the mantle convection-roll system beneath Iceland works:

The convection rolls, once incorporated into a model of Iceland and the layers below it, appear to explain much of the country’s main geological framework. It is easiest to begin with the uppermost layers, since they likely have the most direct influence on the tectonic plate above. Here, a section is shown with reference points A and B. We can then focus on four distinct convection-roll sections.

The first roll, extending from the Kolbeinsey Ridge, corresponds closely with the West Volcanic Zone and the Reykjanes Volcanic Zone, eventually meeting the Reykjanes Ridge system of rolls at the southwestern corner of the country.

The second roll passes beneath the northern part of the North Volcanic Zone and extends toward the center of Iceland, framing the Central Volcanic Zone. It also aligns well with the South Iceland Seismic Zone.

The third convection roll is situated beneath the complex formed by the southern part of the North Volcanic Zone, the East Volcanic Zone, the South Iceland Volcanic Belt, and the Westman Islands volcanic system.

The fourth roll, located near point B, appears to provide the conditions necessary for the Öræfajökull Volcanic Belt.

The volcanic activity of Iceland can be examined by emphasizing the role of the North American side of the plate boundary system. In general, the North American Plate lies west of the mid-ocean ridges, the Kolbeinsey Ridge north of Iceland and the Reykjanes Ridge south of it. Iceland, however, is different, because two regions appear to transfer the volcanic activity eastwards. This is commonly explained simply by referring to a mantle hotspot beneath Vatnajökull, but here the process is interpreted somewhat differently.

According to this interpretation, convection rolls in the mantle layers beneath the tectonic plate contain both upwelling and downwelling sides. At the latitude of Iceland, the volcanic activity shifts eastwards from one side of the convection rolls to the other. Instead of forming a simple linear ridge, volcanic zones develop because the convection-roll structure interacts with the tectonic forces associated with the overall westward drift of the North American Plate. This interaction results in a broader rifting process and the formation of distinct volcanic and seismic zones across Iceland.

What is special about the equatorial section of the Mid-Atlantic Ridge? This segment extends across roughly one third of the distance between South America and Africa along the equator — approximately 20° of longitude within the roughly 60°-wide Atlantic Ocean at equatorial latitudes. The geometrical midpoint of the Atlantic at the equator therefore lies near 21°W, with about 30° extending westward to the coast of South America near 51°W and about 30° eastward to the African coast near 9°E.

When examining the zigzag geometry of this section of the Mid-Atlantic Ridge, an additional symmetry appears. The major deviations toward more northerly and southerly alignments occur at approximately equal distances from this central point, around 9° to either side. If one considers a basic upper-mantle convection-roll width of roughly 1.5°, together with a broader large-scale equatorial spacing pattern of about 30°, the geometry becomes particularly intriguing.

Naturally, this section of the Mid-Atlantic Ridge has been studied extensively using the full range of modern marine geophysical methods. The individual segments and fracture zones are well mapped and documented. Two of the most prominent equatorial fracture zones are the St. Paul Fracture Zone and the Romanche Fracture Zone, which together form a striking en-echelon pattern along the equatorial Atlantic.

To realize the importance of the equator, the most basic map of convection rolls division lines can be added:

We can describe the route across North Iceland with reference to the map and its division lines. Note that the convection roll leading from Reykjanes Ridge main section is slightly coloured with yellow. Imagine setting out from Borgarnes in polygon 1 and driving up across Holtavörðuheiði. The road runs parallel to the division line until reaching the center of polygon 2, where it then turns directly north into Hrútafjörður. From there, the route bends toward the northeast, following the division line until arriving at the northern corner of polygon 3, where the road turns toward Blönduós.

The road onward into Skagafjörður curves repeatedly before turning inland, eventually entering Öxnadalur precisely at the boundary line adjoining polygon 5, the same line that defines the principal axis of the Reykjanes Ridge, continuing all the way to Akureyri.

From Akureyri to Mývatn, the main direction once again runs parallel to a division line, crossing one polygon, and thereby the other convection roll of the Reykjanes Ridge, from one side to the other (no. 6). To continue from Mývatn toward East Iceland, the route first heads directly east across a small polygon that appears to exert a major influence on the geology of the region. Beyond this point, the road follows the boundary of polygon 7 (the next convetion roll) until reaching the highlands of East Iceland.

Roads are, of course, constructed independently of geology or geological principles. Nevertheless, the landscape itself strongly influences where it is most practical to build them. If the boundaries formed by mantle convection rolls are indeed real, then they reveal themselves in various ways across the terrain. With this in mind, one may consider, while travelling from one region to another, which geological processes and structural influences may have shaped the environment within each individual section of the journey.