By John Donato (founder member of Merlin, now reluctantly retired!)

Several years ago, my wife Sue and I tried to visit Iceland, but that trip was unfortunately cancelled. During May of this year we finally made it, spending a few very enjoyable days on the island with some of our family. Always interested in the local geophysics, and knowing little of Iceland’s geology, I was naively expecting to find a Mid-Atlantic Ridge rift and a possibly thinner oceanic crust – wildly wrong on both counts!

Figure 1(a)  Simplified rift structure map of Iceland.  Slightly modified from Psiĥedelisto and ChaseKiwi, CC BY-SA 4.0.  The black dotted outline shows the area of the deep Bouguer gravity low, taken from Figure 1(b).
Figure 1(b)  Bouguer Anomaly gravity map. The green shading, circled by the black dashed outline, shows the approximate area of a deep Bouguer Anomaly gravity low.  Gravity map taken from Bonvalot, S., Briais, A., Kuhn, M., Peyrefitte, A., Vales, N., Biancale, R., Gabalda, G., Moreaux, G., Reinquin, F. & Sarrailh, M. (2012). Global grids : World Gravity Map (WGM2012). Bureau Gravimetrique International

There is not one rift, but several (Thordarson and Hoskuldsson, 2014, an excellent guide if you are planning a trip).  The rifts cross the island (solid red lines in Figure 1(a)), are linked by cross-cutting fracture zones (dashed red lines), and are overlain by volcanic belts.  Separate rifts are at different stages in their evolution.  The Snaefellsness Volcanic Belt (SVB) is now extinct.  The West and North Volcanic Belts (WVB & NVB) are currently active, with the East Volcanic Belt (EVB) and possibly the Oraefi Volcanic Belt (OVB) representing future rift zone locations.  The rifts become younger to the east, and this is thought to be related to the relative motion of the plates across an underlying mantle plume, causing a series of ‘rift jumps’.

The crust is complex, being an interplay between spreading plate boundaries and a deep mantle plume.  Figure 1(b) shows a Bouguer Anomaly gravity map with a deep gravity low (green shading) located mainly over the south-eastern side of the island.  The low signifies a large mass deficit beneath the island.  This is interpreted (Darbyshire et al., 2000) as related to a local crustal thickening (up to 40km), located over anomalously-low density, warm mantle.  The gravity anomaly, taken together with seismic velocity measurements, offers strong evidence for the presence of the deep mantle plume.

Figure 2(a)  Standing within the surface expression of the rift within the West Volcanic Zone (WVZ)
Figure 2(b)  Fine detail of Pahoehoe Lava on the SE rift shoulder in the WVZ  (the fence post is about 1m high)
Figure 2(c)  Enjoying the warm waters of the Blue Lagoon within the Reykjanes Volcanic Belt (RVB) (I managed to escape the benefits of the obligatory face pack!)

On the touristic side of things, we visited two locations along the onshore continuation of the Reykjanes Ridge (RR). One was at the West Volcanic Zone (WVZ) and the other along the Reykjanes Volcanic Belt (RVB).  At the WVZ, we walked down into the collapsed surface expression of the rift (Figure 2(a)) and found some clear examples of the top-rippled surface structure of pahoehoe lava (Figure 2(b)) located on the south-easterly rift shoulder.  At the RVB, close to the recent lava flows near Grindavík, the road to the Blue Lagoon is now open and we enjoyed the warm waters (Figure 2(c)) heated by the underlying rift.

During our trip, the weather was excellent because Iceland was experiencing anomalously sunny, warm and dry conditions.  This was great for our trip, but possibly another example of the world’s changing climate.

Iceland is a great place to visit.  It has wonderful scenery and geology, friendly locals and great food. Be warned, however, as eating out is expensive, especially if you have two hungry grandsons!

References

Darbyshire, F. A., White , R. S. & Priestley, K. F., 2000. Structure of the crust and uppermost mantle of Iceland from a combined seismic and gravity study. Earth and Planetary Science Letters, pp. 409-428.

Thordarson, T. & Hoskuldsson, A., 2014. Iceland – Classic Geology in Europe 3. Edinburgh: Dunedin Academic Press Ltd.

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