Growth is in. Politicians and leaders (not always the same thing) want their country to grow. Nowadays this normally means economic growth: the wish to become more wealthy. It would make those leaders more popular, and may provide them with more resources to spend on whatever leaders spend money on. In the very different world of the past, population growth was seen as a good thing. More people would make the nation more powerful and help it to win wars. The eastern Roman empire began to fade after the Justinian plague robbed it of its people and left the armies too small to secure its huge territory. Nowadays we see the fallacy in that policy, as unlimited population growth brings its own collapse, as Jared Diamond has pointed out in several case studies. Economic growth too has its issues, as it depletes our limited resources. The Club of Rome calculated already in the 1970’s that our economic growth would exceed the resource capacity of the Earth by the year 2100. Although the details of their models have changed, and modern technology making much more efficient use of those resources, the conclusion still holds. And of course, the ultimate growth is in size of the nation. For a nation to physically grow, it needs to swallow its neighbours, sometimes openly so (Russia), sometimes more circumspect (China).
All these growth areas are linked. The Babylonian empire spend enormous amounts of money it did not have on building its capital, Babylon, still remembered in our language even though nothing remain but some ruins. To finance the building of Babylon, the empire continually conquered other nations in order to take their money. This became a pyramid scheme with inevitable outcomes: their empire did not last long.
Interestingly, in democracies the tendency goes the other way. Swallowing other nations dilutes the voting power of the original population. Therefore, democracies are more likely to fragment into smaller nations than to grow into bigger ones. Germany is a rare example of a growing democracy, helped by the fact it is a federation rather than a monolithic country.
The final way to grow is by physically creating new land. This was pioneered by the Dutch, but is hampered by the fact that most people are reluctant to live below sea level. But sometimes new land comes naturally. An island may appear where none existed before. A lava flow may create new land or raise existing land. The opposite can also occur. Krakatau was lost twice within 140 years. Hunga Tonga is half the island it used to be. India is still losing a few centimeters of length every year, as it rams into Asia with the same effect as a crumple zone in a car: what India is losing in length, the Himalayas are gaining in depth. The Earth gives and the Earth takes away. It too puts limits on our growth.
Iceland may not look like a growing nation. Stuck in the middle of the North Atlantic, is has long reached its own limits to growth. The population remained constant (with ups and downs) from the end of the settlement era (think Eldgja) to the first world war, limited by severely constrained food production. Growth only resumed once improved transportation brought the rest of the world within traveling and trading distance. But Iceland is perfectly placed to gain from the Earth’s bowel movements. 30 million years ago, it did not exist. Everything you see in Iceland was created since. The oldest rocks are a little over 20 million years old. The youngest are from August.
The Mid Atlantic Ridge
Iceland is part of the Mid-Atlantic ridge, the spreading centre from which the ocean formed. It runs from the southern ocean (the oldest part) to the Artic ocean where it is younger than the dinosaurs. It shows the typical pattern of a 500-km wide ridge, rising 4 kilometers above the surrounding ocean, with a rift valley at the crest. There are nine islands or island groups along the Mid-Atlantic ridge, from Bouvet island at the southern end to Jan Mayen in the north (both owned by Norway). The other seven are from south to north: Gough Island, Tristan da Cunha, St Helena, Ascension Island, St Pauls’s rock, the Azores and of course the only independent nation on a mid-oceanic ridge, Iceland. The ridge gives a broken appearance: it consists of short ridges separated by small sideways jumps. The jumps have formed longer faults running perpendicular to the ridge. These stripes disappear north of Africa where the ridge borders the Eurasian plate rather than the African plate. The ridge here has a single large eastward jump, at the latitude of southern Ireland. A little further north the ridge turns towards the northeast and changes appearance. The brokenness disappears and the ridge become smooth looking. There is also no longer a central valley. Eventually the ridge reaches land at the Reykjanes peninsula. The smooth section is called the Reykjanes Ridge.
The Mid-Atlantic ridge is a pure spreading rift, with the offsets acting as transform faults. This changes as it reaches the Reykjanes peninsula: here it becomes a combination of a spreading and transform fault. It re-enters the ocean north of Iceland.
The brokenness shows that the oceanic crust around most of the Mid-Atlantic ridge is brittle and cold. The smoothness of the ridge near Iceland shows that the crust here is much more ductile and warm. It shows the effect of the Iceland hot spot. And because of the warmth, the density of the rock and mantle is a bit less, and this pushes up the ridge. The ridge becomes shallower and eventually surfaces. This is how Iceland first formed.
To understand this more easily, try to find a newspaper. (For younger readers: a newspaper used to be a paper that described what was happening in the world. Nowadays a newspaper tells you what the editor wants you to think about it.) Try to tear the newspaper by pulling both sides apart. Do this against the grain of the paper (as you may know, paper will tear easily only along one direction, the direction of its fibres. Try the other way.) You will get an irregular tear, continuously going off in unwanted directions. Now make the paper moist and try again. It now tears much more regularly.)
The spreading of the ocean floor is directly visible in maps of magnetic anomalies. At irregular intervals the earth’s magnetic field reverses. The direction of the magnetic field is engrained in the ocean floor. Iron particles in the magma acts as mini compasses, and when the magma solidifies this freezes into the rock. So when the Earth field reverses with magnetic north and south changing places, now the frozen magnetic compasses point the wrong way. The iron particles causes their own magnetic field: there is now a magnetic anomaly. When mapping these anomalies, they show up as stripes parallel to the spreading axis. Each stripe is a memory of what the field was when the rocks formed. This was the conclusive evidence that the ocean floor is alive. It is born in the rift, drifts outward, and finally dies in the subduction zone. (The latter is not yet happening in the Atlantic Ocean: it is much too young to subduct.) The fact that the stripes are parallel shows that the spreading rate is approximately constant along the rift. It is around 2 cm per year, or 20 kilometers in a million years.
The migration of the Reykjanes Ridge
The spreading in the Reykjanes Ridge is not entirely symmetric. Currently, the Eurasian plate receives a little more new crust from the ridge than the American plate. Between 6 and 15 million years ago it was the other way around. Asymmetric spreading indicate that ridge is migrating, and the change 6 million years ago may indicate that the ridge jumped from one location to another, with the crust in between changing plates. As a result of the asymmetry, the shallow shelf surrounding Iceland extends 50 kilometres further on the American side of the Reykjanes Ridge than it does on the other side.
But there is something else funny going on south of Iceland. There are ridges in the ocean floor on either side of the rift, forming a ‘V’ pattern like a flock of migrating geese. The show up particularly well on gravity measurements, as shown in the figure. A variety of explanations are available: there is some dispute around their origin. One obvious explanation is that the spreading rate increases towards Iceland. But this is not the case, since the magnetic stripes remain perfectly parallel. Perhaps the Icelandic plume pulsates, an each pulse slow propagates down the ridge, formed a higher ridge in the process. But there is no other evidence for such pulsating behaviour. Others have suggested it is related to relocations of the ridge. The western edge of the band runs roughly along the main direction of the Mid-Atlantic Rift, along the rough fjord-like coast in the far northwest called Vestfirdir (Westfjords) which looks like it should be a part of Greenland. Other segments line up with the two peninsulas on the west side of Iceland, and one extends towards the south coast near Katla. The idea is that these are mini-rifts which begin to form near Iceland and propagate towards the weakest point where the Reykjanes Ridge begins. These mini-rifts compete to become the true Mid-Atlantic Ridge.
This gives a model where 15 million years ago a mini-rift began to move southwards from Iceland, in the process transferring a block of ocean crust from Europe to America. A second mini-rift developed from Iceland 10 million years ago, another followed 6 million years ago and a final one came 4 million years ago. Each rift transferred as much as 10-15 kilometers of crust to the other plate.
Paleo spreading centres
Iceland is a volcanic nation. The map above, taken from the Mammoth guide and made by VC’s Tom Wallace, shows the main volcanoes. The colour of the triangle indicates how likely it is to erupt. Beware of the power of VC: this map was made in 2017, but it shows the Reykjanes Peninsula at enhanced risk – well before it unexpectedly began to shake and erupt after 800 years of solitude. I just mention it. (We also managed to postpone the Reykjanes eruption by a year, as you may recall.) The map shows that the volcanoes cluster. Not all of Iceland is equally affected.
Deleting the volcanoes shows the underlying pattern. There are several main fault lines. The Reykjanes Ridge links to the Reykjanes Volcanic Belt which continues into the West Volcanic Belt. On the other side of Iceland is the East Volcanic Belt which continues into the North Volcanic Belt and continues as the Kolbeinsey oceanic ridge towards Jan Mayen. The WVZ peters out to the north and EVZ peters out to the south. The two are connected by a transform fault, the South Iceland Seismic Belt. There is also a transform fault indicated connecting the two further north but there is not much evidence for this. The enormous Hofsjokul volcano is located on this line, but transform faults do not form volcanic centres. Other descriptions instead pose a Central Iceland Volcanic Zone (CIVZ) which links Hofsjokul and Tungnafellsjökull system. Finally, there are two less active and disconnected volcanic lines, the Öræfajökul Volcanic Belt in the far east and the Snæfellsnes Volcanic Belt in the west. Each volcanic zone consists of several parallel lines of activity, separated by 10 kilometers. Individual volcanoes tend to be some 20 kilometers apart.
Activity varies. The EVZ is the most active, and the two kilometer high bulge in the landscape suggests that this is the current location of the heat source. The NVZ and the extension of the EVZ into the ocean where it peters out (this is actually the region that was land during the ice age) has less extreme activity. The latter region is sometimes called the South Iceland Volcanic Zone (SIVZ). The WVZ is not nearly as active but can do significant eruptions – the next one will come as a surprise. The two isolated belts have very infrequent eruptions. Finally, the Westfjord area (connected to Iceland only by a narrow isthmus) has no volcanic activity but has old many basaltic lava flows. There was a rift here similar to the modern Reykjanes peninsula, between 8 and 15 million years ago but it has long since died.
Age is important. The EVZ became active only about 3 million years ago. Before that, the WVZ was the prime mover. The Hofsjokul region may have acted as an intermediary. All three remain active. The WVZ still accounts for about a third of the spreading rate in Iceland with the EVZ accounting for the remainder, while the parallel fault around Hofsjokul showed some activity during the 1996 Gjalp eruption. The Snæfellsnes Volcanic Belt is older again. It still contains volcanoes but these are more recent activity (form the last million years) sitting in a much older volcanic platform which existed from 16 million years to 6 million years ago. The region re-activated more recently, perhaps related to the ice age.
Iceland is moving. Compared to the hotspot, the area (including the spreading ridge) is moving at around 5 millimeters per year NW. This means that the hot spot appears to be moving to SE. Going back in time, we can put an approximate track across Iceland.
It is to be expected that the volcanic zones migrate with the hot spot. Five million years ago, the WVZ was well placed for the hot spot. Then it slowly moved away, and 3 million years ago it was far enough that the WVZ began to lose magma supply. This was the time the EVZ began to develop as a flank rift, and it gained in importance as the hot spot approached. But even now, the WVZ still accounts for a third of the spreading. Old rifts die very reluctantly. The OVB seems to have developed as a flank rift, situated on the edge of the hot spot bulge. Perhaps on the future it will begin from direct interaction with the hot spot but that is still 1 or 2 million years away. We need to be patient.
So 5 million years ago the paleo spreading centre was somewhere near the tip of the WVZ, and 15 million years ago it was on the Westfjord region. It was never near Snæfellsnes or Reykjanes. So why were or are these regions volcanic?
The answer seems to lie in the ridge propagation. Let’s go back in time. 25 million years seems about right. At that time Iceland was still close to Greenland. The Mid-Atlantic Ridge followed its original course, tracing where the separation had occurred. This follows the western most ridge of the ‘V’, and runs along the Westfjord coast. Of course this wasn’t yet a fjord coast: the fjord formed much later from ice age erosion.
Over time, the hot spot moved east. This made is much easier for the crust to rift in this location, made ductile by the heat. 15 million years ago, the ridge began to bend toward the new location of the hotspot. This may have initiated as a mini-rift running south from the what is now the Snaefelness peninsula. The Atlantic ridge relocated a bit eastward.
5 million years ago the hot spot arrived at what is now the WVZ. This now became Iceland’s main spreading centre. A new mini-rift began to run south from what is now the Reykjanes peninsula. The Atlantic ridge gains jumped by a few kilometers to connect to this mini-rift. The Reykjanes peninsula began to form and the Snaefelnes peninsula died a volcanic death.
Nowadays, the EVZ has become more important. A new mini-rift may be developing, although if past behaviour holds it won’t be from its southern extension but further west, perhaps from the weakness that is the WVZ. In fact the Icelandic shelf already shows a small extension along this line, connecting to another part of the V’.
As the Icelandic spreading centre has moved east, it has created new land Iceland is growing both to the east and west. The spreading is symmetric and creates land in both directions equally. But the host spot is moving east and this introduces an asymmetry. Most of the created land is now on the west side of the host spot. This make the connection to the Mid Atlantic Ridge more and more difficult. It explains why the Reykjanes Ridge has been relocating, accommodating an increasing diversion. The relocations begin on Iceland itself, where new rifts develop and new volcanic centres grow. Now mini-rifts extend into the ocean. It seems this is not a regular process. The ridge 15 million years ago transferred a part of the oceanic crust from Europe to America. But subsequent jumps have been smaller. The Reykjanes Peninsula sticks out almost as far east as does the Snæfellsnes peninsula. From the details of the ridges, it appears that there have been several ridge jumps, but they have jumped in either direction, east or west. At times the ridge even ran further east than it does now. .Every time a jump happened, the spreading took the ridges apart, and as the new rift propagated south, it formed a ‘V’ sign in the ocean – and Icelandic victory, growing new land without causing political problems.
The current Reykjanes Ridge has survived for more than 5 million years. But there is trouble afoot. It is a long way form the tip of the Reykjanes peninsula to the EVZ, and this includes a long section of pure transform zone. A new mini-rift south of Reykjavik may help: it would follow a weakness that has already been created. But it is far from the spreading centre in the ocean, and that means the rocks here are not as warm as they used to be. One day it will become too difficult to break these rocks. When that happens, Iceland will leave the Mid-Atlantic Ridge behind and lead an independent existence. No longer will it straddle the boundary between Europe and America, but it will nail its flag to the European side, following the Faroer.
And when, perhaps 50 or 100 million years from now, a subduction zone forms along the continental shelf north of Scotland, Iceland will be pulled into the abyss and the time of growth is over. There will be limits to its growth.
Albert, October 2022