In the previous post, we read about the birth of Surtsey. It was a famous eruption, which taught us how quickly and unexpectedly new land can form.
We have since seen similar eruptions elsewhere as well. Nishinoshima is a small and isolated Japanese island, 1000 kilometers south of Tokyo. An eruption started just off its shoreline in November 2013. It was a classical surtseyan eruption, and it quickly formed a new island while the eruption became strombolian. Over the next three years, the new island merged with Nishinoshima and more than doubled its size. A look under water explains what happens. Nishinoshima is the top of a very large volcano, of which the flat summit is mostly just below the sea surface. The eruption happened on the summit but beside the highest cone – which was the only part seen above water.
All deep sea islands are volcanic in nature, and all came to be in this way: a series of eruptions building up a precipice until it rises above the waters. The volcano is already very tall by the time it reaches the surface. Many never do, whilst others continue to grow until they tower over the waves. From Anak Krakatau and Stromboli to Mauna Loa, the sea gives us some of our best volcanoes. They can grow higher than those on land, because part of the structure is below the water: the water carries part of the weight. To see such a volcano first reach the light of day is an impressive and memorable sight. It brings to mind Britain’s rousing old
brexit hymn battle hymn, Rule Britannia! – Britannia rule the waves!:
When Britain first, at heaven’s command,
Arose from out the azure main
But there is something funny about Nishinoshima. The summit of the volcano was a large, flat plateau just below the water surface. Why was this? It seems an unlikely coincidence. But there is in fact a reason why there was a plateau, and why it was at this height: you may blame Britannia’s waves. Like an anxious boss, they continuously stamp down on anyone who dares to put a head above the parapet and threaten the pecking order. Rule Britannia depicts an inverted view of the world. In reality, the waves are not there to be ruled – they are the ones in charge. They are the rulers of all, and overpower anything that tries to interfere with their domain.
The sea does not tolerate summits. It demands a level playing field, and it achieves this by attacking heights. The sea is a tyrant that aims to bring down what rises above; it has a powerful attitude, and it gives rise to a plateau of mediocrity. Hence the flat submarine summit of Nishinoshima, located just below the depth to which waves penetrate.
The nations not so blest as thee
Must, in their turn, to tyrants fall
But to a volcano, a level playing field provides an opportunity. Even the smallest eruption can create a notable peak if it happens on a tennis court. Like a celebrity tweet, it draws the attention to content that in other contexts would not seem so notable. This is true for Surtsey, and to surtseyan eruptions in general. Remove the sea, and these eruptions, while still significant, would not seem so outlandish.
There are many examples of submarine plateaus summits surrounding a minor island. Iwo Jima (nowadays known as Iwo Ioto) is one such, and as the underlying volcano inflates, the island grows and grows. Various sources state that the Iwo Ioto plateau is a caldera, but there is not much evidence for that: it seems like a normal wave-battered summit. (If it really were a caldera, the inner regions would have been much deeper.) The Aleutian eruption of Bogoslof is another case. Various peaks around Bogoslof show previous eruptions, and as the eruption center moves around, it almost causes the island to wander. Bogoslof is in a battle with the elements. The waves erode, the eruptions build up, and the winner remains undetermined. It sails close to the wind. But if the eruptions will come fast enough and are voluminous enough, it may rise above the sea far enough to gain some stability. Rule Britannia suggests such a battle between water and volcanoes:
Still more majestic shalt thou rise,
More dreadful from each foreign stroke,
As the loud blast that tears the skies
Serves but to root thy native oak.
A volcanic island grows as the lava flows reach into the surrounding sea. We have seen this happen at Kapoho where a tropical bay became a basaltic desert. At the same time, erosion by the waves reduces the size of the island. If the growth and the erosion balance, at least over the long term, the island keeps a constant size.
Let’s assume that the eruptions create new land at a rate ‘A’, which is measured in square meters per year. We ignore lava that solidifies on land and may build a cone: only the lava that encounters and expels the sea counts. (If the island is very large, sediment carried by rivers will also build new land which should be included in ‘A’. But we are now looking at smaller island which lack such rivers.)
The waves attack the coast and cause it to retreat. Let’s call the speed of coastal retreat ‘C’, measured in meters per year. The island loses a total area each year which is equal to this ‘C’ times the circumference (assuming the rate is the same all around the island – this may not be true). For a circular island, the area lost per year is equal to 2πrC. The island is stable if the gain and loss balance: A = 2πrC.
The value of ‘C’ can be measured. Let’s assume for now that it is equal to 1 meter per year. In that case, if the island is 1 kilometer in radius, it loses an area equal to (2π) times (1000 meters) times (1 meter/yr), which is just over 6000 m2 per year. To be stable, over time eruptions need to create the same amount of new land per year. If the island erupts once per century, each eruption would need to form 100 times mores, or 0.6 km2 of new land. If it erupts more than this, it can maintain a larger island – if less, smaller. A four times larger eruption rate will maintain an island with a four times larger radius (and thus 16 times larger area). The radius of the island will roughly be given by the equation r=A/2πC. If you know r and C, you can calculate A: you know how much new land the lava must have created over time.
But what if there are no further eruptions? In that case, erosion will continue to eat away the island. The length of time that the island will survive is equal to the radius divided by the erosion rate: t = r/C. An island with a four times larger radius will exist four times as long. For C=1 meter/yr, an island 1 kilometer in radius can expect to exist for 1000 years. Most of the area of the island is lost early on: after 500 years, half the life expectancy, the island is already reduced to a quarter of its original area. The island spends most of the years of decline as a small remnant.
So to survive, a volcanic island needs continuing eruptions. A monogenetic cone (erupting once) will succumb, whilst a regularly erupting cone can keep the waves at bay.
All their attempts to bend thee down
Will but arouse thy generous flame
But in the long run, all islands fail. No volcano lasts forever: eventually the waves will win and the sea prevail. One can hear the sea singing the words of Rule Britannia
All thine shall be the subject main,
And every shore it circles, thine.
After this introduction, let’s see how it applies to Surtsey. The island formed in a single event lasting three years, very similar to Nishinoshima. How is it faring in its battle against the sea? And what lies beneath?
The map above shows the location, off the coast of southwest Iceland. It is a continuation of the eastern rift zone but it is not yet rifting. Give it another million years for that. The main island in the area is Heimaey. There are a number of much smaller islands, of which Surtsey, at the end of the arc and furthest from the main land, is the largest and youngest. It formed in the 1963-1967 eruptions when several eruption centers were active. The eruption started out as a shallow underwater, phraeto-magmatic event. Once the cone was secured against intruding sea water, the eruption became strombolian, producing basaltic lava. The type of eruption is important: surtseyan explosion produce weak conglomerates of tephra which can erode fast, while strombolian eruptions produce tephra, tuff, and lava flows. Hardest of these is the tuff. Note, however, that the important factor is the strength of the rock at sea level. Having harder rocks higher up the cone does not help if it is all undermined from below. The sea can be a sneaky neighbour.
During the Sursey eruptions, there were satellite eruptions which also attempted to form islands: Surtla, 2.5 km to the E-NE, Syrtlingur in the same direction but closer, and Jólnir on the opposite side flank. Surtla never showed above water, but both Syrtlingur and Jólnir reached 70 m above sea level. Both islands consisted of tephra which was washed away within months.
Surtsey itself was not particularly resistant either, although to its credit it does live in a very hostile environment where the storms, waves and currents are severe. By the end of the eruptions, in July 1967, the island covered an area of 2.65 km2. In July 2012, only 1.31 km2 was left. The eroded debris build up some shallow shelves around the steep cliffs. These will have helped protect the island from the worst of the waves, at least temporarily.
The volcanic field
Look from Surtsey to the direction of the Icelandic coast (see the image at the top of the post), and a plethora of small cones can be seen above the sea. These are remnants of previous eruptions. There are more small remnants than larger islands. The calculation above in fact predicted this, provided each island formed in a single eruption and suffered erosion since. If, in contrast, the islands are stabilised by continuing eruptions, you expect fewer small remnants compared to the number of larger islands. The fact that the there are more small cones than islands tells you that this area is monogenetic. The archipelago of 18 islands and many more rocks act as a monogenetic volcanic center.
So the plethora of rocks shows that the region is very active, but that eruptions move around. Historical evidence is consistent with this. Recent events include an eruption in 1893 south of Hellisey, the much larger 1963–1967 Surtsey eruption, and the damaging 1973 Eldfell eruption on Heimaey. The majority of rocks, stacks and islands are close to Heimaey and this is clearly the focal region where most of the eruptions occur. But eruptions do not recur: there is no large volcano and each location indeed only erupts once.
Under water, the island chains sits on a 50 to 100-meter deep shelf, highest around Heimaey, build up by the volcanic activity. The linear chain of islands gives the impression of a central conduit (Heimaey) with dikes feeding the more distant eruptions. Surtsey is the most distant of these. The chain follows the direction of least resistance along the Icelandic spreading axis (although it is not itself a spreading ridge). This is what would be expected from a dike system. However, in that case you would also expect that activity along a dike is preceded by inflation and earthquakes at the central system, and no such events were reported at the time of Surtsey. The actual origin of the magma could also be further away, on the main land: the Surtsey lava had similarities to Eldgja. But again, there is no evidence for this. At Surtsey, the earliest lava was the most evolved and thus likely dated from an earlier intrusion. Later lava was much more mantle-like. But all this says is that the magma came from deep. It does not tell which pathway was followed.
Monogenetic fields occur where the crust is weak and instead of one single magma chamber there are a number of smaller, short-lived chambers. Magma finds easy pathways to the surface, and there is no need or advantage in using a previously prepared conduit. We may thus be looking at a large number of small magma chambers, most of which are around Heimaey but some further away, along the axis. The magma chambers are in pressure equilibrium with the much deeper magma from the mantle. During an eruption, the pressure in the upper chamber drops as magma escapes, and this sucks in the deeper magma, over a time scale of months to years. This continues until the pressure difference between the upper and deeper chamber drops below a critical value. This model explains why the field is monogenetic, why eruption last years, and why the magma quickly becomes very primitive. It does not explain why this is happening at this location, southwest of Iceland. Perhaps this is the way that spreading centres develop: they start out with a line of weakness, where the sides are slightly pulled apart, allowing these magma chambers to form, whilst the weak crust lacks the strength of a captain Picard, feels that ‘resistance is futile’, and quickly gives in to the rising
Surtsey exemplifies the weakness of volcanic rock. By the end of the eruption, it consisted for about 70% of lava flows and 30% very porous tephra. The lava reached thickness up to 100 meter near the eruption center, but at the coast the flows were typically only 1 meter thick. Both the tephra and the lava were extraordinary weak, and rapidly retreated under the onslaught of the Atlantic ocean. The erosion was strongest on the southwest side where most of the waves came from. The opposite side was more protected, both by the main land and by the dominant wind direction.
But over time, the island hardened and adopted more of a USS Enterprise attitude. The tephra reacted with hydrothermal water, and this turned into a much denser and harder tuff. By 1985, the remaining tephra had been removed and the erosion began to attack the tuff. But the hardened tuff was far more resilient against the sea.
The images here show how Surtsey developed during the eruption and since. The coast retreated fastest in the west and southwest, where the waves were most severe. Over time, the southwest side became a straight line, perpendicular to the prevailing wave direction. In places, it had been pushed back by 300 meters. On the west side, the sea was eating into the side of one of the craters. Before 1985, this side was tephra but later the waves encountered the far harder tuff and the erosion slowed down dramatically. There is now a 130-meter tall cliff here, protected by the hardness of the rock and by a boulder beach. The beach is clearly visible at the bottom of the cliff, and it causes the waves to break and lose some of their power. Beaches are a good investment for a volcanic island.
On the south and east side, the sea is eating into lava beds. Here the cliffs are not as tall, and frequent collapses have created large boulder beaches. The boulders are rapidly rounded by the waves. The much harder tuff in the west has produced a much less pronounced boulder beach. The tougher the cliff, the less effective its beach will be. Strength can sometimes be counterproductive.
The waves and currents are transporting the boulders along the coast – after all, rounded boulders roll rather easily. They have collected on a spit which developed on the north side after 1970. This is the only part of the island which grew after the eruption had ended. The volcanic sand and boulders give the spit an inhospitable appearance! The time series of images show that the spit migrated eastward in later years. This was because of the tuff. On the west side, boulder formation became much reduced after 1985, and thus the spit lost some of its supply on that side. On the east, boulders continued to be available. But in recent years, especially since 2012, the spit has narrowed and moved a bit westward as the supply on the east side also no longer keeps up with spit erosion.
There is a very different landscape underneath the sea. The satellite volcanoes which formed during the Surtsey eruption are still there. They eroded very rapidly to a depth of 25 meters (Surtla had only just reached that height), but the erosion slowed down after that. As of 2007, the plateaus of Surtla, Jólnir and Syrtlingur were 51, 43 and 34 m below the surface. The lesser depth of Syrtlingur reflects the fact that it is shielded from the stronger waves on the southwest side. Their summits have also become wider, because some of the debris from Surtsey found its way to the submarine peaks of Syrtlingur and Jólnir.
Surtsey is surrounded by a plateau, 20-30 meters below sea level, with very steep edges going down to the original seabed, 120 meter deep. The plateau extend furthest from the current island on the southwest side, where it approximately follows the original, 1967 extent of the land. The plateau is covered with debris and boulders, debris from the erosion above the water line.
The erosion rate of Surtsey has been very changeable. The highest rates were seen during the eruption, when the unconsolidated lava and tephra retreated by 30-100 meter per year. The eastern lava cliff retreated by 100 meter in 1966 due to a single severe, easterly storm. After the eruption ended, the southwestern lava cliffs continued to erode, at first at a rate of 30 meters per year, later reducing to around 12 meter per year (average over the period 1967-2012). The more resistant northwestern side eroded at 30 meters per year during the eruption, but this rapidly became less as the tephra hardened and at present it retreats at no more than 20 centimeters per year. Tuff is tough.
In 50 years, the island has halved in area. But this has not happened at a constant rate. Between 1967 and 1985, 1 km2 was lost, or roughly 0.05 km2/yr. Between 1985 and 2015, 0.5 km2 went, or less than 0.02 km2/yr, which is a much slower rate. The erosion is meeting more resistant rock, and is slowing down, and as the island becomes smaller, the waves attack a smaller circumference as discussed above.
What does the future hold? If the erosion rate had been constant, the whole island would be gone within 200 years, by 2150-2200. But the rate is getting smaller. The erosion will continue to remove the lava shields. In another 50 years, those lava flows will be largely gone. The spit will also have disappeared, and only the double cone will be left. The tuff of these two cones will be around for much longer. At an erosion rate of only 20 centimeters per year, they should be able to last two thousand years, perhaps longer.
In the interlude above, we used a parameter C, which is the speed at which the coast retreats each year through erosion. But this is clearly not particularly constant. At Surtsey, the speed varies along the coast line, and it also changed with time, between 100 meters per year and 20 centimeters per year. Clearly, the assumption that C was ‘a constant’ is a little too simplistic!
Surtsey is probably a typical example of the destruction of a volcanic island. The highest erosion rates occur during and shortly after the eruption, when the waves attack loose tephra and soft lava. Decades later, the rates slow down by a factor of 100 or more as the tephra has hardened. This means that a volcanic island has a rapid growth spurt during an eruption, followed by a fast retreat, but is much more stable (and much smaller) a few decades after such an eruption. With a rate of 12 meters per year over the first 30 years, an island can only survive this phase if it starts out with an area of at least 0.1 km2. Eruptions smaller than this will create a rather ephemeral island. If the eruption is larger, it can last until the erosion drops by a factor of 100, and it will survive, perhaps as a stack, for very much longer. The current rate of 20 cm/yr appears typical for later phases. For the much larger and older Bouvet island we estimated a similar rate, of around 10 cm/yr.
Wave erosion is very important to volcanic islands. But it is not the only type of erosion. Especially if the volcano builds up a cone, it will attract rain, and this will attack the island from above. However, this mainly reduces the height and makes the cone much steeper; it is much less effective in reducing the size (and the sediment an even temporarily enlarge the island). Rain does not in itself reduce the longevity. However, steep cones can become unstable, and lead to land slides which can remove parts of an island altogether. Hawaii has suffered tremendous landslides, which have taken big bites out of the coast line and left debris on the ocean floor one hundred kilometers away. Such slides are rare: on Hawaii, they happen perhaps once every 100,000 years. But if they put the coast back by 10 kilometers, that is similar to what an erosion rate of 10 centimeters per year would do over the same time. Rare but catastrophic events can significantly increasing the total erosion rate. Surtsey is unlikely to suffer such a land slide, as it lacks a steep cone. An extreme storm could however have an impact.
The Surtsey volcanic field
Looking at the Vestmannaeyjar archipelago, cones seem to be sticking out above the sea everywhere you look. Each of them once was an island like Surtsey, now reduced to a central tuff cone. There are 18 islands in the archipelago, and another 25-30 rocky outcrops. Assuming that the cones survive for a few thousand years, this suggests one Surtsey-like eruption per 1 or 2 centuries. But Heimaey itself has lasted much longer. It contains both the oldest (10,000 years old) and the youngest (1970’s) rocks of the archipelago. This is the only island in the archipelago with repeating eruptions, needed for longevity. All others had only a single eruption, as a monogenetic cone. It does not mean that Heimaey is different. This is the centre of the field where eruptions are most frequent. The island was able to grow large enough that other eruptions occur on it before the island has had time to disappear. This has made the island stable. All known vents (10 in total) on the 13 km2 large island are in fact also monogenetic.
Under water, there are many more remnants, perhaps some 70 volcanic cones in total. After the waves have removed the upper parts, 50 meters below the sea the cones can last a long time, suffering only a bit of erosion from currents. If we assume as eruption rate of one or two per century, the number of volcanic cones suggests that the archipelago is around 10,000 years old.
This is uncomfortably young. A volcanic area does not pop up out of nowhere in such a short time. And rifts don’t move much on such time scales either: they extend by at most a few kilometers, far smaller than the archipelago is long. You would expect the volcanic environment to have been very similar even as long as 100,000 years ago. What happened to the volcanic cones that formed before 10,000 years ago?
The likely answer lies in another factor that we have ignored. Wave erosion happens at sea level. But sea level has not been constant. 20,000 years ago, it was over 100 meter lower than it is now and the region of the Vestmannaeyjar archipelago would have been dry land. When the sea came in, as the ice age was melting, it would have attacked any volcanic cones from ground level up. An erosion rate similar to what it is now would have quickly removed the entire volcanic history of the region. After that, volcanic building work had to start from scratch. This is the reason for the short history shown by the archipelago. It doesn’t mean it is new, just that someone erased the earlier writings in the book of history. This was a monogenetic volcanic field long before the oldest surviving cones formed.
(Of course, the area would also have been covered by ice age glaciers, and they may have done even more damage. There is normally more than one party trying to rewrite history.)
What will happen in the future? At the surface, not much will change. New islands will come, old ones will go. Only Heimaey will survive. But below the surface, the series of eruptions will slowly build up the sea floor. Eventually, there will be a ridge here extending from the shore outward, some 30 meters below sea level. Eruptions appear to be too infrequent or not voluminous enough to get beyond that. It will not be another Reykjanes peninsula: it will stay under water. But in another 100,000 years, it may get close to the surface. But no further. Any higher, and the jealous sea will strive to bring it down. Like Britannia, Vestmannaeyjar will be ruled by the waves.
Albert, September 2018