The secret lives of volcanic islands (2023)

Land is precious. The saying goes ‘Buy land – they don’t make it anymore’. We need the land to live on, to grow our crops, to socialise, work, educate, and all other things that makes human life worth living. Even our touch phones won’t work under water. With a growing population, the pressure on the available land increases every year. So do buy – it is an investment. But be careful what you buy. If you live in New Mexico, check whether the land also includes the water rights. If you live in Florida, a 100-year mortgage may find the rising sea overtake you before it is paid off. And buying land close to a youngish looking volcano could be considered a gamble. Especially if that volcano is underneath you.

The expression leaves it unclear who the ‘they’ are. Who made the land? Can we take it into our own hands? The person who invented the expression had probably not been to the Netherlands! But it is also possible that the person had discounted land created below sea level as not worth buying. Expanding a country the Dutch way comes with risks. It is a risk that more and more nations will need to take to keep up with the rising sea. Nowadays, it takes effort to keep what you have.

The ‘they’ mostly refers to nature. The Earth does build new land. Can we enlist it? A cheap way to expand the available land would be to get a willing volcano to kindly steer some lava towards the coast. Trust is important: lava going the wrong way could undo all the good work. But if you can’t trust a volcano, who can you trust?

A less (or more) intrusive way would be to keep the lava (magma) underground but use it to inflate the land, thus pushing back the sea. Iwo Jima has been doing this for centuries. What could possibly go wrong? Of course, you may suddenly find your nice newly inflated land exploding and covering most of the neighbouring countries as a deep layer of volcanic ash – liability insurance is recommended. And what goes up can come down: Naples has had its impressive ups followed by meters of decline, with a major Roman resort now 12 meter deep under water. I expect they did not see that one coming! People are remarkably poor at understanding risks.

Back to valuable land. Strangely, the most valuable land is also the most isolated. All sea-facing land comes with a 12 nautical mile territorial claim on that sea, plus a 200-mile exclusive economic zone. Where such claims overlap, a line is drawn through the middle. Not everyone agrees (China draws it claimed border on the beaches of the neighbouring countries) but this is the UN convention of the law of the sea. (There is an exception for continental shelfs.) Now, an island far away can become valuable property even if the land is barely habitable. An island has coast on all sides, and therefore it has a very large economic zone. There are some restrictions: the island should be above water at all times, it should have a natural origin, and most of all, it should exist. Not all islands meet these requirements.

New islands

Qeqertaq Avannarleq was discovered in 2021 by a team visiting Oodaaq, the (reportedly) most northerly island in the world, located off the northern coast of Greenland. The team landed their helicopter on the island, and send the coordinates back to the world. Very quickly the comments came back that they were in the wrong place. They had ended up on the wrong island, 750 meters north of where they were supposed to be. Their newly and accidentally discovered island consisted of silt and gravel, 50 meters across and up to 4 meters high. It may be the result of storm debris, and could be destroyed by one as well. But for now, this is the most northerly island in the world. It extend Greenland’s territorial waters by 5.6 km². Not bad for an island that is only 0.0018 km² in size! The economic zone is now 24 km² larger.

Whether this will be accepted depends on the longevity of the island. There is an ongoing discussion whether Qeqertaq Avannarleq (‘northern island’) should be called an island. Several similar islands have been discovered in this region in the past, to disappear again later. The off-shore region has a moraine bank under water, and shifting ice can push up the bank to form an island, and similarly make it disappear later. A shifting sand or gravel bank may not deserve the name of ‘island’. But it is fun to find new land.

Phantom islands

Ephemeral islands have appeared in many places. They were found, mapped, and disappeared leaving people to wonder whether they had ever existed.

The north coast of the Yucatan borders the Gulf of Mexico. There are a few reefs and islands about 100 km off the coast: the beautiful Scorpion Reef, the Cayo Arenas, Triangulas and Cayo Arcos, forming an archipelago of low sandy islands. They are rather hard to reach and have no facilities whatsoever – they are for the ‘adventurous visitor’. A bit further from the coast is the very similar island of Bermeja, discovered in 1539 and described as having ‘red-blond soil’ (which explains the name) and as a bit less than 10 by 10 km in size. It is located at 22°33′ N, 91°22 E in the Gulf. At least is used to be.

But after its discovery, no one ever saw it again. The 1921 Geographic Atlas of the Mexican Republic still included the island, but by that time other cartographers had given up on it. It was reported to have sunk ’60 fathoms’. In the late 20^th century, the Gulf of Mexico became an important source of oil. The ownership of that oil was negotiated between the US and Mexico. The region north of the Yucatan was put on hold until 2009, pending searches for the missing island. Had it sunk a bit, as sandy islands sometimes do? Had one of the hurricanes knocked it back to below sea level? This was now important to know, since if the island (or its remnants) could be found Mexico had a claim to a significant larger area in the Gulf. Searches were made both in the late ‘90s and in 2009. Bathymetry indicated only deep sea sediment at this location: there was no evidence for any shallow remnant. The island had never existed and Mexico had to give some of the oil wealth of the Gulf.

What happened? There are two main possibilities. The discovery may have been one of the other islands to the south, and the ship had its own location wrong. Those things happened: this was the 16th century when deep sea location were in part guess work. The original record may also have been intentionally misleading, to put the competitors off the track and make them avoid a supposedly dangerous region. You do not want to run aground in a dark night far from any help. Those things, publishing deliberately misleading maps, happened too. There are other proposals around but those are best dismissed out of hand. No, the CIA did not blow up the island to extend the US rights further into the Gulf. The 2008 investigators concluded that the original descriptions of Bermeja were so precise that it is likely that the island did exist. Just not in this location.

Phantom islands happened more often. Benjamin Morrell, a 19th-century sealing captain, published a memoir called Narrative of Four Voyages. Although some of the content was correct, much of it was fiction. Several phantom islands owe their existence (or rather phantomness) to Morrell. These include Byres Island and Morrell Island, located northwest of Hawai’i. The International Date Line was drawn to go around these islands, until in 1910 their non-existence was discovered and the date line could be re-straightened. False facts are nothing new, and 19th century readers were just as prone to believing made-up stories as the current twitter generation.

The Aurora islands are another example of disappearing islands, but different since they were seen several times. They remain a bit of a mystery. The origin is in 1502 when Amerigo Vespucci joined a Portuguese expedition off the Atlantic coast of Brazil. The following is quoted from wikipedia:

we found ourselves in latitudes so high that the midday fix was 52° above the horizon and we could no longer see the stars of the Little Bear nor the Bg Bear constellations. This was the 3 April 1502. That day a storm blew up so strong that it made us furl all our sails and run with bare masts before strong winds from the south-east, enormous seas and stormy gusts. Such was the tempest that all the fleet was greatly fearful. The nights were very long, and the one of 7 April was of fifteen hours duration since the sun was at the end of Aries, and in this region it was winter, as Your Majesty can calculate. In the middle of this storm of 7 April we sighted a new land, which we sailed alongside of for almost 20 leagues, finding the coast wild, and we did not see any harbour or people. I believe because the cold was so intense that none of us could remedy it or bear it.

There is some confusion here: the 15-hour length of the night at that latitude actually is in early May or August, not April, so the stated month is clearly wrong in spite of the precise detail. The statement that it was winter and the claimed cold also agrees better with a later time of the year. But clearly, any land 100 km long is worth claiming. The land was found again in 1772, and at that time the longitude was noted and it was stated to consist of three separate islands. Their existence was confirmed in 1796 and again in 1856. And they were never seen again. The location is midway between the Falklands and South Georgia where only empty sea exists. In the late 1800’s, maps stopped including the Aurora Islands. How could this have happened? The original discovery reported by Vespucci is a bit vague. Was he perhaps much further east and was this South Georgia? Was it the Falklands? Or perhaps the southern coast of Brazil? Or, dare I say it, was it made up? The vagueness and errors in the report sound more like hear-say, which Vespucci perhaps wrote up and tried to pass off as his own discovery. Once the island was ‘known’, it became self-fulfilling: people seeing something in the distance where the maps said there should be an inhospitable and dangerous coast, would identify it as that land and stay well away. The later reports may even be from icebergs: they can appear like land from a safe distance.

These are a few examples of the many phantom islands, islands which at one time existed on the maps but could not be relocated later on. A brilliant overview can be found at http://andrewpekler.com/phantom-islands/. It is wel worth spending time there exploring a phantom world.

But some islands really do appear and disappear. These are volcanoes at work, often in secret and unobserved, their handiwork like a Banksy appearing overnight.

The deep sea

Before and island can emerge from the deep sea, major building work is required. The ocean is kilometers deep, and any island needs to grow to at least that height before it can make territorial claims. There are two ways this can happen. The first is buckling of the oceanic crust, and it happens at a subduction zone. The descending plate creates a deep trough, easily spotted on maps of the sea floor. But as it descends, it locks to the other plate and pulls it back. That plate buckles and can rise above water. Kodiak Island off the coast of Alaska is an example. Another example is provided by Chichijima Island, part of the Ogasawara (or Bonin) Islands south of Japan.

The other way is volcanic. There are again two ways this can happen. The most familiar one is where a patch of excess heat in the mantle melts part of the upper mantle and grows a volcanic island. This can happen anywhere on Earth but is easiest on a spreading ridge where there is a ready-made transport route from the mantle to the surface, and only little extra heat is needed. Elsewhere more serious heat is needed: Hawai’i is a good example of such a hot spot.

The second volcanic way is again related to subduction. When the subducted plate gets deep enough, the water-saturated crust wets the mantle just above it and thus lowers its melting point. Melt accumulates and begins the arduous journey to the surface. This forms a volcanic ridge a few hundred kilometers from the subduction. The map of Japan above shows this volcanic ridge well separated from the buckled crust. The melt inflates the whole length of the ridge but only the individual volcanoes manage to puncture the sea surface. If the melt occurs below continental crust, you can get an elevated and mountainous ridge, with volcanoes on top (or just beside the elevated ridge– magma can take funny paths). Indonesia is good example.

So now you know why Kodiak Island has no volcanoes!

Tonga: Late’iki

Volcanic islands can seemingly appear out of nowhere. We are oblivious to the building work that prepared for the emergence. A familiar example is Surtsey which suddenly formed in November 1963. (That was on the shallow sea surrounding Iceland, so not an example of a deep-sea island.) The first sign was a sulfur smell, and shortly after a column of black smoke heralded the birth of new land. That was 60 years ago and the island is still there, qualifying as a territorial marker under the law of the sea. Neptune, of course, ignores this law and claims it all as his own anyway. Perhaps Surtsey tried to escape from the rule of Neptune into the freedom of the air.

Surtsey is just one example among many. Reports of eruptions breaching the surface are far from uncommon. Hunga Tonga is such an example. Before its spectacular destruction last year, it had existed only for 6 or 7 years. The island had formed from an eruption in between two smaller islands, which became incorporated in the new island. The four-part full name abbreviated as HTHH still refers to those two fragments.

Hunga Tonga is part of a volcanic arc caused by the Tonga Trench (a name that has a ring to it). The map shows the main Tonga islands located on the buckle ridge, and the separate line of seamounts and volcanic islands. The ridge is paradise. The volcanic arc is the brutal world beyond paradise, outside of the eastern gate. (Actually it is to the west – the analogue does not quite work in this case.) It is a place best studied from a safe distance. Surtseyan eruptions are common here. Hunga Tonga had them in 2009 and again in 2014/15, whilst Home Reef (what’s in a name?) erupted in 2006 and 2022, and Late’iki erupted in 2019. Definitely not a place to call home. It is a place to stay away form, and that s what people did.

In spite of this, Late’iki is worthy of attention. The intermittent island has erupted ten times since 1851. Islands were formed in 1967, 1979 and 1995. It had to go through this island birth process three times as the islands of 1967 and 1979 lasted only a brief time before Neptune reclaimed the land. 1995 was more successful and the island that was formed in that eruption remained. The eruption started on June 6 and by the end of the month, the island was 300 meter long and 50 meters tall. This was the real thing. The previous eruptions had build tephra cones, presumably because the eruption itself had remained under water. In 1995 the eruption became exposed and now the new land was made out of lava – rather sterner stuff than tephra. On 14 October 2019 a new eruption was observed by a passing ship, and yet another island formed: New Late’iki.

As we know from Hunga Tonga, the volcanic arc is not well monitored. Even in today’s world, the Tonga volcanic arc remains a good location for a secret eruption. Satellites provide the best images but they may be available only one for every few days, and of course clouds may get in the way. The images above show satellite images of the events of 2019. In the first image, the Late’iki island of 1995 is clearly visible, surrounded by the cyan ring indicating shallow water. The eruption disaster struck on October 15. The image captured a big explosion near the island. By the end of the month, a new island had appeared, in an arc around where Late’iki had been – but no longer was. The 1995 creation had been destroyed in this eruption. The new island was not nearly as sturdy as the old one. Over the next months it rapidly reduced in size and it was last seen on Dec 9. The next image, Dec 14, shows shallow ocean but no land. (Images from Plank et al. 2020).

The eruption had been much more significant than it seemed at the time. The quick dispersal of the new island showed it consisted of debris. The arc suggests it was debris from Late’iki. The eruption had blown up the entire island! It hadn’t been a large island, but still. Any eruption wiping an entire territory of the face of the sea is worth noting. In hindsight, this seems like a dress rehearsal for the Hunga Tonga, the loudest eruption since Krakatoa!

Volcanic islands live and die by the power of the earth and sea. They are a battle ground. It is always worth checking in the morning what may have appeared overnight. To quote Carl: I love the smell of a fresh volcanic island in the morning.

Didicas volcano

The ocean north of the Philippines is dotted with small islands. In 1857, an eruption added one to the collection. The activity lasted four years and by the end the new island was reported as 200 meters tall. It didn’t last. Within decades the island was gone – surprising, in view of the height. The island reformed in 1952 and grew even taller than before: 240 meters. Since 1979 it has been quiet. But the Philippines newest island still exists.

Krakatoa

The most famous of all is Krakatoa. Before the cataclysmic eruption, it consisted of four separate islands: the main, large one, two flanking islands and one standing rock called Polish Hat. When the island exploded, the flanking islands survived, as did the south end of the main island. Polish Hat was demolished, in spite of not being near the actual explosion. It may have been destroyed by the massive tsunami, which (scaling from Hunga Tonga) may have been over 100 meter tall in this location. The net number of islands thus did not change.

Polish hat remains a bit of an unknown. We have no detailed description other than that it consisted of glassy, greenish-black rock, identified as a dark andesitic obsidian. Verbeek spend a few hours (at most) on Polish hat in July 1880. No images or even drawings of it remain. It really has become the phantom of Krakatau.

Once Anak Krakatau appeared, there was one more island than there used to be. Its subsequent collapse in 2018 left the new island itself in place. It is now regrowing its cone (and its danger). Over the next centuries, Anak Krakatau may rejoin with the surviving side of the original island. Volcanic islands are dynamic. Here today, gone tomorrow, back next week.

Limits to the growth

These island forming events are quite common. The destruction events are almost as common: most new islands will exist intermittently and go through phases where they are present and where they have become phantoms. But that constant turn-over is surprising. The islands form in the deep ocean (the exception is Krakatoa which is on the continental shelf). Before they can breach the surface, they already have grown kilometers tall. These are not new volcanoes: they are fully grown, mature beings. They are not mayday flies. So why, after growing up several kilometers, do they suddenly balk at the next 100 meters and can’t quite make the jump to the next level? And why do all these volcanoes, in oceans of different depth, get stuck at the same level?

Aerial volcanoes behave differently from submarine ones. Submarine volcanoes are steeper than aerial ones. This has two explanations. The slope is determined by how far lava can flow: the further it gets, the shallower the slope. Under water, lava cools much faster, or at least the surface solidifies. Now it becomes sluggish. The same lava will flow further in air than it does under water. The fact that the water carries a third of the weight of the volcano (through water displacement) helps to stabilize the steeper slope. Therefore, the under-water part of the seamount can be steeper than the part that sticks out above the water. The underlying slope can be hidden by the loose sediment on top which has a much shallower slope. Volcanic islands may have thick aprons formed by debris coming down the slopes. But seamounts often grow steeper towards the surface.

This makes it harder for the volcano to grow once it reaches the surface. In order to grow taller, it needs to grow wider. On a kilometers-tall mount, that requires a lot of lava. The foundations were made too small.

The second explanation has to do with the ocean surface. It creates a hostile environment, with wind and waves and rain and snow. As soon as a volcano reaches the surfaces, it becomes ruled by the waves. Wave erosion attacks the edge and eats away on the new island. It takes tough lava to survive this, and in the end the waves will win. Below the surface, where everything is quiet, there is much less erosion.

Wave erosion attacks from 10-30 meters below the surface to 10 meters above it. This is the danger zone which the island must overcome. It does so by growing wide: the larger the circumference, the better protected the island is. But this runs into the previous problem, that the mounts needs to grow larger foundations.

The effect is that many seamounts end up with a flat top, some 10-30 meter below sea level. This is the effect of wave erosion.

An example is Iwo Jima. The island is in effect the tip of a large cone, and forms a low flattish area, surrounded by a ring which in places sticks out above the water. The flat area was shaped by wave erosion, even though it is now well above sea level. This is not due to eruptions: Iwo Jima only has very minor eruptions which do not add to the height. The area has grown above water not by erupting lava, but by rapid inflation. The magma below the surface is pushing it up.

Some seamounts have flat tops but 100 meters below the surface. They show wave action much deeper than expected. These tops date from the ice age when the sea level was 100 meter lower than today. They can be anywhere from 10,000 to 500,000 years old – that is the age when the top was flattened, and is probably near the time of their most recent eruption. Much lower tops are also possible. Those are Iwo Jima in reverse: once the magmatic heat is turned off, the volcano cake deflates and sinks. Flat-topped seamounts below sea level are called guyots.

So to overcome all these difficulties, a volcano wanting to become a lasting island has to grow fast through the ocean surface. If it is too slow, it will get stuck in a cycle where it builds a small island which the waves take down again before the next eruption. This happens in many places: it is the reason we have so many phantom volcanic islands putting in a brief explosive appearance before being removed again from the aerial world. Neptune’s border control can be harsh.

Collapse

There is one further problem. There is a maximum slope any mountain can have before it fails. The slope depends on how hard the rock is. Volcanic mounts tend to have a lot of low strength material on top of the steeper lava cone. This is not strong and cannot carry much weight. A steeper slope has to be stronger. Growing a mountain from the top makes it steeper, as shown by the concave shapes. Lava flowing into the sea can cause very steep slopes because the lava solidifies quickly under water. Flank eruptions give a shallower, more stable shape but does not make islands from a seamount. If the top-heavy mountain already grew on a slope, as Anak Krakatau did, it gets even worse. The side of the mountain can collapse with potentially devastating consequences. In 2018, Anak Krakatau’s entire aerial cone slid into the sea; the displaces sea water caused a large tsunami. I have a suspicion that the main explosion of its daddy, Krakatoa, similarly coincided with a flank collapse, in the same direction as its descendent.

Island volcanoes can be intrinsically unstable. All the work the volcano does in growing above water may eventually be in vain. Neptune reclaims it’s own.

The end

This is the ephemeral landscape of ocean islands. They come and go at a whim, always at risk of explosion and collapse. It is a dangerous world out there. Explorers may indeed suddenly find an island no one had ever seen before, or fail to find one that existed a week ago. But explorers explore to become finders. They need to discover to justify the effort. And so they found and recorded finds which in hindsight are highly dubious. Sandy island off the northwest coast of Australia, found by Cook, never existed. Neither did the Aurora islands. But they could have existed. And they may still do.

Albert, April 2023

And some volcanoes are true phantoms. Read the story of the Florida volcano