So we all know that volcanic islands exist, but how do they form? If you’re answer is “it’s simple, the lava just keeps piling up until it reaches the surface and then bam you’ve got an island” then you’re wrong and in for a fun surprise: it’s more complicated than that! Island building is important to look at because: many of today’s tropical paradises lie atop these processes, eruptive events can be hazardous to shipping or tourists, and because it’s there. Really, though, only the last reason is 100% free of BS. In reality, these events are relatively rare and have only been observed on a couple of occasions (and only one has been extremely well documented). Moreover, the hazards posed are few because the activity is usually fairly easy to spot once you’re near it and tourists, to this point, have not been a problem (although at Myojinsho, in 1952, 31 people died when their research vessel was destroyed). Still, to neglect these events is to miss a very exciting piece of volcanology. Thus, in this article we are going to cover two sections: the process of island formation (mechanics) and specific examples from the past.
So the submarine volcano has been erupting for years, centuries, millennia, or longer. Eruptions have been docile and characterized by quiet effusion of basalt pillows. These pillows make up the base of islands and thus, by far, make up the largest section in volume and height (although most submarine volcanoes don’t make it to the surface). Over the years this particular pile of pillows has built itself up into quite a formidable mound and is now approaching the surface of the water. As you should know, the high pressure imposed on the lava by the overlying water column has stifled any explosive activity relating to dissolved/exsolved gas or thermal quenching. Unfortunately, the volcano is approaching the surface where the pressure is dropping enough to allow for some changes in the behavior of our eruptions. After emerging from the deep-water stage, the volcano enters the shoaling stage in which the pillow flows become more vesicular and begin to shatter to form what are called hyaloclastites. Hyaloclastites form by the shattering of magma due to the exsolution of volatiles or, more commonly, by simple thermal fragmentation (the extremely hot magma shatters when cooled rapidly…like if you baked a glass and then threw it into frigid water).
As the lava gradually moves closer to the water surface, gaseous explosions become more common, but are largely subdued by the overlying pressure. These explosions (more accurately called “flashes”) are caused by explosive exsolution of gas and flash boiling of water in small sections where the pillow lava has cracked to expose fresh lava to the water. These flashes may occur in rapid succession at shallow depth. The water surface above the vent may display turbulence from shockwaves, rising water vapor, and extreme convection of the water. Steam explosions with tephra jetting may occur which, depending on the frequency of such events, leads rapidly to the formation of subaqueous tephra mounds and to the next stage: emergent/island stage.
Here we encounter a type of eruption that is not discussed anywhere else in this website; it is called a surtseyan eruption. The surtseyan style of eruption is a relatively new category, being named after the 1963-67 eruption of Surtsey, in Iceland (offshore). Surtseyan eruptions actually fall into the general heading of phreatomagmatic volcanism. Phreatomagmatic volcanism, which is discussed in greater detail elsewhere, simply refers to the explosive interaction of water and lava. Surtseyan typically refers to the water-rich explosive eruption of barely or nearly submerged volcanic vents. Surtseyan eruptions are truly spectacles to behold for their mere violence. These explosions are characterized by tephra jets which are precisely the way they sound: a jet of water-laden ash shoots skyward hundreds of meters like a rocket and then collapses back down due to it’s weight. These eruptions don’t usually produce massive eruption columns because the saturated jets are far to heavy to be carried upward by convecting air currents. The tephra jetting is accompanied by deep rumbling and forms a tephra cone around the vent. Lightning, whirlwinds, and tephra-hail (hail with an interior composed of ash) are common around such eruptions.
As the island continues to be built, the eruption becomes drier. Commonly, the tuff cone around the vent is built above sea level while the vent remains below. Periodically, the vent is flooded with seawater, which causes the eruption to turn surtseyan. Once the water is forced out, however, the eruption becomes dry again and resembles a wet plinian eruption. These styles of eruption alternate at various intervals depending on the size of the tuff cone and the size of the eruption. Finally, the island will be built high enough to no longer allow flooding. The heat of the vent causes groundwater to be driven away and keep the eruption dry, and thus the eruption becomes plinian/subplinian, vulcanian, strombolian, or hawaiian. The island continues to be built until the eruption finally commences. If the eruption picks back up at a later time, it will likely be water-rich due to the reintroduction of subsurface water around the vent.
Once the island is built, however, it is by no means a permanent feature. More often than not, the new island disappears within months of its appearance due to the action of wave erosion. You will note that the island is almost entirely composed of volcanic ash, which is light and very easily eroded. Once the eruption ends, the main geologic process occurring on the island is the removal of its beaches, which move inward until the island is gone. To make an island semi-permanent, either a massive quantity of ash needs to be erupted and vegetated or the island has to be covered with basalt lava flows which are highly resistant to erosive wave action.
So why does Surtsey get the special treatment you ask? Because it is by far the best example documented today. The island spawned it’s own style of eruption…the surtseyan eruption, and so it deserves a little credit. The eruption occurred for roughly three-and-a-half years, along five fissure vents, beginning in 1963. When the initial activity began, Surtsey resided 100m below sea level and went unnoticed until it came within 10m of the surface. At that point, it became hard to miss with three different dark-black eruption columns rising over 60m (color was probably due to basaltic composition ash which was saturated with water). The eruption was silent, and surrounding the base of the plumes were large circles of agitated water, turned white by the churning vapor bubbles and explosion-induced shockwaves. One day later, Surtsey had built itself above sea level. Joining Surtsey in this shoaling stage were the vents of Surtla, Syrtlingur, and Jolnir whose tephra mounds approached the surface, but never broke through due to the erosive action of waves and the apparently lower effusion rates (compared to Surtsey). During an overflight of these vents during their shoaling stage, noted volcanologist Sigurd Thorarinsson reported that the vents lay only several feet below the surface and displayed rapid fiery flashes at half-second intervals (remember the processes that occur during island formation as you read through here). He noted, as well, that the vents were producing numerous shockwaves and occasional bursts of ash from the water, reaching as high as 52m. Some of the observed agitation, in addition to being caused by the aforementioned processes, may have been caused by lower energy tephra jets that did not have the required power to reach or punch through the surface of the water. Steam rising from near the tephra mounds indicated that the accumulating ash remained hot enough to boil the surrounding water.
New behavior did not end once Surtsey had established it’s above water tephra mounds, however. Two types of eruptions were dominant, though both explosive. One was a continuation of what was observed during the shoaling stage, though to a more extreme degree. During this event, sea water had managed to flood the crater, resulting in an enormous explosion of tephra jets. These tephra jets contained numerous lava bombs (clumps of fluid or nearly-fluid lava) and were dark in color. As the jets rose, however, the plume turned to a light gray color as the water vaporized to steam. The amount of energy involved in these eruptions can hardly be appreciated without seeing pictures, and even then, the impression is probably deceptively weak. The second style occurred when the vent had effectively blocked the surrounding seawater. This style of eruption was more plinian in nature and resulted in a sustained eruption column reaching several kilometers in height. Unlike the other style, these eruptions lasted for longer periods of time (up to several hours) and were, in total, much more energetic.
During these eruptions, weather phenomena were common due to the convection of air, the friction of ash particles, and the addition of large amounts of water to the atmosphere. These phenomena included lightning, whirlwinds, and tephra-hail. The collapse of some heavier jets produced pyroclastic flows which rolled out over the sea.
It is important to note during this description, however, that though Surtsey is an excellent example, it is by no means the only one. The events that occur during island formation are dependent, not only on the access of water to the vent, but also to the temperature, composition, and gas-content of the magma involved. Also a factor is the geometry and composition of the vent. If the vent is narrow, then the amount of magma involved in an explosion is naturally going to be lower. If the vent is composed of unconsolidated ash particles, as it usually is, water will be able to move through it toward the vent more rapidly. Variations from the above example have been observed (in both intensity and style), but won’t be elaborated upon here for the sake of space.
Most new islands don’t last long, however. The action of waves is just too much for islands composed purely of ash. Look at Graham Island…or Julia Island…or Ferdinandea Island. Actually it’s all the same island…or rather, a lack of one. In 1831, this resurfacing volcano emerged from the ocean and started a political firestorm in Europe. Immediately off the coast of Sicily, the new island was immediately claimed by the British and named. Not wanting to miss out, the French got in on the action by claiming it and naming it Julia for the month in which it appeared. Upset by the foreigners just offshore, the Kingdom of the Two Sicilies (there was no country Italy) claimed it and named it after their king. Perhaps feeling left out of the fray, Spain decided to jump in and claim it too. Things got a little hot on the continent, but cooled quickly when the island was gone six months later.
Islands can certainly become permanent features, which should be obvious from the number of volcanic islands in the Pacific and Mediterranean. Metis Shoal, in Tonga, was another emerging/disappearing volcano which erupted rather frequently. Finally, after centuries of here-and-gone-again antics, the volcano generated a sizeable lava dome in 1995. Whether or not the volcano will become a permanent feature is uncertain, but it will have a good chance now that it sports a dense shield from the waves across its slopes. Volcanic islands aren’t only eroded, though; they also sink. Through a process called isostatic adjustment (don’t worry about the terminology) the island literally sinks into the crust due to its large weight. The process is a slow one, but one that is occurring everywhere. For example, the Hawaiian chain actually extends much further if you look at a map of the seafloor. Beyond the islands lie the Emperor Seamounts, which form a chain much longer than the still-exposed islands. These seamounts are ancient islands that have sunk under their own weight, and this is the destiny of all of the Hawaiian islands. Not to worry, however; they are still being formed. Still active are Kilauea and Mauna Loa on the Big Island; unknown to many, however, a new island is forming at the front of the chain at a volcano called Loihi. It’ll be a while before it reaches the surface, but it probably will eventually, and will either form its own island or join the Big Island to make it larger.
Over the course of this article, we’ve looked at how volcanic islands form, from the base up. Pillows to tephra jets, and then finally displaying more typical behavior once establishing themselves, the development of islands is no simple matter. Neither are they permanent features, as we’ve discussed. Though they disappear, more are constantly being formed in one stage or another. This makes the subject important and extremely interesting to study. I hope you’ve enjoyed this look at how these volcanoes act and encourage you to continue reading on the subject if it interests you. For further information on the subject, I would suggest consulting scientific journals and textbooks.