From time to time when we discuss our beloved volcanoes, we get a mild-mannered enquiry – ”Excuse me, but what’s effusive? And that cryptodome you are all speaking of, what is that?” I thought I’d take the time to jot down a few notes trying to explain what is meant by the various types and what characterises each one of them.
First of all, the term eruption is sometimes used to describe any type of volcanic activity that results in a visible emission of ash and/or steam. In a comment on FB, Dr Boris Behncke recently pointed out that an eruption is when magma reaches the surface. From this definition, we can immediately distinguish between two main types – eruptions and failed eruptions.
When magma moves through the crust, it does so along conduits. These can be pre-existing or the magma may make new ones. If such a conduit is oriented sideways, i.e. the magma does not move upwards but sideways, we speak of it being either a dyke or a sill. The difference is that a dyke also extends in an up-down dimension like a book standing on its end and may cover quite large distances as evidenced by the 2014-5 Holuhraun eruption. The extension of a sill on the other hand is flat or horizontal like a book lying down.
Magma tends to collect in what with time becomes vast underground chambers. Logically the term for these is magma chamber. If there is no eruption for a long time, and we are speaking of a period of time many thousands if not tens of thousands of years, depending on the size of the chamber, the magma cools and solidifies to a point where it no longer is able to erupt. Once solidified, it is often referred to as a pluton. The Yosemite National Park in California is a splendid plutonic landscape where millions of years have eroded away the overlaying layers to reveal the ancient, solidified bodies of magma.
Because magma, being much hotter and gassy, has a lower density than that of the deeper bedrock, it moves or floats upwards to a point known as the point of neutral buoyancy where it’s density is at equilibrium with the surrounding rocks. On Earth, this point is reached at about four km below the surface. Sometimes, magma is energetic enough or contains so much volcanic gases under high pressure that it may move above this point. Most of the time, this does indeed lead to an eruption. But if it does not, i.e the magma moves to within ~1 – 1.5 km of the surface but does not erupt and begins to solidify, we speak of a cryptodome (which is similar to a lava dome, only that it is invisible below ground).
When magma reaches the surface, i.e. comes into contact with air as it moves above ground, the result is an eruption. The type of eruption, i.e. the manner in which the magma behaves as it comes up out of the ground and in our somewhat muddled terminology becomes lava = magma that has been erupted, is determined by two main considerations – its chemical composition and its relative content of volcanic gasses; primarily water but also carbon dioxide and sulphur dioxide.
Magma is usually described as being Basalt, Andesite, Dacite or Rhyolite, even if there are other sub-species such as Thrachytes and Phonolites. This division is based on the content of Silicon Dioxide, SiO2. Basalt has the lowest content of silica, in the region of ~40-50%. Andesite usually lies between ~55-60%, Dacite around 65% and Rhyolite greater than 70%. Intermediate magmas/lavas are describes as being basaltic-andesitic, andesitic-dacitic or rhyodacite. Generally speaking, erupting basalt has a temperature in the region of 1200C +/- 100C, whereas erupting rhyolite is cooler at about 800-850C.
While all types contain varying amounts of volcanic gasses, basalt generally contains more sulphur dioxide as it often comes more or less directly from the Earth’s mantle. Basalt usually does not contain much water and when it comes to how explosive an eruption is, water is by far the biggest factor. The greater the water content, the more explosive the magma tends to be. Because of their chemical composition, the most evolved magmas – dacite and rhyolite – can contain huge amounts of water.
The final aspect of how explosive an eruption can become is how “runny” the magma is. Just like hot syrup easily runs out of a warm bottle, hot basalt is far more runny than the sticky stodges at the other end. Rhyolite is extremely non-runny or viscous, as runny as thick oatmeal porridge in comparison to the more syrupy basalt if we continue the analogy.
If the magma erupted is basaltic, it is very runny and usually does not contain vast quantities of volcanic gas. At the vent, there is usually some spattering fountains of fire as the gas propels the erupting magma some tens to maybe hundreds of metres into the air. As the magma does not travel far in this manner, it forms a spatter cone in which it pools and sooner of later overflows, breaching the walls of the spatter cone. We get rivers of molten rock that can travel upwards of ten kilometres. The Hawaiians refer to this kind of lava as “Pahoe” or ropy. There is a second type of basaltic lava flow, the Hawaiian “A-aa”, which is more viscous and does not form rivers but up to 5-10 metres thick, somewhat blocky and slow-moving flows that spill forwards seemingly inexorably accompanied by a glassy crackle. Effusive eruptions are sometimes also referred to as Hawaiian.
But it is not only basalt that erupts effusively! Sometimes andesite, dacite and even rhyolite does not contain enough gas to erupt explosively, but oozes out of the vent just like the last dregs of toothpaste forced out of its tube. Commonly, this happens towards the end of an eruptive cycle and the extruded dome is often destroyed, either as it collapses to form a deadly pyroclastic flow such as the one at Unzen on June 3rd 1991 that killed 43 people including renowned volcanogists Katia and Maurice Kraft plus the American St Helens survivor Harry Glicken, or as the next eruptive cycle commences such as at Kelut in 2014.
The main types of explosive eruptions are usually sub-divided into Strombolian, Vulcanian, Peléan, Sub-plinian, Plinian, Ultra-plinian, Sub-glacial and Sub-acquatic (or Surtseyan). As the name implies, explosive eruptions are explosive and range from the very common small Strombolian type that is usually contained within the crater with spatter reaching the upper slopes to the extremely rare and violent ultra-Plinian ones which launch hundreds of thousands of tons of volcanic material per second as high as 30 km or more above the summit.
Strombolian eruption. The name derives from Stromboli; the poetically named “Lighthouse of the Mediterranean”, located among the Liparian Islands northeast of Sicily. Strombolian eruptions are characterised by short-lived explosive events that send a plume of ash and rocks many hundreds of metres above the summit from where they rain down on the uppermost slopes. During daytime, the eruptions appear to be grey but at night there is usually an intense reddish glow. The material that rains down on the upper slopes may coalesce to form lava flows and if such a flow encounters water, may cause what appears to be a pyroclastic flow as sometimes happens at Etna during the winter months. Apart from Stromboli itself, the Showa crater of Sakurajima is also known for its frequent bursts of Strombolian explosions as is Klyuchevskoy in Kamchatka where the eruptions typically cause lava flows that may travel all the way down the 4 km high edifice.
Vulcanian eruption. The term was coined by the Italian Giuseppe Mercalli who witnessed the 1888-90 eruptions of Vulcano, also one of the Liparian Islands. The Vulcanian eruption is characterised by a series of intermittent and irregularly spaced explosions in the crater as pulses of magma arrive which sends an eruption column kilometres into the air above the summit. As the eruption is not continuous, the column ranges in colour from the dark grey of magma-rich explosions to almost white when there is very little magma and thus mostly water.
The next step up in volcanic violence is the Peléan eruption, named after Mt Pelé in Martinique. This type of eruption is characterised by large pyroclastic flows, usually the result of the collapse of a recently extruded lava dome or spine, another characteristic of this type of eruption. The pyroclastic flows travel not only down the edifice of the volcano but may inundate the surrounding landscape as well. A good, recent example of this is the Soufrière Hills volcano on Montserrat, the pyroclastic flows of which caused first the evacuation of the capital Plymouth and later its complete destruction.
Sub-plinian, Plinian and Ultra-plinian. The term was coined in honour of Pliny the Younger who witnessed and described the 79 AD eruption of Mt Vesuvio. This is the most violent type of eruption where the main eruption launches an plume reminiscent of a Stone Pine tree tens of kilometres in the air within minutes. If the eruption column reaches an altitude of between 10 and 19 km, it is referred to as Sub-plinian, between 20 to 30 km Plinian and above 30 km as Ultra-plinian, the exact heights given for each types varies from source to source. The amount of magma ejected is in the case of the Sub-plinian eruption tons to tens of tons per second whereas the Plinian eruption launches hundreds to thousands of tons per second. In the case of an Ultra-plinian eruption, it may involve ejection rates well in excess of 100,000 tons per second and may reach as high as the stratosphere.
After a series of phreatic to phreatomagmatic precursor eruptions that in themselves can be quite spectacular – when Mt Pinatubo erupted in 1991 there were several Plinian precursor eruptions before the main one – there is a main eruption which lasts typically several minutes to several hours. Once the Plinian phase is over, the volcano usually shifts to Vulcanian or Peléan activity and this is usually the most destructive phase of the eruption with massive pyroclastic flows, such as Vesuvius in 79 AD and Pinatubo in 1991, that may continue for days or even weeks, and pyroclastic base surges such as Taal in 1965 where these were first identified.
Sub-glacial eruptions are as the name implies eruptions of a volcano covered by a glacier. As with all other eruptions, the smaller ones are far more frequent. These eruptions are not energetic enough to break through the several hundreds of metres to kilometres thick glacier but result in glacial melt, sometimes causing huge pits to form on the surface of the glacier, that result in a glacial flow or jökulhlaup. These are the most treacherous and in olden days, the deadliest ones as there was no visible warning that a hlaup was imminent. This caused several areas around primarily Katla to be abandoned and people avoided travelling there as much as possible. Sometimes, an eruption breaks through the glacier if it is either powerful enough (Katla 1918) or occurs in a spot where the glacier is thinner and weakened (Gjalp 1996).
If the covering glacier is several kilometres thick, the glacier won’t be breached. Instead the erupted magma collects around the vent to form a steep-sided, flat-topped mountain, a tuja which is revealed once the glacier eventually melts and recedes. There are plenty of such formations outside of Iceland; the type was first recognised in North America (in British Columbia and the Cascades.)
Just as the subglacial eruption occurs beneath a glacier, the Sub-aquatic eruption begins below the surface of a large body of water but may rise above the water to become sub-aerial as happened at Surtsey in 1963. This type of eruption is also referred to as Surtseyan.
Problems With the Current, “Magma-centric” Definition
At the beginning of this article, I referred to the current definition of an eruption; it begins when magma reaches the surface. On Saturday, September 27, 2014, at around 11:53 a.m. JST, there was a phreatic explosion at Mt Ontake which killed 54 people hiking to the summit of Japan’s second-highest volcano. As no magma reached the surface, it was a phreatic explosion that pulverised rock and remobilised old ash, this was not a volcanic eruption according to the definition.
This is where, in my opinion, the current definition is not only wrong but dead wrong! It is the result of a matter-centric view that ignores the driving force behind all of Earth’s geology; energy. Energy retained in the form of heat within our planet since it was formed ~4.5 BY by the collision of millions of asteroids and planetessimals. Energy resulting from the radioactive decay of Uranium 238 and Thorium 240.
One of the driving forces of our Universe is entropy; equalisation of energy potentials. The core of our planet is ~6,000K hot and with a mean radius of 6,371 km, this means that there is a temperature gradient of 1K per km. It is this energy difference that drives volcanism and plate tectonics, hence we should think of eruptions in the form of energy, and not matter (e.g. magma reaching the surface). Matter is only the vector.
If we do, we get a far better picture and understanding of what is going on. The lowest level of eruption would be gas emissions or volcanic degassing which would range from the carbon dioxide bubbles of the Laacher See to sulphur fumes of Solfatara in the Campi Flegrei. The next level would be hydrothermal energy such as Geysir or Old Faithful. From there we move on to the phreatic explosive events when superheated steam; a liquid under high pressure and temperature, reaches the critical point and flashes into steam explosively such as at Mt Ondake. If we adopt this energy-system instead of the current magma-centric view, Mt Ondake would have been listed as “active” and perhaps, 54 people might not have needlessly died. And with Solfatara listed as active, would those apartment blocks on the crater rim have been given the go-ahead?