Lately Taal volcano has been showing unsettling signs of a possible imminent eruption, including high sulphur dioxide emissions, small steam-driven explosions, tremor, and somewhat surprisingly, deflation. I think that this is no conventional magma intrusion but rather a very rare type.
First of all I should briefly review what happened in 2020.
The events of 2020
Taal is a volcano located in Luzon, the largest of the Philippine Islands. It is also located within the Macolod Corridor, an area of tectonic extension that runs in a NE-SW direction. As such Taal is able to send dykes to the northeast, towards Laguna de Bay, which may have happened in 1749, and also to the southwest towards Balayan Bay, which happened in 1911, and in 2020.
After several years of slow deflation, the volcano started to inflate in early 2019 leading to the eruption a year later. On January 12, 2020, the volcano snapped, columns of ash and fire fountains erupted from a fissure extending from the Main Crater to the northern slopes of the Volcano Island. A volume of 0.03 km3 of tephra was erupted.
The main event was the dyke intrusion however. After the eruption, a large dyke started propagating southwestwardly, which cracked open the ground and produced numerous earthquakes. A later publication, that I will link at the end of the article, estimates that 0.53 km3 of magma were drained away from the storage of the volcano during the intrusion.
Volcanoes do not simply erupt, they also intrude a lot of magma underground, to make dykes, sills, layered intrusions, batholiths. They are the creators of new ocean crust. Therefore there are enormous magmatic structures that are invisible to the eye, hidden deep underground. The mountain is just the tip of the iceberg so to speak. This is exactly what happened in 2020. The enormous dyke will probably solidify into rock, accommodating some of the extension that takes place in the Macolod Corridor.
The amount of deflation that took place in 2020 was very large, 0.53 km3. It would take a long time to replenish this volume. For example in 2019 Taal inflated with an estimated 0.045 km3 of magma. At such a rate it would take 12 years to refill the deflation of 2020, and considering that the inflation of 2019 represents a surge far above the long term supply, then it would take much longer probably.
It seems highly unlikely that Taal could refill and reach overpressure so soon, so that the explanation for the current unrest may lie elsewhere.
The current unrest extends all the way back to the 2020 events really. The volcano has been steaming ever since. The crater lake inside the 2-kilometre wide Main Crater has been continuously acidificating, probably due to the sulphur dioxide emissions.
The situation started to escalate rapidly in March 2021 with sulphur dioxide emissions that went over 1000 tonnes/day. Tremor, long period earthquakes, steaming, and recently steam explosions, are signs that something is wrong under the volcano and escalating. Sulphur dioxide emissions reached 22,628 tonnes/day on July 4 which is extraordinarily high for a non-erupting volcano.
PHIVOLCS also reports something quite unusual:
“Based on ground deformation parameters from electronic tilt, continuous GPS and InSAR monitoring, Taal Volcano Island has begun deflating in April 2021 while the Taal region continues to undergo very slow extension since 2020.”
I wish there was more information regarding the deformation, cause I find this statement quite enigmatic. It doesn’t fit well with the more conventional forms of intrusion. Typical dykes can produce inflation around the intrusion, and deflation around the magma chamber feeding it. However dykes also produce a lot of volcano-tectonic earthquakes and cracks, and I don’t see any clear signs of this happening right now. A sill usually produces one elliptical area of inflation which doesn’t fit either. And also typical dykes or sills require overpressure which is unlikely to have been reached so soon after the 2020 draining.
One possibility is that magma is melting its way upwards making a pipe shaped conduit, a plug, like lava lakes or domes, but I don’t see much reason for any deflation to take place in such a situation though.
There is one rare type of intrusion that could perhaps fit the current situation better than the others, a ring dyke.
This is a variety of magma intrusion that takes place along the edge of a caldera in a ring-like shape. There are two varieties.
When the roof of magma chamber is pushed up the dykes can intrude with an inward dip. The volcanoes of Galapagos do this a lot. They commonly erupt from ring fissures just outside the edge of a caldera. They dip inward towards the magma chamber under the caldera.
However the roof of the magma chamber can also drop down, and sometimes collapse like a piston, here a dyke can form that is vertical or outward dipping. This a much rarer situation. The best known example is the Loch Bà ring dyke, in Scotland. “The finest ring dyke known to science”.
Scotland has several ancient volcanoes that date back to the initial opening of the North Atlantic 60 million years ago. These volcanoes have been eroded down to their ancient magma chambers thus showing a section the interior of the otherwise invisible and unknown internal structure of volcanoes. One of these is the volcano of Mull Island which contains the Loch Bà ring dyke. It is a truly massive intrusion that is up to 400 meters thick! And much thicker ring dykes, over 1 km, are known elsewhere. A normal dyke is only 1-4 meters thick or so. The Loch Bà intrusion extends almost all round the 8 kilometre diameter caldera, and formed when it collapsed down by several hundred meters.
Why could this be happening at Taal? The 2020 draining could have made the roof of the magma chamber under the Main Crater unstable, thus providing a trigger for the formation of a ring dyke. The Main Crater itself may have collapsed in earlier historic times, in 1754, when it is briefly stated in one of the reports that the volcano suffered a tremendous erosion in its elevation, although without more information this statement is rather ambiguous. In 1911 the Main Crater also deepened and the Crater Lake formed, however the volume lost can be easily explained by excavation during the enormous steam explosion that took place back then.
A ring dyke could explain the contradictory signs, that the Volcano Island is deflating but at the same time magma intruding. It could also explain the extremely high sulphur dioxide emissions of up to 22,600 tonnes/day, because ring dyke intrusions are often enormous.
The current situation may also be mirroring the events of 1749-1754. A large dyke intrusion, towards the northeast from Taal, took place in 1749, with fissure eruptions inside the larger caldera, and with the formation of cracks and grabens all the way to Laguna de Bay. It was followed by some years of strong steaming until the major eruption of 1754, which lasted 7 months and had multiple episodes of eruption of varying intensity and style.
The two eruptions of 1749 and 1754 seem somehow related, one possible explanation is that the drainage during the initial event led to the roof of the magma chamber starting to subside, which over time evolved into ring dyke intrusion and the major eruption of 1754.
It does seem worthy of consideration that this is happening right now. Confirmation that such an intrusion is taking place would be if an arcuate fissure opens up along the sides of the Main Crater, or if caldera collapse takes place, particularly if collapse events are detected, earthquakes in which the roof of the chamber suddenly drops.
I should note that if any caldera collapse takes place it will be related to the Main Crater which is a 2 km wide structure, a very small caldera. The larger 20-12 kilometre caldera seems to be inactive, the magma chamber that once existed underneath may have been destroyed during the collapse, and it is unclear whether it exists anymore or not, and I would guess it no longer does. However even the smallest caldera is capable of terrible destruction. Caldera collapses bring out the worst face of a volcano.
If a ring dyke intrusion is taking place it may not necessarily lead to a major eruption, intrusions do not always lead to eruptions and the same thing perhaps applies to ring dykes, although not much is known about their behaviour.
Fire and water eruptions
When I did my Big Basalt Blasts series of articles, ring dykes may have been the missing piece in the theory that I was trying to put together. Some of the eruptions that I covered show signs of being related to the ring structures of the caldera, in particular the eruptions of Mijakeyima in 2000, Askja 1875, and Kilauea in 1790, which erupted, and/or degassed intensely, along ring fractures.
A ring dyke can explain the high sulphur dioxide emissions of Mijakeyima that lasted a few years and at their peak reached up to 50,000 tonnes/day. The massive size of the intrusion could also provide enormous amounts of heat to the hydrothermal system and trigger giant steam explosions. The ring dyke itself might drive magmatic eruptions, particularly when the roof of the chamber collapses and pushes magma up the dyke, nucleates gas bubbles, makes the magma fizzy, light, and explosive.
The basaltic explosive eruptions of Kilauea and Masaya, that I talked about in the Big Basalt Blasts series, often have such sharp contrasts between hydromagmatic and magmatic styles. Deposits are often made of alternating layers of greyish mud, and black scoria/pumice. The 1754 eruption of Taal was also this way. If Taal does go for a major eruption it is possible that we will see such hydromagmatic-magmatic episodicity show up.
The current unrest of Taal is somewhat unusual in that it combines deflation with extremely high SO2 emissions. One possibility is that a ring dyke intrusion is taking place together with subsidence of the roof above the central magma chamber of Taal
Even if such an intrusion is taking place it would be hard to known whether the situation will gradually calm down with nothing serious happening, or whether it will progress into a major explosive eruption like that of 1754.
PHIVOLCS is already reporting damage to crops and throat irritation due to the high SO2 emissions which are creating acid rain and volcanic fog. The situation is already becoming quite problematic. If Taal does go into a major eruption there will also be the major hazards of pyroclastic flows (base surges), which are like a breath of hell, that tears down trees and houses and kills almost every living being unfortunate enough to be found on their path, and there may also be tsunamis on the lake, volcanic bombs, and copious showers of pumice, ash, and mud.
Volcanoes are not friendly neighbours, and much less Taal.
The 2020 Eruption and Large Lateral Dike Emplacement at Taal Volcano, Philippines: Insights From Satellite Radar Data:
Some of Taal’s eruption history, particularly 1749, 1754 and 1911:
Big Basalt Blasts series I: