Guest post from Héctor (DustDevil). There will be a further update either later today or tomorrow as events have changed since Héctor wrote this post.
The Main Crater of Taal seems to have calmed down, doesn’t it? However fissures are breaking the ground apart, the river dries, Lake Taal falls and strong earthquakes keep coming. What exactly is going on right now at Taal behind all of these changes?
As I have mentioned a couple of times in the commentary section of the previous post a dyke intrusion seems to be occurring. This is not something you are going to see in the news, at least not the word “dyke”, and at least so far. However, for someone with a particular and deep interest in the workings of rift zones and basaltic volcanoes (like me) what was happening seems obvious.
And don’t let the enormous caldera of Taal fool you, it is a basaltic volcano after all! Might have done a couple of large silicic eruptions in the past with ignimbrites but currently, it has more in common with a shield volcano.
The Macolod Corridor
To understand Taal knowing about the setting is necessary.
Taal is located in the Macolod Corridor, an area varied volcanism, basalts, andesites and dacites are found here. It spans a 60 km long area of southern Luzon, Philippines. Even though small in area there is a number of volcanoes, you find 2 large calderas, Laguna de Bay and Taal plus a number of stratovolcanoes (Banahaw, Makelunyo, Makiling) and monogenetic vents: maars, cones and domes.
The secret to the productivity of this region is rifting, you can think of it as a small continental rift. The rifting nature of the Macolod Corridor is well agreed upon, but how this affects Taal and its eruptive history has never really been considered.
A rift volcano
Taal works with dykes, these happen when magma forces open a crack and flows into it, the pressure of the magma and volatiles keeps expanding the crack and so on as long as conditions are right. It turns into a fiery sword slicing through the ground. When it touches the surface, magma comes out though this won’t always happen.
Iceland has an important place in this blog so you are probably already familiar with how important this is in a rift, dykes fill the space opened by the separating tectonic plates.
In Taal it is similar, major events such as the eruptions in 1749 and 1911 included large cracks opening up parallel to the Macolod Corridor and areas of the ground subsiding. This is what concerns most right now because it is what is currently taking place.
The 1749 and 1911 were devastating and if you want to know more about their deadly effects, once again this article by Henrik contains a lot of information on them: http://www.volcanocafe.org/the-tiger-in-the-smoke-taal-the-new-decade-volcano-program-8/
When a volcano intrudes a dyke but it stays deep below the surface, the ground above is pulled apart, it snaps. You get a graben and cracks open which is what happened in 1749, 1911 and now.
As magma drains away from the magma chamber and into the dyke, the caldera subsides, water flows into the new volume lowering Lake Taal’s water level, it is possible that is the main cause why river Pansipit is currently drying up. However, deformation from the dyke and ash blockage is also to consider which have also been proposed by fellow readers.
Current situation and the future
PHIVOLCS currently maintains Taal at alert level 4 and reports that new cracks keep opening. These seem to be located to the southwest of Taal Lake, coincident with the area of earthquake epicentres. As previously mentioned the earthquake swarm and ground cracking is very likely the result of a dyke intrusion.
The situation plays by the rules of basaltic volcanoes, and don’t get me wrong, rhyolite dykes (and of other magmas) do exist but they are not as frequent. The dyke (usually) starts to intrude under the summit of the volcano, same goes for Taal, this phase involved a subplinian eruption from the Main Crater of Taal and lava fountains on the north flank of volcano island, this happened through the afternoon of January 12, with the lateral vents opening the 13th. Hard to say exactly when (future scientific analysis will have to constrain this) the dyke intrusion propagated to the southwest from the summit to outside the caldera.
PHIVOLCS still warns of a POSSIBLE hazardous eruption. So let’s consider what possible scenarios could lead to another violent eruption:
– Vents open along the dyke: Taal doesn’t seem to erupt outside the Caldera, which is good news to the local population. Bad news is that fissure eruptions are a bit unpredictable and that new or previous vents in the Volcano Island area can open or reactivate, even if unlikely.
– Caldera faults rupture: When magma leaves into a dyke intrusion you drain the magma chamber of a volcano and it can reach the point of collapsing. From a tectonic viewpoint, this happens when the caldera/ring faults give way to the strain they are under, and this is quite sudden! Remember the collapse events of Kilauea? The Talisay (Taal) Caldera is unlikely to undergo major collapse, but the Main Crater in Volcano Island is a smaller caldera structure and more vulnerable.
If this happens we are in uncharted territory but one can picture water making its way to the magma via collapse. Worst case? Probably a VEI 4.
However, it is also likely that dyke intrusion will eventually stop without any these scenarios taking place. The earthquake swarms have slowed down over the last 2 days which could be a sign of the intrusion coming to an end, but since there are ways the situation can escalate again anyone violating the evacuation ordered by PHIVOLCS are putting themselves under grave hazard and in case a paroxysm occurs there is no telling how much time people will have to run, and it may not be enough.
Following Lurking’s advice, do not be there!