In my previous article I wrote that Taal most likely would not continue to erupt for a long period of time, based on the previous history of the volcano. Obviously Taal decided to go for something not recorded in the historic annals.

It is therefore time to take a deeper look at what is going on using an array of different data.

But, before that I need to straighten out two different things that have been circulating in the media, and on various internet sites.

The first part is that the BBC have misunderstood the volcano completely, they went so far that they called Taal “a small baby volcano”. It is kind of refreshing compared to the Daily Mail that calls every volcano a “supervolcano doom eruption”, but it is dangerously wrong and diminishes the possibility for a future mass-evacuation.

It is though equally wrong, Taal is after all a massive caldera volcano that have had confirmed VEI-7 eruptions, and numerous other caldera-extending events. There is a reason that Taal ended up on the New Decade Volcano Program.

The second thing is that some people have stated that the eruption is over. Yes, the volcanic activity on the surface has dropped significantly at the surface with steam and intermittent explosions creating columns reaching up to 1500 metres in height. But compared to the beginning there is definitely less surface activity. But as we will see below the activity has instead increased below surface.

The signals from the deep

INSar indicates WSW/ENE compression, but in reality, it is caused by magmatic central uplift pulling the rock in this direction towards the middle. INSar indicates that this is extending all the way to the ocean coastline.

At the same time there is according to INSar data a band of uplift ranging also in WSW/ENE. This is indicative of a dyke extending through the caldera trending in this direction.

GPS data corroborates this general trend, but interestingly the uplift pattern is broader than INSar gives at hand, in other words the entire shebang is uplifting, but there is a marked concentration along the dyke-line.

Water depth measurements in the lake shows us that the general water level is decreasing on all measured points around the lake. So much so that the river that normally is the outlet for the lake has dried up.

Parts of the dried-up river can be explained by uplift caused by the dyke, but it does not explain where the water has gone.

During peak eruption the SO2 gas release was roughly 6500 metric tons per day, it then dropped to around 500 metric tons, before it rose sharply to the current 4500 metric tons per day.

Finally, we come to the seismic activity. It has been fairly constant since onset of eruption, but in the last couple of days it has increased markedly. The current count for today was more than 700 earthquakes in total and 26 of those was of volcanic type.

The volcanic type earthquakes ranged from M2.2 up to M3.8 during the last day. There have also been a number of tornillo type tremor events indicating increasing pressure in the magma reservoir.

Interpreting the signals

Collectively from INSar, GPS and seismic data we can infer that a dyke has formed transecting the caldera and that the central magma reservoir is being fed by this dyke, the general broader uplift indicates that horizontal pancake like sills have formed (see image for a visual explanation of pancake sills).

Due to the expanding dyke numerous faults and fissures has formed causing widespread destruction of houses and infrastructure.

From the seismic data we can surmise two things. One is that a ringfault has either started to form or, has reactivated from a dormant state. The ringfault runs partly along the caldera rim and partly outside of the caldera wall, indicating a complex magma reservoir topology.

Secondly it is evident that magma is influxing at a steady rate, larger than what erupted during peak eruption and that this influx has not stopped. Even though surface activity has decreased.

The cause for the drop in the caldera-lake water level is more uncertain, but it is likely that the water has infiltrated via fissures and faults into the active vents at Volcano Lake and was turned into steam. Another explanation might be that the water has disappeared due to evaporation, but I have so far not seen any data that the lake temperature has increased.

Possible consequences

DOST-PHIVOLCS has stated that evacuation of up towards 1 million people might be necessary, but the result of the ongoing smaller evacuation has been so-so, indicating that a larger eruption might cause a high toll.

They have also warned that a larger explosive eruption might be hours to days away. The data analysed above is amply corroborating their assessment.

So far, the eruption ranges from a large VEI-3 to a small VEI-4, as such this is not uncommon for a Taal eruption.

Question is more about what is in the near future for Taal. I see three different options.

Option one is that the intrusion will decrease rapidly and that the current eruption ends in a few days. The intruded magma will cause further eruptions over the next couple of decades as the magma surfaces due to heat driven buoyancy. In other words, that Taal still is able to take care of business in the known historical way.

The second option is that the partially blocked pathway to the main vent will reopen, or that new vents will form, and that a second more vigorous eruptive phase will follow. Judging from the rate of intrusion and the size of the magma reservoir this second phase could be likely to reach VEI-4 or VEI-5 status.

The third option is that the intrusion of fresh hot magma continues without a corresponding eruptive outlet putting further strain on the various faults and fissures under the lake, or further invigorating the ringfault.

This could in turn lead to a larger eruption causing either a central caldera collapse, or a caldera extending event, partially fuelled by the lake water.

Depending on how much of the magma inside the reservoir will be reheated and made eruptible depends on how much hot fresh magma intrudes into it, the more that ends up above the eruptible limit, the worse the eruption would become.

If the entirety of the known magma reservoir would become eruptible (500km3) we would typically see a mid-range VEI-6 if it is at the centre of the caldera, or a large VEI-6 to a small VEI-7 if a ringfault caldera extension event occurs.

Option 3 is so far not likely; it is just a possibility further down the line. In my initial article about the eruption I believed that we would see option one as the by far most likely thing to happen. Now that more data has arrived, I am starting to think that option two is more likely, let us say that Taal currently hovers in between option 1 and option 2.

Also, remember that the level of fissuring and fracturing indicates that Taal might not be able to withstand the pressure involved in an option 3 scenario since something would break prior to it happening.

A word on volcano tourism to Taal

As everyone knows I love to watch volcanoes erupting. I am aware about the dangers involved, and I have the knowledge to make judgement calls on what is reasonably safe to do. Most people do not have that knowledge, instead they rely on so called “volcano-guides” to keep them safe.

In almost all cases the people working as guides do not know enough to keep you safe during an eruption, they are just out for your buck.

So, let me instead state this. There is not enough money on the planet to convince me to go inside the Taal caldera. This is based on the signals that I see coming out of the volcano.

CARL REHNBERG