The Lowering of Taal Lake and Rock Deformation: Result of Magma Upwelling

In my previous post, I described three possible scenarios that might explain why the Pansipit River dried up so quickly in one day. In an interview between our kabayan, Noli De Castro and Dr. Renato Solidum, USEC and director of PHIVOLCS, the director mentioned that the level of crater lake is lower now than the surrounding lake. So this means there is a change in the water gradient.

Where does water go by virtue of gravity? It goes from high elevation to low elevation. Think of crater lake as the sink and water is going into that sink. So as response to that shift in water gradient (or slope), Taal lake’s level has lowered which can then result to drying of Pansipit River and possibly, other streams around the lake.

The ground deformation around Taal is already happening as evidenced by fissures, ground subsidence, and streams that dried up. This ground deformation is a result of the magma still rising within the volcano. When we say ground deformation – we mean that some of the rocks (+ top soils) are fracturing or fissuring, some are subsiding (sinking) while others are going up or emerging in response to the forceful entry of the molten magma underneath. Think of rocks as movable blocks that respond to force applied from different directions, but this time, the force is mostly coming from below. With the rocks subsiding or emerging within and around Taal, the structures (houses, bridges, buildings) over these rock “blocks” are also affected and being shifted accordingly, rivers included. We’ve seen reports of houses that are sinking, fissures that are widening, and rivers and coastal areas that are drying in some areas.

My hypothesis 2 wherein I mentioned that some of the fissures could be diverting the water from the lake and Pansipit River was also based on my experience in the past as a government geologist. In February 25, 2008, I got sent to Lanao Del Norte along with my fellow geologist and geologic aide to investigate reports from the locals of Brgy. Inasagan, Iligan, Lanao Del Norte, mentioning that springs started appearing inside peoples’ houses and at the foot of hills behind the small village. When we got there, we were invited inside the houses to check the springs. Below are some of the photos we took during the one-day fieldwork:

Indeed, a newborn spring! What’s amazing about this spring is that it was gushing right out of this house’s living room floor.
Still within the same house, another spring had developed on the kitchen floor.
The source of the water was so robust, small creeks were flowing in between houses.
I borrowed the styrofoam boat from one of the kids in barangay Inasagan to see how fast the creek flows. I don’t remember the figure exactly but the springs are turning the community into a valley, with houses as its islets. That’s how robust the water source is at the foot of the hills.

Based on our interview with the locals, water started gushing out of their floors nine days after continuous rainfall. Also, there was a weak intensity earthquake felt before that nine-day deluge. The small village in Brgy. Inasagan, Iligan, rests at the foot hills of Lake Danao, a high elevation lake that separates Lanao Del Norte from Misamis Oriental hinterlands. Based on our data, we concluded that new fractures, possibly formed during the earthquake, may have developed underneath Brgy. Inasagan and became pathways for water coming from Danao Lake. With the high difference in elevation of lake Danao and Iligan, water would be forcibly gushed out of these fractures and as a result, develop new springs and creeks.

A new creek formed between these houses in Inasagan. The source of the water is from a new spring at the foot a hill behind these houses.

I’m not saying that the same thing could be happening or will happen around Taal Lake. Both lakes are different in terms of the mechanism that formed them. Both lakes’ geology and topography (which relates to their shape) are also different. What I’m trying to say is that if my second hypothesis regarding the drying up of Pansipit River turns out to be true, springs can develop in other areas around Taal that are previously “dry”. Maybe these aren’t just too obvious now because of the ash cover. But if these springs develop along foot slopes and are robust enough, then you’re looking at new creeks which will run its course where there is lower elevation. Taal is starting to do major makeover of the topography (land shape relating to slope and elevation) – this is what we call “cut and fill” process in geology. Valleys are being formed and then filled with volcanic fragments around volcanoes. Now, to map out these new springs, if they are visible now, is a colossal task for PHIVOLCS for sure, what with all the things happening, our heroes in the agency are overstretched. Mindanao earthquakes recently happened and now Taal is at Level 4. They have so much on their plates right now.

What we can do to help? Again, if you observed new springs in your property, kindly report them to PHIVOLCS for their future investigation and evaluation.

Finally, it’s hard for PHIVOLCS scientists to predict when Taal will blow up and cause base surge, if it ever does. Volcanoes are highly dynamic and complex land forms that are constantly changing and shifting rocks around in response to physical and chemical changes underneath. Taal’s past eruptions can only tell us so much. After every eruption, the shape of the magma chamber underneath changes too. Our scientists at PHIVOLCS are working overtime to monitor and interpret data from the volcano. Meanwhile, geologists like me are waiting with scientific interest as Taal unfolds itself.

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