Palawan Continental Terrane. “Palawan?” According to Google Translate, it means “Sleeper Fish.”
Sleeper gobies are members of the Eleotridae fish family, found predominantly in the tropical Indo-Pacific. There are approximately 35 genera and 150 species.
Interesting… sort of. The Palawan Continental Terrane is actually a fairly sizable chunk of material that has perplexed a few researchers as to where it came from, or how it originated. Before I yammer about that, let me point out what that word actually means… not Palawan, but “Terrane.”
A Terrane is a geologic term for a somewhat contigious block(s) of material that operate/move over geologic timescales as one unit. The boundaries are not really clear enough to call it a crust block or microplate, or microcontinent… though each of those could eventually wind up being a terrane once they get to a resting place, or are plastered onto a continent. Essentially, the material in the Terrane is related to all the other material in origin, chemical make up, and destination. Usually a Terrane originates from one crust block/plate and winds up attached to or sutured onto another. The Wrangellia Terrane is where I learned the term… that’s the region plastered to the North American craton east of where the recent Queen Charlotte quakes occurred at. If you think of bugs and windshields, you get the general idea of how terranes work and accumulate.
From the name “Palawan Continental Terrane” you would assume that it originated from some continent somewhere. According to Knittel et al., it’s a piece of the rifted margin of SE China. So where is it now? Well, it makes up a significant chunk of Mindoro in the Philippines. Mindoro is a collection of three uniquely different chunks of material. The other parts are the Philippine Mobile Belt that the Palawan Continental Terrane is sutured to, as well as a third unit that is made up of metamorphic material and a section that might indicate an ophiolitic unit… complete with gabbros. From Wikipedia: “a section of the Earth’s oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crust”
Okay… so it’s a slow motion collision in process. More correctly, part of a slow motion collision in process. Why part? Well, this affects Taal and Laguna de Bay.
Mukasa et al. points out that other researchers have pinned their origin as products of the subducting plate at the Manila trench, and then further notes that the geochemistry of them has changed as they have grown older. Specifically, they differ from the other volcanoes in the northern part of that chain. (East Bataan Lineament). The reason for this, according to the authors, is the incorporation of Palawan Continental Terrane material into the magma production.
This could explain how Taal and Laguna de Bay could have become capable of making large calderas. By its nature, continental material is more silica rich than oceanic crust. The general thought is that leading shards of this material are intruding into and being caught up in the melt formation process.
As cbus20122 notes:
…It’s amazing that such a large eruption [Pinatubo] only produced a comparative blip of a caldera when looking at the other volcanic areas on the map…
Pinatubo, being on the West Bataan Lineament, is more north and further away from this source of silica rich magma.
Right next to the Taal/Laguna de Bay region is the Macolod Corridor. From the abstract of a pay to play paper by Förster et al (1990):
an approximately 40 km wide zone of still active intense Quarternary volcanism which perpendicularly crosses the Island in a NE-SW direction … we believe that the corridor is a pull-apart zone formed by a diffuse system of NW-SE oriented shearing.
And of course… a plot of sorts. Not my usual, I wanted to focus on quake depths in relation to the major players. This was put together with DivaGIS. Red Quakes are greater than 90 km deep, Blue quakes are less than 90 km deep. Somewhere around 125 km is where melt percolates off of the subducting slab. The majority of the deep spike is under and just to the southwest of the Taal parent caldera. (The one the island of Taal sits in). There are a few deep quakes up around Pinatubo, but nothing like the area around Taal. These are quakes from the USGS list back to 1975 and greater than magnitude 4.5 or so.
An after the fact addition: If you note my first graphic, there is a region that forms a “T” with the Manilla Trench that I called “Old South China Sea Spreading Center.” A closer look at this was done in “Basement structures from satellite-derived gravity field: South China Sea ridge” by Braitenberg et al (2005). You may find it of interest. It is what put the Palawan Continental Terrane where it is.
“The Macolod Corridor: A rift crossing the Philippine island arc” Förster et al (1990).
In summary, Taal and Laguna de Bay fall into the “Large Caldera” category of volcanic systems. Both are on the margin of the McCleod Corridor which is part of the Philippine mobile belt. The McCleod Corridor is seemingly a nascent spreading region. What exact dynamics are in play is hard to nail down since like much of the Western Pacific, the Philippine islands are a geological train wreck with many tectonic forces competing for dominance. The presence of the Palawan Terrane could contribute to a higher silica content of the magmas feeding the volcanoes in the area since it is continental crust in origin. Don’t let the4 fact that much of it is underwater fool you. The Jan Mayen microcontinent is fully underwater except for a volcanic feature known as Jan Mayen island.
The important part of the animated plot is to note just how prolific the deep quakes are in the Benioff zone under the Taal area compared to Pinatubo.