Many thousands of years ago, a massive volcanic eruption began in the north central Philippines.
It was so energetic that it caused a plume of ash to shoot more than 45 kilometers into
the atmosphere.
Simultaneously, pyroclastic flows raced across the landscape, burying everything in a vast
area in superheated rock.
These pyroclastic flows traveled 20 then 30 then 50 kilometers, until finally stopping
after having traveled a whopping 66 kilometers.
Then, due to the large amount of erupted material, a 24 by 12 kilometer section of land collapsed
downwards, forming a massive caldera.
What had just occurred was a VEI 7 eruption from the Laguna Caldera, which is the closest
volcano to the densely populated city of Manila.
Today, this city along with Rizal and Bulacan is built on top of pyroclastic flow deposits
from this massive volcano.
Although it has not erupted in a long time, this unique volcano could erupt again.
The Laguna Caldera is located a mere 37 kilometers southeast of downtown Manila.
Its massive complex is located within the large Laguna de Bay, which has three distinct
lobes much like the footprint of a Dinosaur.
The middle of these three lobes represents the Laguna Caldera.
Before I begin, I want to note that the similarly named Laguna Volcanic Field is located to
the south.
This is a separate volcanic complex which is not related to the Laguna Caldera.
As a whole, the geologic setting of the Laguna Caldera and Taal calderas are quite similar.
In this section of the Philippines there is a small rift zone where the crust is spreading
apart to the northwest and southeast.
This rifting from the area termed the Macolod Corridor has allowed for large volumes of
magma to intrude into the crust, forming a chain of volcanoes.
Despite the vast size of the Laguna Caldera, very little is known about its geologic history.
Its earliest documented eruption occurred 1 million years ago, which most likely formed
a smaller caldera which was more recently absorbed by a subsequent caldera forming eruption.
This eruption produced a large volume of andesite and dacite lavas, and distributed significant
quantities of ash over a wide swath of the Philippines.
Over the next several hundred thousand years, there is evidence that a number of smaller
eruptions took place, which likely produced a number of temporary islands and volcanic
cones out of basaltic rock.
Some larger eruptions also occurred due to the interaction of shallow magma with the
surrounding lake water.
One such eruption which occurred within the last 100,000 years carved out a 3.5 kilometer
or 2.2 mile wide circular crater.
Today, the western edge of this vast explosion crater can be found on the southeastern edge
of Talim island.
The modern large caldera which we see today formed approximately 28,000 years ago, although
we are not truly sure how large its eruption was.
In all likelihood, this megaeruption ejected between 70 and 130 cubic kilometers of volcanic
rock, and may have been witnessed by early humans who lived in the region.
Since then, no eruptions are confirmed to have taken place.
However, this caldera complex has not been well studied, and we don't know the age of
its associated maar explosion craters.
It may have erupted in the last 10,000 years but we simply lack the evidence to make such
a claim as more studies are needed.
Today, fumaroles associated with this volcano can be found at the southern end of Mount
Sembrano, which are actively venting volcanic gasses from a still ever present underground
magma chamber.
As a final note, there is an interesting yet unproven theory regarding the Laguna Caldera.
To the southwest, the Taal volcano has produced eruptions which are seemingly too large, as
it ejected more magma than its magma chamber holds.
The theory suggests that during Taal's very large eruptions it has somehow stolen a quantity
of magma from Laguna's underlying magma chamber.
Adding more evidence to this theory is the fact that during a major eruption from Taal
in 1749, a 70 kilometer long fissure formed which linked the two volcanoes.
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