Origin and geologic features of Taal Lake, Philippines

Taal Lake was formed by a series of catastrophic volcanic eruptions and other geologic processes whose character slowly evolved as the large basinal depression and the lake took form. During the early stage ofTaal's life, large scale magmatic activity was very common in this part of Luzon including the caldera-forming eruptions that created parts of Laguna de Bai and the smaller scale eruptions that formed the cones and volcanic peaks in the area. Around Taal, large eruptions were carving large bowl-shaped depressions on one part while depositing thick pyroclastic rocks on more distant sites. During the last stages of the large scale magmatic eruptions, a lake must have been already present, leading to the phreatic eruptions that created the crossbedded, base-surge pyroclastics found on the upper layers of the terrain around the lake. These phreatic eruptions created smaller circular depressions that later coalesced to form the present caldera. With much of the magma exhausted after periods of protracted eruptions, the scale of volcanism was reduced and mostly centered in the region where Volcano Island is now located. Combined with the hills near Balite and Bilibinwang, development of Volcano Island formed an east-west trending ridge that divided the lake in two. Tectonic uplift and erosion on the northern shores of the young lake and the presence of deeper craters on the south led to contrasts in the depths of the depressions. Smaller scale geologic processes marked the last stage in the development of the lake. Landslides modified the steep slopes, and large blocks of land were shifted vertically and horizontally along faults by slow tectonic movements. More sediments shed from the small region drained by the lake were being deposited and volcanism continued mostly on Volcano Island. Numerous large circular craters on the shores of the lake suggest the explosive origin of the whole Taal Lake depression. The flat-bottomed lake is shallower by about 100 m on the north. Faults are found mostly on the northern slopes, where large-scale landslide blocks are also common. Landslide features are also found underwater particularly near the steep shores. Most of the historic volcanism had been centered on Taal Volcano Island, and these are often of the explosive phreatic type reflecting the influence of the lake on magmatic activity. The resources and the people residing near the lake are still under the threat of the same active geologic processes that created Taal Lake.