GeoPics

Geopics # 22

"The Pillow Basalts of Montalban, Rizal"

Rodriguez, Rizal is known for its hiking destinations, caves, and the famous Wawa Dam. However, did you know that there are remnants of underwater volcanism located in the same area?

These are pillow basalts, which are exposed on the right bank of the Wawa River (part of the Marikina River). Pillow basalts are a type of volcanic rock that forms when lava erupts underwater from an underwater eruption. The outer layer cools rapidly, creating a glassy surface, while the interior remains hot and expands, forming rounded shapes that resemble pillows.

These structures can be spherical, bulbous, or tubular, often with a rounded top and a downward-extending tail. Pillow basalts usually contain few gas bubbles (vesicles), which are small cavities left by gases. Those that remain often display a radial pattern due to the quick cooling from all sides.

Some pillow basalts show reddish discoloration due to the oxidation of primary iron-bearing minerals, indicating the formation of ferric iron oxides, most notably hematite. The greenish coloration is attributed to the metamorphic development of secondary hydrous mineral phases, such as chlorite or epidote.

The Pillow Basalts are part of the Montalban Ophiolitic Complex—a section of the Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level. This is previously known as the Angat Ophiolite, as described by Karig in 1983. The Montalban Ophiolitic Complex is the basement rock (the oldest layer of rock) of the Southern Sierra Madre, and its age is said to be early Late Cretaceous.

References

Mines and Geosciences Bureau Philippines. (2010). Geology of the Philippines. General Books.

U.S. National Park Service. (n.d.). Pillow Basalts (U.S. National Park Service). Www.nps.gov. https://www.nps.gov/articles/000/pillow-basalts.htm

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