GeoPics

30/03/2026

27/12/2025

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

11/12/2025

Geopics # 21
SummerSlam Destination # 3

"The volcanoes of Bataan"

Bataan is renowned for its significant historical role during World War II, most notably as the site of the Bataan Death March. Furthermore, in addition to its historical importance, the province boasts numerous natural attractions, including pristine beaches, scenic waterfalls, and volcanoes, such as Corregidor Island, Mount Mariveles, and Mount Natib.

Both Mount Mariveles and Mount Natib are classified as Quaternary composite volcanoes, indicating their formation during the Quaternary period and their characteristic layered structure formed by successive eruptions of lava and pyroclastic material. As two of the principal peaks that comprise the Bataan Volcanic Arc, these volcanoes are geologically related and collectively shape the topography of the Bataan Peninsula. Although no eruptions have been documented in recorded history, geological evidence confirms episodes of volcanic activity in the past, contributing to the region’s rich volcanic landscape.

Note: This was taken during our Summerslam last April 8-10, 2025 at Morong, Bataan 🐬😎🏄🏻🏖️🌴🌞⛱️🐋🐚

References:

Benguet Corporation. (2024, August 21). Exploration, Research and Development - Benguet Corporation. Benguet Corporation. https://benguetcorp.com/about-us/exploration-research-and-development/

Global Volcanism Program. (n.d.). Global Volcanism Program | Mariveles. Smithsonian Institution | Global Volcanism Program. https://volcano.si.edu/volcano.cfm?vn=273081

Global Volcanism Program. (2024). Global Volcanism Program | Natib. Smithsonian Institution | Global Volcanism Program. https://volcano.si.edu/volcano.cfm?vn=273082

Lagmay, A. M. F., Rodolfo, R., Cabria, H., Soria, J., Zamora, P., Abon, C., Lit, C., Lapus, M. R. T., Paguican, E., Bato, M. G., Tiu, G., Obille, E., Pellejera, N. E., Francisco, P. C., Eco, R. N., & Aviso, J. (2012). Geological hazards of SW Natib Volcano, site of the Bataan Nuclear Power Plant, the Philippines. Geological Society, London, Special Publications, 361(1), 151–169. https://doi.org/10.1144/SP361.13

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

Volentik, A. C., Connor, C. B., Connor, L., & Bonadonna, C. (2009). Aspects of volcanic hazard assessment for the Bataan nuclear power plant, Luzon Peninsula, Philippines. Cambridge University Press EBooks, 229–256. https://doi.org/10.1017/cbo9780511635380.010