Science and Environment

The geology of the panghilod, the traditional Filipino stone tool

STAR SCIENCE - STAR SCIENCE By Leopoldo P. De Silva Jr., Ph.D. -
Even before the introduction of loofas and other body scrubs in today’s beauty bars, there has always been the lowly panghilod. To the young and uninitiated, a panghilod is a piece of rock or stone used in scrubbing out dirt and grime from the skin during bathing. The type of rock used for this purpose varies, but the panghilod generally possesses three important attributes: small enough to be hand-held; a naturally polished or smoothened surface; and flat or oblate in shape.

These are the same characteristics exhibited by pebbles found on stream beds, the natural source of panghilod. Pieces of rocks found on stream beds were derived from rock exposed upstream. Weathering causes these large bodies of rocks to break down into smaller pieces. They are then removed (erosion) and transported by river water. In the process of transport, the pieces of rock are tumbled and abraded until the surface is smoothened and the shape flattened. Rocks with similar characteristics can also be observed along rocky shorelines where constant wave action causes smoothening of the surface without having to transport them long distances. In either case, the natural processes responsible in making a panghilod involve hundreds even thousands of years. Should you choose to use the panghilod anytime in the future, remember that you are literally holding a piece of Earth’s history in your hands.

But what type of rock is more commonly used as panghilod? To answer this question, we need a little understanding of petrology (study of rocks) and geology of the Philippines. There are three basic rock types: igneous, metamorphic, and sedimentary. An igneous rock results from the solidification of molten material known as magma. The solidification process involves the formation of solid phases known as minerals. If the rate of cooling or solidification is rapid, minerals are not given time to grow, resulting in a very fine-grained texture (mineral grains are so small that they cannot be seen). This texture is typical of volcanic rocks. Volcanic rocks form from lava (magma extruded to the surface). If magma, however, solidifies beneath the surface of the Earth, the rate of cooling is relatively slow, allowing the formation of larger crystals or a coarse-grained texture. The resulting igneous rock, called a plutonic rock, is composed of large interlocking grains of minerals. What about sedimentary rocks? Sedimentary rocks are literally recycled rocks. They form from previously existing rocks. Rocks exposed to the surface are mechanically or chemically broken down to pieces, transported and then deposited. These fragments of the original rocks are called sediments. Thus, a panghilod is an example of a sediment. Compaction resulting from burial and cementation (pore spaces filled with cementing material) transforms the loose sediments into a solid rock known as a sedimentary rock. Rocks buried deep beneath the surface are subject to extreme temperature, pressure and chemical conditions. These may cause the rocks to undergo changes in the solid state (no melting involved), forming a rock type known as a metamorphic rock.

The choice of rock type to use as a panghilod depends on mineralogy (type of minerals present in the rock) and availability. Sandstone, for some people, is an ideal panghilod. This type of sedimentary rock is composed of compacted and cemented sand-sized sediments. The surface of the rock tends to be grainy, quite similar to sandpaper. The thought of rubbing sandpaper against the skin is, of course, not very pleasant; it is, however, very effective in getting rid of dead skin (exfoliation). The disadvantage of using sandstone as a scrubbing agent is the tendency of this rock to be composed of very hard minerals like quartz. The mineral quartz is so hard that it can scratch steel. Processes involved in the formation of sedimentary rocks (e.g., weathering, erosion, and transport) tend to favor the enrichment of very stable and hard minerals. Therefore, sandstones are not the rock of choice for people claiming to have soft "baby" skin. They, however, are ideal for getting rid of tough and stubborn skin such as calluses.

Metamorphic rocks are usually associated with very old terranes similar to what we find in Palawan and Mindoro. Most of the Philippine islands are relatively young (geologically speaking), less than 30 million years old. Thus, metamorphic rocks are exceedingly rare to be used as panghilod. The most abundant and widely distributed rock type in the Philippines is volcanic rocks. For those who are familiar with the theory of Plate Tectonics, the Philippines is an example of an Island Arc: an arcuate group of islands formed through submarine volcanism. Philippine rivers, the natural source of panghilod, are littered with volcanic rocks. In fact, most of the panghilods I have seen so far are volcanic rocks. I have yet to hear somebody boasting of an imported panghilod. We simply use what is readily available.

To this day, many swear to the effectiveness of the panghilod as a stone implement for bathing. Although I have not yet consulted an anthropologist, I suspect that the use of the panghilod pre-dates the Spanish period. In Africa, paleontologists believe that the first stone tools used by the ancestors of modern humans were stones similar to the panghilod. Of course, they think that these river pebbles were used as a means of defense against predators (convenient things to throw). This just adds merit to the use of the panghilod. You’re clean and you’re safe. Scrub away…
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Dr. Leopoldo P. de Silva Jr. is an assistant professor of geology at the National Institute of Geological Sciences, College of Science, University of the Philippines, Diliman. He specializes in micropaleontology and paleoceanography. E-mail him at [email protected].

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