The Beach: An Origin Story

If you’re one of the millions of beach lovers in the country, chances are you’ve been to many different ones. Have you ever wondered why Boracay’s sand is white and powdery fine while Puerto Galera’s beaches are white but a bit more coarse? Why the beach in Botolan, Zambales has black sand? Or why the shores of Mabini, Batangas have more gravel rather than sand? The answer lies mainly in the geology and geomorphology of the country’s vast coastline.

What are the different coastal landscapes in the Philippines?

Owing to its geologic history, the Philippine archipelago has a variety of coastal landscapes. The islands are generally hilly or mountainous which are typically dissected by short steep rivers with associated small fan-deltas along the coast. Additionally, volcanoes abound in the country and contribute to its mountainous landscape.

Rocky cliffed coastlines are abundant with pocket beaches in small embayments (A, B). Occasionally, the steep coastline is broken by a relatively broad coastal plain associated with a delta or a strandplain where beaches can be extensive and well-developed (C). Muddy coastlines (D) are more common in embayments within the internal seas. These are associated with mangroves but numerous muddy coastlines have already  been converted to fishponds.

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Coral reefs, mangroves, and seagrass meadows abound in the Philippines.  These three (3) ecosystems are typically associated with each other.  Most of the large islands in the country are lined with fringing reefs except in areas close to river discharge points. Mangroves and seagrasses are also found in muddy coastlines. Many small islands have uplifted reefal limestone edifices or are low-lying sandy cays. Thus, a large proportion of Philippine beaches have materials derived directly from the adjacent coral reef.

Where does all the sediment come from?

Beach sediments are derived from a wide variety of sources including nearby cliffs, offshore sources (i.e. coral reefs), and a combination of both:

(a) River-derived sediments known as terrigenous, siliclastics or lithics

Sediments originating from land are produced by processes such as weathering and soil erosion, landslides, and volcanic eruptions. These are transported by rivers to the coast. Beach sediments reflect the characteristics of the parent or source rock. Sediments may also come from a cliff or bluff erosion. Repeated pounding of waves may cause a portion of a cliff or bluff to collapse breaking it down into smaller pieces and transported by waves and currents onto nearby beaches.

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(b) Offshore sources

Sediments may come from deposits in the littoral depths, or from biogenic processes. Siliciclastic materials can come from deposits within the shoreface (e.g., longshore bar) that can be transported onshore by waves. ‘Biogenic’ sediments are particles produced by living organisms. These are mostly coral reef-derived materials and are known as carbonates since they are composed of calcium carbonate (CaCO3). Some examples are coral fragments, flakes of calcareous algae Halimeda and foraminifera.

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(c) Mixed

Beach sediments can also be a combination of siliciclastic and carbonate materials. Immediate sources can be sandflats or mudflats where mollusks are abundant or where a thriving seagrass community is present. Mixed sediments are also common in areas where coral reefs and rivers co-exist within the same coastal cell.

Why do beach sediments have different colors?

The color of beach sediments is determined by its source rock. Siliciclastics (river-derived) are typically darker than carbonate (coral-reef derived) materials. Siliciclastics are mostly derived from volcanic rocks having hues of gray, brown, and black. Moreover, degree of coloration reflects the abundance of certain minerals (i.e. darker sediments contain higher levels of iron or chromium). The black sands in Caba, La Union, for example, are iron rich; hence, are called magnetite sands. Some siliciclastics are light-colored due to the abundance of quartz and plagioclase crystals; giving the sediments a glassy appearance.  Beaches fed by rivers draining Mt. Pinatubo such as in Cabangan, Zambales have glassy sediments.

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Carbonates are typically light-colored materials. Instead of glassy sediments, carbonates are composed of coral fragments and reef-associated organisms such as molluscs, echinoderm, foraminifera and Halimeda flakes. The individual components reflect the biogenic sources of beach materials. The abundance of coral fragments in beach sediments, for example, suggests the presence of branching corals in the nearby offshore area.

carbonates

Beach sediments can also be a combination of siliciclastic and carbonate materials. Immediate sources can be sandflats or mudflats where mollusks are abundant or where a thriving seagrass community is present. Mixed sediments are also common in areas where coral reefs and rivers co-exist within the same coastal cell.  verde.jpgmalay.jpgcaramoan.jpg

Why do beach sediments have different sizes?

Beach sediments have different grain sizes depending on their proximity to its source and exposure to waves. There is greater concentration of large clasts near river mouths, while it becomes finer as one moves farther away from the river. Additionally, sediment sizes correlate well with wave energy: coarse-grained (fine-grained) beaches with steep (gentle) slopes are usually associated with high-energy (low-energy) setting.

Beaches and their grains.

Boracay 2010

Do we really need to know the sources of sediment and their transport along the coast? YES! Where the sediment comes from to your favorite beach, and where it goes, is important in the understanding of its stability. We all want our favorite beaches to remain “unchanged.” Certain changes in the source and transport of sediment to and from a coastal segment could mean the disappearance of sand, which not only could affect tourism, it also means possible loss of property and higher risk to high energy wave events such as storm surges.

Source: Sagip Baybay PH Module 1

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