How Exactly Did the Chocolate Hills Come About?

The Philippines stand out as a nation with amazing biodiversity. I’ve had the opportunity to visit many parts of the country and write about some of it right here on Esquire (see here, here, and here). But even the rocks the country is built from are highly unusual, fascinating, and diverse.

Anyone who has traveled the country will know that the landscapes of one part of the Philippines look very different in other parts of the archipelago: the rock spires of El Nido, the towering volcanoes of Negros Island, and the unique caldera of Taal volcano all impress the expert and enthuse the geological layperson.

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One of the rightfully most famous landscapes of the Philippines are the Chocolate Hills in Bohol, a field of oddly regular rounded cones, which turn chocolate-brownish in the dry season, hence the name. The Chocolate Hills recently hit the news again after photographs of a somewhat unsightly resort built at the foothills of some of the Chocolate Hills surfaced. Up to their true form, it only took hours before witty and humorous memes showing a variety of photoshopped modified Chocolate Hills started making their way around social media. I can’t compete with so much creativity, but I can explain the way the Chocolate Hills formed over geological time.

A diagram showing the factors shaping the Chocolate Hills. From Salomon, 2011

Photo by Jean-Noel Salomon.

A very poetic scientific paper by French geologist Jean-Noël Salomon entitled A Mysterious Karst: The ‘Chocolate Hills’ of Bohol (Philippines) describes in detail the way the Chocolate Hills have formed. Let me summarize the insights from this article for you.

The center of Bohol is primarily made up of limestone, which was formed from sediments during the last 60 million years, roughly since the end of the time of the great dinosaurs. Algae with calcite in their body walls and single-celled organisms called Foraminifera contributed most to this limestone. The limestone was later raised by geological processes, which is not uncommon in the Philippine archipelago: The country is not located on a single tectonic plate, like more boring countries, but sits on the "Philippine Mobile Belt," a jigsaw puzzle of small tectonic plates between the larger Eurasian and Philippine Sea plates. The active geological processes in the area are responsible for both the many volcanoes in the country, as well as for submerging and uplifting pieces of land, as it happened with the limestones of Bohol.

The distribution of the Chocolate Hills in the center of Bohol. From Salomon, 2011

Photo by Jean-Noël Salomon.

Once the limestone was exposed to the surface of the planet, rain and water eroded parts of it away. The resulting landscape is called karst, and the process is called karstification. The word karst, as you might have guessed by its harsh sound, is originally German and was initially used to describe the eroded limestone landscapes on the Slovenian/Italian border. There are many karst landscapes worldwide, but what makes the Chocolate Hills unique are their particularly regular geometric shapes.

This has to do with conditions which are ideal for making karst. There are many impurities and layers of gravels in the central Bohol limestone, and the wet tropical climate further promotes its dissolution. The rainfall in Bohol is relatively evenly distributed over the course of the year, and while the wind direction changes between Amihan and Habagat times, the winds in central Bohol are rarely ever very harsh. The moderately thick soil and the rich tropical vegetation at the base of the Chocolate Hills (probably much thicker and forest-dominated in pre-human times) further made the karstification conditions in Bohol very regular in time and space. The carbonic acid, and the organic acids produced by the tree cover might have further contributed to karst formation. Also, the groundwater level in this part of Bohol is rather low, at about 30 meters, which also helps in karstification.

The underwater landscapes in Bohol are beautiful

Photo by Klaus Stiefel.

Hence, central Bohol was handed all the aces by geology, climate, and botany when it comes to regular karstification. The result is what we now call the Chocolate Hills.

The term geologists use for the conical Chocolate Hills is "mogote." The mogotes in Bohol are 40 to 120 meters high, and 65 to 80 meters in diameter. There are 1776 of them. Caves are frequently found at the base of the hills as a result of the water flowing down their cones.

The tarsier, one of the smallest living primates, is another reason to visit Bohol

Photo by Klaus Stiefel.

The native Boholano legend explains the making of the Chocolate hills with the tears of a giant, Arogo. Saddened by unreciprocated love, he cried, and where his tears were drying, the hills grew. Will Argo cry some more when he sees what humanity is doing to the hills in 2024? Just as we can’t compete with the meme makers, we can’t really go into politics, but everyone who appreciates the Philippines’ natural beauty will hope that it will be preserved for future generations, be it birds, sea slugs, or karst hills.