The Chocolate Hills in Bohol are one of the Philippines’ most famous natural wonders, but how did these 1,200+ cone-shaped hills come to exist? Their formation is rooted in tectonic activity, marine geology, and erosion. Let’s explore how the Chocolate Hills Formed: A Tectonic and Geological Perspective.
📍 Location: Bohol, Philippines
🧭 Formed by: Tectonic uplift, karst erosion
🏞 Number of hills: Over 1,200
🏛 Status: National Geological Monument
📅 Best time to visit: Dry season (Dec–May)
Tectonic Uplift and Marine Origins
Millions of years ago, the area that is now Bohol was underwater, part of an ancient seabed. The Chocolate Hills were originally coral reefs and marine limestone deposits, formed over thousands of years by the accumulation of coral, shells, and other calcium carbonate-rich materials.
Due to tectonic movements, these seabeds were gradually uplifted above sea level as the Earth’s crust shifted. This process, known as orogenic uplift, was caused by the collision of the Philippine Sea Plate and the Eurasian Plate, which continues to shape the Philippine archipelago today.
Weathering and Erosion
Once exposed above sea level, the uplifted coral limestone was subjected to erosion over thousands of years. Rainwater, which is slightly acidic due to dissolved carbon dioxide, slowly dissolved the limestone through a process called karst weathering. This created the conical, symmetrical shapes of the Chocolate Hills.
Formation of the Unique Mound Shapes
Unlike typical hills formed by volcanic activity or sediment deposition, the Chocolate Hills are karst formations—similar to other limestone landscapes found worldwide, such as the Guilin Hills in China. The combination of rainwater erosion, underground river systems, and gradual weathering sculpted these hills into their distinctive, rounded forms.
Why They Turn ‘Chocolate’ Brown
The hills are covered in grass, which dries up and turns brown during the dry season (December to May), giving them their iconic “chocolate” appearance. In the wet season (June to November), they are lush and green.
However we visited the Chocolate Hills in February and they were very much green and lush!
Heritage and Protection
The Chocolate Hills are a National Geological Monument of the Philippines and are protected under environmental laws. They are also a candidate for UNESCO World Heritage status, recognising their unique geological significance and the need for conservation efforts.
Summary
- Originally coral reefs and marine limestone, uplifted by tectonic activity
- Shaped over millions of years by erosion and karst weathering
- Their rounded, symmetrical forms are due to gradual limestone dissolution
- Covered in grass, which turns brown in the dry season, giving them their chocolate appearance
- Protected as a National Geological Monument
These incredible formations are evidence of the Earth’s dynamic geological processes and remain one of the most unique landscapes in the world!
