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The Chocolate Hills: A Geological wonder

Writer's picture: Tom McAndrewTom McAndrew

Source: Tom McAndrew CGeog ('Matcha' Chocolate Hills - not the dry season so not the brown shade that gives them their name!)


The Chocolate Hills of the Philippines are one of the most fascinating geological formations in the world. Found in the Bohol province, these hills consist of over 1,200 conical mounds spread across approximately 50 km2. During the dry season, the grass covering the hills turns brown, giving them the appearance of giant chocolate mounds, multiple of which form a tray or box of chocolates (use your imagination!) hence their name. But beyond their aesthetic appeal, the Chocolate Hills are an important subject of study for geologists and geographers alike. Understanding their formation involves examining karst landscapes, limestone weathering, and tectonic activity.


The Geological Setting


The Chocolate Hills are a classic example of a karst landscape, which forms due to the dissolution of soluble rocks such as limestone, dolomite, and gypsum. Karst landscapes are characterised by unique landforms such as sinkholes, caves, and underground rivers, all of which are present in Bohol. The foundation of these hills is marine limestone, deposited over millions of years when the region was submerged under an ancient ocean.


The Formation Process


The formation of the Chocolate Hills is believed to be a result of several geological processes occurring over millions of years. The primary theories explaining their formation include:


1. Limestone Deposition


Around 2 to 5 million years ago, the area that is now Bohol was covered by a shallow sea. Over time, the skeletal remains of marine organisms such as corals, shellfish, and plankton accumulated on the seabed. These calcium-rich remains compacted and solidified to form limestone, a sedimentary rock that is highly susceptible to chemical weathering.


2. Tectonic Uplift


Due to the movement of the Earth's tectonic plates, the seafloor was gradually uplifted, exposing the limestone to surface conditions. The Philippines is located along the Pacific Ring of Fire, a region known for frequent tectonic activity. The uplift process resulted in the emergence of landforms, setting the stage for further erosion and weathering.


3. Weathering and Erosion


Once the limestone was exposed, it underwent extensive weathering and erosion. Chemical weathering, particularly carbonation, played a significant role in shaping the Chocolate Hills. When rainwater, which contains dissolved carbon dioxide, came into contact with the limestone, it reacted to form a weak carbonic acid. This acid slowly dissolved the limestone, creating a rugged karst landscape with depressions and caves.

Over time, differential weathering led to the formation of the conical hills. Certain areas of limestone were more resistant to erosion due to variations in composition and density, leaving behind the distinct rounded mounds while the surrounding material was gradually worn away.



A cross section of a 'Chocolate Hill' (Source: J Saloman, 2011)
A cross section of a 'Chocolate Hill' (Source: J Saloman, 2011)

4. Subaerial Erosion and Soil Development


As the hills continued to be exposed to the elements, additional processes such as rainwater erosion and biological weathering further sculpted their shape. The thin layer of soil that formed on the surface allowed vegetation to grow, primarily grass species that thrive in nutrient-poor conditions. This vegetation protects the hills from rapid erosion, helping maintain their iconic conical shape.





Source: J Saloman, 2011
Source: J Saloman, 2011


Karst Landscapes and global comparisons


The Chocolate Hills are part of a broader category of karst landscapes found worldwide. Similar formations can be seen in China’s Guilin region and the limestone hills of Vietnam’s Ha Long Bay. However, the Chocolate Hills are unique due to their uniformity and sheer number. Most karst landscapes are irregular and jagged, while Bohol’s hills are nearly symmetrical and evenly distributed.


Human and Environmental Impact


Although the Chocolate Hills are a protected natural site, human activities such as quarrying, deforestation, and tourism development pose potential threats. Mining limestone for construction materials can alter the delicate karst system, leading to increased erosion and loss of biodiversity. Additionally, climate change may impact weathering rates and vegetation cover, potentially altering the landscape over time.

On the positive side, the Chocolate Hills are a major tourist attraction, contributing to the local economy. Sustainable tourism efforts are in place to ensure that visitors can experience the site without causing significant environmental damage. Conservation measures include designated viewing areas, controlled access, and educational programs to raise awareness about geological preservation.




Why study the formation of the Chocolate Hills?


The Chocolate Hills are a remarkable example of how geological forces shape our planet over millions of years. Through limestone deposition, tectonic uplift, chemical weathering, and erosion, these unique landforms have emerged as a globally recognised natural wonder. Their presence not only provides insight into karst landscapes but also highlights the delicate balance between nature and human activity. For geography students, studying the Chocolate Hills offers a valuable lesson in geomorphology, environmental conservation, and the dynamic processes that continue to shape Earth’s surface. This a way of going above and beyond A Level or IB syllabi, thus embedding and deepening knowledge previously learned. They can also make the synoptic links with the CIE International A-Level 'Rocks & Weathering' module or any other syllabus' unit that is themed around Geology and weathering. Links can further be made with tourism topics and the sustainable management of a tourist site.

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