#raock

Chocolate Hills

At the center of the island of Bohol in the Philippines sits this fascinating National Monument and UNESCO National Geological Monument – the Chocolate Hills.

When the plants on the slopes of these hills dry out in the fall, they turn brown, giving the hills a color and shape that resembles a Hershey’s Kiss candy.

The rocks are limestones – the remnants of coral reefs deposited in shallow, tropical, Pacific Ocean waters. They contain fine-grained and broken up fossils of a variety of species, including coral, mollusks, and foraminifera.

The shape is the result of a unique type of karst erosion known as conical karst. There are a few examples of it around the world, but this park is almost certainly the best example. The internet disagrees on the number of hills in the area, but its somewhere between 1100 and about 1800.

Karst topography forms from the dissolution of limestones when they’re exposed to flowing water or rain. These limestones are fractured into roughly square patches (the fractures are breaks in rocks called joints). Those cracks let water flow through them and erode downwards, shaping the hills. The high temperatures in this area then lead to high evaporation rates, so once the limestone erodes and is sculpted into the cone shape, the water evaporates and forms an extra- hard shell that establishes the shape and holds the hill together against further erosion.

-JBB

Image credit: https://www.flickr.com/photos/storm-crypt/5051559138