Karsting away in Raja Ampat, Indonesia
Zodiac cruising through the karst landscape of the Piaynemu Islands, West Waigeo, in Raja Ampat, Indonesia.
I started to write this blog post last April, while happily cruising through Island Southeast Asia, on board SH Minerva. My whole voyage contract was for two months: starting in Solomon Islands at the beginning of April, then continuing through Papua New Guinea, Indonesia, and Philippines, then finishing in northern Japan at the end of May.
But life is super busy when you work on an expedition cruise ship, and I never finished the post. So here we go, a vignette from an amazing day in the famously beautiful islands of Raja Ampat, Indonesia, just off the northwest coast of the island of New Guinea.
It is a super interesting place for a geologist! These distinctive landforms are part of what we call a karst landscape. Karst is simply limestone that has been uplifted and weathered by rainwater.
Most limestone is created through biological processes in the shallow ocean, and is composed of the hard calcite parts deposited by organisms such as coral and molluscs. Those hard calcite parts might be microscopic (making very fine-grained grey limestone), or they may be in big chunks, so you can see fossils of those corals and shells in the rock.
(Here I am on the zodiac explaining that to my guests – thanks to my colleague and roommate Qi for this pic!)
Limestone’s main mineral, calcite, is soluble in rainwater (in fact, rainwater combines with CO2 in the atmosphere, forming very weak acid). Not all limestone is formed from broken shells – but the limestone that is may be very porous. Therefore, when limestone gets uplifted (for example by tectonic processes), it is susceptible to erosion by that slightly acidic rainwater, which not only runs over it but also through it. (That’s why most cave systems are in limestone!)
So that’s a brief explanation of how this unusual, scenic landscape of Raja Ampat – these evenly scattered island mounds, nestled in a turquoise coralline sea – formed. This is classic karst landscape!
In the case of Raja Ampat, these limestone deposits are of varying age – some is very young (Miocene) and some is well over 300 million years old. (That is way before mammals existed – way before even dinosaurs existed!)
The photos I am showing here are all from the Piaynemu Islands, off the western part of Waigeo Island, one of Raja Ampat’s four main islands. The distinctive karst landscape here, characterized by these even-sized mound-shaped islands, is a result of another geological complication: there are two fracture sets running through the rocks. These two fracture sets are perpendicular to one another, so they form a shape like a tic-tac-toe board. Rainwater running through the fractures gradually dissolved them, over the years making them wider and wider, and eventually leaving the centres as these evenly sized and evenly shaped islands.
Super cool! An example of how underlying geology influences our landscape! Here’s a fun little video of me driving the zodiac at Piaynemu Island, and explaining that geology to my guests – thanks to my roommate and colleague Qi (I had no idea she was filming this!)
Later on our voyage we passed through the Philippines. One of our stops was on Bohol Island, where we visited the famous Chocolate Hills. You can see here how similar the karst geology is – all of these evenly spaced mounds (there are over a thousand of them!) But, in this case, sea level is much lower relative to the land level – so rather than being individual islands like they are at Piaynemu, these karst mounds form individual hills.
I work super hard as a guide and lecturer on expedition cruise ships (there is a lot more mundane work to do than most of you would ever see!) But I sure do appreciate the opportunity to see and know these very interesting places – as well as all of the first-hand learning I get to do in my fields of interest (geology/anthropology/history, especially through the South Pacific and Patagonia). If you would like to know more about what I am up to, you can explore this website more (especially the What’s happening! page, or go to Fun! to see my exact itineraries) or you can simply Contact me here.
Jacqueline Windh 