Brothers' Blog: Building an Earth Cache

What is karst?

El Torcal (Antequera - Spain)

Limestone pavement in Dent de Crolles, France

Karst is a type of landform made by water eroding layers of bedrock, usually carbonate rock, such as limestone or dolomite. Most karst features appear as sinkholes, but karst can create caves, natural bridges, and other very interesting shapes.

When did karst start?

The maximum extent of glacial
ice in the north polar
area during Pleistocene time.

The karst at this location started in the Pleistocene era. The Pleistocene era is from 12,000 to almost 2.6 million years ago, from when the ice from the ice age started melting, to when it finished melting. The name Pleistocene comes from the Greek words for “most”, and “new”. The Pleistocene era ended the same time as the Paleolithic age (the time when humans started using stone tools and started planting seeds instead of catching food.)

Where is karst?

Karst is found in Africa, Europe, North and South America, Asia, and Oceania (Australia, new Zealand, and Papua new guinea)

Here in Japan, the most famous karst spots are:

Karst pinnacles at Akiyoshidai

Akiyoshi Plateau,
Atetsudai and Taishakudai Plateaus
Shikoku Karst
Hiraodai Plateau
and Okinoerabujima Island

But the karst here in Okinawa is not the same as it is in other places. In other places karst makes bridges, caves, ravines, and there’s even a lake in Italy that’s made by a karst sinkhole. But in Okinawa, it mostly looks like bowls in the ground. At this spot on the beach though, you can see at least three different types of karst: Caving, Horizontal Tubing, and Vertical Tubing. The karst here is made by rain coming down from the sky and mixing with the red clay and then running down the hill to the shore.

How is karst formed?

Karst is usually the result of slightly acidic water acting on weak bedrock like limestone or dolostone. The slightly acidic water starts to wear away the surface in the cracks or bedding planes (which is the line between the layers or ‘beds’ of rock.) Once the karst forms an opening, it allows for more water which makes more karst and it goes faster and faster.

The main way karst happens is when rainwater combines with carbon dioxide (CO2) and makes carbonic acid (H2O+CO2=H2CO3… this is where we get the term “acid rain”.) When it hits the ground and mixes with the CO2 from the dirt (or in this case, red clay,) it gets even more powerful. When the acidic rain water gets to the limestone, it settles in the cracks or dips in the rock and begins to eat away at the rock creating these bowl shaped hole formations in the limestone.

The chemical reaction taking place between the carbonic acid and the limestone is like this:

H₂O (water) + CO₂ (carbon dioxide) → H₂CO₃(carbonic acid)

CaCO₃(calcium carbonate) → Ca²⁺ (calcium)+ CO₃^2–(carbonate)

CO₃^2– (carbonate)+ H₂CO₃(Carbonic Acid) → 2 HCO₃^–(bicarbonate)

CaCO₃ (calcium carbonate)+ H₂CO₃ (carbonic acid) → Ca²⁺ (calcium)+ 2 HCO₃^–(bicarbonate)

Tidal Karst on Okinawa

So here's the cache part... To successfully log this cache you need to complete a few scientific measurements.

1: Near where you parked, you will see this locally famous karst which shows horizontal tubing. Measure the width and length and calculate the approximate volume of rock removed from this example from karst (volume of a cylinder is v= π r²h).

2: Travel to this spot on the beach to see a good example of vertical tubing. Find this tube I'm in and measure the diameter and height.

3: Travel to this spot on the beach to see a really good example of what "Bedding Plane" means and to see the caving that happens when water gets inbetween these layers of rock. Go to the spot where you see me measuring, and measure for yourself the height to the bottom of the caving, the height to the top, and just to double check your answers, the height of the caving itself (the height of the cave + the height to the bottom should equal near the height to the top of the caving.)

4: Finally, take a picture of your favorite karst feature on this beach. Mine is the one where the rock looks like the head of a moray eel sticking out of the side of the cliff. Here's pictures with and without my head in his mouth: