In the field with a geologist – nature’s underground treasures

This post is also available in: Dutch.

After last month’s introduction it is time to enter the dark world of wonder in the Hato caves. Mind blowing as it may be, the moment you enter the cave you are inside a fossil coral reef! We are not completely sure of the age of these rocks, but it is at least 900.000 years. Although the cave now lies some 60 meters above sea level, around 400.000 years ago the sea lapped at that same spot. Two factors interacted to cause that situation, as well as the fact that we can now enter the cave without getting our feet wet.

Gastropod (snail) fossils - proof of a marine origin of the rocks the caves is located in.

Sea level changes

First of all we have to take the ice ages into account. At this moment in time the earth is capped, both at the south and north poles, by a layer of ice. However, during most of geological history the earth was virtually ice free. Around 2.6 million years ago the last of the 5 known ice ages kicked in, with dramatic effects on sea levels from that time on. Periods with lots of ice – the glacial periods – resulted in low sea levels, while the periods in between – the interglacials – were times of high water. Basically, the sea level yoyo-ed up and down. The other factor I mentioned above comprises the fact that the Island was, and still is, uplifted slowly but steadily, because of forces acting from within the earths crust. The combination of both resulted in several limestone terraces at different elevations, which are beautifully visible on the Hato plain. The airport is located on the lower, youngest, terrace, and both the middle and higher terraces are clearly visible. During every period of relatively stable sea levels a new reef system developed, which turned into a terrace after drying out. The remnants of the oldest, the so called highest terrace, are only visible south of the road to Westpunt, opposite Tera Kora. But the higher terrace is the one that’s important in this story, because it is the one in which the Hato cave has been carved out.

Water at work - dissolution and crystallization in action

Cave formation – war of acids

Those terraces are basically fossil coral reefs, and corals are colonial organisms that build their intricate apartment complexes out of calcium carbonate extracted from sea water. This building material has one chemical enemy, which is acid. Acid dissolves the calcium carbonate, freeing up carbon dioxide (that doomsday gas that is causing climate change as well). Ordinary rainwater is somewhat acid, because the carbon dioxide in the air reacts a little bit with the raindrops while they’re falling down to earth, producing carbonic acid. This means that wherever rainwater comes into contact with limestone, slowly but surely the rocks are being dissolved (unpainted local houses as well). Because the fossil reefs are also very porous, rainwater will not flow over the surface towards the sea, but will penetrate the pores. And that’s how caves come into being. This process of dissolution created the wonderful chambers we can now walk and crawl around in.

soda straw stalactite

When there’s a hole it needs to be fixed

The moment the Hato caves became a little dryer, after the sea level dropped during another glacial period and the island was pushed up a bit more, the process seemingly reversed. Instead of dissolution, calcium carbonate crystals were deposited within the chambers in dramatic shapes, called speleothems. It still has everything to do with water, though. Rainwater still penetrated the pores and tunnels within the terrace, and was still dissolving the calcium carbonate along its track. But the moment a drop of water emerged at the ceiling of an underground chamber, a little carbon dioxide escaped from the solution, so the water became over-saturated with calcium carbonate. A tiny bit of this stayed behind as small crystals. At first a so called soda straw stalactite formed, but over time this fragile straw developed into one of the majestic dripstone pillars the cave is famous for.

dripstone pillar - the marriage of a stalactite and a stalagmite

The Hato caves, quite special

The Hato cave system is not the only cave system on the Island. What is special about it, though, is the fact that it has been carved out into the higher terrace, while most other caves are younger middle terrace members. The Hato caves are also multi-level, indicating variable groundwater levels during their formation. The others are all single level caves, hence much simpler in structure.

A visit to the Hato caves will introduce you to the wondrous speleothems. Look for those soda straws, there are still a few in action, as well as the stalactites (hanging down from the ceiling), stalagmites (the other half of one drip-trajectory, on the floor), massive flowstones which formed while sheets of water flowed down from above, and the fusion of the stalactites and stalagmites into monumental pillars.

Next month we’ll take a look at some of the activities the park has to offer. Stay tuned!

Leon Pors

About Leon Pors