Station: [45] Dripstone

The formation of dripstones – or, more generally, of cave sinter – reverses the karstification process. The lime had dissolved in the water, but in certain conditions, the minerals once again precipitate and form new layers of rock.

This is how it happens: the solubility of the lime is strongly dependent on the carbon dioxide content of the water. And topsoil contains very high levels of carbon dioxide. So in places where the water from rain or other precipitation has to percolate through layers of soil before it reaches the rock, it can absorb very high levels of lime.

Subsequently, the water may enter a cavity where the carbon dioxide concentration is roughly the same as in the outside air. At that point, CO2 is released from the water into the air inside the cave. As a result the concentration of lime in the water becomes too high. The lime precipitates, and equilibrium is re-established. These processes are diffusion-controlled – which means they’re slow. The calcium and carbonate ions have enough time to form crystals and hence dripstones – stalagmites and stalactites.

So dripstone deposits in caves tell us that when they were being formed, there was vegetation cover above, through which the water seeped, absorbing carbon dioxide on its way into the cave.

Dripstones are not only beautiful to look at. Science is able to determine their age fairly precisely based on isotope distributions, tell us when they were formed and how long they’ve been growing. For example, take the two dripstones in the display case on the right, which have been cut open. The thin sections display black discolouration. Those are layers of soot from fires people lit in the cave during Celtic times. At some point, the people disappeared, and the stalactites and stalagmites simply continued to grow.