Underground Rivers and Haystack Hills
Jeff ClarkLawrence University
Location
Various but focused on Puerto Rico and Wisconsin
UTM coordinates and datum: none
Setting
Climate Setting: Tropical
Tectonic setting: Island Arc
Type: Process
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Description
We start by gearing up for the expedition that awaits us. We don the standard gear - hiking boots, long pants and long sleeve shirts for protection against thorny vegetation and the craggy limestone - and then we strap on climbing harnesses and helmets preparing for the forthcoming descent. We snap the life jackets shut, double check all of our straps and make sure that our water proof headlamps are functioning. We then hike a kilometer through dense tropical vegetation until we reach the river gorge. All around us are dome shaped hills, some of which have deep conical depressions in the middle. From here we rappel 40 m down the near vertical limestone cliffs into the river below. As we float along the river, it seems to disappear around the bend. As we get closer, we see that river literally runs into the rock just ahead and we follow (insert picture). Once inside the cave we pull ourselves ashore and turn on our headlamps. As we look around we see an underground world replete with dozens of small side passages of pure white limestone polished by running water, and impressive rooms of stalactites and stalagmites.
Our route to the Camuy Caves in north central Puerto Rico was unconventional, but exhilarating. How did these caves and this unusual landscape form? Why to rivers here disappear underground and then reappear kilometers away? This type of landscape is called karst. Karst forms by a chemical weathering process called dissolution, which is essentially the same process by which salt dissolves into water. In this case limestone, which is calcium carbonate (CaCO3), is "readily" dissolved by rainwater. Note that rate of dissolution is very slow compared to salt in water, but quite fast relative to silicate rocks which are the most common on Earth's surface. Rainwater itself is slightly acidic (pH~5.5) because of atmospheric CO2 dissolving into rainwater and this natural acidity helps dissolve the rocks. Over millions of years rainwater eats away at the limestone through this process of chemical weathering, which leads to impressive landscapes and beautiful cave interiors.
Water will follow any cracks or joints in the bedrocks and this is where chemical weathering is focused. A regional system of joints over time will yield a fairly regular field of dome shaped hills (create a diagram). Some of the most impressive examples of this topography can be found in SE Asia and has been famously depicted by artists for thousands of years (copyright picture?). Both SE Asia and Puerto Rico are in tropical climates, which are conducive to the development of karst due to the abundant rainfall and warmer temperatures (as with many chemical reactions, dissolution grinds to a halt as the temperatures approach freezing). Nonetheless, even temperate settings like Wisconsin have some karst features. There too you can find small caves, sinkholes and disappearing streams. Groundwater readily moves through the rock fractures that have been widened by dissolution over the years. This creates a complex groundwater system that can provide abundant water, but is also highly susceptible to contamination by agriculture, landfills, and septic systems.
Even in these temperate caves you can find the spelothems (cave deposits like stalactites and stalagmites) that characterize world-famous cave systems like Carlsbad Caverns. These interior features form as water percolates through the rock, dissolves some of the limestone, and carries the ions in solution. When the water reaches air space (like a cave) the water can evaporate and thereby increase the concentration of the ions in the solution. If enough water evaporates then some of the calcium carbonate will precipitate out and will gradually build a new calcium carbonate structure - perhaps a stalactite. Some of the water can drip down to the floor of the cave where again evaporation can take place and ultimately calcium carbonate can precipitate - perhaps forming a stalagmite. The process operating over thousands of years can build limestone columns that run floor to ceiling. Stalactites and stalagmites are not only hauntingly beautifully, they are also sensitive indicators of past climate over the past 500,000 years. Because they are built gradually over time and because they incorporate isotopes of oxygen and carbon that were once in the atmosphere and overlying soils, speleothems provide us with a relatively high resolution, near continuous record of environmental conditions going back hundreds of thousands of years.