How Caves Form
CAVES, KARST and GROUNDWATER
WHAT IS KARST?
A map showing the density of caves by county within the United States reveals that, for the most part, caves are located within specific areas or regions. Many of these regions share some similarities in the type of rock within the area as well as how water flows through the region. These regions where caves form often develop a "look" to the landscape which actually has a name. The name for such landscapes is karst.
Karst landscapes may be characterized by the presence of sinkholes/dolines, disappearing streams, closed basins, caves, and pits. Another important factor for a karst landscape is subterranean drainage. There need not be limestone within a karst landscape, but the rock generally does need to be soluble. Within the United States, 20% of the land surface is karst.
KARST & AQUIFERS
Another common feature of karst landscapes is the presence of an aquifer. Think of an aquifer like a huge bucket of water lying under the ground. This bucket is filled with water. Springs on the surface would be similar to holes in the bucket - water flows out wherever the water table intersects the surface. Wells would be similar to giant drinking straws stuck into the bucket. By sucking on the straws (pumping on the wells), we draw water out of the aquifer. The water leaving the aquifer will lower the water table.
So how does the water get into the aquifer? Gravity is doing a good bit of the work. As rain falls on the surface, the water will seep into the ground. Some of that water will run off on the surface to fill up creeks, rivers, ponds, or lakes. Some of the water will soak into the soil but will be utilized by plants or animals and will not get very far underground. The rest of the water will get past the surface and past the plants and start making its way deeper into the earth.
While water is passing through the rock, it will also move through two distinct areas or zones within the aquifer. The first zone the water passes through is called the zone of aeration. This is the area above the water table where the majority of pores or spaces within the rock are filled mostly with air. The next zone where the water passes through is called the zone of saturation. This is the area under the water table where the rock is completely saturated with water. Between these two layers is the capillary fringe. A capillary is similar to a very small drinking straw. The capillary fringe is the boundary where the attractive forces between the molecules of water and rock will cause the rock to "suck" up water into the "straw", thus forming the capillary fringe.
What does this have to do with caves? The cave passages containing air would be within the zone of aeration. The zone of saturation falls somewhere below these passages. Cave forming processes may occur within any of these zones, wherever water has been flowing. But there is some chemistry involved in how caves form.
While it is true that some caves can be formed by the action of waves (sea caves) or even lava (lava tubes), we will deal with those caves formed by water dissolving rock or solution caves. The term dissolution refers to the chemical weathering or "dissolving" of limestone or other soluble rocks by water. Water, by itself and with enough time, could eventually carve out a small opening in rock. For a large cave system to form, however, water needs some additional help, which it gets from acids within the water. We will discuss two types of acids, carbonic and sulfuric, which are common in some types of groundwater. We will then discuss how these acids form caves.
CARBONIC ACID, SULFURIC ACID & CAVES
As rain falls through the atmosphere, and especially as it moves through the soil, the water mixes with carbon dioxide gas to create a weak solution of carbonic acid. This acid is many times more efficient than water at dissolving rock. An even more powerful acid to dissolve rock is sulfuric acid. This acid can be formed either by water coming into contact with rocks containing sulfide minerals, or by bacteria within the groundwater which break down rock as a normal process of their life cycle. But even a weak solution of carbonic acid, or the much stronger sulfuric acid, will not make a cave unless it can get underground. One way water moves deeper into the earth is through cracks or fractures which geologists call joints or faults. These fractures allow water to move further into the rock. The fractures widened over time as the acid dissolved away the soluble rock. What once were small cracks eventually widened into larger voids or cavities. Some of these cavities widened into larger rooms or caves. And some of the caves eventually connected with other caves to form caverns.
Up to this point, we've discussed karst, aquifers, and acids, all of which contribute to the formation of a cave or cavern. But what about the cave formations - the stalactites, stalagmites, soda straws, fried eggs, bacon, flowstone, and many other types of speleothems? These formations are created by a process which involves three steps.
First, and as explained above with carbonic acid and caves, as rain passes through the soil layers, it will mix with carbon dioxide gas. The carbon dioxide gets in the soil from decaying plant and animal remains. A weak solution of carbonic acid is formed from this mixing. As the mixture moves toward the cave, the acids in the water will dissolve minerals from the rock through which it passes. These minerals are carried within the solution and into the cave.
A great deal of a cave's chemistry is driven by equilibrium - all things must be equal. When they aren't equal, then that's when things happen. In the case of water, once the solution reaches any cavity filled with air, a number of different things can happen. One thing is that the amount of carbon dioxide carried by the water will often be higher than the air within the cave. If this is the case, the water's carbon dioxide will be released to the cave air in an effort to equilibrate between the two. When the carbon dioxide gets released, the solution must also deposit some of the dissolved minerals. This mineral deposition is what forms all of the different cave formations.
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