There are several different types of caves, the most common being solution caves. These caves are formed by the dissolving of the rock along and adjacent to joints (fractures), faults, and layers in the rock. The processes involved are both chemical corrosion and physical erosion. Solution caves are most often found in rock types such as limestone, marble, dolomite (both, close relatives of limestone), gypsum and halite, and are associated with karst landscapes.
The reason these rocks dissolve is because rainwater is acidic and when it mixes with the soil it becomes undersaturated. Even though the acid is weak, it is strong enough to dissolve the limestone over extended periods of time. As we know from wells, there is a lot of water underground, and there are several ways that acids can form in that water. While weak acids are by far the most common, but there are exceptions to that rule. Limestone caves are found in dozens of National Park Units including Oregon Caves National Monument and Preserve (Oregon), Big Bend National Park (Texas), and the Chesapeake and Ohio Canal National Historic Park (Washington DC and Maryland).
Gypsum and halite dissolve much more easily than limestone. They are usually found in deserts or drier environments because wet climates cause the rock to erode away quickly. Small caves in these rock types can be found in some of our desert national parks such as Death Valley National Park (California and Nevads) and Mojave National Park and Preserve (California). Caves in dolomite often appear very similar to limestone caves and limestone and dolomite are often found near each other or are even layered together like cake and icing. Caves in dolomite are common at Ozark National Scenic Riverways (Missouri) in the Ozarks. Marble is the metamorphosed version of limestone. Metamorphosed rock is rock that has undergone additional heat and pressure deep underground to change it into a different type of rock, which in this case turns limestone into marble. Chemically, marble is essentially the same as limestone and can be dissolved in the same way. Marble caves often have beautiful bands and patterns in the cave walls. Marble caves are found in Sequoia & Kings Canyon National Parks (California) and Yosemite National Park (California).
Weathering and Erosion
When caves form the acid that makes them is usually carbonic acid. It is a common and very weak acid. Carbonic acid is found in sodas, beer, and all carbonated beverages. This acid forms when carbon dioxide (made of one Carbon and two Oxygen atoms and known as CO2) dissolves in water. CO2 is everywhere. It’s part of our atmosphere and soil and is constantly released into the environment by animals, including humans, when we exhale. It mixes with rainfall and snow melt most often in the soil. There, due to high pressures that can form within the soil, a lot of CO2 ends up dissolved in water to make carbonic acid. The acidified water will keep percolating downward until it reaches the limestone.
Solution caves form when this acidified water has a way into the rock. This is usually through cracks, fractures, weak spots, or open places within the limestone. In a small crack the water cannot move far. It sticks to the rock wall of the crack just like it sticks to the underside of your arm in the shower. Once the cracks enlarge and is a bit bigger, water can flow through. This brings in more acidified water and the cave begins to grow more quickly. Active streams in caves make passageways and continue to enlarge them. Bigger underground streams or rivers usually make bigger cave passages. Streams also bring in sediments that can abrade and scour the soft limestone removing more rock and making the cave larger again. Eventually the stream exits the cave and returns to the surface as a spring. Many caves end where the river that made them flows back again onto the surface. Cave springs are important for human use. One such spring at Lincoln’s Birthplace National Historic Site (Kentucky) provided Honest Abe with his very first drink of water.
Water can enter a cave at one point or at multiple points. When the water enters at one location this is usually as a sinking stream, where an entire creek or stream diverts underground and into a cave passage. Sometimes there are entrances into the cave system where the water goes in. But other times the water can get into the cave through small cracks as mentioned earlier, but where there are no holes large enough for a person to enter.
When the water enters at multiple locations this is usually through sinkholes. These are circular depressions in the earth that can vary in size from a few feet to a few miles across. Any rainfall, snow melt, or water in a sinkhole drains out from the bottom of the sink and into cave passages below. Sinkholes enlarge as water carries sediments and dissolved rock downward. Sometimes caves with water from sinkholes form multiple passages that join together like branches on tree. This pattern is also seen in many surface streams.
Collectively caves, sinking streams, sinkholes and other such features form what is called karst. Karst is a type of landscape and topography formed in areas with limestone or the other soluble rocks.
Cave Growth and Development
Geologists have determined that cave passages with active streams can grow a maximum of about 1 mm per year. While that does not sound like much growth, in a thousand years the cave passage is a meter or more than 3 feet wide. In two thousand years it will be approximately 6 and ½ feet in diameter. Many caves are hundreds of thousands or even millions of years old. In that long geologic lifespan, a lot of cave passages can form.
Steep and Deep
An important factor in a cave's development is the gradient or the vertical distance from where the water enters the cave to where a spring returns it to the surface. The steepest caves are vertical pits requiring the use of ropes to descend and to return to the surface. Often waterfalls cascade down into the pits. These are common in many regions and underground waterfalls can be found in NPS units such as Cumberland Gap National Historical Park (KY, TN, and VA), Ozark National Scenic Riverways (Missouri), Mammoth Cave National Park (Kentucky), Oregon Caves National Monument & Preserve (Oregon), and Sequoia & Kings Canyon National Parks (California). Sometimes a cave is very steep and is nothing but a series of pits and falls. In other caves, there may be a pit or two and then the cave becomes more horizontal and less steep. Many other caves have no pits at all and are horizontal. They can be traversed by walking, crawling and some occasional scrambling or rock hopping.
Flat and Flooded
The flattest and most low-gradient solution caves of all are those filled with water. These caves are found in river valleys and coastal areas and adjacent lowlands and exploration requires cave diving. Cave diving is a popular, but sometimes dangerous sport. Divers often find caves by swimming underwater and upstream of springs. Water-filled cave passages can be very extensive. The second longest cave in the world is found in Quintana Roo, Mexico and it is almost entirely filled with water It has been explored and mapped by divers over many decades and using many different entrances into the cave system. Underwater cave passages in our National Park System can be found at Buffalo National River (Arkansas), Jewel Cave National Monument (South Dakota), and in many other parks.
An Underground Labyrinth
Some solution caves are formed as mazes with many junctions and parallel passages on all sides. Maze like passages form when the water making the caves is diverted to new paths. This might happen if there is a rock collapse in the cave or if sediment depositsbuild up to the point that they plug a cave passage. In some instances, a large flood will fill the cave passage, but excess water will force its way into the passage. In these situations, the acidic water will find any weakness in the rock and erode a new route forward for the water. Mazes can add to the length of a cave. Lilburn Cave in Kings Canyon National Park is the longest cave in California at 22 miles. It is notoriously mazy due to big Spring floods from snowmelt. It can take years for cavers to learn routes through this cave due to the hundreds of passage junctions.
Rising Waters—Hypogene Caves
In rare instances, the water that makes a cave does not flow down through the limestone, but rather is rises upward, sometimes from deep within the Earth. Caves formed in this manner are known as "hypogene" caves. Water rising upward from deep underground can have unusual chemistry that includes compounds that form sulfuric acid and much higher concentrations of CO2, making stronger carbonic acid. These compounds can come from nearby volcanic activity or from the sulfur in oil and gas deposits deep underground. Hypogene caves come in all sizes just like regular stream caves, but our national parks are home to some of the longest hypogene caves on Earth. These caves are Jewel Cave in Jewel Cave National Monument (South Dakota) at over 200 miles and Wind and Lechuguilla caves at Wind Cave National Park (South Dakota) and Carlsbad Caverns National Park (New Mexico), respectively. Both are just over 150 miles in mapped length. Due to their deep origins all these caves have unusual minerals and cave formations not usually found in regular stream caves. A good example is sunflower-yellow tyuyamunite, an unusual Uranium and Vanadium mineral found in the walls of Lechuguilla Cave in Carlsbad Caverns National Park (New Mexico).
Find Your Park—Solution Caves
The following is a partial list of National Park Service units that include solution caves: