Caves and Sinkholes
The term “cave” refers to a natural opening, usually in rocks, that is large enough for human entry. There are several different types of caves: volcanic, glacier, sea and solution. Caves are natural openings in the ground extending beyond the zone of light. Occurring in a wide variety of rock types and caused by widely differing geological processes, caves range in size from single small rooms to interconnecting passages many miles long. The scientific study of caves is called speleology. It is a composite science based on geology, hydrology, biology, and archaeology.
Caves have been natural attractions since prehistoric times. Much of the evidence of early man’s interest has been discovered in caves scattered throughout the world. Fragments of skeletons of some of the earliest manlike creatures (Australopithecines) have been discovered in cave deposits in South Africa, and the first evidence of primitive Neanderthal Man was found in a cave in the Neander Valley of Germany. The Cro-Magnon man of Europe created remarkable murals on the walls of caves in southern France and northern Spain where they took refuge more than 10,000 years ago during the chill of the ice age.
Cave formation
Caves begins when rainwater absorbs carbon dioxide as it falls through the atmosphere. Rain water must have carbon dioxide to become acidic. It must be acidic to chemically react to the limestone bedrock. Rainwater is then absorbed by the soil in the ground. As rainwater comes through the soil it absorbs more carbon dioxide that is being produced by plants. When the rock is limestone or dolomite caves can form. The acidic water reacts chemically with limestone and slowly a larger and larger space will form. This happens because the rocks are made of calcium carbonate. This is what you call chemical erosion.
As acidic water passes through fractures, crevices, and cavities it dissolves the limestone very slowly, and it enlarges the network of passageways. Most solution caves require more than 100,000 years to grow large enough for humans to be able to pass through. Folds and faults within the earth’s crust, are immediately obvious in caves. Cave walls, ceiling, and floor can show bends, fractures, and folds in rock layers. Cave passages cut through the rock layers and make rock identification easy. As the space becomes larger water can flow through. As water flows through erosion washes away rock and sand. This is what makes a cave larger and forms an underground stream. Finally over hundreds of thousands of years or even millions of years the cave is formed.
Types of Caves
A simple classification of caves includes four main types and several other relatively less important types.
Lava caves are tunnels or tubes in lava formed when the outer surface of a lava flow cools and hardens while the molten lava within continues to flow and eventually drains out through the newly formed tube.
Sea caves are formed by the constant action of waves which attacks the weaker portions of rocks lining the shores of oceans and large lakes. Such caves testify to the enormous pressures exerted by waves and to the corrosive power of waves carrying sand and gravel.
Glacier caves are formed by melting water that excavates drainage tunnels through the ice. Of entirely different origin and not to be included in the category of glacier caves are so-called "ice caves," which usually are either solution caves or lava caves within which ice forms and persists through all or most of the year.
Solution caves are formed in carbonate and sulfate rocks such as limestone, dolomite, marble, and gypsum by the action of slowly moving ground water that dissolves the rock to form tunnels, irregular passages, and even large caverns along joints and bedding planes. Most of the caves in the world-as well as the largest-are of this type.
About 10% of the earth's land surface consists of soluble limestone. When limestone interacts with underground water, the water dissolves the limestone to form karst topography - a series of caves and underground channels. Karst topography is named for the Kras plateau region of eastern Italy and western Slovenia (Kras is Karst in German for "barren land").
The underground water of karst topography carves an impressive series of channels and caves that are susceptible to collapse from the surface. When enough limestone is eroded from underground, a sinkhole (also called a doline) may develop. Sinkholes are depressions that form when a portion of the lithosphere below is eroded away. Sinkholes can range in size from a few feet or meters to over 100 meters deep. They've been known to "swallow" cars, homes, businesses, and other structures. Sinkholes are common in places like Florida where they're often caused by the loss of groundwater from pumping. A sinkhole can even collapse through the roof of an underground cavern and form what's known as a collapse sinkhole, which can become a portal into a deep underground cavern.
1. What are the properties of a cave?
2. Explain how weathering plays a role in the development of caves.
3. In what ways have caves expanded our understanding of the earth?
4. Karst topography creates what type of caves and why is this so?
5. How are sinkholes and caves related?
6. What are some reasons that you believe that earlier humans being sought refuge in caves?