Geoscience Resources Multiple choice
Geologic Factors in the Formation of the Stone Forest

When rain falls, some of the water runs off, some evaporates, and the remainder soaks into the ground. The amount of water that follows each of these paths varies greatly as a function of factors such as:

  1. slope angle,
  2. the nature of the surface material,
  3. intensity of rainfall,
  4. and the type and amount of vegetation

Porosity is the percentage of open space in the rock or surface material. Permeability is a measure of the material's ability to transmit fluids. If the pores are very small or if they are not connected to form a network, the material will have a low primary permeability. If the material is fractured (broken) the permeability will be increased.

Some of the water that soaks into the ground adheres to particles in the soil - the belt of soil moisture. Water not held in this belt migrates downward until it reaches a zone in which the pore spaces in the material are completely filled with water - the zone of saturation or the vadose zone. Pores in the overlying zone of aeration are filled with a mixture of air and water. The contact between these two zones is the water table.

Rarely is the water table a flat, planar surface. The water table tends to rise beneath hills and to subside beneath a valley. If water is withdrawn from the zone of saturation at a greater rate than it is replenished, the water table will migrate downward with time. If, on the other hand, the rate of replenishment exceeds the rate of removal, the water table will migrate upwards. If the water table intersects the surface of the Earth, a spring will form.

A small quantity of the water found beneath the water table is juvenile water that has migrated upward from deeper within the Earth. Most, however, originates from the downward migration of rain water.

Tarbuk and Lutgens (1998) estimate that approximately 14% of the fresh water of the hydrosphere is stored as water beneath the Earth's surface.

Minerals vary widely in their chemical response to groundwater. Calcite, for example, is highly soluble in a weak acid such as H2CO3 - carbonic acid. Carbonic acid can form by a reaction between water and carbon dioxide. Rainwater is a weak acid (pH ~ 6). Thus, limestones which consist primarily of calcite (CaCO3) are vulnerable to chemical attack by groundwater. Dolomite (CaMg(CO3)2, on the other hand are considerably less soluble.

Are You Prepared To Continue?

    A familiarity with topics such as acids and bases, chemical equilibrium and the solution of a solid material is really required to understand the genesis of the Stone Forest and related features. Take a few minutes for a self-quiz which is designed to see whether you should proceed.

    1. Chemical equilibrium is static. That is, once a system reaches equilibrium, the processes involved cease and nothing will disturb the equilibrium condition.

      True
      False

    2. Calcium Carbonate (the mineral calcite) __________ in an acidic solution.

      goes into solution
      precipitates

    3. Calcium Carbonate goes into solution in sea water.

      True
      False

    4. Carbon dioxide (CO2) reacts with _______ to form carbonic acid.

      water
      air
      silicates - rocks

    5. Any process that reduces the amount of CO2 in the environment favor the precipitation of calcite.

      True
      False

    6. Consider the system H2O + CO2 = H2CO3. The system is at equilibrium. You add additional CO2. What do you predict will happen?

      the system will not accept additional CO2 because it is in equilibrium
      the system will react to use up enough CO2 to restore equilibrium
      the system will react to restore equilibrium by forming more H2 to balance the additional CO2

An understanding of the solution and precipitation of calcite (and other, related carbonate minerals) is a key to understanding the formation of the Stone Forest and you should spend some time reviewing this material before going further. This is especially true if you missed more than 2 of the questions on the self-quiz.

Caverns

Most caverns are created at or just below the water table in the zone of saturation in limestones. If the water table is stable, large openings can be created by dissolving the limestone. If the water table drops, the area of active cave formation will move lower into the bedrock and the upper openings are abandoned as they are now in the zone of aeration. Water dripping through these abandoned passages may deposit CaCO3 in various forms collectively referred to as speleothems.

Time did not permit a visit to one of the numerous caves in the Stone Forest region so a few pictures from the Shenandoah Caverns in Virginia will have to suffice as a brief introduction to cave formations.

Shenandoah Caverns, VirginiaCave Formations

Ribbon or Bacon RockFlow Stone

The Virtual Cave offers you an opportunity to explore caves from the "comfort of your keyboard". If your have a high resolution monitor (1024 x 768 or higher) you can begin with their Image Map to begin your exploration. Otherwise, you will have to hunt and peck around the list that the document provides.

Karst Topography


Sinkhole Lakes in Central Florida
A topography consisting of numerous depressions called sinkholes (depressiona produced in a region where soluble rock has been removed by groundwater) is called Karst. The name is derived from the Kras Plateau in Slovenia where this type of topography is particularly well developed. Typically, Karst develops in areas of limestone bedrock although any soluble material (such as gypsum or anhydrite [calcium sulphate] could support the development of this type of surface.

While some sinkholes form slowly by solution of the underlying carbonate rock, other sinkholes develop as a result of the collapse of surface or near-surface material. There are two basic types of sinkhole-forming collapses:

  1. bedrock collapses, and
  2. regolith or soil zone collapses.

Imagine a situation in which a cavern begins to form at depth. Continued percolation of groundwater through the network of pore spaces or along fractures enlarges the cavern upwards. Eventually the roof of the cavern approaches the surface and a sinkhole may form. Watch the following animation to see one way in which a sinkhole can form.

Science Links
Return To Table of Contents