Cavern Geology Lesson Plan 1: What is a Cavern

OBJECTIVE

Students will learn to identify the different types of caves.

BACKGROUND INFORMATION

Cave or Cavern?
Is there a difference between a cave and a cavern? This is a frequently asked question, and many people use the terms interchangeably. However, there is a difference. A cave is any cavity in the ground that is large enough that some portion of it will not receive direct sunlight. There are many types of caves (discussed in this lesson plan). A cavern is a specific type of cave, naturally formed in soluble rock with the ability to grow speleothems. So, although a cavern can accurately be called a cave (since it is a type of cave), all caves cannot be called caverns.

Caves
Caves can be classified into two main categories known as primary and secondary. This classification is based on their origin. Primary caves are developed as the host rock is solidifying. Examples of primary caves include lava tubes and coral caves (descriptions follow). Secondary caves are carved out of the host rock after it has been deposited or consolidated. Most caves fall in the secondary category. However, some primary caves may later be enlarged by the forces associated with secondary cave development.

Listed here are the main types of caves, and how they are formed:

Coral Caves
When colonies of coral in shallow water expand and unite, they form lacy or bulbous walls around an open area. When the shoreline is pushed up or the sea level falls, the cave is exposed. Waves and wind erode the coral, enlarging the cave, sometimes even destroying it.

Eolian Caves
Also known as wind caves. Wind erodes away the weak areas in sandstone cliffs.

Glacier Caves
Long tunnels form near the snouts of glaciers between glacial ice and the underlying bedrock. Water from the surface drains down through crevasses in the glacier. It enlarges the crevasses and melts away the ice at the base of the glacier.

Ice Caves
There are two types of ice caves:

The first is carved out of glaciers or snowfields by water and/or wind.
The second is a rock cavity containing ice formations. As moisture in a cave is frozen it clings to the walls and continues to build up. When slight melting occurs or water enters the cave, it runs along the walls creating formations similar to calcite speleothems.

Volcanic Caves
There are various forms of volcanic caves. They are all created from flowing lava and the effects of volcanic gases. Categories of volcanic caves include lava tubes, pressure-ridge caves, spatter cone chambers and blister caves.

Sea Caves
Waves eroding away weak areas along sea cliffs create these caves which can be any size from crevices to large chambers.

Talus Caves
Boulders pile up leaving passages underneath and between them.

Tectonic Caves
A massive movement of bedrock separates rocks along joints or fractures. The cave created in this fashion is usually a small, high, narrow fissure consisting of a single passage. The ceiling is often a flat section of rock that did not move, or moved in a different direction. Massive, brittle rocks such as sandstone and granite are the best rocks for tectonic caves; however, they can also occur in basalt and limestone.

Solution Caves
This is the category of caves that is classified as caverns. They are formed by the dissolution of soluble rocks such as limestone (calcium carbonate), dolomite (calcium magnesium carbonate), gypsum (calcium sulfate dihydrate) and salt (halite). When researching caves, the dissolution of limestone is usually the example given. For the purpose of this lesson we will also use limestone as our host rock. However, the conditions are basically the same for other soluble rocks.

Cavern Genesis

Many people think that caverns are created by limestone bedrock being dissolved slowly by water. But limestone is not much more soluble than glass in water. So water alone could not form caverns. However, when ground-water contains an acid in solution it can and does make caverns.

Older traditional theories identified dissolved carbon dioxide as the probable cavern forming acid. When carbon dioxide dissolves into water it forms a weak acid known as carbonic acid. It is the same acid that gives carbonated beverages the bubbles and the pleasant “bite” on the tongue. It is however a very weak acid and would not normally be able to explain most of the worlds caverns. Another gas, hydrogen sulfide, seems much better suited and is gaining wide spread acceptance as the probable primary source of the cavern development acid.

Hydrogen sulfide is available from many natural sources. It is generated by the anaerobic (oxygen free) decomposition of organic materials such as leaves, oil, coal and other natural deposits. It is also formed by the oxidation of sulfide deposits such as iron pyrite when it is exposed to weathering. It is produced in large quantities during volcanic actions and from geo-thermal springs and features.

When an underground source of hydrogen sulfide wells up and mixes with down-percolating oxygenated water from the surface the mixture of water, hydrogen sulfide and oxygen forms sulfuric acid.

Sulfuric acid has no trouble dissolving limestones, (common limestone, dolomite and marble), creating caverns. At first it slowly enlarges the cracks and joints of the bedrock and over time through a combination of dissolution and breakdown of weakened areas caverns are formed. The more hydrogen sulfide the larger the cavern. Some form very slowly over long periods of time with weak but long lasting sources of hydrogen sulfide while others form relatively quickly and sometimes gigantically with strong but short lived sources. Most, if not all, major cavern systems form this way. It may be possible that a few systems were formed by, or at least modified, over time by the action of weak carbonic acid while still others were formed by corrosive underground acidic brines but the large majority were formed by sulfuric acid

Carbonic acid does play a major part in how caverns look. It is mostly responsible for the “decorations,” such as stalactites, stalagmites and flowstones that are formed which make caverns so delightful to look at. The solution of carbonic acid and limestone (calcium carbonate) forms calcium bicarbonate which when reaching the cavern interior can deposit calcite (calcium carbonate) that makes the formations within the cavern, and can even entirely fill the cavern.

Cavern Geography

Because the dissolution of limestone takes place under the surface in the bedrock, a cavern may not have an entrance. Most entrances are formed after a cavern is formed. Entrances may be created by a valley enlarging and exposing a section of the cavern on the side of a hill, the collapse of a sinkhole, or the intrusion of man’s developments into the surface.

Limestone caverns are commonly found in karst terrain. Karst comes from the Slav word krs meaning crag or stone. The word became a term to describe the rocky region along the Dalmatian coast of the Adriatic Sea. Karst terrain is generally characterized by bare rocky ground, sinkholes, underground rivers and the absence of surface streams and lakes. It should be noted that a karst area does not necessarily contain a lot of caves and at the same time an area with extensive caves may have little karst terrain.

Experiments & Activities

Grades K – 4 “Dissolution Experiment”

Materials

  • 27 or more sugar cubes
  • toothpicks
  • A clear fish tank with straight sides
  • 1 spray bottle with water
  • 2 lbs. modeling clay

Procedure
Stack the sugar cubes against the inside of the fish tank. Make the structure at least four cubes high and wide, and three deep. For a more realistic look, a few columns may have one or two more cubes. Explain how the sugar cubes represent large areas of limestone and the spaces between the cubes represent the natural cracks and fissures in the limestone through which water travels.
Cover the sugar structure with approximately 1/8-inch of clay. Make sure there are no gaps. The clay represents the surface dirt.

Using the toothpick, poke several holes through the clay into the sugar. This will allow water to seep through.

If needed, you can explain dissolvability by dissolving salt or sugar in a glass of warm water.

Grades 5 – 8 “Dissolution Experiment”

Materials

  • white vinegar
  • shallow pan
  • eye droppers
  • samples of rocks, one of which is limestone.

Procedure

Explain that it is sometimes possible to tell what minerals rocks are composed of by their physical properties such as appearance, grain size, hardness, texture, color, and whether they can be dissolved.
Remind the students of the dissolution process of limestone explained in “Background Information.” Explain that vinegar is an acid stronger than carbonic acid, but weaker than sulfuric acid.

Place the rock samples in the shallow pan. Using the eye dropper, start coating the rocks with the vinegar. This is known as the “Bubble Test.” The limestone piece will bubble as it dissolves while the other rock(s) will not. (This is a vivid demonstration, be prepared to allow each student enough time to try it).

Ask the students to identify which rock is limestone.

Grades 9 – 12 “Dissolution Experiment”

Materials

  • two beakers
  • 20% solution of hydrochloric acid (also known as muriatic acid)
  • a sample of limestone
  • a sample of non-carbonate rock

Procedure

Partially fill two beakers with the acid.
Weigh and measure each rock.
Place them in separate beakers and watch what happens. The limestone will fizz and the non-carbonate rock will not react.
Check the beakers every 15 minutes. (The dissolving action will stop if either the acid or the calcite is no longer available to react. If this happens, either add more acid or break the limestone to expose more calcite.) In about one hour the limestone sample will be noticeably smaller.
After drying, the samples can be weighed and measured again to determine the changes.
Note: appropriate laboratory safety procedures should be followed when working with hydrochloric acid.