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.