Caves are natural openings in the earth that are large enough for human exploration. They can form in different types of rock and by different processes, but the most common and largest caves are those formed by the chemical reaction between groundwater and limestone or dolomite. These caves are called solution caves, and they are part of the karst terrain, which is characterized by a rough and jumbled landscape of bare bedrock, sinkholes, and springs.
Solution caves develop through several stages, depending on the movement and chemistry of water, the structure and composition of rock, and the erosion and uplift of the surface. One of these stages is called the decoration stage, which is when various features or formations are created inside the cave by the precipitation of minerals from water. These features are collectively called speleothems, and they include stalactites, stalagmites, columns, flowstone, draperies, helictites, cave pearls, and many others.
How do speleothems form?
Speleothems form by similar processes in most solution caves. The water that seeps or flows into the cave has dissolved carbon dioxide from the soil or atmosphere, which makes it acidic. As a weak acid, the water can dissolve some of the limestone or dolomite rock as it passes through cracks and pores. This water becomes saturated with calcium carbonate, which is the main mineral in these rocks.
When this water reaches the air-filled cave, it loses some of its carbon dioxide and becomes less acidic. As a result, it cannot hold as much dissolved calcium carbonate, and some of it is deposited on the cave walls, ceilings, or floors as calcite crystals. The shape and size of these crystals depend on various factors, such as the rate of water flow, the amount of evaporation, the temperature, the pH, and the presence of other minerals or organic matter.
Some of the most common speleothems are stalactites and stalagmites, which are icicle-like formations that hang from the ceiling or grow from the floor of the cave. Stalactites form when water drips from a small hole or crack in the ceiling and leaves behind a ring of calcite around the edge. As more water drips, more rings are added, forming a hollow tube that grows downward. Stalagmites form when water drops from a stalactite or from the ceiling directly onto the floor and builds up a cone-shaped mound of calcite. Sometimes stalactites and stalagmites meet and form a column that connects the ceiling and floor.
Other speleothems have different shapes and names, such as:
- Flowstone: A smooth or wavy sheet of calcite that covers large areas of walls or floors, formed by thin films or streams of water flowing over them.
- Draperies: Thin sheets of calcite that hang from the ceiling or walls like curtains, formed by water flowing along an inclined surface.
- Helictites: Twisted or curved formations that defy gravity, formed by water seeping through tiny pores or channels in the calcite.
- Cave pearls: Small spherical formations that look like pearls, formed by water dripping onto grains of sand or other nuclei and coating them with layers of calcite.
- Frostwork: Delicate needle-like growths of calcite or aragonite (a related mineral), formed by evaporation in areas with high airflow.
- Popcorn: Small knobby growths of calcite on walls or ceilings, formed by water seeping out of the rock or splashing from drips.
- Dogtooth spar: Spear-shaped crystals of calcite that line small pockets or cavities in the rock.
The term “decoration stage” of cave development is related to all but one of the following terms:
A) Carbon dioxide and calcite.
B) Speleothems and stalactites.
C) Frostwork and popcorn.
D) Boxwork and dogtooth spar.
The correct answer is D) Boxwork and dogtooth spar. These two features are not related to the decoration stage because they do not form by precipitation from water. Instead, they form by dissolution or erosion of rock.
Boxwork is a rare feature that makes some caves unique. It consists of thin blades or fins of calcite that project from the walls or ceilings in a honeycomb pattern. Boxwork forms when cracks in limestone are filled with calcite before the cave is formed. Later, when groundwater dissolves the surrounding limestone, it leaves behind the calcite blades as a network of boxes.
Dogtooth spar is a common feature in some caves, especially those in quartzite or granite rocks. It consists of large crystals of calcite or quartz that fill fractures or vugs (small cavities) in the rock. Dogtooth spar forms when groundwater rich in calcium or silica flows through the rock and deposits the minerals in the cracks or vugs. The crystals grow outward from the walls, forming sharp points that resemble dog teeth.
The decoration stage of cave development is when various speleothems or cave formations are created by the precipitation of minerals from water. The most common mineral is calcite, which forms stalactites, stalagmites, columns, flowstone, draperies, helictites, cave pearls, frostwork, and popcorn. Other minerals, such as aragonite, gypsum, or iron oxides, can also form speleothems with different colors or shapes. Some features that are not related to the decoration stage are boxwork and dogtooth spar, which form by dissolution or erosion of rock.
The decoration stage is one of the most fascinating and beautiful aspects of cave exploration. It reveals the diversity and complexity of nature’s artistry and the history and chemistry of cave formation. According to Britannica, there are more than 17,000 known caves in the world, each with its own unique speleothems and secrets.