We should view caves dialectically. They have their frightening aspects, as exploring the underground world is truly an adventure, but they are also just normal geological phenomena, and many people have actually visited caves. I hope that through the following discussion, everyone can gain a preliminary and clear understanding of caves.
Speaking of caves, we must start with karst landforms: Karst is the name of a plateau in the northwest of the former Yugoslavia, an area covered with limestone. Scholars from Yugoslavia were the first to study it, and by international convention, it was named after the place. In China, it is synonymous with karst topography.
So, do karst caves represent all caves? Clearly not—they are just one type of cave. Let’s look at the definition of caves by the International Union of Speleology: natural underground spaces that humans can enter. This distinguishes them from man-made mines, tunnels, etc.
With ongoing research into caves, people have developed a fairly comprehensive classification system. Let’s give a brief introduction.
Classified by the formation of underground space: primary caves and secondary caves
Primary caves are underground spaces that formed at the same time as the surrounding bedrock. Generally, they are not related to groundwater. The main types include lava tubes (commonly known as volcanic caves) and travertine caves. Volcanic caves form due to the temperature difference between the inside and outside of flowing magma—the exterior cools quickly, forming a hard shell, while the central magma remains hot and continues to flow, gradually creating passages.
Secondary caves refer to those where rocks formed first and were later shaped into caves by various external forces. Let’s take a look at what these external forces are and what kinds of caves they form...
Caves formed by dissolution include karst caves and semi-karst caves. Karst caves are very common in China and are currently the main focus of speleological scientific expeditions and research.
Semi-karst caves, such as the conglomerate karst caves found across Sichuan.
Caves formed by mechanical erosion include sea caves created by wave erosion, aeolian caves in arid regions, and seepage caves in soil layers.
Classified by spatial morphology (i.e., orientation): horizontal, inclined, vertical, and composite
Classified by cave scale into four grades:
Small: length less than 50 meters, vertical depth less than 50 meters
Medium: length 50–500 meters, vertical depth 50–200 meters
Large: length 500–5,000 meters, vertical depth 200–1,000 meters
Giant: length greater than 50,000 meters, vertical depth greater than 1,000 meters
Classified by hydrological conditions into dry caves and water caves; water caves are further divided into outflow caves and inflow caves.
The most popular classification method today is by scale. A caver’s achievement is also reflected by the data on the length or depth of the explored cave.
The Formation of Caves:
Water and soluble limestone are the prerequisites for the formation of karst caves. Water dissolves the soluble bedrock into calcium bicarbonate. Year after year, the cave passages expand until they can no longer support the weight of the ceiling, leading to collapse. Collapse only changes the shape of the space, not its size. The collapsed material is then carried away by water flow, which continues to dissolve the bedrock. This cycle repeats, forming a cave system. The chemical reaction formula below illustrates the principle of cave formation, but it is reversible.
CaCO3 + H2O + CO2 → Ca(HCO3) (Rock dissolves to form a cave)
Ca(HCO3) → CaCO3 ↓ + H2O + CO2 ↑ (Precipitation forms stalactites)
Karst caves also go through stages of formation, development, and decline. The formation of collapse sinkholes is a manifestation of cave development.
Classification of Stalactites:
In karst caves, the most direct resource is their stalactites. So, how do we distinguish among the dazzling variety of forms?
Gravity water deposits: These are the most common and numerous types of cave deposits. They also have more specific subcategories...
Drip water deposits: These include soda straws, stalactites, stalagmites, and columns. Soda straws grow downward, are very thin, hollow, with white and transparent walls. Stalactites are the thicker, grown forms of soda straws, often cone-shaped. Stalagmites grow upward from the ground under the effect of dripping water, have no central hole, and form in a stacked cap-like shape. Columns are formed when stalactites and stalagmites join together. Flowing water deposits—those that flow down from the cave ceiling are called flowstone from above, such as draperies and cave curtains. Those that flow down cave walls are called wall flowstone, such as veil stones and cave waterfalls. Those produced from the cave floor are called floor flowstone, among which flowstone dams are the most noticeable.
Pool water deposits: These take forms such as rimstone, crystal flowers, cave pearls, moonmilk, and raft crystals. Rimstone often forms rimstone dams, which resemble flowstone dams, but should be distinguished based on their different formation processes.
Non-gravity water deposits: These are formed from capillary water or film water that seeps out from cave walls or stalactites. Examples include rare formations such as helictites (twisted crystals) and cave hair. Their appearance is quite intriguing, adding much charm to the cave—I like to call them “cave sprites.” They generally appear in more enclosed sections of caves.
It’s worth noting that some soda straws and stalactites may change direction after growing for a period of time, due to the influence of wind. So don’t be fooled by nature’s tricks.
Synergistic deposits: These are formed through the interactive cooperation of two or more different types of water movements. Examples include pool basins, palm-like stalagmites, and crystal cones.
For those forms not detailed above, you can usually infer their names from their appearances. After distinguishing various deposit types, let’s discuss why stalactites come in colors like white, black, and brownish-red.
The color of stalactites is closely tied to their formation process. Among the possible colors of calcite are white, yellow, and black. Gypsum can also appear in white, yellow, or brown hues. Brownish-red colors are usually caused by the presence of trivalent iron. If water has passed through coal seams, it can result in an entirely black color. Other color origins may involve redox reactions, weathering, or human damage. Ordinary cavers don’t need to know the exact causes, so we won’t go into too much detail here.
When people discover stalactites, age is often a common question. Stalactites can only form after a cave passage has emerged from a fully water-filled state, so they are generally much younger than the age of the cave itself. Generally speaking, the oldest stalactites in caves are around 350,000 years old but under 1 million years, and these usually show severe weathering and erosion. Those aged 150,000–300,000 years tend to be darker in color (brown or gray-black), with slightly weathered surfaces that haven’t yet flaked off in layers but have stopped growing. Stalactites under 100,000 years old are lighter in color and have smooth, solid surfaces. The youngest generation is between 2,000 and 20,000 years old. In some caves, scientists have monitored that stalagmites typically grow at a rate of about 0.6–1.2 meters every 10,000 years, and among all types of stalactites, stalagmites have the fastest deposition rate. Therefore, any kind of damage is irreversible, irreparable, and unforgivable.