Caves

Most rocks have pores and cavities within them. Generally, however, these are not large enough for a person to explore. Caves are cavities or hollow spaces inside a rocky mass that are large enough for a person to explore. There are many types of caves formed through different processes : some are small, and it is difficult for man to penetrate into; others, on the contrary, stretch underground for tens or hundreds of kilometres, reaching depths of over 2,000 m. Formation processes control lenght, development and shape of a cave, and also the difficulties that will arise when exploring them. Most of the longest and deepest caves do not consist in an isolated cavity, but they form a system, which at times may be very complex, made of rooms, sinkholes, shafts, meanders, canyons, interconnecting galleries, which are arranged to form a system or karst complex. Large quantities of underground water move through karst systems, caves may therefore be classified in different sub-areas, i.e. occupied by air and completely dry or scoured by streams, they may be flooded at times, or permanently invaded by fresh and salt water.

Processes to form a void

There are many mechanisms leading to the formation of cavities and voids underground. Some caves are classified as “primary” because they are generated at the same time as the rocks containing them, others instead form subsequently, due to rock weathering that modify their original characteristics.

First a void and then the rock

Caves formed by biologic processes. Among the primary caves, the most widespread are caves forming when corals and coralline algae grow forming encrusted coral reefs: the growth of these organisms is neither uniform nor homogeneous and voids of various shapes and sizes are created, which are often large enough for a man to visit. All those who have done scuba-diving near a coral reef must have observed these caves. Most are practically anfractuosities or recesses and, at times, small irregularly shaped channels or galleries are formed, through which it is possible to cross the reef from one side to the other.

First the rock and then a void

Unlike primary caves formed at the same time as the rock in which they are formed, the origins of secondary caves imply different processes acting on rocks that already exist. At times these processes take place many millions or tens of millions of years after the formation of the rock. Eolian cave. These caves do not have a great lenght, no more than a few metres, and are formed by the abrasive action of the wind and due to particular weathering processes in arid and desert zones, or near the coasts on tender rocks that are particularly flaky…

Most of the longest and deepest caves in the world are formed by chemical corrosion processes in rocks particularly water-soluble thanks to their mineral composition. These corrosion processes are known as karst processes.

Water and rock. All minerals are more or less soluble in water, but some are much more soluble than others, and require a very short time span to dissolve (in a geological sense, naturally), while others require much longer periods of time, and are therefore considered practically insoluble. Rocks made of the most soluble minerals are the ones in which karst features develop more easily, even though the karst process is a complex one in which rock composition is only one of the many factors involved in the development of the phenomenon.

When studying the solubility of the main minerals forming the most diffused rocks of the Earth’s surface, it can be observed how the solubility of the different minerals varies greatly. For this reason, rocks such as rock salt (made of sodium chloride NaCl, or halite), one of the most water-soluble minerals, are practically absent in humid climates as they dissolve rapidly. In quartzite rocks, made of quartz (SiO2), one of the minerals most resistant to weathering, karst features may develop only in particular climatic conditions and in areas in which waters had a very long time, i.e. millions of years, to dissolve the rocks (e.g. the Amazon Tepuy quartzite cave systems).

Not only water. In nature, however, things are not that simple. In fact, natural waters are never pure waters, but they are water solutions containing various ions dissolved within them that can increase the aggressiveness and corrosive action on some types of rocks, thus complicating the simple dissolution reaction. The process is well known to those who have to clean the bathroom at home. In order to remove the calcareous incrustations that ruin all the bathroom fittings (geologically speaking, these are calcium carbonate crystals, CaCO3, calcite; rocks that prevalently contain calcite are called limestones), we use aqueous solutions enriched with acids that increase their corrosive streght, such as hydrochloric acid (also known to housewives as muriatic acid), or acetic acid, that are present in many house-cleaning products. These substances make the removal of incrustations easier in two ways: on one hand they increase the solubility of calcite, on the other they greatly accelerate the speed of reaction (that is very rapid and violent, in fact bubbles form because of gases freed when these products are used). Even pure water could obtain the same results, but the time taken would be decidedly beyond the scale of human observation…. and by far too long for the housewives’ patience! In fact natural water behaves in the same way as the detergents, but since the acid solution is much more diluted and with much weaker acids, chemical reactions are much slower, at least on a human-observation scale. Nature, unlike housewives, has no hurry, and the results are even more spectacular!

Carbon dioxide: allied with karst

Among the substances that can increase the corrosive power of natural water, carbon dioxide (CO2) has the most important role. It is already present in meteoric waters, as it is one of the gases of the atmosphere, however its percentages are very low (0.03 atm). Its concentration increases greatly in waters that cross through thick layers of soil covered with dense vegetation. Enrichment with CO2 and other organic acids produced by vegetation and biological activity of water coming into contact with rock can lead to a remarkable increase in the solubility of minerals like calcite and dolomite.

Rocks that are most suited

Calcite and dolomite (calcium carbonate and calcium carbonate and magnesium, respectively) are minerals that are very abundant on the Earth’s surface and are the main constituents of particular sedimentary rocks, known as carbonatic rocks, such as limestone and dolomite rocks. These rocks are not the most prone to karstification (gypsum and rock salt are much more soluble), however they are the most common rocks in which karstification processes take place, the most widespread and the ones able to support the existence of large volume rooms and galleries without collapsing, unlike other “weaker” rocks.

Underground water

Water commonly found underground creating caves is mainly of meteoric origin, however other kinds of water may be mixed to it in various ways. The following can be found: “Connate” water, i.e. ancient water that was trapped in a sedimentary rock during its formation, and generally very rich in salts, and therefore potentially very aggressive; deep so-called “juvenile” water, produced by magma activity, often very hot and aggressive; or meteoric water that reaches the deeper layers where it is heated and enriched with salts and acids and then comes out at the surface again through faults, generally with the characteristics of hydrothermal water.

Hot waters, cold waters

The possibilty of water to corrode the rocks it comes into contact with depends on the chemical composition of both water and minerals building rocks. However other factors intervene to make reactions complex. In particular, temperature is a fundamental factor that acts in two ways that are apparently in contrast one with the other. In fact, the quantity of CO2 that can dissolve in water depends on temperature: the lower the temperature, the greater the amount of CO2 that can dissolve.

Aggressive waters, saturated waters

Water that comes into contact with rocks initially is undersaturated, i.e. it can dissolve the minerals of the rock and progressively be enriched with the ions freed in the dissolving reaction, till it reaches a point of saturation, i.e. the water solution contains the maximum quantity possible of a particular ion in certain temperature conditions, atmospheric pressure or content of other acids. When this condition of saturation is reached, water no longer has a chemical effect on rock, and therefore it can only affect rock with mechanical erosion processes (as in the case of surface free flowing water).

A bit of geology

The fundamental ingredients to produce karst processes are abundant water rich in CO2 and organic acids, and a favourable type of rock, but in order to have long and deep cave systems, these conditions are not sufficient. Carbonatic rocks, which are most favourable for karst process, are generally very compact rocks. The granules forming them are very densely packed, the mechanisms by which they were formed led to a primary porosity (i.e. a percentage of voids) and permeability (i.e. a percentage of interconnecting voids that allow the passage of water) of these rocks that are very low.

Chemical deposits

All caves are filled with more or less large amounts of chemical deposits, minerals and physical deposits, sediments of various types which are mostly transported by water into caves. These, as a whole, are known as speleothemes, and are a very precious database regarding the geological, environmental and especially climatic evolution of the past. Chemical sediments form when water saturated with calcium carbonate is subjected to variations in temperature or CO2 content, or gets concentrated due to evaporation, thus becoming oversaturated.

Physical deposits

Physical deposits include a large variety of materials that accumulate in caves thanks to gravity (deposits, blocks abd boulders due to break- down) or transported by water. Sediments may be autochthonous, produced in the cave (as in the case of blocks and boulders due to breaking down, or the clay formed by the insoluble minerals in the calcareous rock), or allochthonous, transported into the caves by different agents, generally by water. The material transported by water can be distinguished because its rounded characteristics…

A large number of factors influence the formation of cave systems : chemical factors and climate control the dissolving capacity of water; geological factors control the type of rock, the geological structure and the characteristics of fracturing and jointing, which in their turn control underground water circulation and the development and trend of the cave zone; topographic factors such as altitudes gradient and the presence of deep valleys control the prevalently vertical or horizontal development of karst systems. Caves therefore show different characteristics depending on the conditions and the environment in which they were formed.

Tropical caves. In tropical environments, all caves have similar characteristics. They are often arranged in vast underground systems, generally with a prevalently horizontal development, often drained by veritable underground rivers, and cave passages are very large and rich with speleothemes. Thanks to high temperatures, dissolution reaction is fast: since water filtering underground is very aggressive due to the presence of CO2 and organic acids deriving from the dense plant overgrowth, it rapidly dissolves large quantities of limestone near the surface, soon becoming saturated or oversaturated, thus forming large quantities of speleothemes, which are a peculiar features of tropical karst.

High mountain caves. On the extreme opposite hand, caves in high mountain areas have a prevalently vertical trend due to the energetic potential created because of the large differences in altitudes and a geological structure that is generally complex, with big shafts, which are often very deep (at times over 600 m). Due to the slower dissolution speed as a result of low temperatures, circulating water may remain aggressive even at deeper levels, thus creating deep and vertical systems. Because of low temperatures, speleothemes are on the contrary very rare. On the surface, vertical shafts and sinkholes are often peculiar features. Their formation is often due to the conjoined action of karst processes and corrosion processes controlled by the presence of ice and snow.

Surface karst landscape

Karst landscapes have two peculiar characteristics which make them immediately identifiable even where rocks are covered by soil and vegetation: particular forms of dissolution on the surface and the practically total absence of watercourses on the surface, as all the water or most of it, rapidly is swallowed into the depths. This characteristic makes the work of speleologists particularly important, because karst zones are generally characterized by water supply problems…

Underground landscape

Observing a vertical cross section of a karst system it is possible to point out different zones, depending on the presence of water and on how water moves within. The catchment zone is the one closest to the surface where surface water and meteoric water sink and seep deep underground. Catchment may take place through a large number of fractures, thus being diffuse, or through concentrated sinking input points, such as dolines, where surface water can collect and concentrate: in this way the corrosive action of water concentrates on a small number of fractures that are then widened preferentially, thus leading rapidly to the creation of underground karst galleries.

Base level

The peculiarity of the base level is that it is not a fixed unchanging level, but it varies in (geological) time. It generally tends to lower progressively, as valleys deepen getting more and more entrenched. When a valley cuts the phreatic zone of a karst system the cave conduits that were previuosly filled with water get empty, letting water drain out. Thus karst springs are formed. These are normally found close to the base level, on valley floors – at times directly feeding water courses, at times creating pocket valleys.

Caves have a shape

Cave morphology is often complex and difficult to describe, however there are few elementary shapes: galleries, shafts, meandering channels,or canyons, rooms. The trend of galleries is prevalently horizontal or slightly inclined, and generally galleries are large (if the size is small they are often called “passages”, but this definition is purely speleological and not geologic. From the point of view of the origin, there is no difference between a “passage” and a gallery). The diameters of the galleries can be impressive. The largest gallery in the world is in Deer Cave in Sarawak – its average diameter is over 80 m.

Different zones, different forms

Cave morphologies are controlled by the zones in which they have been formed: in the vadose zone mechanical erosion features prevail (such as canyons, gorges, meanders and vertical shafts) and collapses, such as rooms; while in the phreatic zone corrosion features prevail. Finding typical phreatic zone features in a vadose zone (or more rarely vice versa) is a precious clue in order to reconstruct the evolution and the geological history of a cave. Most underground caves form in the phreatic zone, in particular at its top (near to what is called the water table, or, less correct, the piezometric surface)…

There are many different reasons that drive some men and women towards speleology: for some it is the sporting or technical aspect, for others it is the urge for adventure and ‘strong’ emotions or the curiosity to know ‘what lies beyond’, or even scientific research. Often it is a mixture of all these put together or yet some other reason. In any case, the aim of a speleologist is rarely just visiting an underground environment, be it sub-aerial or flooded, (we call him/her a caver), but it is the exploration of new conduits and galleries and the conjunction of the caves, to be able to reconstruct one large karst system, that is vaster and deeper, to be able to understand how these caves formed and evolved and to discover the potential of the system and how much vaster and deeper it may become (we call him/her a caver),. Man, however, is not suited to the cave environment, so in order to explore it he must be aware of certain specific techniques and equip himself adequately. Since we are unable to move about in the dark, at least two sources of light are necessary, the main one usually being an acetylene lamp. It is essential to protect oneself from cold and mud by wearing clothing made of pile and appropriate jump-suits. At times the use of a wet suit is required to cover parts that are very wet without risking hypothermia. Generally, boots or mountain climbing boots are the required footwear, while rubber gloves must be worn to protect one’s hands from rock and rope abrasion. To cope with the vertical parts, static ropes with a 10 or 9 mm diameter are used, along with a climbing harness (similar, with some modifications, to those used for climbing) and suitable equipment for climbing up and descending ropes.

Many dangers are present when exploring a cave, but actually all can be foreseen and overcome with a correct technical preparation and the right equipment: you cannot stand in as a caver!

Contrary to what is usually thought, no caver has ever died trapped in a narrow passage or under a collapsed cave roof (which, on the contrary, might happen in a mine, where the hollow is man-made, and is therefore unstable): the main risks are falling stones (always caused by the passing of explorers) and water. Since flood propagation in a karst system can be very rapid at times, it is possible that, in conjunction with an external rain event, galleries that are normally dry might get flooded, even completely: this is one of the most frequent causes of entrapment inside caves of imprudent cavers (often with a limited knowledge of the underground system),which requires the intervention of a rescue team of cave divers and adds another, at times, gruesome anecdote to the literature on this subject even though, fortunately, the majority have a happy ending. An example is the incident in the French cave of Vittarelle, where some cavers were trapped for days on board a small inflatable boat, in a chamber that was rapidly turning into a lake: the rise of the waters stopped when the boat was just a few metres from the roof…). In caves, however, water leaves unmistakable and evident traces, so that those who normally visit this type of caves know its behaviour and can foresee it easily: it is unnecessary to underline that before venturing into complex cave systems, especially if they are close to the springs, it is essential to collect information from the local caving groups.

Tracers and dyes

At times, the fact that a cave and a spring belong to the same system can be perceived immediately, especially in the case of the so-called hydrogeological tunnels, where the course of the underground waters can be followed physically by cavers from the sinkholes to the spring. Other times, on the contrary, the relationship between caves and karst springs is not obvious; it may happen that the closest springs, that rationally seem to be the most likely to be connected to a karst system, do not in fact belong to it.

The age of caves

Chemical deposits in caves offer extraordinary research possibilities to those who are engaged in reconstructing the geological history of the past. These can, in fact, be easily dated with a method based on the decadence of certain isotopes of the radioactive ‘family’ of 238U. The latter, in fact, decays into a series of elements: 234Th, 234Pa, 234U, 230Th, up to 206Pb, which is stable. Cave speleothemes contain uranium, which substitutes calcium in the crystalline network of calcite, but they do not contain thorium.

Caves remember the past

Quando il livello di base di abbassa, per l’erosione continua delle valli, le gallerie singenetiche si trovano sospese al di sopra del livello di base, e lentamente si svuotano, mentre iniziano ad allargarsi nuove gallerie più in profondità. I sistemi carsici tendono sempre a mettersi in equilibrio con il livello di base, ma se le variazioni di quest’ultimo sono rapide, o, meglio, più rapide della capacità di adeguamento delle grotte, si può registrare un certo disequilibrio, con la presenza di sorgenti di troppo pieno o sorgenti sospese, e di tratti allagati anche al di sopra del livello di base.

Cave evolution

Caves are formed progressively in relatively long geological periods and evolve continuously: their history depends on many factors, among which the amount of water (depending mainly on climate), the way in which the latter enters the system, the variations of the base level and of the surface topography. Modifications in the topography can change the hydraulic supply of a cave causing, for example, the transfer of phreatic conduits to vadose zones, or bring about variations in position and functioning of springs, and much more…

Landscape evolution

Generally, the evolution of karst systems is similar to that of the mountain massif in which they are found. The general tendency is a gradual deepening of the cave systems as a consequence of the deepening of the base level of the valleys. But this is not always the case: the base level can also rise, bringing about the flooding of galleries that previously were fossilized. This has taken place, for example, in all the caves of coastal areas where, in the course of the last 2 million years, continental glaciations determined fluctuations in the sea level.

Caves and climate in the past

A study of sediments transported within caves, their characteristics, their composition and their content in fossils allows the reconstruction of the variations of the environment on the surface and of the climate: in particular, remains of soil formed in tropical climates can be interesting, as well as sediments related to cold climates, such as material deriving from glacial or periglacial deposits. Speleothemes, on the other hand, are formed prevalently in warm climates and are therefore very important climate markers.

An example close by

Even caves close to the big prealpine lakes (Maggiore, Como and Garda) have experienced a similar evolution: Lake Como, for example, is set in a deep canyon whose formation dates back more than 5 million years and hence is not of glacial origin (like its fellow-lakes, Lake Maggiore, Lake Iseo and Lake Garda). At present, Lake Como is over 400 m deep, which means that its bed is 200 m below sea level, but the bottom of the canyon, filled with sediments, is 700 m deeper.

Immense reserves

The phreatic zone in a karst system can grow in size and depth depending on the geological structure. At times the phreatic zone may be quite thin or absent as in karst systems that are perched above the base level, at times the phreatic zone may be hundreds of metres deep and thus provide an immense and precious water reserve. The more superficial area of the phreatic zone, known as the epiphreatic zone, varies during seasons, and can rise various tens of metres during rainy periods.

Springs

Ground water coming out on the surface is called a spring, if the origin of the water is unknown, or if it comes from an unconfined catchment area. It is called a resurgence if it is on the contrary the re-emergence of a watercourse that sank upstream, as the already mentioned case of the Timavo river. Springs can be classified in various ways, depending on the flow rate, the constancy of the discharge, or the geological characteristics that determine its formation. Many springs are characterized by a perennial flow, even though there can be remarkable differences in the discharge, depending on precipitation.

How underground waters move

Karst springs greatly depend on surface rainfall for different reasons. Considering that the speed of waters in the vadose zone is as high as that of surface streams, the increase in the hydraulic head when great quantities of water enter a cave can exert a high pressure on waters in the phreatic zone that are literally pushed out with a piston flow, a term that clearly explains the mechanism involved. The flood wave that follows the arrival of infiltration waters will reach after a period of time equal to the time it takes to physically transport these infiltrated waters…

Karst aquifers

Karst aquifers provide a very important water resource in many parts of the Earth: karst terrains, in fact, are, by nature, lacking in surface waters, and all water circulation occurs underground. However, these resources are very difficult to utilize and to protect. Karst aquifers, in fact, due to some of their characteristics, are particularly vulnerable to pollutants and excessive exploitation. An excessive and uncontrolled utilization of the reserves of deep phreatic zones can be a hazard for these kinds of aquifers.

Cave environment, dark and mysterious, has always stimulated man’s fantasy, provoking a mixture of curiosity and fear. Man has envisioned cave inhabitants as being arcane and fantastic, often related to the netherworld and worship of the dead: in western cultures they were seen as evil and devilish, but for many others such as the oriental ones, they were (and in many cases still are) positive supernatural beings that gave protection and brought good luck. Further studies and knowledge on this particular environment made us understand that caves are not home to devils or dragons, but to a microfauna of tiny and shy beings, difficult to observe but interesting for studies on evolution and management of the environment.

Special biologists. Biospeleology is a zoology branch that studies animals, big and small, that live in caves, their life cycle and how they have adapted to life in environments with particular characteristics. Man’s interest in cave inhabitants dates back to a very long time ago: in a cave on the Pyrenees (France) a bison bone was found on which 15.000 years ago an ancestor of ours had drawn an insect that is easy to find in our caves nowadays: a grasshopper which belongs to the Troglophilus species (trogo means cave and philo means friend). Anyway the first scientific descriptions of cave animals date around 1500, and only in 1700 researchers started to be interested systematically in this kind of environment. Biospeleology was born in 1907 thanks to the work done by a Rumanian naturalist called Racovitza, that started off modern day-type studies. Initially biospeleology focused on animals that lived in caves, but as they progressed in their studies, researchers have realized that for smaller animals (such as insects, spiders and other Arthropods) small cracks or tiny shaded valleys have the same environmental characteristics as caves. This way the term biospeleology has broadened to include the study of all types of organisms that live in environments similar to caves.

Temporary and permanent guests

Biospeleologists divide cave inhabitants into 3 big categories: troglossens, troglophiles, and troglobes. These are difficult sounding names that classify animals that live by chance in a cave (troglossens) or by necessity (troglophiles), or animals that live out their entire lifecycle in a cave (troglobes). The latter have adapted so well to cave life that that they could not survive on the outside. Troglossens are animals that end up living in a cave by chance, possibly because they fell inside a well or in a crack or were dragged into a cave by an overflowing creek or water infiltration.

A particular environment

Cave environment has certain physical and morphological characteristics that make it very particular and different from any other environment on earth. These characteristics don’t allow for all animals to survive, but only those that have developed particular evolutionary modifications. The hypogeous ambient can be divided into different under-ambients: surface ambient, endogeous ambient (soil), superficial subsurface (rock cracks and holes) and deep subsurface.

Evolution

Cave environment is hard and selective and a very few organisms have adapted by undergoing specific morphologic and metabolic modifications. Changes aren’t immediate, but come as the result of an evolution that can take up to millions of years, starting from species that lived outside and that for many reasons ended up trapped and isolated in underground cavities. Species that have been used to living longer in underground conditions are the ones that show more specific modifications. In the absence of light, eyes are a useless instrument: species that have lived in caves the longest can be recognized by the fact that they are eyeless...

Miniature tigers

Because of the lack of vegetables, in the hypogeous environment the food pyramid is structured differently: the alimentary base is represented by autotrophic bacteria, which are organisms that are able to produce organic substance not from light but directly from mineral substances, such as nitrobacteria (that use nitrogen), sulphur bacteria (that use sulphur) and many others. There is a bacteriophage fauna too, that lives in mud and feeds on bacteria.

Lifestyles

The cave environment offers very little in the way of food. The trick in underground life is energy saving, so the best adapted organisms are those that need less energy to reproduce and live. For this reason, most of the cave inhabitants have very slow metabolisms: slow growth, small dimensions, a long life cycle, they are slow and don’t move very much, sexual maturation occurs very slowly and very often they maintain the typical characteristics from their youth, they reproduce very little, they are satisfied with minimum quantities of food and normally they use very little oxygen.

Bats: small devils that need protection

The typical cave animal is the bat. In many caves in tropical areas, bats form colonies made up by thousands or millions of individuals. The show offered by these animals is one of the most fascinating in nature: at dusk they move about the sky, “drawing” a black snake in search of insects. This animal has very particular characteristics: it is the only mammal able to fly, with real wings and muscles unlike other “flying” mammals, such as some squirrel species that simply have skin membranes. But their most particular characteristic is the echolocation system.

Dragon tales

The animal man’s fantasy located in caves, as a guardian of huge treasures or kidnapped princesses, is the dragon. But are they only legends? Very often are exaggerations of reality and legends on dragons aren’t an exception. Dragons are generally provided of wings, just like bats (and devils too in the end): maybe these small cave inhabitants scared them so much, that in some way, they seemed to be bigger than what they actually are. Many palaeontology findings (such as the dinosaur ones) have contributed surely on making up legends on dragons.

Proteus

It’s an amphibious, a distant salamander relative, that represents one of the most surprising adaptations of life in caves. It lives in caves in the Oriental Mediterranean area, in Dalmazia, Slovenia and in the Carso triestino and goriziano part. It is 20-30 centimetres long, pinkish colour and extended form, with a long tail and 4 small paws (that have 3 anterior fingers and 2 posterior), that aren’t though able to sustain it, so the proteus can’t walk but can swim. When it is born it has developed eyes, but these, during the growth, regress completely.