Stalactite - an overview | ScienceDirect Topics

Abstract
Stalactites and stalagmites are the most common speleothems. Stalactites are centimeter to meter in scale, hang from the ceiling and grow toward the cave floor. Stalagmites grow from the cave floor upward and are commonly fed by water dripping from an overhead stalactite. The most common variety of stalactites is the tubular soda straw, which is characterized by a central hollow tube and a translucent wall structure. Stalagmite morphologies are mostly determined by the drip rate, with candle-shaped stalagmites fed by relatively slow drips and dome-shaped stalagmites fed by fast drips. These speleothems archive Earth's climate changes from the Palaeozoic to the Present.
Introduction
Stalactites and stalagmites are the most common speleothems, the morphology of which is basically controlled by dripping; therefore, both speleothems can be considered as gravitational forms. Stalactites are centimeter to meter in scale, hanging from the ceiling and growing toward the cave floor. Stalagmites grow from the cave floor upward and are commonly fed by water dripping from an overhead stalactite (Fig. 1). Stalagmitic flowstones are a particular type of stalagmite formed by a thin flowing film of water itself fed by groups of dripping stalactites, and coat the cave floor and walls. When a stalagmite and the overhanging stalactite merge, they form a column (Fig. 1). Most stalactites and stalagmites are composed of calcite, a few of aragonite, the rhombohedral and orthorhombic phases of calcium carbonate (CaCO3), respectively. Rare stalactites and stalagmites consisting of huntite (a Mg-carbonate), halite (NaCl), gypsum (CaSO4·2H2O), and even opal (amorphous hydrated SiO2) have been found.
Figure 1. Stalactites, both soda straws and cone stalactites, candle-shaped stalagmites, columns (stalactites and stalagmites merged), and stalagmitic flowstone coating the cave floor. The plastic containers host glasses onto which in situ calcite precipitation experiments have been carried out to determine the processes that influence the development of different crystals.
Stalactites and stalagmites likely started to develop in caves when the first carbonate rocks had been subaerially exposed and eroded well over 1 billion years ago. Most speleothems that have been extensively studied date from the Quaternary, and the genesis of these is commonly driven by the process of degassing, which occurs when drip waters having a high carbon dioxide concentration (pCO2) interact with the cave atmosphere that has a relatively low pCO2. It is, therefore, believed that occurrence of stalagmites and stalactites greatly increased since the rise of vascular plants in the Devonian, which led to an acceleration of chemical weathering, greater availability of soil CO2, and a decline in global atmospheric CO2 concentration (Alonso-Zarza and Tanner, 2010). Chemical weathering of Ca-bearing silicate minerals by acidic waters generated in peat soil, for example, is very effective in yielding calcite stalactites and stalagmites in caves cut into granite and gneiss; in such cases karst dissolution does not play a role in the genesis of these speleothems, but the presence of vascular plants does. The focus of the following sections is, therefore, on the genesis, structure, and chemical properties of calcium carbonate stalagmites and stalactites since the Devonian.
4.3.2.2 Ice Speleothems (Stalagmites, Stalactites, and Columns)
Ice stalagmites and stalactites are common occurrences in cave entrances during the cold season in mid-to-high-latitude and/or altitude caves. They form as drip water freezes and usually melt as the temperature of the water feeding them becomes positive. In caves with perennial ice, these stalagmites can be perennial, the negative temperatures preserved throughout the year in the vicinity of underground glaciers helping their survival. However, differences exist between stalagmites, on one hand, and stalactites and columns, on the other, as the latter melt earlier and usually completely, the warm water leading to their detachment from the ceiling and collapse.
The dynamics of these ice formations have been studied in great detail in Scărișoara Ice Cave, Romania (SIC), by Viehmann and Racoviţă (1968), Racoviţă et al. (1987), and Racoviţă (1994), and it is summarized below, as a “template” for other caves, as well.
In Scărișoara Ice Cave, the dynamics of ice speleothems were studied separately for stalagmites, ice massifs, and ice crusts on the cave floor (Fig. 4.3.6), on both annual and subannual time scales.
Fig. 4.3.6. Seasonal variations (10 years average) of the upper face of the ice block in SIC (A), ice massifs (B), ice stalagmites (C), and floor ice crust (D). The ice massifs are similar to the one visible on the left in Fig. 4.3.1.
For all these speleothems, a maximum was reached between March and June, with the larger ice bodies having a delayed onset of melting. Ice crusts of the floor (Fig. 4.3.6D) grow outwards from the edges of the ice block, thinning with increased distance, their dynamics being controlled by the inflow of cold air which sweeps the upper surface of the ice block and leads to water freezing. However, this genetic mechanism renders them extremely vulnerable to melting, a process which initiates once the inflow of cold air ceases in early spring (Figs. 4.3.2 and 4.3.6D). The melting of the larger ice forms, both stalagmites and ice massifs (Fig. 4.3.6B and C), is delayed by the larger thermal inertia of the ice (Perșoiu et al., 2011). However, regardless of their size and shape, all ice forms in caves (including the large ice blocks) reach a minimum at the end of the melting season, just before the onset of freezing, usually in November (Figs. 4.3.2 and 4.3.6). Both growing and melting of ice is a complex process, controlled by the variable interplay between air temperature and precipitation amount and distribution, thus there is no clear correlation with either of these two (Perșoiu et al., 2011). The effect of precipitation amounts reaching ice caves on ice dynamics is strongly dependent of air temperature during winter, with water input superimposed on below 0°C conditions leading to rapid ice build-up, while the same input occurring during periods with positive temperature anomalies resulting in ice loss. However, in summer, temperature doesn't play too important a role, as the latent heat of the ice prevents temperature inside the caves reaching above 0°C. Thus, water input is the main factor leading to ice ablation, the heat delivered to the ice by inflowing warm water being the main factor in the ablation of ice.
A peculiar type of subannual ice dynamics of ice is that of the “thermoindicator” speleothems (Viehmann and Racoviţă, 1968, Fig. 4.3.7). These form in winter, during periods of alternating cold and mild weather, with the translucent, bulky sections developing when temperatures are between 0°C and − 2°C, and drip water will freeze slowly, expelling gas and calcite impurities. When temperatures drop below − 3°C, dripping water will tend to freeze quickly, incorporating both air bubbles and in situ precipitated cryogenic cave calcite (Žák et al., 2008), hence the semiopaque appearance of the ice.
Conclusions
Radiometric dates of stalagmites, stalactites, and flowstones are a valuable tool to establish upper limits of sea-level position through time. Establishing the timing of speleothem growth helps to set bounds on the timing of cave emergence above sea level. Dates bounding erosional unconformities in speleothems formed during seawater submergence provide further constraints on the timing of sea-level highstands and the elevation of the speleothem in the cave provides an important limit on the position of sea level at that time. Organisms that encrust or bore into cave deposits can be studied to provide a complementary age on the timing of cave submergence. In particular environments, it is also possible to capitalize on the occurrence of phreatic overgrowths that form at the sea surface to establish the timing and position of sea level. In this last case, instead of providing a bracketing age, that is an age that represents a minimum or maximum estimate on the timing of sea level passing through the elevation of the cave, the age of the phreatic overgrowth represents the exact time at which sea level was at that position. In all cases, these age constraints provide important benchmarks to test existing models and reconstructions of sea level through time.
Our understanding of the timing and magnitude of sea-level changes can be greatly enhanced by further study of both emergent and submerged caves. In particular, submerged caves have the potential to deliver rare information on the timing of sea-level lowstands, and the timing of sea-level changes during glacial periods in addition to providing bracketing ages for sea-level highstands. Existing data from submerged speleothems and associated cave deposits are scarce due to practical difficulties entailed in locating and recovering samples from underwater caves. In fact, this raises an ethical question of how this line of research should be pursued in light of the risk involved with cave diving to collect these valuable specimens. Perhaps there is scope to conduct future explorations or even sampling of submerged cave systems with remotely operated underwater vehicles.
Alternatively, one could also turn to uplifted areas to access speleothems in caves that were once submerged. Coral terraces exposed by rapid tectonic uplift have been exploited to access this portion of the record at localities such as Barbados and Huon Peninsula, Papua New Guinea (Chappell and Polach, 1991; Fairbanks, 1989). Unfortunately, there remain few examples of radiometrically dated speleothems from similarly uplifted environments to constrain former sea-level oscillations. Cave exploration in such environments certainly has the potential to yield an important and largely untapped resource for additional constraints on past sea-level change.
26.5.1.4 Forms and Dynamics of Ice
Congelation (ice of lake, stalactites) and sublimation ice are in the cave. The temperature in the cave during the summer does not exceed 3°C, and ice and hoarfrost in the heart of the cave do not melt throughout almost all of the summer. Even in the summer, the lake is sometimes covered with ice.
Although in the summer of 1980, the air temperature in the caves of the Ichalkovsky pine forest was negative (ranging from − 0.40°C to 6°C), now because of the winters' warming air temperatures, the air temperature during the summer months is positive, which has led to a decrease in the amount of permanent ice in the caves. Some years the presence of ice in the caves is seasonal.
Subaerial Helictites
Helictites are elongated speleothems that, unlike stalactites, may grow in any direction. Upward-growing helictites have sometimes been called heligmites, but there is little logical basis for this distinction, because helictites do not occur as separate “up” and “down” forms. Helictites may be straight, smoothly curving, or even spiral (helical, the root meaning of helictite), but in most cases they twist and turn erratically. Accordingly, the alternative names erratics, eccentrics, or eccentric stalactites have been used by some authors. Helictites are usually composed of calcite or aragonite, more rarely other minerals. They occur in a great range of sizes, from hair-thin and a fraction of a centimeter long to several centimeters wide and more than a meter in length. All share the common characteristic of a narrow central canal of capillary size.
Because helictites are so conspicuous and unusual in appearance, many (often fanciful) theories have been proposed for their origin. By growing artificial helictites of sodium thiosulfate, Huff (1940) demonstrated that hydrostatic pressure feeding capillary flow was the true mechanism. In natural carbonate helictites, the tip is extended by deposition of calcium carbonate around the central pore as the outflowing moisture evaporates or loses carbon dioxide. Moore (1954) subsequently explained helictite curvature by a combination of effects of impurities, crystallographic-axis rotation, and stacking of wedge-shaped crystals. These factors take precedence over gravity because the rate of flow is too slow to form a hanging drop at the tip. Increased flow can cause helictites to convert to soda-straw stalactites, and decreased flow, vice versa.
Varieties including filiform (hair-like), vermiform (worm-like), and antler` (forking) helictites have been defined on the basis of shape and size. Aragonite helictites may be beaded (Fig. 1), consisting of a string of conical beads of radiating fibrous crystals. The larger ends of the cones may face either the attached end of the helictite or the free end, but the orientation is usually consistent within each chain. This bizarre-looking beaded structure is relatively rare and has never been explained.
Subaerial helictites
Helictites are elongated speleothems that, unlike stalactites, may grow in any direction (Fig. 1). The upward-growing helictites have sometimes been called heligmites, but there is little logical basis for this distinction, because helictites do not occur as separate “up” and “down” forms. Helictites may be straight, smoothly curving, or even spiral (helical, the root meaning of helictite), but in most cases, they twist and turn erratically. Accordingly, the alternative names erratics, eccentrics, or eccentric stalactites have been used by some authors. Helictites are usually composed of calcite or aragonite and very rarely other minerals. They occur in a great range of sizes, from hair-thin and a fraction of a centimeter long to several centimeters wide and more than a meter in length. All share the common characteristic of a narrow central canal of capillary size.
Fig. 1. Dense growth of helictites on cave ceiling.
(Photograph courtesy of Kevin Downey.)
Because helictites are so conspicuous and unusual in appearance, many (often fanciful) theories have been proposed for their origin. By growing artificial helictites of sodium thiosulfate, Huff (1940) demonstrated that hydrostatic pressure feeding capillary flow was the true mechanism. In natural carbonate helictites, the tip is extended by deposition of calcium carbonate around the central pore as the outflowing moisture evaporates or loses carbon dioxide. Moore (1954) subsequently explained helictite curvature by a combination of effects of impurities, crystallographic-axis rotation, and stacking of wedge-shaped crystals. These factors take precedence over gravity because the rate of flow is too slow to form a hanging drop at the tip. Increased flow can cause helictites to convert to soda-straw stalactites and decreased flow, vice versa.
Varieties including filiform (hairlike), vermiform (wormlike), and antler (forking) helictites have been defined on the basis of shape and size. Aragonite helictites may be beaded (Fig. 2), consisting of a string of conical beads of radiating fibrous crystals. The larger ends of the cones may face either the attached end of the helictite or the free end, but the orientation is usually consistent within each chain. This bizarre-looking beaded structure is relatively rare and has never been explained.
Karstification
The process of karstification is responsible for the formation of stalactites, stalagmites, columns, and other speleothems by accumulation of dissolved material sourced by the rock dissolved (Fig. 8). Degassing is also involved in the formation of speleothems as water filters through the rocks and enters the cave environment that causes a release of CO2 from the water that results in calcium carbonate precipitation. This process does not happen in flooded caves.
Fig. 8. Speleothem formation.
Younger (lotic) cenotes are interconnected with the groundwater through fractures and dissolution features, while in the older (lentic) cenotes water could be stagnant due to sedimentation and blocking of those fractures (Schmitter-Soto et al., 2002). The latter ones are often turbid, thermally, and chemically stratified, oxygen rich in the fresh water body and oxygen depleted in organic debris rich horizons, either on the cave floor or in the density-stratified halocline.
The ring of cenotes (Fig. 9) consists of a semicircular alignment of cenotes with a diameter of 180 km and with an average altitude of 13 m. It is a geological feature that functions as a source of water and as nodes in the underground river system that channels water toward the coast (Marín et al., 2000; Perry et al., 1989; Steinich and Marín, 1997; Steinich and Marin, 1996; Perez-Ceballos et al., 2012). It extends southeast from Celestun to Dzilam in the northeast forming a large arc across the middle of the state of (Alcocer et al., 1999). Numerous productive and domestic activities take place around the RC in the absence of wastewater treatment or sewage systems (Arcega-Cabrera et al., 2014).
Fig. 9. Cenotes in the “ring of cenotes” and the rest of the Yucatan Peninsula.
Mammoth Cave minerals
Mammoth Cave is not noted for extensive displays of speleothems, such as stalactites, stalagmites, columns, and flowstone. These depositional formations are confined to parts of the cave system where the clastic capping bedrock is thin or has been removed and carbonate-saturated ground water can descend. The Frozen Niagara section, shown to visitors, contains a significant display of these speleothems (Fig. 7).
Various evaporative minerals can be seen in parts of the cave where the clastic caprock above remains intact. These include gypsum crystals, needles, cotton, flowers, massive crusts, and loose deposits resembling drifted snow (Fig. 8). Less common are epsomite and mirabilite crystals. The origin of the sulfate is pyrite in the overlying beds as indicated by recent isotope studies. Aborigines gathered and used mirabilite as a laxative and perhaps as food seasoning, gypsum as a token of manhood or possible body paint ingredient, and selenite crystals as ceremonial objects.
Abstract
Speleothems are mineral formations occurring in limestone caves, most commonly as stalagmites and stalactites or slablike deposits known as flowstones. Stalactites (which hang from the ceilings of caves) often have a hollow core, with growth occurring around this central orifice, whereas stalagmites are solid and grow incrementally at the drip site. Thus, stalagmites are generally selected for paleoclimatic analysis. The extensive distribution of karst landscapes means that studies can be undertaken on a worldwide basis. Speleothems are primarily composed of calcium carbonate, precipitated from groundwater that has percolated through the adjacent carbonate host rock. Certain trace elements may also be present (often giving the deposit a characteristic color), and one of these, uranium, can be used to determine the age of a speleothem, as discussed in the succeeding text. Seasonal variations in the trace element composition of dripwaters may also be used to identify annual layers. Deposition of a speleothems results from evaporation of water or degassing of carbon dioxide from water droplets. Evaporation is normally only an important process near cave entrances; most speleothems from deep within caves therefore result from the degassing process. Water that has percolated through soil and been in contact with decaying organic matter usually accrues a partial pressure of carbon dioxide exceeding that of the cave atmosphere. Thus, when water enters the cave, degassing of carbon dioxide occurs, causing the water to become supersaturated with calcite, which is thus precipitated.