FORM AND STRUCTURE OF THE HOT-SPRING DEPOSITS
Liberty Cap and the Devil's Thumb (fig. 8) are the two best examples of a cone-type hot-spring deposit at Mammoth Hot Springs. Hot-spring cones form by travertine deposition where water persistently emerges at a single point rather than along a crack in the terrace. As long as water flows up this point of weakness in the terrace, the cone continues to grow, but if the water finds a more convenient underground channel or if the orifice of the spring is sealed over by travertine, the cone becomes dormant. Also, as a tall cone such as Liberty Cap increases in height, the flow of water from the top may eventually stop, because there is insufficient artesian pressure to continue lifting the hot water over the newly deposited lip of the pool.
Hot-spring runoff over steep banks typically results in the formation of multitudes of small scalloped deposits. In places where the runoff slopes are gentle or where ir regularities in the slope allow small pools to form, travertine precipitates around the edge of the slowly rising pool to produce larger scallop-shaped deposits called terracettes (figs. 9, 10). New Highland Springs are characterized by spectacular overhanging terracette deposits (fig. 11). Hot-spring water overflowing the lip of the deposit typically forms a mass of stalactites that gradually fills in the base of the projecting terracette.
Collapse features are fairly common on most of the Mammoth terraces. Water flowing through near-surface channels weakens the overlying travertine until the under cut deposits collapse under their own weight (fig. 12). The surface expression of a collapse feature is commonly circular; however, several collapse or slump features may coalesce to produce a linear slump feature such as is found along the surface projection of the underground Hot River.
Collapse features usually range from 1 to 6 m in depth and width. The largest and most conspicuous ones occur on Hotel Terrace (pl. 1), one of them opening into McCartney's Cave. This cave is named after James C. McCartney, who in 1877 reportedly escaped from Nez Perce Indians by hiding in the cave for 3 days (Baggley, 1933). The walls of the cave, which extends about 50 m northeast down an average slope of about 40°, are composed of horizontally stratified travertine with lenses of soil and sand (Guptill, 1890; Condon, 1954).
Three large tension fractures, one of which is shown in the lower left corner of plate 1, and several smaller sub-parallel fractures occur on Pinyon Terrace. Similar tension fractures probably gave rise to slumping that eventually resulted in large blocks of travertine cascading down the face of Terrace Mountain to form the landslide deposit at Silver Gate (see fig. 21).
Several parallel tension fractures occur southwest of and along the same trend as the White Elephant Back fissure ridge (pl. 1). Spring MHS30 (fig. 13) is a small linear pool fed by several springs that reach the surface along a segment of one of the tension fractures. This relation between a fissure ridge and a tension fracture suggests that the many fissure-ridge deposits on the Mammoth terraces may have developed from hot-spring waters flowing to the surface along preexisting linear vertical planes of weakness.
The most outstanding topographic features on the upper Mammoth terraces are the numerous fissure ridges (pl. 1). A few, such as the old inactive ridge located just north of the Devil's Kitchen Springs (fig. 14), stand out in bold relief above the surrounding terraces; others have been partially buried and nearly obscured by later travertine deposits (fig. 15).
In addition to their topographic relief, most fissure ridges are characterized by a fissure that extends along the top of the ridge for nearly its entire length (see fig. 15). Width of the crack ranges from a centimeter to as much as 0.3 m. Most of the cracks probably widen when a ridge becomes topheavy and pulls apart along the lubricated plane of weakness at the center. The width of some fissures, however, may depend on the thickness of the dense, vertically banded calcite layers (fig. 6) that are deposited on the interior walls of the fissures. Winkler and Singer (1972) calculated that salt, sprinkled on snow- and ice-covered marble steps in winter, is capable of exerting a pressure of up to 670 kg/cm2 upon recrystallization (when oversaturated by a factor of two) and thus causing hairline cracks in the marble to widen. Similar calculations based on the average chemical analyses of Mammoth Hot Spring water from table 2 suggest that the thermal water may be oversaturated with calcium carbonate by a factor of three (J. M. Thompson, oral commun., 1975). With this amount of supersaturation, a pressure exceeding 770 kg/cm2 could be exerted on the interior walls of a fissure ridge, forcing the fissure to widen gradually.
A few of the fissure ridges in the Highland Terrace area, such as the Devil's Kitchen Springs and Orange Spring Mound (fig. 16), appear to be composed of coalesced cone-shaped hot-spring deposits. Devil's Kitchen Springs contains 28 small cones rather than a fissure marking the axis of its ridge, whereas Orange Spring Mound, which appears to be a large cone-type hot spring, has the medial fracture line of a fissure ridge extending along its longer axis; however, three distinct and partly coalesced hot-spring cones (one of which is named Tangerine) do occur along the projection of the fracture of Orange Spring Mound. This ill-proportioned fissure ridge undoubtedly results from the greater duration of hot-spring activity near its West end. A few other fissure ridges such as White Elephant Back Terrace (figs. 13, 18) display the same asymmetry to a lesser degree.
An important function of many fissure ridges is that they form dams or barriers behind which later travertine deposits accumulate. These may eventually overflow the ridge (fig. 15). Several of the large flat travertine terraces at Mammoth Hot Springs (pl. 1) apparently owe their origin to this mechanism, and deposits in areas such as the confined Glen Springs (fig. 17) may in the distant future build up to form a large flat terrace.
White Elephant Back Terrace contains small cavelike openings along its northwestern side called "The Grottos" (H. M. Majors III, unpub. data, 1962) (fig. 18). Such caves develop because of the inverse solubility of calcium carbonate (CaCO3). That is, a given volume of cold water can take more CaCO3 into solution than the same amount of hot water. As hot water flows down the sides of a fissure ridge, it begins losing CaCO3 through deposition, but more important, the temperature of the water decreases with increasing distance from the hot-spring orifice. Eventually the temperature and dissolved CaCO3 content of the water reach a point at which the cooled runoff is able to dissolve the old travertine deposits and thus create an underground channel beneath the flank of the ridge. A larger solution cave, Stygian Cave, similar to the Grottos, lies at the base of the Squirrel Springs fissure ridge. In winter, the icicles formed by spring water dripping into the cave may be "beautifully colored with tints varying from white to light blue and amethyst" (N. W. Scherer, unpub. data, 1932) (apparently owing at least in part to algal growth). Stygian Cave also contained carbon dioxide gas; Joyner (1928b) once counted 53 asphyxiated birds in the cave during a 3-month period. No study has been made recently; however, such caves remain particularly dangerous.
A similar type of solution cave may occur in the center of a fissure ridge, giving the impression that part of the ridge is merely a shell with a hollow interior. One such cave, known as the Devil's Kitchen (pl. 1), has a 3-m-long entrance (along the crack of the ridge) through which early visitors to the park were able to descend 10 m by means of a ladder and explore the cavern for a distance of about 22 m. Unfortunately, the Devil's Kitchen had to be closed to the public in 1939 because of carbon dioxide gas and lack of oxygen (Haynes, 1949).
Calcite "ice" and fossiliferous travertine
A rare type of travertine deposit found on the Mammoth terraces is called calcite "ice." Calcite "ice," also called "hot-water ice" by Allen and Day (1935), is a thin crust of calcite that gradually forms on the surface of some stag nant hot-spring pools. Usually the delicate calcite layer breaks up and settles to the bottom of the pool; however, if the pool dries up, the calcite crust may persist for a short time (fig. 19).
Another unusual travertine texture results from the growth of microorganisms. The significance of microorganisms or algae in promoting precipitation of travertine was discussed by Weed (1889) and later by Allen (1934). Weed concluded that algae play a prominent role in travertine deposition. However, Allen indicated that the amount of calcium carbonate actually deposited by algal growth at Mammoth Hot Springs is very small. Samples containing microorganism fossils (fig. 20) are found at only a few locations on the Mammoth terraces, which bears out Allen's conclusion.
Last Updated: 20-Nov-2007