HEATING THE HOT SPRING

By H. W. Lix,
Acting Park Naturalist,
Hot Springs National Park.

Practically every visitor who comes to Hot Springs National Park, Arkansas, is curious about the origin and mechanics of the hot springs. Until several centuries ago, supernatural agencies were credited with responsibility for such phenomena as large streams of hot water gushing out of the earth. Evil monsters were a favorite explanation of incomprehensible occurrences in nature. Later, the water of springs, and expecially of hot springs, was believed by many to be forced up by the winds produced by immense subterranean fires. Misconceptions of volcanoes gave credence to this idea of fires beneath the earth's surface. Today, with the development of geology, we know that hot springs may have one or a combination of various sources of heat. By interpreting the nature and relationships of rocks and their fossil and mineral contents, in the light of what is happening on the earth's surface today, geologists have gradually pieced together a remarkable geological history of the earth that extends back immense ages of time. An explanation of the hot springs logically begins with a summary of the geological history of this region, including the processes that eventually produced our present topographical features.

Hot Springs National Park is in the Ouachita (Wash-i-taw) Mountains of the Interior Highlands. This mountain system extends from Little Rock, for about 220 miles westward into Oklahoma. The average width is about 50 miles. The novaculite uplift portion of the Ouachita Mountains is divided into a series of seven mountain ranges and four wide flat intermontane basins in which flow the larger streams. One of these ranges, known as the Zigzag Mountains, contains Hot Springs National Park. Adjacent to the park on the southwest, and extending into it in the valley between West Mountain and Hot Springs Mountain, is the Mazarn Basin.

All of the rocks of the Ouachita Mountains, with the exception of several igneous intrusives of moderate extent, are of marine sedimentary formations. They were deposited as sediments of the floor of a sea during the Paleozoic era of geologic time. During most of the Paleozoic era, this present mountainous region was submerged by the waters of the Ouachita Embayment which was a westward-extending arm of the Appalachian geosyncline, an ancient sea located roughly between Louisiana and New Hampshire. To the south of this gradually sinking Ouachita Embayment area of western Louisiana, Texas, and southern Arkansas and Oklahoma, lay the land of lofty mountains known as Llanoris. The eroding rocks of northern Llanoris were carried northward by rivers and streams into the Ouachita Embayment and deposited there as gravel, sand, mud, and chemical precipitates. Throughout the hundreds of millions of years believed to have passed during the Paleozoic era, these sediments continued to accumulate until they reached the remarkable thickness of over 30,000 feet. This deposition all occurred in comparatively shallow water, accompanied by a gradual corresponding sinking of the ocean floor.

In late Pennsylvanian time there came a period of mountain-making. Compressive forces from the ocean to the south and southeast were applied against the thick sediments and raised their surface above sea-level. In this elevation, the rocks were compressed so severely that they now occupy but one-half their original surface area. A width of 100 miles or more was reduced to the present average 50-mile width of the Ouachita Mountains. This compression produced a series of folds, or anticlines and synclines. Where they were unable to withstand the strain produced by the enormous pressure, the rocks fractured, and often adjacent masses moved up and dawn, respectively, along the break, or fault plane.

The rocks of the Hot Springs district have remained above sea-level during most or all of the time since their emergency near the end of the Paleozoic era, and have been subjected to subaerial erosion from that time. This erosion did not proceed steadily. At least twice, this area was reduced to low-lying land of such low relief that erosion practically ceased. Areas at this stage are known as peneplains. There always followed an uplift, which resumed the erosion process. The first of these peneplains, now evidenced by the higher mountain crests, occurred at the end of the Jurassic period; and the second, evidenced by the present intermountane basins, developed in early Tertiary time. Horizontal layers of sediments beneath an inland sea were lifted many thousands of feet above sea-level, compressed and folded into one-half their original space. The lofty ridges were cut down until they are now valleys. This is a part of the history of the hot springs of Hot Springs National Park. With this brief geologic background, the springs themselves can be discussed more easily.

In the immediate vicinity of Hot Springs National Park, the following seven rock formations, with their respective ages, are found:

The relative positions of these formations and their pertinent structure are shown in the accompanying diagram. This drawing includes the hypothetical hot igneous intrusive and is adapted for illustrating the theory outlined below.

It is believed by probably the majority of geologists that the highly fractured Bigfork Chert formation is the aquifer upon which rain water falls and flows between the collecting area and the spring outlets. The Womble shale below, and the Polk Creek shale above, retain the water within the chert formation. Analysis of the hot spring water would indicate that the water is at least chiefly meteoric (rain) in origin. The minerals in solution, with possibly one exception, are those which would be expected in rain water that has flowed through rocks of the kind that occur in the spring area. Furthermore, the geologic structures required by the meteoric theory seem to be present.

GEOLOGIC CROSS-SECTION.

From the collecting area, the water flows southeastward in the Bigfork formation beneath the lowest part of the trough of the fold (syncline) which lies below West Mountain. The water is then forced upward by hydrostatic pressure along the western part of the upturned fold (anticline) which makes up Hot Springs Mountain. The springs emerge from the base of the Stanley shale and the top of the Hot Springs sandstone. This requires the water to pass through the Polk Creek shale, Missouri Mountain shale, Arkansas Novaculite, and most of the Hot Springs sandstone. The logical assumption is that this transfer is accomplished along a crack, or fault, at the site of the springs. Indications are favorable for the existence of this fault.

The abnormally high temperature of the spring water is its most unusual characteristic. To account for the source of heat, we must rely on hypotheses because of a lack of any definite diagnostic criteria. The generally accepted belief today seems to be that the spring water, somewhere in its underground course, passes near a hot igneous intrusive that has not been exposed at the earth's surface. Possibly the water, is heated chiefly by rising hot vapors emanating from this cooling mass of igneous rock. This possibility is supported by the trace of boron which has been found in the water. Steam would be a principal constituent of these vapors and in condensing would add to the meteoric water of the springs. Some geologists attribute all or most of the water to this juvenile source. It may be that the water merely approaches near enough to the buried igneous mass to heat the water by conduction and convection.

There are several areas of exposed igneous rock and numerous dikes in the vicinity of Hot Springs National Park. These are supposedly related to the larger intrusives. This igneous rock material is believed to have been intruded into the sedimentary formations near the end of the Lower Cretaceous epoch. Presumably any heat of such ancient igneous activity would have been dissipated before now. Consequently, it is not maintained that the exposed igneous masses represent a part of the identical magmetic body which supposedly heats the spring waters. Probably the chief objection to this theory explaining the origin of the springs is the apparent inadequacy of the collecting basin to supply the large volume of water which the springs produce. The fact that part of the collecting area has an elevation lower than that of some of the springs has also been used as evidence against this theory. However, it has not been shown that an adequate supply of water at sufficient elevation does not exist.

It has been suggested that subterranean chemical activity produces the heat, but this is not borne out by analysis of the water. It would be expected to find unusually large amounts of mineral matter, or minerals different from those ordinarily extracted from the containing rocks by solution in water. Radioactivity has also been repeatedly suggested, as a source of heat. There has been no definite evidence to substantiate this hypothesis. The amount of radioactivity of the water is not very large, and there is no correlation between the temperatures and amounts of radioactivity found in the different springs. Compression of rocks during periods of intense stress, such as obtains during periods of mountain-making, produces heat, but any such connection between the hot springs and the orogeny of this region is too remote.

The temperature of the earth increases with depth. Assuming an average thermal gradient of 10 Fahrenheit for every 65 foot descent, and assuming a mean atmospheric temperature of 60° Fahrenheit, the line of flow of water would have to descend almost 5,400 feet, or over a mile. Allowing for a certain amount of cooling in ascending, this depth would be increased considerably. This hypothesis is generally mentioned merely as a possibility.

The group of hot springs which are extensively used for bathing, is the chief feature ef Hot Springs National Park that attracted more than 175,000 visitors last year. The hot waters are generally believed to possess curative properties for the treatment of various diseases. Approximately 750,000 baths are administered annually by the bathhouses licensed to use the hot springs water. There are forty-seven known springs, including two hot wells. The total flow of hot water is approximately 1,200,000 gallons per day. The temperature varies from about 112.5 degrees to 148 degrees Fahrenheit, with an average of about 143 degrees. Generally classed as "mineral springs", the hot springs produce remarkably pure water. The amount of mineral matter in solution varies with individual springs from 231 parts per million to 310 parts per million, with an average of about 280 parts per million. The following, in parts per million, is an analysis of a typical sample of the hot water:

The forms of combination of the most abundant constituents are calcium carbonate (CaCO₃) and silica (SiO₂). Considerable thicknesses of these minerals, making up the rock known as tufa, were deposited around the vents of the springs. The het water has also been found to contain measurable amounts of the radioactive gas known as radon. The amount of this gas varies considerably between different springs. This radioactivity varies from fifty-two one-thousandths of a millimicrocurie per liter to nine and one-tenth millimicrocuries per liter, with an average of forty-six hundredths of a millimicrocurie per liter. Some of the tufa was found to contain a small amount of radium. The effect of this radioactivity from a medical standpoint has not been determined.

Hot Springs National Park consists of approximately 1,000 acres of steep mountain ridges attaining an elevation of 1,200 feet above sea-level, and narrow V-shaped valleys having an elevation of 600 feet above sea-level. The springs are in an area of about twenty acres at the base of one of these ridges known as Hot Springs Mountain. The geology has been studied in detail by various geologists. Further clarifying data could possibly be derived from deep drilling and a study of the rainfall in the collecting areas, and of the course of underground water movement in the relevant rock formations.

This discussion has been an interpretation of various references published on the subject. The following bibliography will indicate the sources of the general information for this paper:

Branner, G. C. - An Outline of the Physical Features of Arkansas; Arkansas Geological Survey publication; 1927.

Bryan, K. - The Hot Springs of Arkansas; Journal of Geology; vol. 32; No. 6, 1924, pp 449-459.

Cron, F. W. - Mineral Water at Hot Springs Arkansas; Military Engineer; March-April; 1939.

Croneis, C. G. - Geology of the Arkansas Paleozoic Area; Arkansas Geological Survey; Bulletin 3; 1930.

Fenneman, N. M. - Physiography of the Eastern United States; McGraw-Hill Book Company, Inc.; 714 pp: 1938.

Honess, C. W. - Geology of the Southern Ouachita Mountains of Oklahoma; Oklahoma Geological Survey, Bulletin 32, 1923; Pt. 1.

Miser, H. D. and Purdue, A. H. - Hot Springs Folio; Geological Atlas of the United States. U.S. Geol. Survey: 1923.

Miser, H. D. - Structure of the Ouachita Mountains of Oklahoma and Arkansas; Oklahoma Geological Survey; Bulletin 50: 1929.

Schlundt, H. - The Radioactivity of the Spring Water on the Hot Springs Reservation, Hot Springs, Arkansas; American Journal of Science; Vol. 30; July, 1935.

Weed, W. H. - Geological Sketch of the Senate Document No. 282, Hot Springs District of Arkansas; 1902; pp. 79-94.