The thermal springs are situated in the Ouachita Mountains of central Arkansas. The springs emerge in a gap between Hot Springs Mountain and West Mountain in an area about long by wide at altitudes from . The springs predominantly are composed of hot water from thousands of feet underground mixed with some shallow cold ground water. Currently, there are 43 thermal springs in the park that are presumed to be flowing. Thermal water from 33 of the thermal springs is collected and monitored at a central reservoir, which distributes the combined discharge for public use and consumption. Rock types in the area include
shale units which generally impede ground-water movement, while fractured
chert,
novaculite, and
sandstone units generally support ground-water movement. The water comes from rain which falls in mountains to the north and northeast. Flowing downward through cracked rock at about one foot per year, the meteoric water migrates to estimated minimum depths of and achieves high temperatures in the deep section of the flow path before rising along fault and fracture conduits. Under artesian pressure, the thermal waters rise and emerge through the Hot Springs Sandstone between the traces of two thrust faults, along several northeast-trending lineaments. Some rainwater from near the springs mixes with the deep hot water before discharge. The trip down takes about 4,000 years while the hot water takes about a year to reach the surface. The heat comes from
the natural heating of rocks as depth increases. The composition of the water indicates it is heated rainwater which has not approached a magmatic source, so no volcanic action is involved in the formation of these hot springs. The result is the mildly
alkaline, pleasant tasting solution with dissolved calcium carbonate.
Rock types The exposed rock types in the vicinity of the thermal springs are
sedimentary rocks of
Mississippian to
Ordovician age, with the exception of younger
igneous rocks (
Cretaceous age) exposed in two small areas about southeast of the thermal springs (Potash Sulphur Spring and
Magnet Cove, respectively), and in many small
dikes and
sills. Most dikes are less than wide. There have been 80 dikes noted about southeast of Hot Springs, on and near the
Ouachita River. There is no indication that igneous rock occurs where the thermal springs discharge. The sedimentary rocks in the vicinity of the thermal springs consist of shale, chert, novaculite, sandstone, and conglomerate. • The Stanley Shale is predominantly a
clayey, thinly fissile, black to green shale, with large amounts of sandstone interbedded throughout the formation. The sandstone, when freshly exposed, is a hard, fine-grained,
feldspathic,
silty sandstone, but weathers easily to a soft, clayey porous material ranging from green to brown in color. Almost all of the low-lying areas in the city of Hot Springs are composed of the Stanley Shale, and it surrounds Hot Springs Mountain on the south, east, and west sides. • The Hot Springs Sandstone Member of the Stanley Shale, hereafter referred to as the Hot Springs Sandstone, consists of fine- to medium-grained sandstone with some shale and conglomerate. The sandstone is gray, hard and
quartzitic, reaching thicknesses up to . The shale predominantly occurs at the top of the unit, and the principal bed of the conglomerate occurs at the bottom. • The Arkansas Novaculite consists of lower, middle, and upper members. The lower member is a massive fractured novaculite, and is the dominant member on Hot Springs Mountain, with a thickness of about The middle member is a black clay shale interbedded with novaculite, about thick on Hot Springs Mountain. The upper member is chiefly a massive, highly calcareous light gray to black novaculite. It reaches a maximum thickness of in the area. • The Polk Creek Shale and Missouri Mountain Shale overlie the Bigfork Chert and generally consist of shale with minor thin layers of quartzitic sandstone. The Polk Creek Shale is a black, fissile,
graphitic shale. The Missouri Mountain Shale varies in color, and is soft and
argillaceous. • The Bigfork Chert overlies the Womble Shale and consists almost entirely of chert and silty chert in layers thick, separated by minor thin beds of black shale. The chert is very brittle and intensely fractured from folding. • The Womble Shale is the oldest geologic unit that underlies all other exposed units. It is black, hard, and argillaceous shale. During most of the
Paleozoic Era, what became the Ouachita Mountains was the bottom of a shallow sea, where several sedimentary layers were created. About 500 million years ago a collision of the
South American Plate with the
North American Plate caused the shale and sandstone layers to fracture and fissure, creating mountains of the folded rocks. The thermal springs emerge from the plunging crest line of a large overturned
anticline in the Zigzag Mountains of the Ouachita anticlinorium. The overturned anticline plunges toward the southwest into the Mazarn Basin. There are two recognized major
thrust faults trending nearly parallel to fold axes that define the northern and southern limits of the thermal springs discharge area. The northern fault extends nearly parallel to Fountain Street northeastward about onto the southeast flank of North Mountain, and
dips about 26 degrees north. At the northern extent of the thermal springs, this fault is suggested to form along the bedding contact of the Hot Springs Sandstone and Stanley Shale, with the Stanley Shale forming the hanging wall of the fault. The southern fault extends northeastward about roughly along the axis of the Hot Springs anticline, and dips about 44 degrees north. It has been proposed that a fault splits away from the southern fault, trends west and connects with the northern fault. A natural ravine trends along the location of this fault. Extensive cracks, joints, and fissures in the Bigfork Chert, Arkansas Novaculite, and the Hot Springs Sandstone allow the water to flow in the thermal springs area. Dissolved minerals in the water precipitate to form the white to tan
travertine or "
tufa rock" seen near the openings of the hot springs. == Composition of the water ==