Hot Springs National Park - Geology

Geology

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 1,500 feet (460 m) long by 400 feet (120 m) wide at altitudes from 576 to 683 feet (208 m). 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 a year, the meteoric water migrates to estimated minimum depths of 4,500 to 7,500 ft (2,300 m) 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.