Hot Springs Geology

view of cascade basin from above; park ranger gives talk about water while visitors sit on edge concrete basin
Hot spring water cascade pool at Arlington Lawn, north end of Bathhouse Row

NPS photo by Gail Sears

Up From the Depths


The story of the hot springs in Hot Springs National Park begins with rocks that formed approximately 400 million years ago. Rocks called chert and novaculite were formed in the deep ocean environments of the Carboniferous Period. Sediments that would become shale and sandstone were also deposited in the warm, shallow waters of this environment.

 
Simplified drawing of rock layers, faults, and water movement.
A simplified cross section of Hot Springs’ geology, showing the flow of rainwater in the sub-surface.

NPS Photo

As the Earth’s plates moved and shifted into the supercontinent called Pangaea, the layers of rock were bent and broken (folded and faulted) into what is now known as the Ouachita Mountain Range. This deformation of the rock layers created cracks in the rocks. When water falls as rain on the mountains, it can seep into the ground.

Temperature in the Earth increases with depth below the surface, something known as the
geothermal gradient. This causes the water to heat up as it follows cracks down 6000 to 8000 feet. The folds and faults formed during the building of the Ouachita mountains create a route which allows the water to quickly reach the surface in what is now the historic downtown area of Hot Springs National Park, along Bathhouse Row. When the water emerges through the Hot Springs Sandstone, approximately 4400 years later, its average temperature is 143° Fahrenheit (62° Celcius).
 
Thermal water flows over tufa rock and green algae.
The grey rock that forms when thermal water evaporates and cools is called tufa.  This rock can be observed in several places along Bathhouse Row.

NPS Photo

Like the sugar in sweet tea, the hot water dissolves some of the minerals from the rocks, bringing them to the surface. The cherts, novaculite, sandstone and shales are made of quartz, calcite, and clay minerals. These minerals supply elements, including silica, calcium, bicarbonate, magnesium, and potassium, that can be found in the hot springs water. When the water cools and evaporates, the dissolved elements solidify into a new sedimentary rock called tufa.

Last updated: September 4, 2019

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Hot Springs , AR 71901

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