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.
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).
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: October 2, 2020