Hot Springs National Park is found within the Zig-Zag Mountains, a section of the Ouachita Mountains of central Arkansas and eastern Oklahoma. The name "Zig Zag" comes from the sharply angular folds of the rock when seen from above. The Ouachita mountains were formed during the collision of two tectonic plates around 300 Ma (million years ago), which is called Tectonic Orogeny, or a mountain-building event. As these tectonic plates collided, the layers of rock were bent and broken (folded and faulted). 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.
The area of the park is characterized by steep, mountainous terrain. Although once more jagged, time has reduced the steep peaks of ancient mountains into more gently rolling hills. Visitors will notice that rock layers exposed along bluffs or road-cuts appear to be tilted, or even to stand up straight, rather than lay flat. This tilting of rock layers is a result of extreme forces that "pushed" the rocks into the tilted position, which we can observe today.
Hot spring water cascade pool at Arlington Lawn, north end of Bathhouse Row
NPS Photo/Mitch Smith
The rocks found within the park are approximately 400 million years old and are mostly sedimentary in nature. Rocks like sandstone, shale, chert and novaculite were orginally formed in the deep ocean environments of the Carboniferous Period.
Arkansas Novaculite is a very special rock made up of very small quartz crystals, and is quite dense and hard. The novaculite is well known for its use as a whetstone, or a stone used to sharpen cutting tools such as knives. The sedimentary rocks that were folded and uplifted to form the Ouachita mountains were originally flat-lying. As the land was pushed together, made their way to the top of our present day mountains and can be found in outcroppings throughout the park.
A simplified cross section of Hot Springs’ geology, showing the flow of rainwater in the sub-surface.
NPS Photo
Up From the Depths
Temperature in the Earth increases with depth below the surface, something known as the geothermal gradient. The deeper that you travel towards the Earth's core, the hotter the rocks become.
Remember the foldsandfaults formed during the building of the Ouachita mountains? They create a route which allows rainwater to travel down 6,000 to 8,000 feet below the surface, slowly heating up as it travels deeper and deeper. The water travels for 4,000 years before hitting a fault line and relatively quickly, or about 400 years, reaching the surface in what is now the historic downtown area of Hot Springs National Park, along Bathhouse Row.
So, in short, when rain falls on the recharge zone, it follows the faults and cracks to a depth of 8,000 feet and then re-emerges approximately 4400 years later, at an average temperature of 143° Fahrenheit (62° Celcius).
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/Mitch Smith
The hot water that emerges at the park contains a variety of dissolved minerals that come from the water's interaction with rocks both deep within and near the earth's surface. Much like dissolving sugar in sweet tea, the hot water dissolves some of these minerals from the rocks, bringing them to the surface. These minerals include silica, calcium, calcium carbonate, magnesium, and potassium.
Of these, one of the most noticeable minerals is the calcium carbonate. When the hot, thermal water reaches the surface it cools. As the cooling occurs, calcium carbonate, also known as limestone, is deposited. Visitors will notice a light grey, "spongy" looking rocks near some of the park's display springs, espcially at the Hot Water Cascade on Arlington Lawn. This limestone rock is also referred to as tufa.
