Geology and Ecology of National Parks

Hot Springs National Park

Hot Springs National Park in Arkansas primary resource is the hot springs that flow from the southwestern slope of Hot Spring Mountain.

Check out the National Park Service Tour of Bathhouse Row!

 

Hot Springs National Park was once known as Hot Springs Reservation. It was set aside in 1832 to protect the Park's primary resource, the thermal hot springs.  This type of Reservation was an early version of the National Park idea, as it was the first area in the United States to be set aside for its natural features by the federal government. In 1916 the National Park Service was formed and in 1921, Hot Springs Reservation changed its name to Hot Springs National Park. This change made it the 18th National Park in the Service.

Spring Tulips at Hot Springs National Park

Spring Tulips at Hot Springs National Park

(Credit: NPS. Public domain.)

Hot Springs National Park is best known for the 47 hot springs that emerge from Hot Springs Mountain at an average temperature 143° Fahrenheit.  The National Park Service, USGS, and other collaborating science agencies work to understand the nature and characteristics of these thermal springs--including plumbing framework, mechanism of flow, source of water and heat , and potential impacts on thermal water quality and quantity--so that this resource can be protected and sustained. The fact that most of the flow path of the thermal springs is hidden from human eye beneath the mountains and valleys of the Ouachita Mountains makes gaining knowledge of the springs a challenging task. Hot Springs National Park is the only unit of the national park system that is mandated to give away its primary natural resource to the general public in an unending and unaltered state. The water is naturally potable (good to drink) when it arrives at the surface of Hot Springs Mountain. The water is naturally potable (good to drink) when it arrives at the surface of Hot Springs Mountain. Thousands of visitors highly endorse the excellent quality of the hot springs water and fill vessels to take home. Two bathhouses operate in the Hot Springs National Park where one can fully experience the water, and drinking fountains provide access for people to enjoy the water that has given the park and the city its name.  In addition to the ample amount of thermal water available to the public, the park also has an outstanding geologic and ecologic history that visitors are sure to enjoy.

Hot Springs National Park protects the geothermal spring water and associated land located in the Ouachita Mountains of Arkansas.  The mountains within the park are also managed within this conservation philosophy in order to preserve the hydrological system that feeds the springs. The park and its surrounding mountains exhibit a south-central United States pine-oak-hickory forest ecosystem. The park's vegetation, thermal waters, cold water springs, bathhouses and associated cultural features, foot trails, prehistoric and historic novaculite quarries, and general physiography combine to form an almost 5400-acre area of resource preservation and interpretation that is under the exclusive legislative jurisdiction of the federal government.

Geologic History

Hot Springs National Park lies in the Zigzag range, which forms the southern limb of the Ouachita Mountains. The Ouachita Mountains are part of an ancient chain of folded and faulted rocks that extend from Central Mississippi to Texas. The Ouachitas began their formation during the Carboniferous Period (286 to 360 million years ago) as plates collided along what is now the eastern seaboard and the gulf coast of North America. The orogenic events that shaped these mountains are estimated to have occurred in the Late Pennsylvanian Period or during the Early Permian Period (about 260 million years ago).

While the sedimentary rocks were originally deposited in accordance to the Law of Original Horizontality (sediment is deposited horizontally under the action of gravity), mountain-building processes arranged the strata into what we see today. Folding occurred when the stress of geologic forces created rumples in the rock strata, and normal and reverse faulting occurred as well. The Ouachita Mountains are largely composed of folds and thrust faults, which are created when a reverse fault breaks in a low angle. In the Hot Springs area, small thrusts typically displace strata towards the Southeast, with the highest ridges of the Zigzag range being composed of Arkansas Novaculite. These forces create anticlines and synclines, which can be found throughout the Ouachitas.

Stratigraphy

Hot Springs National Park now lies in the heart of a northern bulge of the Ouachita Thrust Belt, which was once a reentrant in an ocean basin. Before the collision of plates in the Carboniferous Period caused the folding and faulting of what we now call the Ouachita Mountains, ancient seas ruled Central and South Arkansas. Due to the abundant supply of various types of marine sediments, the Ouachitas are composed of sedimentary rocks, with some thermal alteration.

The oldest of the sedimentary rocks date from the Late Cambrian Period (500 million years ago) to the Early Mississippian Period (about 350 million years ago). Exceeding 8,000 feet of thickness, the strata are largely composed of deep marine shales, limestones, sandstones, and cherts. These strata include Hot Springs Sandstone and our famous Arkansas Novaculite. Overlying these strata are the Stanley Shale, Jackfork Sandstone, Johns Valley Shale, and the Atoka Formation, all formed in the Carboniferous Period. Due to the folding and faulting of the Ouachita Mountains, thermal spring water escapes through the Hot Springs Sandstone. In addition to the sandstone and shale one may see in Hot Springs National Park, visitors will also see tufa- a precipitated carbonate rock that forms around the mouths of the hot springs.

Hot Springs National Park Water Cascade

Hot Water Cascade at Hot Springs National Park

(Credit: NPS. Public domain.)

Hydrology

Hot Springs Sandstone of the Early Mississippian Period generally ranges from 10 to 250 feet in thickness. The thermal springs found in Hot Springs National Park originate exclusively from fracture zones in this sandstone. While there are other minor springs, the majority of the springs found within our park originate from the northwest limp and axis of the Hot Springs Mountain Anticline. The hot springs flow upward through large, open fractures and are effectively sealed by the overlying Stanley Shale.

When many people think of hot springs, there is often the association of volcanoes, geysers, and perhaps underground chambers of magma. While these features are often found in association with many thermal springs around the world, this is not the case in Hot Springs National Park. In Central Arkansas, the earth is relatively quiet. There is no evidence of magma beneath the surface to heat the thermal water. Instead, many geologists believe that the right combination of rock types and fractures allow for water to travel from the surface to areas deep within the Earth. As water percolates down into the earth, it is then heated by surrounding rock. In Hot Springs National Park, this process takes approximately 4,000 years! That’s right. The water we collect and distribute today in our park was once rainwater over 4,000 years ago. Thanks to our highly porous Bigfork Chert and Arkansas Novaculite, rainwater can travel down these conduits to depths between 2,000 and 8,000 feet.

A natural thermal gradient heats the water. As the water moves deeper within the Earth, the hotter the water becomes. At great depths, the heated water meets fractures and faults in the Hot Springs Sandstone, and the water is quickly brought to the surface as one of our many thermal hot springs. On average, the water arriving to the surface is approximately 143 degrees Fahrenheit.

Most springs found within Hot Springs National Park are capped by green collection boxes, which can be found throughout the park. Beneath the lids, one will find stainless steel cylinders connected to the springs. While the springs previously flowed without containment, caps are now essential and necessary to preserve the purity and health of our natural thermal water. The thermal water is then routed to a larger reservoir beneath bathhouse row in Hot Springs National Park to be redistributed for public use.

In addition to capping our most valuable resource, our hydrologic technician for the park monitors 35 of the 47 spring boxes for temperature, conductivity, dissolved oxygen, barometric pressure, and pH. Further mineral testing is conducted through the Buffalo National River, and through the United States Geological Survey. Thanks to these efforts, Hot Springs National Park can safely distribute over 600,000 gallons a day for public use.

Purple Flower at Hot Springs National Park

(Credit: NPS. Public domain.)

Biology and Ecology

Hot Springs National Park is home to a variety of flora and fauna. On our 5,400 acres and 26 miles of trails, visitors often encounter white-tailed deer, American Robins, and perhaps even a few species of special concern, such as the Bald Eagle. While there are no federally endangered or threatened species within Hot Springs National Park, we encourage all visitors to be aware of our species of special concern found within the park.

Plant species of special concern in Arkansas:

Lobed spleenwort (Asplenium pinnatifidum)

Ouachita Blazing-star (Liatris compacta)

Hairy-flower Arkansas Bedstraw (Galium arkansanum var. pubiflorum)

Graves’ spleenwort (Asplenium x gravesii)

Dwarf spiderwort (Tradescantia longipes)

 

Animal species of special concern in Arkansas:

Bald Eagle

Ouachita madtom (Noturus lachneri)

Southeastern myotis bat: Not only is the Southeastern myotis bat a species of concern in Arkansas, but also the Northern Long-eared Bat. The biggest threat facing both species is white-nose syndrome, a fungus that has killed nearly 6 million bats in the eastern portion of North America alone. White-nose syndrome is spread in three ways: Bat-to-bat, soil-to-bat, and human-to-bat.

What you can do:

Decontaminate your gear with Clorox or Lysol before and after each cave visit.

Stay out of caves or mines where bats are known to hibernate during winter months.

Honor cave closures.

Report dead or dying bats.

 

Publications:

Kresse, T.M., and Hays, P.D., 2009, Geochemistry, comparative analysis, and physical and chemical characteristics of the thermal waters east of Hot Springs National Park, Arkansas, 2006–09: U.S. Geological Survey Scientific Investigations Report 2009-5263, 48 p.

Bell, R.W., and Hays, P.D., 2007, Influence of Locally Derived Recharge on the Water Quality and Temperature of Springs in Hot Springs National Park, Arkansas: U.S. Geological Survey Scientific Investigations Report 2007-5004, 46 p.

Yeatts, D.S., 2006, Characteristics of thermal springs and the shallow ground-water system at Hot Springs National Park, Arkansas:   U.S. Geological Survey Scientific Investigations Report 2006-5001, 42 p.

Petersen, J.C. and Justus, B.G., 2005, The fishes of Hot Springs National Park, Arkansas, 2003:  U.S. Geological Survey Scientific Investigations Report 2005-5126, 17 p.

Roberts, M. T., Stone, C.G., Blaeuer, M., and Shugart, S., Hot Springs National Park Geologic Excursion Tour Guide: U.S. Department of the Interior, National Park Service.