Geology of Hot Springs National Park
Hot Springs National Park is famous for the soothing and healing properties of the natural hot spring water, so it stands to reason that you ought to find out a little about where it comes from before your visit. It really will help your understanding of the whole place – trust me!
Most of the time we associate hot springs with images of volcanoes, geysers, molten rock and magma, but there are no volcanoes in Hot Springs Arkansas, this has got nothing to do with magma lying just below the surface of the earth, this is simply a combination of old faults and rock types which create the perfect scenario for water to permeate far below the earths surface and become heated by the surrounding rocks.
The thermal springs which supply Hot Springs National Park with the hot springs are actually situated within the Ouachita Moutain Range, central Arkansas. These springs pop up in a space between West Mountain and Hot Springs Mountain and are predominantly composed of hot spring water which comes from deep underground, thousands of feet, plus a little colder, shallow ground water. There are presently 43 thermal springs thought to be still flowing in Hot Springs National Park. The water from 33 of these springs is collected in a central reservoir where it can be monitored and distributed as and when it is needed. The rocks contributing to the mineral properties of the hot spring water include shale, novaculite, chert and sandstone.
The water starts out as rain which falls over the mountains towards the north and northeast of Hot Springs National Park. This flows downwards (well, rain doesn’t often flow upwards does it) through cracked rocks at a rate of about a foot per year, the water migrating to depths of between 4,500 and 7,500 feet reaching incredibly high temperatures. These thermal waters then rise due to the artesian pressure and pop up in the Hot Springs Sandstone which is found between two thrust faults. Some of the rainwater close to the springs mixes in with the hot, spring water before it is discharged. For those of you who like fascinating facts – it takes around 4,000 years for the water to reach the bottom of its journey, and only one year for the hot water to return to the surface, which by my calculations would make the bathing water at least 4,001 years old – fascinating. By the time the water reaches the surface it has an average temperature of 143 deg F.