Our region is home to many unusual geothermal phenomena, including a mysterious hot cave (106°F) about 12 miles south of Lajitas as well as abandoned mercury mines flooded with hot water (113°F) at a depth of about 900 feet around Terlingua. The area also has several miniature geysers that spurt hot water (140°F to 194°F) nearly two feet into the air from a series of small hot springs about 4 miles southwest of Candelaria. These curiosities, among many others, are described by Christopher D. Henry in “Geologic Setting and Geochemistry of Thermal Water and Geothermal Assessment, Trans-Pecos, Texas” published in 1979. It constitutes one of the most comprehensive surveys of hot-water features ever conducted in our area, whose most common sites involve hot springs that occur “where a somewhat fortuitous combination of circumstances allows the hot water to reach the surface,” writes Henry.
In an article titled “Geysers and hot springs” on the EBSCO Information Services website, William R. Hackett describes the water in hot springs “as rain and snowfall, which percolates several kilometers down into the Earth’s crust through permeable volcanic rocks and sediments.” The temperature of these rocks increases due to our planet’s geothermal gradient, which Hackett defines as “the rate at which temperature increases with depth in the Earth.” Henry reports that “[t]hermal gradients are at least 30°C [per kilometer of depth] … and possibly higher along the Rio Grande. The increase in thermal gradient and heat flow may be due to progressive thinning of the crust” beneath the Trans-Pecos region. This rise in the subsurface temperature heats the water, and because the “heated water is less dense,” it “is forced upward along fractures by descending cold water,” writes Henry. Depending on local conditions, “the complete cycle from snowflake to hot spring may take centuries or millennia,” according to Hackett.
Hot springs require very specific physical and thermal conditions. “If there is no focusing pathway to the surface, fluids may remain underground. If the heat source is not hot enough, or if there is too much cool groundwater, then warm springs may result,” according to Hackett. Hot springs are also very fragile. “Any disruption of the delicate plumbing systems by humankind or nature involving the extraction of fluids, injection of fluids, or perturbation of the water table will almost always yield unpredictable and possibly detrimental consequences to nearby thermal features,” writes Hackett.
By the same token, hot springs themselves can have major effects on local environments because their high soil temperatures destroy plant roots and their noxious vapors can form “death traps” for birds and small animals, according to Hackett. But hot springs also “sustain unique ecosystems that include specialized microorganisms and plants … and even entire food chains of colorful algae, flies, and rare spiders … wholly dependent on the supply of warm water.” The ecological importance of hot springs is further evidenced by their often “vivid surface coloration” for which “microorganisms are largely responsible,” writes Hackett, as well as the “birds and mammals that congregate near the year-round open water and green vegetation of hydrothermal features.”
Henry defines three distinct areas in the Trans-Pecos where hot springs occur: “the southern Hueco Bolson adjacent to the Quitman Mountains; the Presidio Bolson and its structural extension to the north, and the Big Bend region.” The most significant site in the southern Hueco Bolson is Indian Hot Springs, “a cluster of seven geothermal springs with high mineral constituents” located along the Rio Grande River about 25 miles south of Sierra Blanca, according to the Texas Historical Commission (THC). The name is apparently derived from the frequent use of the springs by Native peoples before European settlement of the area. Henry measured the temperatures of these springs in 1976 and found the highest recorded temperature of any thermal spring in Texas at 117°F.
The best-known geothermal feature in the Presidio Bolson is Chinati Hot Springs, which Henry refers to by the earlier name of Ruidosa Hot Springs. Located about 8 miles north of Ruidosa, the waters of these springs emanate “from a concrete enclosure on a terrace … evidently from gravels along a small bluff overlooking the terrace” about 10 feet above the adjacent creek. Henry measured a temperature here of 113°F.
The Big Bend region boasts perhaps the most famous hot springs in the whole state known simply as Hot Springs or Boquillas Hot Springs (and formerly as Langford Hot Springs) in Big Bend National Park. Henry describes at least six hot springs here along the Rio Grande on both sides of the border, which mostly “occur in impenetrable cane thickets.” The Hot Springs location inside the national park, however, features a manmade stone tub above the Rio Grande, where the hot water can accumulate before overflowing into the river below. Henry measured a temperature here of 104°F.
Perhaps the most fascinating geothermal features in our region involve two obscure artesian wells drilled by Gulf Oil Corporation back in 1965. Originally intended as part of an exploratory effort to find petroleum, these super-deep, highly prolific wells known as Presidio Trust #1 and Swafford #1 (each with a highest-reported temperature of 180°F) ended up tapping some of the hottest water ever found in the Trans-Pecos by chance. Indeed, as Henry points out: “Some thermal waters do not reach the surface but have been discovered accidentally; thus, there should be many as yet undiscovered thermal convection systems, particularly in [the] Presidio and Hueco Bolsons.”
To learn more, visit www.beg.utexas.edu/publications/geologic-setting-and-geochemistry-thermal-water-and-geothermal-assessment-trans-pecos.
Trey Gerfers serves as general manager of the Presidio County Underground Water Conservation District. A San Antonio native, he has lived in Marfa since 2013 and can be reached at tgerfers@pcuwcd.org.