What is hydrothermal alteration, and why is it important? Most visitors to Yellowstone National Park are only vaguely aware of hydrothermal alteration (chemical and mineral reactions with hot water). Nonetheless, hydrothermal alteration changes the composition of natural thermal fluids and, over time, can substantially alter the make-up, appearance and physical properties of rocks. In addition, without hydrothermal alteration, Yellowstone would not have sinter deposits like at Castle Geyser (Upper Geyser Basin) or Fishing Cone (West Thumb Geyser Basin), travertine deposits like those at Mammoth Hot Springs or mud pots like those at Mud Volcano or Fountain Paint Pots. And rocks exposed in Yellowstone would be far less colorful and varied without changes caused by hydrothermal alteration.
Let’s briefly examine how hydrothermal alteration works, and then look at few examples. The different types of hydrothermal waters in Yellowstone include silica-rich alkaline-chloride water; acid-sulfate waters; calcium-carbonate waters; and gas and steam in fumaroles.
The waters that eventually become hot springs are from rain and snow that recharges rivers, lakes and subsurface aquifers. As these waters flow deep beneath the ground, they are heated due to thermal flux from the magma reservoir beneath the caldera to become hydrothermal fluids.
Mudpots that occur in thermal basins throughout Yellowstone National Park are one of the most visible and colorful manifestations of hydrothermal alteration at the surface. Mudpots are created by acid sulfate fluids that consist of gases derived from boiling and also from deep magma (mainly carbon dioxide and hydrogen sulfide) that mixes with near-surface waters. The hydrogen sulfide reacts with oxygen to form sulfuric acid, creating very acidic waters that aggressively react with rocks and create mudpots at the surface and hydrothermally-altered rocks in the shallow subsurface.
Much is known about subsurface hydrothermal alteration related to geyser and thermal basins in the park due to a research drilling program carried out by the USGS in the late 1960s that sampled rocks as deep as 1,088 feet. Numerous studies by USGS and other researchers have shown two types of alteration: alkaline-chloride water alteration that produced minerals like chalcedony, quartz, chlorite, calcite, rhodochrosite, mordenite, fluorite, pyrite and smectite; and later acid-steam alteration in cores and in surface mudpots produces opal, kaolinite, alunite, pyrite and smectite.
An important exposure of hydrothermally-altered rocks occurs in the Grand Canyon of the Yellowstone, where dramatic yellow, orange and red rocks and sediments are a result of hydrothermal alteration of rhyolite volcanic rocks. The outcrops in the incised canyon provide a unique opportunity to view the exposure of what was once the subsurface portion of a significant thermal basin.
Some studies show that hydrothermal alteration of high-silica rhyolite produces adularia, clay minerals, silica minerals, disseminated pyrite and marcasite, zeolite minerals, iron oxides, native sulfur and sulfate minerals. This is what gives the canyon its spectacular colors.
Downstream a few miles from the Lower Falls at the Seven Mile Hole hydrothermal area, distinct zoning is apparent where the rhyolites in the upper 100 meters of the canyon are altered to opal and chalcedony, kaolinite, alunite and minor barite, and disseminated pyrite and marcasite by acid-sulfate fluids at temperatures below roughly 150 °C. Deeper in the canyon, an assemblage of quartz, illite and adularia formed by reaction of rhyolitic rocks with alkaline-chloride fluids at temperatures above roughly 150 °C.
Hydrothermal alteration is everywhere in Yellowstone. It’s happening right now at the many thermal basins, and evidence of it happening in the past is in the colorful rocks all across the landscape.