The last place to be explored in the current 48 contiguous states happened to be the area where the present-day states of Montana, Idaho and Wyoming converge. In the fall of 1869, three adventurers named Folsom, Cook and Peterson spent a month in what is now Yellowstone National Park. Their information prompted another and larger expedition into the area, led by Henry Washburn. Upon their return various members wrote articles, delivered lectures, and attracted national attention to the wonders they saw. This is when the US Geological Survey, headed by Dr. Ferdinand V. Hayden, became involved and in 1871 launched the most extensive of all the early Yellowstone expeditions.
Hayden’s scientists, photographers, and artists produced a wealth of information confirming the wonders of Yellowstone. Hayden gave Congress his official report and urged them to set aside the Yellowstone area. During a time when the vast wilderness of western America was viewed by most as a territory to be tamed, settled, mined, and ranched, the unique features of the Yellowstone country trumped all. Congress acted quickly and in March 1872, President Ulysses Grant signed into law an act creating Yellowstone National Park, protecting the area for future generations.
Most of the geyser basins around the park owe their existence to a mantle hotspot that caused a catastrophic eruption 640,000 years ago, forming the youngest Yellowstone Caldera. Since that time, over 80 eruptions of rhyolite and basalt have occurred in the past 160,000 years. Glaciers covered much of the area as recently as 13,000 years ago, depositing till and gravels that underlie the geyser basins. These deposits provide a storage area for the water that becomes heated and erupts to the surface as bubbling hot springs and geysers. The major geyser basins are the result of the formation over the past 14,000 years of large hydrothermal (steam)-explosion craters.
The high yellow-brown walls that form the Grand Canyon of the Yellowstone River are composed of rhyolite tuff and lava which erupted about 500,000 years ago early in the post-collapse history of the Yellowstone Caldera. The units have been intensely altered by hydrothermal fluids when a geyser basin formed in the area. The dark cliffs like the one seen to the left of the falls are little altered rhyolites. The top of the canyon, the tree-covered white rim above the brown and yellow cliffs, is composed of lacustrine sedimentary rocks deposited in a lake that once filled the caldera. The lake was eventually drained by erosion from the Yellowstone River forming this magnificent 370m deep canyon.
Native Americans called this river Mi tsi a da zi which means ‘Rock Yellow River’. French fur trappers translated this into ‘Yellow Rock’ or ‘Yellow Stone’, giving the area its present-day name, Yellowstone.
A Supervolcano and the Yellowstone Volcano Observatory (YVO)
Through its volcanic history, three major eruptions have occurred at Yellowstone. The first and largest occurred 2.1 million years ago and ejected 2,550 km³ of material. A smaller eruption was recorded at 1.3 million years ago that ejected 170 km³ of ash and lava. The current Yellowstone Caldera formed 640,000 years ago from the violent and catastrophic eruption of over 1,000 km2 of rhyolitic magma and spread ash over thousands of square miles. By contrast, the 1883 eruption at Krakatau ejected about 21 km³ of volcanic material and the 1980 Mt. St. Helens eruption a mere 0.6 km³. Both these eruptions had some major consequences to the surrounding area along with global consequences. The Krakatau eruption lowered global temperatures and disrupted climate patterns for five years. Imagine, therefore, an eruption emitting a hundred times or even a thousand times more material than historic eruptions. The effects would be devastating.
While the chance of an event of the magnitude of Yellowstone’s past eruptions is very remote during our lifetimes, the Yellowstone Volcano Observatory (YVO) was formed to study this system. Dr. Jacob Lowenstern of the US Geological Survey and Scientist-in-Charge at the YVO states “Any of the previous eruptions at Yellowstone could have devastated global human populations. The YVO was created to monitor and scrutinize the varied signals emerging from restless calderas like the one at Yellowstone. The large thermal and CO2 fluxes here require massive input of basaltic magma originating from the mantle. Overlying the basaltic magma is a high-silica magma reservoir, which caused the cataclysmic eruptions in the past and may pose a volcanic hazard in the future.”
Geophysical imaging reveals that there is a tilted deep mantle plume, whose origin goes back 17 million years, extending 500 km below the Yellowstone Caldera. As the North American Plate moved west at about 4.5 cm per year over this hotspot, a 560 km trail of volcanic fields can be found across southern Idaho and into northern Nevada. All are sites of multiple eruptions. The track starts with a large area of flood basalt and rhyolite volcanism extending hundreds of kilometers out from a center near the Nevada-Oregon-Idaho border. Most of the 234,000 km³ of the Columbia and Steens flood basalts erupted within a 2 million year interval. As the North American plate moved across this mantle plume five more major volcanic areas were left behind along the Snake River Plain, until the plume finally arrived below the Yellowstone area 2.1 million years ago.
The Upper Geyser Basin and adjacent Black Sand and Biscuit Basins contain an amazing collection of hydrothermal features along boardwalks and paths with continuous views of the Old Faithful Geyser.
Yellowstone has more than 300 geysers, two thirds of all those found on earth. Geysers erupt when superheated water is brought violently to the surface. The water starts out cold from snow melt in the mountains and slowly percolates down through permeable rocks and into a shallow magma body, where the water is heated to above 200°C. This superheated water is less dense than the cold water sinking around it and begins its journey back to the surface along fractures in the surrounding rock, most of which is siliceous rhyolite. Some of the silica is dissolved from the rhyolite and as the pressure decreases can be deposited along the fractures constricting the plumbing system, increasing the system’s ability to withstand the pressure needed to produce a geyser. At the surface, silica precipitates, forming siliceous sinter deposits known as ‘geyserite’, giving the geyser basins a seemingly barren landscape.
There are more than a dozen hot pools in the area that offer many dazzling colors from bright yellows and reds to sky blue. Viewing pools such as the Emerald Pool in the Black Sand Basin seems to give a peek into the deep, unknown underground. The temperature of this 7.6m deep pool is 68°C, which is cooler than most of the others in the park. The colors displayed in the pools and springs are primarily produced by thermophiles or heat-loving organisms such as algae, bacteria, and archaea. The color of Emerald Pool (light green) has resulted from the cooler water temperatures allowing yellow bacteria to grow lining the pool. The crystal clear water reflects the blues but absorbs other hues of the color spectrum with the combination of blue and yellow producing the emerald green color. The edges of the pools are cooler yet allowing, in this case, yellow, orange, and brownish thermophiles to grow.