|Introduction||Hotspot and Magmatism|
|Regional Volcanism||Hydrothermal Features|
|Recent and Future Tectonic Events||Summary|
One of the most iconic parts of the United States is Yellowstone National Park. Founded in 1872, Yellowstone was the first national park in the world. Many of its most famous landmarks and attractions were formed due to the caldera of the supervolcano found there. Because of this, plate tectonics are key to understanding the formation and behavior of this region.
|Old Faithful is Yellowstone’s most famous and most popular attraction. Unlike most geysers, Old Faithful erupts at a regular rate, approximately once per hour. Photo by the author.|
Yellowstone was formed due to a hotspot beneath the North American lithospheric plate. Many hotspots are interpreted as forming due to mantle plumes rising from the deep mantle, but geophysical and helium isotope data contradict this model for Yellowstone (Christiansen et al, 2002). Rather, it appears to have an upper mantle origin, possibly caused by the heat flow surrounding a residual slab of the subducting Juan de Fuca plate (Fouch, 2012).
The magma chamber for Yellowstone is relatively shallow, at only about 4.3 km depth. The magma within the chamber is bimodal. The upper part of the chamber is dominated by magma of a rhyolitic composition. This magma is primarily solidified, with melt only comprising approximately 32% overall, with another 8% being water and carbon dioxide (Chu et al, 2009). This rhyolitic “crystal mush” traps basaltic magma upwelling from the mantle, forcing it to erupt in the surrounding margins of Yellowstone, but not within it (Christiansen et al, 2002).
TThe Yellowstone Volcanic Complex has produced three major welded tuff members with minor basalt interbedding between them (Christiansen 2001). The oldest member is the Huckleberry Ridge Tuff, followed by the Mesa Falls Tuff. The Youngest member is the Lava Creek Tuff, which contains the modern-day caldera. The eruption events associated with these tuffs ejected ash into the air which settled and is found in sedimentary deposits throughout western North America.
|Hilltop featuring the Huckleberry Ridge and Lava Creek tuffs lying unconformably atop Cretaceous shales. Photo courtesy of the United States Geological Survey.|
|Map of the United States illustrating the extent of ashfall from the three major eruptions of the Yellowstone volcano. The Bishop ash bed from Long Valley caldera and the ash bed from the 1980 explosion of Mount St Helens shown for comparison. Courtesy of the United States Geological Survey.|
The tuffs and ashfalls can be radiometrically dated, which makes them invaluable for use in stratigraphy for the Pliocene and Pleistocene (Lanphere et al, 2002). Each of these eruptions ejected between 280 and 2,450 cubic km of material (USGS, 2014), enough to refer to them by the colloquial phrase “super eruption”. The last super eruption occurred approximately 640,000 years ago, but lesser eruptions continued to occur until 70,000 years ago (Lowenstern et al, .2006).
The caldera itself actively rises and subsides from time to time. Two resurgent domes which act independently of one another have been identified. It is uncertain why this deformation occurs, but two hypotheses have been put forth. One is that the upwelling is caused by heat flow from the magma chamber, which could indicate impending volcanic activity. The other hypothesis is that the resurgent domes are related to hydrothermal activity in the region (Lowenstern et al, 2006).
The most famous tectonic characteristics of Yellowstone are its numerous hydrothermal features. Heat from the magma chamber rises to warm a briny confined aquifer. The heat is then transferred to fresh groundwater which percolates through fractures in the rock. This superheats the water, causing it to rise rapidly to the surface (United States Geologic Survey, 2014). Approximately half of all known hydrothermal features in the world can be found here (National Park Service, 2014). There are several different types of features, each with its own distinctive appearance and behavior.
If the rising water is unconfined at the surface, the result is a hot spring. The lack of pressure and circulation due to convection prevent the water from erupting. Hot springs are the most abundant hydrothermal feature in Yellowstone (National Park Service, 2014).
|Hot springs in the Fountain Paint Pots. Orange colors are due to thermophilic bacteria in the water, and blues are due to light diffraction. Photo by the author.|
Fumaroles are a result of high heat flow and low water volume in a hydraulic conduit. They are typically found on hillsides, so they contain less water than other hydrothermal features. Extreme heat causes the water to evaporate completely before reaching the surface, so all that escapes is steam (National Park Service, 2014).
|Left: Beryl hot spring. Note the fumaroles on the hillside behind the spring (closeup on the right). Photos by the author.|
Some fumaroles have chemotrophic bacteria living within their conduits. These bacteria consume hydrogen sulfide gas rising from the magma chamber, producing sulfuric acid as a waste product. The acid breaks down the feldspars in the rock, converting them to clay minerals, which mix with water to form mud. Volcanic gases continue to rise through the mud, causing them to bubble. This is known as a mudpot (National Park Service, 2014).
The most recognized hydrothermal features of Yellowstone are the geysers, typified by Old Faithful (see above). Unlike hot springs, geysers have constrictions somewhere within the conduit bringing water to the surface. These constrictions prevent water from flowing freely to the surface. Heat, steam, and liquid water together cause a buildup of pressure until they are able to push past the restriction, erupting to the surface forcefully enough to create a large jet of water which rises high into the air (National Park Service, 2014).
In 2002, an earthquake of magnitude 7.9 occurred in the Denali fault of central Alaska. Even though Yellowstone is 3100 km from the epicenter of the earthquake, it was strong enough to feel and to cause disruptions in hydrothermal activity. Certain hot springs, which had never been recorded as exhibiting geyser behavior, began erupting, spewing steam and boiling water into the air (Husen et al, 2004).
The Yellowstone region itself is also still tectonically active. Between 1000 and 3000 earthquakes occur in and around the caldera each year, though most are unnoticeable, but some, such as one which occurred in 1959, are strong enough to damage roads and structures within the park. Some of these are associated with heat flow from the magma chamber, but many are instead considered to be caused by extension of the Basin and Range Province to the south (United States Geological Survey, 2014).
|Some of the damage caused by the 1959 earthquake. The earthquake caused a landslide, which swept this house and the road into the lake. Photo courtesy of the United States Geological Survey.|
One rumor which has frequently been circulated in recent times is the idea that Yellowstone is due to have another catastrophic eruption in the near future. Evidence for this is suggested to include the seismicity, recent ground deformation, and an increase in helium emissions from the caldera. However, this scenario is considered highly unlikely. Geophysicists at the Yellowstone Volcano Observatory interpret recent earthquakes as being related to overall uplift of the region, while the ground deformation is within standard historical norms. While the helium emissions have increased marginally, it is not the volume of helium being released which indicates volcanic activity, but rather the isotope ratios within that helium (United States Geological Survey, 2014).
Yellowstone was formed as a result of a hotspot rising through the North American plate. Its magma chamber is bimodal, featuring basaltic magma in the lower chamber and rhyolitic in the upper portion. The volcano has had three super eruptions in its history, each ejecting large amounts of ash and rhyolite tuffs. Subsidence and uplift are both common within the caldera in modern times. Yellowstone contains half of all known hydrothermal features in the world. There are several types of these features, each with different characteristics. Earthquakes are common in Yellowstone, and though most are not felt, some can be quite strong. While some people believe that another super eruption is imminent, it is considered unlikely by the scientific community at large.