Yellowstone Handbook 2019

The landscape of Yellowstone National Park is the result of many geological processes. Here, glacial erratics (foreground), ground moraines (midground), and Cutoff Mountain (background) appear near Junction Butte. Geology miles in diameter) is extremely hot but solid due to immense pressure. The iron and nickel outer core (1,400 miles thick) is hot and molten. The mantle (1,800 miles thick) is a dense, hot, semi-solid layer of rock. Above the mantle is the relatively thin crust, three to 48 miles thick, forming the continents and ocean floors. In the key principles of Plate Tectonics, Earth’s crust and upper mantle (lithosphere) is divided into Yellowstone’s Geologic Significance Yellowstone continues today as a natural geologic laboratory of active Earth processes. • One of the most geologically dynamic areas on Earth due to a shallow source of magma and resulting volcanic activity • One of the largest volcanic eruptions known to have occurred in the world, creating one of the largest known calderas • More than 10,000 hydrothermal features, including approximately 500 geysers—the most undisturbed hydrothermal features left in the world • The largest concentration of active geysers in the world—more than half of the world’s total • Mammoth Hot Springs, one of the few places in the world where active travertine terraces are found. • Site of many petrified trees formed by a series of andesitic volcanic eruptions 45 to 50 million years ago What Lies Beneath Yellowstone’s geologic story provides examples of how geologic processes work on a planetary scale. The foundation to understanding this story begins with the structure of the Earth and how this structure shapes the planet’s surface. Earth is frequently depicted as a ball with a central core surrounded by concentric layers that culminate in the crust or outer shell. The distance from Earth’s surface to its center or core is approximately 4,000 miles. The core of the earth is divided into two parts. The mostly iron and nickel inner core (about 750 Geology 107 GEOLOGY The landscape of the Greater Yellowstone Ecosystem is the result various geological processes over the last 150 million years. Here, Earth’s crust has been compressed, pulled apart, glaciated, eroded, and subjected to volcanism. All of this geologic activity formed the mountains, canyons, and plateaus that define the natural wonder that is Yellowstone National Park. While these mountains and canyons may appear to change very little during our lifetime, they are still highly dynamic and variable. Some of Earth’s most active volcanic, hydrothermal (water + heat), and earthquake systems make this national park a priceless treasure. In fact, Yellowstone was established as the world’s first national park primarily because of its extraordinary geysers, hot springs, mudpots and steam vents, as well as other wonders such as the Grand Canyon of the Yellowstone River. GEOLOGY many plates, which are in constant motion. Where plate edges meet, they may slide past one another, pull apart from each other, or collide into each other. When plates collide, one plate is commonly driven beneath another (subduction). Subduction is possible because continental plates are made of less dense rocks (granites) that are more buoyant than oceanic plates (basalts) and, thus, “ride” higher than oceanic plates. At divergent plate boundaries, such as midocean ridges, the upwelling of magma pulls plates apart from each other. Many theories have been proposed to explain crustal plate movement. Scientific evidence shows that convection currents in the partially molten asthenosphere (the zone of mantle beneath the lithosphere) move the rigid crustal plates above. The volcanism that has so greatly shaped today’s Yellowstone is a product of plate movement combined with convective upwellings of hotter, semi-molten rock we call mantle plumes. At a Glance Although a cataclysmic eruption of the Yellowstone volcano is unlikely in the foreseeable future, real-time monitoring of seismic activity, volcanic gas concentrations, geothermal activity, and ground deformation helps ensure public safety. Yellowstone’s seismograph stations, monitored by the by the University of Utah for the Yellowstone Volcano Observatory, detect several hundreds to thousands of earthquakes in the park each year. Scientists continue to improve our capacity to monitor the Yellowstone volcano through the deployment of new technology. Beginning in 2004, scientists implemented very precise Global Positioning Systems (GPS), capable of accurately measuring vertical and horizontal groundmotions to within a centimeter, and satellite radar imagery of ground movements called InSAR. These measurements indicated that parts of the Yellowstone caldera were rising at an unprecedented rate of up to seven centimeters (2.75 in) per year (2006), while an area near the northern caldera boundary started to subside. The largest vertical movement was recorded at the White Lake GPS station, inside the caldera’s eastern rim, where the total uplift from 2004 to 2010 was about 27 centimeters (10.6 in). The caldera began to subside during the first half of 2010, about five centimeters (2 in) at White Lake so far. Episodes of uplift and subsidence have been correlated with changes in the frequency of earthquakes in the park. 108 Yellowstone Resources and Issues Handbook, 2019 On March 30, 2014, at 6:34 am Mountain Daylight Time, an earthquake of magnitude 4.8 occurred four miles north-northeast of Norris Geyser Basin. The M4.8 earthquake was felt in Yellowstone National Park, in the towns of Gardiner and West Yellowstone, Montana, and throughout the region. This was the largest earthquake at Yellowstone since the early 1980s. Analysis of the M4.8 earthquake indicates a tectonic origin (mostly strike-slip motion) but it was also involved with unusual ground uplift of 7 centimeters at Norris Geyser Basin that lasted 6 months. Energy and groundwater development outside the park, especially in known geothermal areas in Island Park, Idaho, and Corwin Springs, Montana, could alter the functioning of hydrothermal systems in the park. More Information Anderson, R.J. and D. Harmon, eds. 2002. Yellowstone Lake: Hotbed of Chaos or Reservoir of Resilience? Proceedings of the 6th Biennial Scientific Conference on the Greater Yellowstone Ecosystem. Yellowstone Center for Resources and George Wright Society. Christiansen, R.L. 2001. The Quaternary and Pliocene Yellowstone Plateau volcanic field of Wyoming, Idaho, and Montana. Reston: U.S. Geological Survey. Professional Paper 729–6. Fritz, W.J. and R.C. Thomas. 2011. Roadside Geology of Yellowstone Country. Missoula: Mountain Press Publishing Company. Good, J.M. and K.L. Pierce. 1996. Interpreting the Landscapes of Grand Teton and Yellowstone national parks: Recent and Ongoing Geology. Moose, WY: Grand Teton Natural History Association. Grotzinger, J.P. and T.H. Jordan. 2014. Understanding Earth. New York: W.H. Freeman and Company. Hamilton, W.L. Geological investigations in Yellowstone National Park, 1976–1981. In Wyoming Geological Association Guidebook. Hendrix, M.S. 2011. Geology underfoot in Yellowstone country. Missoula, MT: Mountain Press Publishing Company. Lillie, R.J. 2005. Parks and plates: The geology of our national parks, monuments, and seashores. New York: W.W. Norton Smith, R.B. and L.J. Siegel. 2000. Windows Into the Earth: The Geologic Story of Yellowstone and Grand Teton national parks. New York: Oxford University Press. Tuttle, S.D. 1997. Yellowstone National Park in Geology of national parks. Dubuque, IA: Kendall–Hunt Publishing Company. Staff Reviewers Jefferson Hungerford, Park Geologist Bob Smith, Distinguished Research Professor and Emeritus Professor of Geophysics, University of Utah Between 542 and 66 million years ago—long before the “supervolcano” became part of Yellowstone’s geologic story—the area was covered by inland seas. Geologic History of Yellowstone Yellowstone Geologic History 542 to 66 Ma Area covered by inland seas 50 to 40 Ma Absaroka volcanics 30 Ma to present “Basin and Range” forces creating Great Basin topography 16 Ma Volcanism begins again in present-day Nevada and Idaho 2.1 Ma 1st Yellowstone super-eruption 1.3 Ma 2nd Yellowstone super-eruption 640 ka 3rd Yellowstone super-eruption 174 ka West Thumb eruption 160 to 151 ka Bull Lake Glaciation underway 21 to 16 ka Pinedale Glaciation maximum Ma = mega annum, or millions of years ago ka = kilo annum, or one thousand years ago # =million years ago WASHINGTON MONTANA can eri nt Am eme h v t r No te mo pla OREGON Yellowstone NP IDAHO 15 Craters of the Moon NM and Pres. 1 2 0.6 6 10 16 16 14 4 7 Grand Teton NP 11 12 WYOMING NEVADA UTAH Sixteen million years of movement of the YellowstoneSnake River Plain volcanic system. The youngest activity of the hot spot is the caldera system in Yellowstone. Geology 109 GEOLOGY Most of Earth’s history (from the formation of the earth 4.6 billion years ago to approximately 541 million years ago) is known as the Precambrian time. Rocks of this age are found in northern Yellowstone and in the hearts of the nearby Teton, Beartooth, Wind River, and Gros Ventre mountain ranges. During the Precambrian and the subsequent Paleozoic and Mesozoic eras (541 to 66 million years ago), the western United States was covered at times by oceans, sand dunes, tidal flats, and vast plains. From the end of the Mesozoic through the early Cenozoic, mountainbuilding processes formed the Rocky Mountains. During the Cenozoic era (approximately the past 66 million years of Earth’s history), widespread mountain-building, volcanism, faulting, and glaciation sculpted the Yellowstone area. The Absaroka Range along the park’s north and east sides was formed by numerous volcanic eruptions about 50 million years ago. This period of volcanism is not related to the present Yellowstone volcano. Approximately 30 million years ago, vast expanses of today’s West began stretching apart along an east–west axis. This ongoing stretching process increased about 17 million years ago and created the modern basin and range topography (north–south mountain ranges with long north–south valleys) that characterizes much of the West, including the Yellowstone area. About 16.5 million years ago, an intense period of volcanism initiated near the borders of presentday Nevada, Oregon, and Idaho. Subsequent volcanic eruptions can be traced across southern Idaho towards Yellowstone. This 500-mile trail of more than 100 calderas was created as the North American Plate moved in a southwestern direction over a shallow body of magma. About 2.1 million years ago, the movement of the North American plate brought the Yellowstone area closer to the shallow magma body. This volcanism remains a driving force in Yellowstone today. ADAPTED FROM SMITH AND SIEGEL, 2000; AND HUANG ET AL., 2015. As t hen o e} sp he r plume } Li thos p he r (pl at e mov e e men t) Crust (0–35 km) Upper Mantle (35–270 km) convection plume cell Lower Mantle (270–2,890 km) GEOLOGY Outer Core Molten rock, or magma, rises in convection cells like water boiling in a pot. A hot spot may arise from a heated plume originating from the mantle-core boundary (left), or one originating from higher up in the mantle (right). The magma reservoirs of the Yellowstone hot spot originate at a much shallower depth than the mantle plume. FREQUENTLY ASKED QUESTIONS: Is Yellowstone a volcano? What is a supervolcano? Yes. Within the past two million years, some volcanic eruptions have occurred in the Yellowstone area—three of them super-eruptions. A “supervolcano” refers to a volcano capable of an eruption more than 240 cubic miles of magma. Two of Yellowstone’s three major eruptions met this criteria. What is the caldera shown on the park map? Will the Yellowstone volcano erupt soon? The Yellowstone Caldera was created by a massive volcanic eruption approximately 640,000 years ago. Later lava flows filled in much of the caldera, now it measures 30 x 45 miles. Its rim can best be seen from the Washburn Hot Springs overlook, south of Dunraven Pass. Gibbon Falls, Lewis Falls, Lake Butte, and the Flat Mountain arm of Yellowstone Lake are part of the rim. Another caldera-forming eruption is theoretically possible, but it is very unlikely in the next thousand or even 10,000 years. Scientists have also found no indication of an imminent smaller eruption of lava in more than 30 years of monitoring. When did the Yellowstone volcano last erupt? Scientists from the Yellowstone Volcano Observatory watch an array of monitors in place throughout the region. These monitors would detect sudden or strong earthquake activity, ground shifts, and volcanic gases that would indicate increasing activity. No such evidence exists at this time. Approximately 174,000 years ago, creating what is now the West Thumb of Yellowstone Lake. There have been more than 60 smaller eruptions since then, and the last of the 60–80 post-caldera lava flows was about 70,000 years ago. Is Yellowstone’s volcano still active? Yes. The park’s many hydrothermal features attest to the heat still beneath this area. Earthquakes—700 to 3,000 per year— also reveal activity below ground. The University of Utah Seismograph Station tracks this activity closely at http://quake.utah.edu. What is Yellowstone National Park doing to stop or prevent an eruption? Nothing can be done to prevent an eruption. The temperatures, pressures, physical characteristics of partially molten rock, and immensity of the magma chamber are beyond human ability to impact—much less control. 110 Yellowstone Resources and Issues Handbook, 2019 How do scientists know the Yellowstone volcano won’t erupt? In addition, Yellowstone Volcano Observatory scientists collaborate with scientists from all over the world to study the hazards of the Yellowstone volcano. To view current data about earthquakes, ground movement, and stream flow, visit volcanoes.usgs.gov/yvo/. If Old Faithful Geyser quits, is that a sign the volcano is about to erupt? All geysers are highly dynamic, including Old Faithful. We expect Old Faithful to change in response to the ongoing geologic processes associated with mineral deposition and earthquakes. Thus, a change in Old Faithful Geyser will not necessarily indicate a change in volcanic activity. 111 GEOLOGY ADAPTED WITH PERMISSION FROM WINDOWS INTO THE EARTH BY ROBERT SMITH AND LEE J. SIEGEL, 2000. Geology ADAPTED WITH PERMISSION FROM WINDOWS INTO THE EARTH BY ROBERT SMITH AND LEE J. SIEGEL, 2000. Volume Comparison of Volcanic Eruptions Magma, Hot Spots, and the cubic miles = volume of material ejected Yellowstone Supervolcano Magma (molten rock from below 1st Yellowstone eruption 2.1 million years ago; 600 cubic miles Earth’s crust) is close to the surface in the greater Yellowstone area. This shal3rd Yellowstone eruption low body of magma is caused by heat 640,000 years ago; 240 cubic miles convection in the mantle. Plumes of magma rise through the mantle, melting 2nd Yellowstone eruption rocks in the crust and creating magma 1.3 million years ago; 67 cubic miles reservoirs of partially molten, partially Mazama (U.S.) solid rock. Mantle plumes transport 7,600 years ago; 18 cubic miles Krakatau (Indonesia); heat from deep in the mantle to the 1883; 4.3 cubic miles crust and create what we call “hot spot” Mt. St. Helens (U.S.) volcanism. Hot spots leave a trail of 1980; 0.1 cubic miles volcanic activity as tectonic plates drift Volume comparison of global volcanic eruptions. over them. As the North American Plate 45-mile-wide Yellowstone Caldera. Since then, 80 drifted westward over the past 16.5 smaller eruptions have occurred. Approximately million years, the hot spot that now resides under 174,000 years ago, one of these created what is now the greater Yellowstone area left a swath of volcanic the West Thumb of Yellowstone Lake. During and deposits across Idaho’s Snake River Plain. after these explosive eruptions, huge lava flows of Heat from the mantle plume has melted rocks in viscous rhyolitic lava and less voluminous basalt lava the crust and created two magma chambers of parflows partially filled the caldera floor and surroundtially molten, partially solid rock near Yellowstone’s ing terrain. The youngest of these lava flows is the surface. Heat from the shallowest magma cham70,000-year-old Pitchstone rhyolite flow in the southber caused an area of the crust above it to expand west corner of Yellowstone National Park. and rise. Stress on the overlying crust resulted in Since the last of three caldera-forming eruptions, increased earthquake activity along newly formed pressure from the shallow magma body has formed faults. Eventually, these faults reached the magma two resurgent domes inside the Yellowstone Caldera. chamber, and magma oozed through the cracks. Magma may be as little as 3–8 miles beneath Sour Escaping magma released pressure within the chamber, which also allowed volcanic gasses to escape and Creek Dome and 8–12 miles beneath Mallard Lake Dome, and both domes inflate and subside as the expand explosively in a massive volcanic eruption. volume of magma or hydrothermal fluids changes The eruption spewed copious volcanic ash and gas into the atmosphere and produced fast, super-hot debris flows (pyroclastic flows) over the existing landscape. As the underground magma chamber emptied, the ground above it collapsed and created the first of Yellowstone’s three calderas. This eruption 2.1 million years ago—among the Sour Creek resurgent dome largest volcanic eruptions known to humans—coated 5,790 square miles with ash, as far away as Missouri. 3rd caldera 640,000 years old The total volcanic material ejected is estimated to have been 6,000 times the volume of material ejected during the 1980 eruption of Mt. St. Helens, in Mallard Lake resurgent dome 2nd caldera Washington. 1.3 million years old 1st caldera A second significant, though smaller, volcanic 2.1 million years old eruption occurred within the western edge of the first caldera approximately 1.3 million years ago. The third and most recent massive volcanic erupThe locations of Yellowstone’s three calderas tion 640,000 years ago created the present 30 by and two resurgent domes. beneath them. The entire caldera floor lifts up or subsides, too, but not as much as the two domes. In the past century, the net inflation has tilted the caldera floor toward the south. As a result, Yellowstone Lake’s southern shores have subsided, and trees now stand in water, and the north end of the lake has risen into a sandy beach at Fishing Bridge. Where to See Volcanic Flows 1 11 2 10 Recent Activity 3 4 5 6 Remarkable ground deformation has been documented along the central axis of the caldera between 7 Yellowstone Old Faithful and White Lake in Pelican Valley in Caldera historic time. Surveys of suspected ground deformation began in 1975 using vertical-motion surveys of benchmarks in the ground. By 1985, the surveys documented unprecedented uplift of the entire cal8 dera in excess of a meter (3 ft). Later GPS measurements revealed that the caldera went into an episode of subsidence (sinking) until 2005 when the caldera 1. Sheepeater Cliff: 7. returned to an episode of extreme uplift. The largest columnar basalt vertical movement was recorded at the White Lake 8. 2. Obsidian Cliff: lava GPS station, inside the caldera’s eastern rim, where 3. Virginia Cascades: ash 9. the total uplift from 2004 to 2010 was about 27 centiflow meters (10.6 in). 4. Gibbon Falls: near The rate of rise slowed in 2008, and the caldera caldera rim 10. began to subside again during the first half of 2010. 5. Tuff Cliff: ash flow The uplift is believed to be caused by the movement 6. West Entrance Road, of deep hydrothermal fluids or molten rock into the 11. Mt. Haynes, and Mt. shallow crustal magma system at a depth of about Jackson: columnar 10 km beneath the surface. A caldera may undergo rhyolite, Lava Creek tuff episodes of uplift and subsidence for thousands of years without erupting. Notably, changes in uplift and subsidence have been correlated with increases of earthquake activity. Lateral discharge of these fluids away from the (11) Between Tower Fall caldera—and the accompanying earthand Tower Junction quakes, subsidence, and uplift—relieve 9 Yellowstone Lake GEOLOGY Lewis Lake Continental Divide Firehole Canyon: lava Lewis Falls: near caldera rim Lake Butte: on edge of caldera, overall view of caldera Washburn Hot Springs Overlook: overall view of caldera Between Tower Fall and Tower Junction: columnar basalt (1) Sheepeater Cliff (7) Firehole Canyon 112 Yellowstone Resources and Issues Handbook, 2019 Yellowstone Volcano Observatory Increased scientific surveillance of Yellowstone has detected significant changes in its vast underground volcanic system. The system is centered on an enormous caldera that is characterized by geologically infrequent but very large volcanic eruptions. To strengthen the ability of scientists to track and respond to changes in Yellowstone’s activity, the Yellowstone Volcano Observatory (YVO) was created here in 2001. YVO is a cooperative partnership among the US Geological Survey, National Park Service, University of Utah, University of Wyoming, University NAVSTAR Consortium, and State Geological Surveys of Wyoming, Montana, and Idaho. The observatory is a long-term, instrument-based monitoring program designed for observing volcanic and seismic activity in the Yellowstone National Park region. The principal goals of the Yellowstone Volcano Observatory are to • assess the long-term potential hazards of volcanism, seismicity, and explosive hydrothermal activity in the region; • provide scientific data that enable reliable and timely warnings of significant seismic or volcanic events and related hazards in the Yellowstone region; • notify the NPS, local officials, and the public in the event of such warnings; • improve scientific understanding of tectonic and magmatic processes that influence ongoing seismicity, surface deformation, and hydrothermal activity; and Future Volcanic Activity Will Yellowstone’s volcano erupt again? Over the next thousands to millions of years? Probably. In the next few hundred years? Not likely. The most likely activity would be lava flows, such as those that occurred after the last major eruption. A lava flow would ooze slowly over months and years, allowing plenty of time for park managers to evaluate the situation and protect people. No scientific evidence indicates such a lava flow will occur soon. To monitor volcanic and seismic activity in the Yellowstone area, the Yellowstone Volcano Observatory (YVO) was established in 2001. YVO is a partnership of scientists from the US Geological Survey, National Park Service, University of Utah, University of Wyoming, University NAVSTAR Consortium (UNAVCO), and the state Geological Surveys of Wyoming, Montana, and Idaho. YVO scientists monitor Yellowstone volcano with a realtime and near real-time monitoring network of 26 seismic stations, 16 GPS receivers, and 11 streamgauging stations. Scientists also collect information is on temperature, chemistry, and gas concentrations at selected hydrothermal features and chloride effectively communicate the results of these efforts to responsible authorities and to the public. Current real-time-monitoring data are online at volcanoes.usgs.gov/yvo/ monitoring.html. concentrations in major rivers. A monthly activity summary, real-time monitoring of seismicity and water flow, and near real-time monitoring of ground deformation can be found at the Yellowstone Volcanic Observatory website. More Information Chang, W., R.B. Smith, J. Farrell, and C.M. Puskas. 2010. An extraordinary episode of Yellowstone caldera uplift, 2004–2010, from GPS and InSAR observations. Geophysical Research Letters 37(23). Christiansen, R.L. 2001. The Quaternary + Pliocene, Yellowstone Plateau Volcanic Field of Wyoming, Idaho, and Montana. USGS Professional Paper 729–6. Christiansen, R.L. et al. 2002. Upper-mantle origin of the Yellowstone hotspot. Geological Society of America Bulletin. October. 114:10, pgs. 1245–1256. Christiansen, R.L. et al. 1994. A Field-Trip Guide to Yellowstone National Park, Wyoming, Montana, and Idaho—Volcanic, Hydrothermal, and Glacial Activity in the Region. US Geological Survey Bulletin 2099. Cottrell, W.H. 1987. Born of Fire: The Volcanic Origin of Yellowstone National Park. Boulder: Roberts Rinehart. Hiza, M.M. 1998. The geologic history of the Absaroka Volcanic Province. Yellowstone Science 6(2). Lowenstern, J. 2005. Truth, fiction and everything in between at Yellowstone. Yellowstone Science. 13(3). Morgan, L.A. et al. (editors). 2009. The track of the Yellowstone hot spot: multi-disciplinary perspectives on the origin of the Yellowstone-Snake River Plain Volcanic Province. Journal of Volcanology and Geologic Research. 188(1–3): 1–304. Geology 113 GEOLOGY pressure and could act as a natural pressure-release valve balancing magma recharge and keeping Yellowstone safe from volcanic eruptions. • Smith, R.B. et al. 2009. Geodynamics of the Yellowstone hot spot and mantle plume. Journal of Volcanology and Geologic Research. 188:108–127. Smith, R.B. and J. Farrel. 2016. The Yellowstone hotspot: Volcano and Earthquake Properties, Geologic Hazards and the Yellowstone GeoEcosystem. In Abstracts of the 13th Biennial Scientific Conference on the Greater Yellowstone Ecosystem. p. 56 Yellowstone Volcano Observatory. 2010. Protocols for geologic hazards response by the Yellowstone Volcano Observatory. US Geological Survey Circular 1351. Staff Reviewers GEOLOGY Jefferson Hungerford, Park Geologist Bob Smith, Distinguished Research Professor and Emeritus Professor of Geophysics, University of Utah 114 Yellowstone Resources and Issues Handbook, 2019 Grand Prismatic Stream is one of more than 10,000 thermal features in Yellowstone. Research on heat-resistant microbes in the park’s hydrothermal areas has led to medical, forensic, and commercial uses. Hydrothermal Systems Hydrothermal areas in Yellowstone National Park. Thermal Areas Caldera Geology 115 GEOLOGY Yellowstone was set aside as the world’s first national park because of its hydrothermal wonders. The park contains more than 10,000 thermal features, including the world’s greatest concentration of geysers as well as hot springs, mudpots, and steam vents. Research on heat-resistant microbes in the park’s thermal areas has led to medical, forensic, and commercial uses. Oil, gas, and groundwater development near the park, and drilling in “Known Geothermal Resources Areas” identified by the US Geological Survey in Island Park, Idaho, and Corwin Springs, Montana, could alter the functioning of hydrothermal systems in the park. So in 1994, the National Park Service and State of Montana established a waterrights compact and controlled-groundwater area to protect those areas from development. Under the Surface The park’s hydrothermal system is the visible expression of the immense Yellowstone volcano; it would not exist without the underlying partially molten magma body that releases tremendous heat. The system also requires water, such as ground water from the mountains surrounding the Yellowstone Plateau. There, snow and rain slowly percolate through layers of permeable rock riddled with cracks. Some of this cold water meets hot brine directly heated by the shallow magma body. The water’s temperature rises well above the boiling point, but the water remains in a liquid state due to the great pressure and weight of the overlying water. The result is superheated water with temperatures exceeding 400°F. The superheated water is less dense than the colder, heavier water sinking around it. This creates convection currents that allow the lighter, more buoyant, superheated water to begin its journey back to the surface following the cracks and weak areas through rhyolitic lava flows. This upward path is the natural “plumbing” system of the park’s hydrothermal features. As hot water travels through this rock, it dissolves some silica in the rhyolite. This silica can precipitate in the cracks, increasing the system’s ability to withstand the great pressure needed to produce a geyser. The silica coating the walls of Old Faithful’s geyser tube did not form a pressure-tight seal for the channel of upflow. Lots of water pours through the “silica-lined” walls after an eruption stops. Amorphous silica is a lot less strong than the rock it might coat. The pressure in the geyser tube is not contained by the strength of the wall; rather, the water pressure in the tube is contained by the greater pressure of colder water outside of the tube. At the surface, silica precipitates to form siliceous sinter, creating the scalloped edges of hot springs and the seemingly barren landscape of hydrothermal basins. The siliceous sinter deposits, with bulbous or cauliflower-like surfaces, are known as geyserite. GEOLOGY Hydrothermal Activity The park’s hydrothermal areas are dispersed across 3,472 square miles (8,991 km2) making it challenging to coordinate a systematic monitoring program. Therefore, park geologist use remote sensing, groundwater flow studies, measurements from individual features, and collaboration with many other researchers to gather reproducible data over many years. The variety and duration of monitoring helps to distinguish human influences from natural changes, and define the natural variability of the hydrothermal system. This distinction is essential for Yellowstone to successfully protect the integrity of the system as a whole. Hydrothermal variability is easiest to see in individual features. On March 15, 2018 Steamboat Geyser began a period of more frequent eruptions after three and a half years of dormancy. The world’s tallest geyser erupted 31 times in 2018, and 8 more times by March 5, 2019. Eruption intervals ranged from four to 35 days. The Upper Geyser Basin also experienced some increased activity around the Geyser Hill area in the fall of 2018. This includes new erupting vents splashing water on the boardwalks, surface fractures, and a rare eruption of Ear Spring on September 15, 2018. The eruption ejected a variety of foreign objects; coins and trash dating back to the 1930s. The Old Faithful eruption interval is 93 minutes as of March 2019. These highly visible changes receive public attention, but do not necessarily represent changes in the entire hydrothermal system. New research methods FREQUENTLY ASKED QUESTIONS: Why are geysers in Yellowstone? Yellowstone’s volcanic geology provides the three components necessary for the existence of geysers and other hydrothermal features: heat, water, and a natural “plumbing” system. Magma beneath the surface provides the heat; ample rain and snowfall seep deep underground to supply the water; and underground cracks and fissures form the plumbing. Hot water rises through the plumbing to surface as hydrothermal features. What exactly is a geyser basin? Is Yellowstone’s geothermal energy used to heat park buildings? A geyser basin is a geographically distinct area containing a “cluster” of hydrothermal features that may include geysers, hot springs, mudpots, and fumaroles. These distinct areas often, but not always, occur in low places because hydrothermal features tend to be concentrated around the margins of lava flows and in areas of faulting. Yellowstone National Park’s hydrothermal resources cannot be tapped for geothermal energy because such use could destroy geysers and hot springs, as it has done in other parts of the world. Where can I see mudpots? Dogs have died diving into hot springs. They also disturb wildlife and are prohibited from