A widely accepted measure of the size of an eruption is the volume of lava ejected as pumice and ash (aka. as tephra) during an explosive phase or the volume of lava extruded during an effusive phase. Eruption volumes are commonly expressed in cubic kilometers (roughly equivalent to a quarter cubic mile). This diagram shows the relative sizes of a few example eruptions. In any given time period, there are many more small eruptions than there are large eruptions and the largest caldera forming eruptions (like Long Valley Caldera, Yellowstone, and Toba) are very infrequent. The smallest eruption shown is the Lassen Peak eruption of 1915 in California. The largest is the Toba eruption in Sumatra that occurred 74,000 years ago. (Image Credit: USGS)
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Yellowstone, one of the world's largest active volcanic systems, has produced several giant volcanic eruptions in the past few million years, as well as many smaller eruptions and steam explosions. Although no eruptions of lava or volcanic ash have occurred for many thousands of years, future eruptions are likely. In the next few hundred years, hazards will most probably be limited to ongoing geyser and hot spring activity, with occasional steam explosions and moderate to large earthquakes. To better understand Yellowstone's volcano and earthquake hazards and to help protect the public, the US Geological Survey, the University of Utah, and Yellowstone National Park formed the Yellowstone Volcano Observatory, which continuously monitors activity in the region.
Each year, millions of visitors come to admire the hot springs and geysers of Yellowstone, the Nation's first national park. Few are aware that these wonders are fueled by heat from a large reservoir of partially molten rock (magma), just a few miles beneath their feet. As this magma, which drives one of the world's largest volcanic systems, rises, it pushes up the Earth's crust beneath the Yellowstone Plateau.
Stresses in the crust produce movements on faults, causing earthquakes to occur. Thousands of small quakes are recorded each year by the seismographic network of the Yellowstone Volcano Observatory (YVO). Faults and fractures also allow surface water to penetrate to depth and become heated, rising again to produce hydrothermal features, such as geysers. Steam and hot water carry huge quantities of thermal energy to the surface from the magma chamber below. Continuing up and down ground motions on the Yellowstone Plateau reflect the migration of both hydrothermal fluids and magma below the surface.
Ground motions, earthquakes, and hydrothermal activity are all current manifestations of volcanic activity at Yellowstone. In the not so distant geologic past, Yellowstone has produced many major volcanic eruptions, which have repeatedly reshaped its natural wonders.
Caldera Forming Eruptions
The Yellowstone region has produced three exceedingly large volcanic eruptions in the past 2.1 million years. In each of these cataclysmic events, enormous volumes of magma erupted at the surface and into the atmosphere as mixtures of red-hot pumice, volcanic ash (small, jagged fragments of volcanic glass and rock), and gas that spread as pyroclastic (fire-broken) flows in all directions. Rapid withdrawal of such large volumes of magma from the subsurface then caused the ground to collapse, swallowing overlying mountains and creating broad cauldron shaped volcanic depressions called calderas.
The first of these caldera forming eruptions 2.1 million years ago created a widespread volcanic deposit known as the Huckleberry Ridge Tuff, an outcrop of which can be viewed at Golden Gate, south of Mammoth Hot Springs. This titanic event, one of the five largest individual volcanic eruptions known anywhere on the Earth, formed a caldera more than 60 miles (100 km) across.
A similar, smaller but still huge eruption occurred 1.3 million years ago. This eruption formed the Henrys Fork Caldera, located in the area of Island Park, west of Yellowstone National Park, and produced another widespread volcanic deposit called the Mesa Falls Tuff.
The region's most recent caldera forming eruption 640,000 years ago created the 35 mile wide, 50 mile long (55 by 80 km) Yellowstone Caldera. Pyroclastic flows from this eruption left thick volcanic deposits known as the Lava Creek Tuff, which can be seen in the south facing cliffs east of Madison, where they form the north wall of the caldera. Huge volumes of volcanic ash were blasted high into the atmosphere, and deposits of this ash can still be found in places as distant from Yellowstone as Iowa, Louisiana, and California.
Each of Yellowstone's explosive caldera forming eruptions occurred when large volumes of rhyolitic magma accumulated at shallow levels in the Earth's crust, as little as 3 miles (5 km) below the surface. This highly viscous (thick and sticky) magma, charged with dissolved gas, then moved upward, stressing the crust and generating earthquakes. As the magma neared the surface and pressure decreased, the expanding gas caused violent explosions. Eruptions of rhyolite have been responsible for forming many of the world's calderas, such as those at Katmai National Park in Alaska, which formed in an eruption in 1912, and at Long Valley, California.
If another large caldera forming eruption were to occur at Yellowstone, its effects would be worldwide. Thick ash deposits would bury vast areas of the United States, and injection of huge volumes of volcanic gases into the atmosphere could drastically affect global climate. Fortunately, the Yellowstone volcanic system shows no signs that it is headed toward such an eruption. The probability of a large caldera forming eruption within the next few thousand years is exceedingly low.
More likely in Yellowstone than a large explosive caldera forming eruption is eruption of a lava flow, which would be far less devastating. Since Yellowstone's last caldera forming eruption 640,000 years ago, about 30 eruptions of rhyolitic lava flows have nearly filled the Yellowstone Caldera. Other flows of rhyolite and basalt (a more fluid variety of lava) also have been extruded outside the caldera. Each day, visitors to the park drive and hike across the lavas that fill the caldera, most of which were erupted about 160,000 years ago, some as recently as about 70,000 years ago. These extensive rhyolite lavas are very large and thick, and some cover as much as 130 square miles (340 square kms), twice the area of Washington DC. During eruption, these flows oozed slowly over the surface, moving at most a few hundred feet per day for several months to several years, destroying everything in their paths.
Today, most of the landforms within the Yellowstone Caldera reflect the shapes of these young lava flows. Cliffs surrounding the Upper Geyser Basin near Old Faithful Geyser are the cooled steep flow fronts of once slow moving rhyolite lavas. Some narrow ridges and valleys on the Canyon-Norris road are corrugations on the surface of a 110,000 year old rhyolite flow. These roughly concentric ridges formed as the thick, pasty lava slowly oozed northeastward, wrinkling its surface. Within the caldera, rivers and streams commonly occupy the gaps between individual lava flows, and springs emerge at the edges of flows.
Any renewed volcanic activity at Yellowstone would most likely take the form of such mainly nonexplosive lava eruptions. An eruption of lava could cause widespread havoc in the park, including fires and the loss of roads and facilities, but more distant areas would probably remain largely unaffected.
From 1000 to 3000 earthquakes typically occur each year within Yellowstone National Park and its immediate surroundings. Although most are too small to be felt, these quakes reflect the active nature of the Yellowstone region, one of the most seismically active areas in the United States. Each year, several quakes of magnitude 3 to 4 are felt by people in the park.
Although some quakes are caused by rising magma and hot ground water movement, many emanate from regional faults related to crustal stretching and mountain building. For example, major faults along the Teton, Madison, and Gallatin Ranges pass through the park and likely existed long before the beginning of volcanism there. Movements along many of these faults are capable of producing significant earthquakes. The most notable earthquake in Yellowstone's recent history occurred in 1959. Centered near Hebgen Lake, just west of the park, it had a magnitude of 7.5. This quake caused $11 million in damage and killed 28 people, most of them in a landslide that was triggered by the quake.
Geologists conclude that large earthquakes like the Hebgen Lake event are unlikely within the Yellowstone Caldera itself, because subsurface temperatures there are high, weakening the bedrock and making it less able to rupture. However, quakes within the caldera can be as large as magnitude 6.5. A quake of about this size that occurred in 1975 near Norris Geyser Basin was felt throughout the region.
Even distant earthquakes can affect Yellowstone. In November 2002, the magnitude 7.9 Denali Fault earthquake struck central Alaska, 1900 miles (3100 km) northwest of Yellowstone. Because this quake's energy was focused toward the active Yellowstone volcanic and hydrothermal system, it triggered hundreds of small earthquakes there. The region's hydrothermal system is highly sensitive to quakes and undergoes significant changes in their wake. Earthquakes may have the potential to cause Yellowstone's hot water system to destabilize and produce explosive hydrothermal eruptions.
The large magma reservoir beneath Yellowstone may have temperatures higher than 1475 degrees F (800 degrees C), and the surrounding rocks are heated by it. Because of this, the average heat flow from the Earth's interior at Yellowstone is about 30 times greater than that typical for areas elsewhere in the northern Rocky Mountains. As snowmelt and rainfall seep deep into the ground, they can absorb enough of this heat to raise the temperature of the ground water close to the boiling point. Geyser basins and other thermal areas in Yellowstone National Park are places where hot ground water has risen close to the surface. Research drilling at Yellowstone in the 1960s confirmed that the ground water beneath many of the park's thermal areas is very hot. At Norris Geyser Basin, water temperatures as high as 460 degrees F (238degrees C) were recorded at depths of only 1090 feet (332 m).
Because the boiling point of water increases with increasing pressure and pressure increases with depth, deep water can be hotter than boiling water near the surface. If the pressure that confines this deep water is reduced quickly, pockets of water may suddenly boil, causing an explosion as the water is converted to steam. Such activity drives the eruptions of geysers, like Old Faithful, which are repetitive releases of plumes of steam and water. Rarely, steam explosions are more violent and can hurl water and rock thousands of feet. In Yellowstone's geologic past, such violent events, called hydrothermal explosions, have occurred countless times, creating new landscapes of hills and craters.
A recent and notable hydrothermal explosion occurred in 1989 at Porkchop Geyser in Norris Geyser Basin. The remains of this explosion are still clearly visible today as an apron of rock debris 15 feet (5 m) across surrounding Porkchop's central spring. In the 1880s and early 1890s, a series of powerful hydrothermal explosions and geyser eruptions occurred at Excelsior Geyser in the Midway Geyser Basin. Some of the explosions hurled large rocks as far as 50 feet (15 m).
Much larger hydrothermal explosions have occurred at Yellowstone in the recent geologic past. More than a dozen large hydrothermal explosion craters formed between about 14,000 and 3000 years ago, triggered by sudden changes in pressure of the hydrothermal system. Most of these craters are within the Yellowstone Caldera or along a north-south trending zone between Norris and Mammoth Hot Springs.
The largest hydrothermal explosion crater documented in the world is along the north edge of Yellowstone Lake in an embayment known as Mary Bay. This 1.5 mile (2.6 km) diameter crater formed about 13,800 years ago and may have had several separate explosions in a short time interval. What specifically triggered these very large events is not firmly established, but earthquakes or a pressure release caused by melting glaciers or rapid changes in lake level may have been a significant factor.
These very large and violent hydrothermal explosions are independent of associated volcanism. None of the large hydrothermal events of the past 16,000 years has been followed by an eruption of magma. The deeper magma system appears to be unaffected even by spectacular steam explosions and crater excavations within the overlying hydrothermal system.
Although large hydrothermal explosions are a feature of Yellowstone's recent geologic history, most explosions in historical times have been relatively small and have left craters at most a few yards across. For example, in early 2003, a long linear fissure appeared on a hillside above Nymph Lake, north of Norris Geyser Basin, venting steam and throwing bits of rock onto the surrounding hillside. Although most hydrothermal explosions in the park are small, their remains can be noticed by observant visitors and attest to the nearly continuous geologic activity at Yellowstone.
How Dangerous Is Yellowstone?
None of the events described above -- cataclysmic caldera forming eruptions, lava flows, large earthquakes, or major hydrothermal explosions -- are common in Yellowstone. Although visitors to Yellowstone National Park may never experience them, some hazardous events are certain to occur in the future. Fortunately, systematic monitoring of Yellowstone's active volcanic and hydrothermal systems, including monitoring of earthquakes and ground deformation, is now carried out routinely by YVO scientists. This monitoring will allow YVO to alert the public well in advance of any future volcanic eruptions. Currently the ability to reliably predict large quakes or hydrothermal explosions, events more likely than a volcanic eruption, remains a challenge. However, changes in the patterns of ongoing seismicity or other indicators of possible geologic unrest are quickly reported to officials responsible for public safety in the National Park Service and other agencies.
Through continuous monitoring and research, YVO is greatly improving understanding of Yellowstone's volcanic, earthquake, and hydrothermal hazards. The work of USGS scientists with YVO is only part of the USGS Volcano Hazards Program's ongoing efforts to protect people's lives and property in all of the volcanic regions of the United States, including California, Hawaii, Alaska, and the Pacific Northwest.
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