By Andrea Thompson, LiveScience Staff Writer
posted: 08 November 2007 02:00 pm ET
Yellowstone’s ancient volcanic floor has been rising since mid-2004 because a blob of molten rock the size of Los Angeles infiltrated the system 6 miles beneath the surface, scientists say, but there is no risk of an eruption.
Yellowstone National Park is the site of North America's largest volcanic field, which is produced by a hotspot, or gigantic plume of hot, molten rock, that begins at least 400 miles (643 kilometers) beneath Earth's surface and rises to 30 miles (48 kilometers) underground, where it widens to about 300 miles across.
Occasionally, blobs of magma break away from the top of this plume and rise up to resupply the magma chamber beneath the park's "caldera," a 40-mile by 25-mile bowl-like depression and volcanic leftover whose walls you can see in the northwest part of the park.
These rising blobs of magma can sometimes push on the caldera floor, causing it to rise. Scientists monitoring the Yellowstone caldera think that's exactly what has caused the caldera floor to rise by almost 3 inches (7 centimeters) per year over the past three years—more than three times faster than it has more typically risen since observations began in 1923.
"Our best evidence is that the crustal magma chamber is filling with molten rock," said study leader Robert Smith, a seismologist at the University of Utah. "But we have no idea how long this process goes on before there either is an eruption or the inflow of molten rock stops and the caldera deflates again."
Smith and his colleagues, whose work is detailed in the Nov. 9 issue of the journal Science, say that there is no indication that the caldera will produce an eruption anytime soon.
"There is no evidence of an imminent volcanic eruption or hydrothermal explosion. That's the bottom line," Smith said. "A lot of calderas worldwide go up and down over decades without erupting."
Other well known calderas include California's Long Valley and Italy's Campi Flegrei, near Naples, which have both been known to rise and subside again for even tens of thousands of years without erupting.
The recent uplift of the Yellowstone caldera, recorded by GPS and satellite radar measurements, is considerably faster than the previous record of 0.8 inches (2 centimeters) per year from 1976 to 1985.
Smith and his teams used computer simulations to determine changes in the shape of the magma chamber, which acts as a sponge-like body that holds magma between areas of solid rock. They found that the magma pushing up on the caldera is likely about 38 miles long and 13 miles wide, about the area of the city of Los Angeles, but only tens to hundreds of yards thick.
The magma that accumulates in the chamber powers Yellowstone's famous geysers and hot springs, the largest hydrothermal field in the world.
The northeastern part of Yellowstone Caldera, with the Yellowstone River flowing through Hayden Valley and the caldera rim in the distance
|Elevation||10,308 feet (3,142 m) at Mount Sheridan|
|Topo map||USGS Yellowstone National Park|
|Age of rock||70,000 – 2.1 million years|
|Last eruption||640,000 years ago|
The Yellowstone Caldera is a volcanic caldera in Yellowstone National Park in the United States. It is located in the northwest corner of Wyoming, measuring about 55 kilometers (34 mi) by 72 kilometers (45 mi). The caldera was discovered based on geological field work conducted by Bob Christiansen of the United States Geological Survey in the 1960s and 1970s. After a BBC television science program coined the term supervolcano in 2000, it has often been referred to as the "Yellowstone supervolcano."
Yellowstone, like the Hawaiian Islands, is believed to lie on top of one of the planet's few dozen hot spots where light hot molten mantle rock rises towards the surface. The Yellowstone hot spot has a long history. Over the past 17 million years or so, successive eruptions have flooded lava over wide stretches of Washington, Oregon, California, Nevada, and Idaho, forming a string of comparatively flat calderas linked like beads, as the North American plate moves across the stationary hot spot. The oldest identified caldera remnant is straddling the border near McDermitt, Nevada-Oregon. The calderas' apparent motion to the east-northeast forms the Snake River Plain. However, what is actually happening is the result of the North American plate moving west-southwest over the stationary hot spot deep underneath.
Currently, volcanic activity is exhibited only via numerous geothermal vents scattered throughout the region, including the famous Old Faithful Geyser, but within the past two million years, it has undergone three extremely large explosive eruptions, up to 2,500 times the size of the 1980 Mount St. Helens eruption. The three eruptions happened 2.1 million years ago, 1.3 million years ago, and the most recent such eruption produced the Lava Creek Tuff 640,000 years ago and spread a layer of volcanic ash over most of the North American continent. Smaller steam explosions occur every 20,000 years or so; an explosion 13,000 years ago left a 5 kilometer diameter crater at Mary Bay on the edge of Yellowstone Lake (located in the center of the caldera). Additionally, non-explosive eruptions of lava flows have occurred in and near the caldera since the last major eruption; the most recent of these was about 70,000 years ago. Craters of the Moon National Monument in Idaho is the result of volcanic activity between 11,000 and 2,000 years ago.
The volcanic eruptions, as well as the continuing geothermal activity, are a result of a large chamber of magma located below the caldera's surface. The magma in this chamber contains gases that are kept dissolved only by the immense pressure that the magma is under. If the pressure is released to a sufficient degree by some geological shift, then some of the gases bubble out and cause the magma to expand. This can cause a runaway reaction. If the expansion results in further relief of pressure, for example, by blowing crust material off the top of the chamber, the result is a very large gas explosion.
A full-scale eruption of the Yellowstone caldera could result in millions of deaths locally and catastrophic climatic effects globally, but there is little indication that such an eruption is imminent.  However, the system is not yet completely understood, and the study of Yellowstone is ongoing. Geologists are closely monitoring the rise and fall of the Yellowstone Plateau, which averages +/- 1.5 cm yearly, as an indication of changes in magma chamber pressure. Major eruptions of the Yellowstone hotspot appear to occur roughly every 700,000 years. The Lava Creek Tuff eruption 630,000 years ago was last major eruption.
The source of the Yellowstone Hot Spot is not without controversy. Some geoscientists theorize that the Yellowstone Hot Spot is the effect of an interaction between local conditions in the lithosphere and upper mantle convection (G.R. Foulger:  and ). Others prefer a deep mantle origin (mantle plume). (See list of off-line references in mantleplumes.org/CRB.html). No theory is close to airtight. Part of the controversy is due to the rather sudden appearance of the hot spot in the geologic record. Additionally, the Columbia Basalt flows appear at the same approximate point in time, causing speculation about their origin.
Labels: Yellowatone Magma
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