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The Yellowstone Super Volcano

Yellowstone National Park in northwest Wyoming is a picturesque land of geysers, hot springs, waterfalls, mountains, and lakes. But just in case you don’t have enough to worry about, it is also the largest supervolcano in North America and among the top three largest in the world. The term “supervolcano” refers to a measure of volume of material erupted and explosiveness. See comparison charts for volume here and the volcanic explosivity index here. You will see that the Yellowstone supervolcano is at the maximum end of both scales.

The U.S. Geological Survey (USGS) map below shows the general setting of the Yellowstone volcano and caldera. Calderas are created following an eruption when the volcano collapses in on itself.

Yellowstone-caldera-overview

According to the Yellowstone Volcano Observatory, run by the USGS and the University of Utah, during the past 2 million years the Yellowstone super volcano has had three of the world’s largest volcanic eruptions:

Eruption of the >2450 cu km Huckleberry Ridge Tuff about 2.1 million years ago created the more than 75-km-long Island Park caldera.

The second cycle concluded with the eruption of the Mesa Falls Tuff around 1.3 million years ago, forming the 16-km-wide Henrys Fork caldera at the western end of the first caldera.

Activity subsequently shifted to the present Yellowstone Plateau and culminated 640,000 years ago with the eruption of the >1000 cu km Lava Creek Tuff and the formation of the present 45 x 85 km caldera. Resurgent doming subsequently occurred at both the NE and SW sides of the caldera and voluminous (1000 cu km) intracaldera rhyolitic lava flows were erupted between 150,000 and 70,000 years ago.

Yellowstone is presently the site of one of the world’s largest hydrothermal systems including Earth’s largest concentration of geysers. As such, it could be one of the largest sources of geothermal-produced electricity, but that’s not likely to happen.

The USGS map below shows the coverage of ash deposits from the three major eruptions, compared to the 1980 eruption of Mount St. Helens. The map also shows the extent of the Bishop Tuff which erupted from the Long Valley volcano in California 760,000 years ago.

Yellowstone map 2

The Yellowstone super volcano is the youngest of a series of such volcanoes that have erupted over the past 17 million years. The older volcanoes trace a line running up the Snake River Plain. The graphic below shows the location and age of these volcanoes. Notice also the parabolic shape of earthquake epicenters (red dots).

Yellowstone map 3

The theory of this volcanic region is that there is a stationary “hot spot” in the mantle that periodically breaks the surface with an eruption. Eruptions occur in a linear pattern showing that the continental crust is moving over the hot spot at about 2.8 cm/yr at an azimuth of about 247 degrees according to Smith et al. “The Yellowstone hotspot has been the source of voluminous rhyolite tuffs and lavas with eruptions often having volumes of hundreds to thousands of cubic kilometers and representing some of the largest Quaternary eruptions on Earth.”

A similar hot spot occurs under Hawaii. In Hawaii, the magma is basaltic which is very fluid so eruptions are relatively tame: volcanic explosivity index (VEI) 0 to 1. In Yellowstone, however, the magma is rhyolitic, very thick and viscous. That makes for violent explosions (VEI 8, the maximum) which produces ash rather than lava flows.

Geophysical investigations show that the Yellowstone magma chamber is 6- to 16 km deep beneath the caldera. Under that, the feeder zone to the magma chamber extends 660 km into the mantle transition zone.

What is happening now?

The National Park Service assures us, “There is no evidence that a catastrophic eruption at Yellowstone National Park (YNP) is imminent. Current geologic activity at Yellowstone has remained relatively constant since earth scientists first started monitoring some 30 years ago. Though another caldera-forming eruption is theoretically possible, it is very unlikely to occur in the next thousand or even 10,000 years. Scientists have also found no indication of an imminent smaller eruption of lava.”

On the other hand, National Geographic news of January 19, 2011 reports:

Yellowstone National Park’s supervolcano just took a deep “breath,” causing miles of ground to rise dramatically, scientists report.

But beginning in 2004, scientists saw the ground above the caldera rise upward at rates as high as 2.8 inches (7 centimeters) a year.

The rate slowed between 2007 and 2010 to a centimeter a year or less. Still, since the start of the swelling, ground levels over the volcano have been raised by as much as 10 inches (25 centimeters) in places.

“It’s an extraordinary uplift, because it covers such a large area and the rates are so high,” said the University of Utah’s Bob Smith, a longtime expert in Yellowstone’s volcanism.

Scientists think a swelling magma reservoir four to six miles (seven to ten kilometers) below the surface is driving the uplift. Fortunately, the surge doesn’t seem to herald an imminent catastrophe, Smith said.

(Related story with 3D model of magma chamber: “Under Yellowstone, Magma Pocket 20 Percent Larger Than Thought.”)

“At the beginning we were concerned it could be leading up to an eruption,” said Smith, who co-authored a paper on the surge published in the December 3, 2010, edition of Geophysical Research Letters.

“But once we saw [the magma] was at a depth of ten kilometers, we weren’t so concerned. If it had been at depths of two or three kilometers [one or two miles], we’d have been a lot more concerned.”

Here is a graphic from Smith, showing topographic swelling caused by magma pressure at Yellowstone; rather impressive:

Yellowstone map 4

Apparently, all is (relatively) quiet on the western front, but who knows when the pimple will pop.

Reference:

Smith, R.B., et al., 2009, Geodynamics of the Yellowstone hotspot and mantle plume: Seismic and GPS imaging, kinematics, and mantle flow, Journal of Volcanology and Geothermal Research 188 (2009) 26–56. http://www.uusatrg.utah.edu/PAPERS/smith_jvgr2009complete.pdf

See update:

https://wryheat.wordpress.com/2011/04/13/yellowstone-super-volcano-update/ 

Some Fallout from Icelandic Volcanoes

A story on the Scientific American website proclaims “Ice cap thaw may awaken Icelandic volcanoes.” “A thaw of Iceland’s ice caps in coming decades caused by climate change may trigger more volcanic eruptions by removing a vast weight and freeing magma from deep below ground, scientists said on Friday.”

This contention is based on the fact that rocks melt at a lower temperature under lower pressure. The question is, will removal of some or all of the ice cap in Iceland result in a pressure load decrease that will make a significant difference in melting temperature and therefore produce more magma?

The relationship between pressure and melting temperature of basaltic lava, the type in Iceland, was estimated by the Carnegie Geophysical Laboratory to be:

Tm = 1391.5 + 0.01297 * P

Tm is the melting temperature in degrees Centigrade, and P is the pressure in bars (or atmospheres, 1 atmosphere = 1.01 bars or about 14.7 pounds per square inch at sea level). We need be concerned only with the last term of that formula which says that a one atmosphere change in pressure results in a 0.013º C change in melting point (rounding the number to two significant figures).

The pressure of one atmosphere is about equivalent to the pressure or weight of 10 meters of ice, so one meter of ice would result in a temperature change of about 0.0013º C. The thickest ice in Iceland is about 500 meters. Complete removal of that ice would lower the melting point of rock about 0.65 ºC, not very significant considering the base melting point is nearly 1400 ºC. The earth’s normal geothermal gradient (the change in temperature with depth), is about 20- to 30º C around most of the planet, and about 40º C at tectonically active spots like Iceland. The removal of 500 meters of ice giving a temperature difference of 0.65 ºC is the equivalent of a depth difference of about 16 meters. Big deal. It is very unlikely that this small pressure difference would stimulate additional volcanic activity.

Ash is formed when magma is rapidly cooled and fractured by steam. Removal of the surficial water source could result in less ash formation and make the Icelandic volcanoes behave more like the Hawaiian volcanoes.

Reference:

Change of Melting Point of Diopside with Pressure, by Hatten S. Yoder Jr., Geophysical Laboratory, Carnegie Institution of Washington, © 1952 The University of Chicago Press.

Another type of fallout:

The banning of air travel in Europe, which is causing economic chaos, was based on predicted damage to aircraft from ash clouds. The ban was based on an advisory from the British Meteorological Office’s London Volcanic Ash Advisory Centre. That organization based their advisory on a computer program originally developed to monitor nuclear fallout. The computer model predicted dangerous concentrations of ash at various places around Europe. The Met office failed to send up any weather balloons to check actual conditions. Because there is a valid possibility that flying through an ash cloud can damage aircraft, the major airlines made test flights in the last few days to check conditions and found no danger. This is another example showing that relying on computer modeling, rather than real data, can cause unnecessary economic loss and concern.