Tuesday, September 11, 2018

Kiss Your Ash Goodbye: The Yellowstone Supervolcano, Part I, A Vulcanology Primer

By Stephanie Osborn
Images in this article are public domain unless otherwise noted.

What is a supervolcano?

“The term ‘supervolcano’ implies a volcanic center that has had an eruption of magnitude 8 on the Volcanic Explosivity Index (VEI), meaning the measured deposits for that eruption is greater than 1,000 cubic kilometers (240 cubic miles).” ~U.S. Geological Survey

That said, often volcanic eruptions with a slightly lesser VEI of 7 are also considered supervolcanic. This is because the VEI does not take density of ejecta into account. Magma chemical composition varies, depending upon the source of the melt. This can produce lava with varying densities.

Crater Lake, in Mount Mazama, with Wizard Island cinder cone.

What is the Volcanic Explosivity Index?

It is a means of ranking a volcanic eruption, similar to the Richter or moment magnitude scales for earthquakes. According to the U.S. Geological Survey, “The Volcanic Explosivity Index (VEI) is a relative measure of the explosiveness of volcanic eruptions. It was devised by Chris Newhall of the United States Geological Survey and Stephen Self at the University of Hawaii in 1982.” It categorizes eruption characteristics, such as volume of ejecta, eruption cloud height, etc. Again quoting the USGS, “The scale is open-ended with the largest volcanic eruptions in history (super-eruptions) given magnitude 8.” Since it is open-ended, some geologists do estimate that a very small number of eruptions in geologic history may have reached a 9, though such a rating is currently unofficial.

The Volcanic Explosivity Index.

Are all supervolcanoes explosive?


There are supervolcanoes known as “traps” which tend to be nonexplosive. These are typically long cracks — sometimes fields of parallel cracks — from which vast quantities of lava (“flood basalts”) flow over the surrounding terrain. The term refers to the step-like terrain common to such features. One of the best known in the geological community were the Deccan Traps. This eruption occurred some 60 million years ago in the Deccan Plateau region of what is now India. The residual lava beds originally may have covered some 580,000sq.mi. (1.5million km²) more than 2x the size of Texas. Multiple flows over time covered the area in ~6,600ft (2,000m) thick basalts. It is one of the largest volcanic features on Earth.

How strong is a supervolcano?

That depends on the type of supervolcano and your definition of “strong.” There are two types of supervolcano:

1) Megacalderas, or “massive eruptions”:
These are cliff-edged craters, usually (though not always) NOT surrounded by a mountain, where the violence of the eruption emptied the magma chamber. The overburden collapsed into the chamber, leaving a sinkhole-like depression.

Lake Toba — the lake IS the caldera.

2) Traps, or Large Igneous Provinces:
As already mentioned, these are huge regions of lava flow resulting from flood basalt eruptions, often hundreds or thousands of square miles with volumes on order of millions of cubic miles. The lavas are normally laid down in sequential eruptions over millions of years.

Siberian Traps lava flow.
Image credit Benjamin Black via USGS.

While traps are considered supervolcanoes, usually it is the megacaldera which is being referenced, due to its violence.

What is the difference between traps and megacalderas?

Traps tend to be effusive and megacalderas tend to be eruptive. This is not always true but usually is. The difference lies in the chemistry of the melt.

Effusive flows tend to have thin, runny lava (low viscosity), usually basaltic in composition. Dissolved gases escape quickly. This produces dramatic lava fountains and swift flows. Example: Kilauea.

Eruptive flows tend to have thick, viscous lava (high viscosity), usually granitic in composition. Dissolved gases are held in the melt.  Pressure builds, and an eruption ensues when the containment (volcanic vent/neck/chamber) fails. Example: Mt. St. Helens.

If the melt chemistry changes for any reason, a trap can become eruptive, or a megacaldera can become effusive, at least temporarily.

How strong is a supervolcano? (Take two)

When Mt. St. Helens erupted, it released thermal energy equivalent to approximately 24 megatons (MT). 7 MT of this was expended in the blast alone. The St. Helens eruption was a VEI 5.

As previously mentioned, the Volcano Explosivity Index is logarithmic. A supervolcano eruption is VEI 7-8. This is 2-3 orders of magnitude stronger than St. Helens. A supervolcano, therefore, would release an estimated 2,400-24,000 MT (2.4-24 gigatons (GT)) of thermal energy. If we scale the blast size up proportionally, this would result in a blast equivalent to approximately 700 MT to 7 GT.

How many supervolcanoes exist?

That depends on who you talk to, and what criteria they are using. Some say as few as half a dozen, others as many as 20 or more.

Keep in mind, there may also be ocean-floor volcanos of which we’re unaware.

A Partial List of Known Active/Dormant Supervolcanoes Currently In Existence

• Aira Caldera/Sakurajima, Kagoshima, Japan
• Baekdu Mountain, China/North Korea border
• Campi Flegri/Phlegraean Fields, Naples, Italy
• Cerro Galan Caldera, Catamarca province, Argentina
• Kurile Lake/Kurilskoye Lake, Kamchatka, Russia
• La Pacana, Zapaleri tripoint, Chile/Bolivia/Argentina
• Lake Toba, North Sumatra, Indonesia
• Long Valley Caldera, Mammoth Mountain, California, USA (south of Mono Lake)
• Macauley Island, New Zealand
• Mount Aso, Kumamoto Prefecture, Japan
• Tambora, Sumbawa, Indonesia
• Taupo Caldera, North Island, New Zealand
• Thera/Santorini, Santorini, Greece
• Valles Caldera, Santa Fe, New Mexico, USA
• Yellowstone Caldera, Yellowstone National Park, Wyoming, USA

To obtain a copy of Kiss Your Ash Goodbye: The Yellowstone Supervolcano by Stephanie Osborn, go to: