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

Excerpted from Kiss Your Ash Goodbye: The Yellowstone Supervolcano, © 2018

By Stephanie Osborn

http://www.stephanie-osborn.com

Images in this article are public domain unless otherwise noted.

What Would Happen If Yellowstone Erupted Today?

That would depend on the type of eruption. Because, you see, sometimes Yellowstone vomits, sometimes it coughs, and sometimes it only sneezes.

The Sneeze: Hydrothermal/Phreatic Eruptions

A hydrothermal or phreatic eruption is by far the most common occurrence at Yellowstone. Put simply, it’s a cross between a boiler explosion and a geyser, and at least at Yellowstone, often leaves new geysers or hot springs in its wake. The USGS’s definition is as follows: “[A hydrothermal eruption is an] explosion that can occur when hot water within a volcano’s hydrothermal (hot water) system flashes to steam, breaking rocks and throwing them into the air.” And often throwing them a good distance...though NOT nearly as far as a standard volcanic eruption can chuck a lava bomb.

It does happen, and has happened in living memory — Porkchop Geyser in the Norris Geyser Basin was formed in this fashion in 1989. More historically, Excelsior Geyser generated a large, violent hydrothermal eruption in 1888, which was captured on film; it had been erupting in this fashion off and on through the 1880s-90s. Duck Lake, the ENTIRE LAKE, is the crater formed by such a hydrothermal eruption.

Hydrothermal explosion at Biscuit Basin in Yellowstone National Park.

Credit: USGS

IMPORTANT NOTE: Even when not in an active eruption, Yellowstone’s hydrothermal features are dangerous; some 22 people have died from falling into scalding hot springs since records began in 1870. One of the most recent, which occurred in 2016 in or in the near vicinity of Porkchop, resulted in the partial dissolution of the body before it could be recovered, thanks to the fact that the spring was essentially boiling acid; park rangers said had it taken much longer to recover the body, there would have been no body left to recover. (What was the victim going to do, according to his sister, who saw the whole horrible thing? He was going to take a hot tub bath in it. He fell in while trying to take the temperature of the water, when the thin, fragile mineral deposit on which he stood gave way.)

Yellowstone is far from the only volcano which generates such eruptions; I have been present for numerous such eruptions at Mount Saint Helens. There, they are usually called phreatic eruptions, and may carry some ash along with the steam vents. They are fairly common occurrences, and take place anywhere there is snow, ice, or other groundwater source in close proximity to magma.

Hydrothermal eruptions at Yellowstone are localized to the vicinity of the explosion, and do not affect the park as a whole. They can present a danger to bystanders, however, if observers are in the way of either the large rock ejecta or the scalding and often highly acidic water/steam.

This is THE MOST LIKELY TYPE OF ERUPTION to occur at Yellowstone.

The Cough: Ordinary Eruptions

The last “ordinary” eruption to occur at the Yellowstone hotspot happened only 70,000 years ago — yesterday, geologically-speaking. These tend to form fairly common lava flows, and may be somewhat eruptive (though trap-type flows have also occurred, according to geological evidence). It would devastate the park, and possibly some of the surrounding land, depending on exactly where and how the lava surfaced, but would not necessarily induce a supervolcanic eruption if there is insufficient pressure, or if the “surface shadow” of the underground melt is not large.

This is the second most likely type of eruption to occur at Yellowstone.

Yellowstone caldera’s northwest rim at Madison Junction in Yellowstone National Park. 

Note the presence of two overlapping lava flows from “ordinary” volcanic eruptions. 

Credit: USGS.

The Vomit: Super-eruptions

Super-eruptions take a fairly long time to develop, but as they progress, they accelerate. It can take anywhere from millennia to mere decades for the magma chamber to inflate and the signs to manifest, but once the chamber roof cracks all the way through from the surface, the rest takes place in a matter of minutes to seconds.

Once the cap has broken up, a giant plume develops some 12-20mi (20-32km) high. The initial plume collapses to form a giant pyroclastic flow that wipes everything out within 60-100mi (100-160km). Anything closer than 125-200 mi (200-325 km) is buried under vast amounts of ash and roofed/covered structures collapse within hours to days; aircraft can no longer fly over western USA/Canada, rapidly progressing to all of North America as the ash cloud spreads. Vehicles stall when air intakes choke on ash. Mud flows form in rainy areas. The first cases of silicosis develop.  The ash cloud sweeps worldwide, reaching Europe in ~3 days. Global temperatures drop, and Earth enters a volcanic winter.

Modeled Extent of Ashfall in a Modern Super-eruption

Even Chicago would get up to an inch or more of ash, and the East Coast gets a solid dusting. The west coast of USA, Canada, and northern Mexico get no favors, either. The grain belt would be devastated in a matter of hours.

Some FEMA researchers estimate the US could take as much as $3 trillion in damage/loss. Other experts say that as much as 2/3 of the USA could be rendered uninhabitable.

The modeled extent of ashfall, 

with depth decreasing with increasing distance, 

of a modern-day Yellowstone supereruption. 

Credit: USGS.

Don’t Panic!

A supereruption is the LEAST likely type of eruption to occur. There are far too many panic-mongers out there.

The 2018 “Incident” That Wasn’t

While I was writing the ebook, I was pinged on social media about A DREADFUL SITUATION! OH NOES! HUGE FISSURE OPENS UP INSIDE YELLOWSTONE SUPERVOLCANO, AREA AROUND FISSURE CLOSED! ERUPTION IMMINENT!!! or impressions to that effect.

Well, I took one good look at the article’s source and snorted. And then I got busy researching.

It turns out there was a JOINT FRACTURE in the rock above Hidden Falls in GRAND TETON NATIONAL PARK, NOT Yellowstone National Park, and some SIXTY MILES (100km) from Yellowstone. The crack was HORIZONTAL (not vertical, therefore NOT able to reach down toward a magma chamber, which does not currently have sufficient melt in it to even inflate, anyway), and was considered a rock fall hazard, hence the real reason why the immediate area was closed by the Park Service. Cause of the fracture was most likely the usual freezing/thawing process that causes such things in mountain ranges.

Hidden Falls in Grand Tetons National Park.

Credit: National Parks Service.

The 2004 Incident

In 2004, there was a small kill-off of animals in Yellowstone, notably about half a dozen bison, in a hollow along the Gibbon River inside the park. This, as well as the slight inflation of the Yellowstone Lake bed, resulted in significant concern among both geologists and the public that the magma chamber was inflating and the caldera becoming active.

A nursery group of bison cows and calves makes its way through Lamar Valley in Yellowstone. Credit: National Parks Service/Neal Herbert.

However, that proved not quite the case.

You see, the caldera area “breathes.” Elevations go up and down by small amounts. Sometimes this is because the magma chamber is inflating, and sometimes it isn’t. In this case, the DEEP chamber (NOT the shallow chamber, from whence an eruption comes) had in fact had fresh magma pumped in, but the quantity was small, and it periodically slows, stops, or even reverses. Scientists believe this is because the “fresh” magma, upon reaching the bottom of the upper chamber, then flows away through deep horizontal vents (rather like what Kilauea is doing now), to cool and form plutons — small to very large bodies of underground, intruded, solidified igneous rock.

Often this ground swell is also because there is a tremendous quantity of ground water heated by the (mostly-solid but still hot) upper magma chamber. And when water is heated, it expands. As the heated ground water ebbs and flows, the ground in the area swells and shrinks. This, in turn, tends to change the activity of the hydrothermal features, increasing or decreasing with the pressure of ground water.

That last bit is important, because the animal-kill event occurred shortly after a cold front passage, when the air was frigid and still, and the animals took shelter in a low-lying area near the river, where those same hydrothermal features would help keep the air warm, and make foraging for food easier. Hydrothermal features such as those found nearby are also known to emit toxic volcanic gases such as sulfur dioxide, hydrogen sulfide, and carbon dioxide, often in significant quantities, WITHOUT the need for an eruptive event — pretty much continuously. And these gases, mostly being heavier than air, tend to flow along the ground and collect in low-lying areas. Normally, the winds in the region keep the air sufficiently stirred that concentration is not particularly dangerous, but certain meteorological conditions — such as those brought about by the cold front passage — can result in air that is sufficiently still to allow the vapors to collect and concentrate, and this appears to be what happened in 2004.

And this has happened before, several times in recorded history. This does NOT mean that the caldera is preparing to erupt. Nor does it mean that such an eruption, IF it took place, would automatically be a supereruption, as “ordinary” eruptions are more common, based on geologic studies.

But What if it DID erupt? 

What do you do?

Kiss your ash goodbye...?

No, seriously. What do you do?

In all seriousness? Unfortunately, there are some natural disasters for which it simply isn’t possible to do a lot of advance preparation. They are simply too big, too widespread, and too variable, to make plans. Supervolcano eruptions are one of these. Physicist Michio Kaku said it best, I think...

“All you can do is run.”

~Michio Kaku

To obtain a copy of Kiss Your Ash Goodbye: The Yellowstone Supervolcano by Stephanie Osborn, go to: bit.ly/Kin-KYAGTYSV.

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

Excerpted from Kiss Your Ash Goodbye: The Yellowstone Supervolcano, © 2018
By Stephanie Osborn
http://www.stephanie-osborn.com
Images in this article are public domain unless otherwise noted.

Geological History — Known Yellowstone Eruptions

There have been 3 known “Yellowstone” eruptions with detectable welded-ash (which forms a rock called “tuff”) strata, all of which occurred in approximately the same location:
1) Island Park/Huckleberry Ridge
2) Henry’s Fork/Mesa Falls
3) Yellowstone/Lava Creek

A map depicting the ash bed strata for all three major Yellowstone supereruptions, 

along with the Long Valley eruption’s Bishop ash bed, 

and the Mt. St. Helens ash fall, for comparison.

Credit: USGS.

The Island Park/Huckleberry Ridge Eruption

The Huckleberry Ridge Eruption is the oldest eruption at the current location, some 2.1 million years ago. The caldera formed by this eruption is known as the Island Park Caldera; the stratum of tuff (also “tufa”; a kind of rock composed of welded ash — upon landing, the ash was still hot enough to be partly molten, and the particles literally stuck together, or “welded,” into a single rock layer; it is often porous, fine-grained, but may contain larger, pebble-like particles, especially close to the eruptive source) is the Huckleberry Ridge Tuff.

Etruscan paving stones composed of tuff from the Italian peninsula.

Credit: Patafisik, https://commons.wikimedia.org/w/index.php?curid=6918192, public domain.

One of the world’s largest calderas, the Island Park Caldera is at a minimum 50x40mi (80x65km) up to possibly as large as 60x37mi (95x60km) and possibly up to ~0.6mi (1km) deep. This would have been bigger than the state of Rhode Island. The eruption was 2,500x greater than Mt. St. Helens.

Exposed Huckleberry Ridge tuff strata 

(there were several ashfalls in quick succession here) 

along the Gardner River near Osprey Falls, 

above Mammoth Hot Springs in WY.

Note vehicles for scale.

Credit: https://www.lpi.usra.edu/education/EPO/yellowstone2002/workshop/huckleberry/index.html 

The Henry’s Fork/Mesa Falls Eruption

The Mesa Falls eruption occurred 1.3 million years ago and produced the Henry’s Fork caldera along with the Mesa Falls Tuff. The Henry’s Fork megacaldera is approximately 18x23mi (11x14km) in dimension, though some argue for a rounder shape, anywhere from 10-20mi (16-32km) in diameter.
This was “only” a VEI 7 eruption, but partly due to its density, and partly its overall size, it is considered a supervolcano eruption.

Exposed Mesa Falls tuff at south rim of the Island Park Caldera/Henry's Fork Caldera overlap; 

near Ashton, ID. Note vehicle for scale.

Credit: https://en.wikipedia.org/wiki/Mesa_Falls_Tuff#/media/File:Ashton_Quarry.jpg 

The Yellowstone/Lava Creek Eruption

This eruption is the most recent supereruption, and the only one dubbed “Yellowstone”; it occurred “only” ~630,000-640,000 years ago and produced the current Yellowstone caldera, creating the Lava Creek Tuff formation. The current caldera measures 53x28mi (85x45km). The current caldera rim ranges from some 100ft (30m) tall up to nearly a third of a mile (500m) high. This was without doubt a VEI 8 eruption.

Tuff Cliff in Yellowstone National Park, 

showing an exposed section of the Lava Creek tuff.

Credit: https://en.wikipedia.org/wiki/Lava_Creek_Tuff#/media/File:Tuff_cliff_yellowstone_national_park.jpg 

NOTE: It is essential to realize that the maps depict THE EXTENT OF THE TUFF STRATUM FOR EACH ERUPTION, and do NOT indicate the full extent of the ASH FALL. As aforementioned, tuff is formed when the falling ash is still hot enough to be partly molten, so the particles stick together when they contact. Ash can and does fall far beyond the extent of the formation of tuff — the ash plume from a Yellowstone super-eruption would be caught up in the jet streams and swept worldwide.

Other Yellowstone Activity

So. Three honkin’ big eruptions from the Yellowstone hotspot, huh? Well, really, that’s not a huge record. There’s not that much to worry about, is there?

Except for the fact that the Yellowstone hotspot has been busy. And it’s been around for MILLIONS of years.

Yellowstone hotspot eruptions can be tracked from their current location in the corner where Wyoming, Montana, & Idaho meet, all the way back in a southwest direction nearly to the northeast corner of California — as many as a dozen or more! The oldest known eruption dates to at least 16-18 million years ago.

Location of some of the previous Yellowstone-hotspot calderas, with ages indicated.

Credit: Kelvin Case at English Wikipedia, CC BY 3.0, 

https://commons.wikimedia.org/w/index.php?curid=29981303 

A different version of the map, showing other areal features, 

including a few members of the Cascade volcanic chain, 

and the Columbia Flood Basalts, a trap eruption which may or 

may not be associated with the hotspot.

Credit: Lori Snyder, Department of Geology, University of Wisconsin-Eau Claire, via USGS

https://people.uwec.edu/jolhm/eh2/erickson/history.html 

Also realize that the Rocky Mountains orogeny (mountain-building) ended some 55 million years ago, meaning they were already formed before the Yellowstone hotspot got to them...yet, aside from some resurgent domes, etc., there are essentially NO mountains in the hotspot track — at least within the megacalderas.

The mantle plume/hotspot is NOT moving relative to the Earth’s surface overall, nor with respect to the Earth’s core. Plate tectonics creates the appearance that it is moving, when it is really the North American plate moving across the hotspot. The track of past calderas punched through the plate is therefore an inverse record of the direction of movement of the North American plate. The North American Plate is moving roughly southwest to west-southwest, with slight changes in direction over time. This accounts for the direction and slight curvature of the caldera track.

To obtain a copy of Kiss Your Ash Goodbye: The Yellowstone Supervolcano by Stephanie Osborn, go to: http://bit.ly/Kin-KYAGTYSV.

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

Excerpted from Kiss Your Ash Goodbye: The Yellowstone Supervolcano, © 2018

By Stephanie Osborn

http://www.stephanie-osborn.com

Images in this article are public domain unless otherwise noted.

The Yellowstone Hotspot and Structure

NOTE: I am aware that there is current research claiming that the subducted Farallon Plate is the source of the Yellowstone melt, as well as its long-lived behavior and track. However, based on what I know, I am skeptical.

For now, in the absence of more definitive results, and given the fact that the ancient plate is now pretty much crammed almost as far under the eastern North American plate as it can get and still be under it, and given detailed information on a mantle plume of some substantial size, it is my considered opinion that more than likely, the Farallon Plate had only marginal, if any, effects on Yellowstone. That said, it may possibly be at least part of the reason why said mantle plume is anything but vertical. (I do discuss all this in a bit more detail in the ebook.)

I also note that the current accepted model is “mantle hotspot,” with plenty of data to support it. If that should change, I will add an update.

Overview

The Yellowstone supervolcano is a very long-lived system. Unlike most more ordinary volcanoes, which are supplied with magma via such relatively shallow means as tectonic plate subduction and subsequent melting, Yellowstone is apparently produced by a large and powerful mantle plume; the reason for the plume is unknown. As the melt in the plume rises, it pushes on the overlying crust, “puddling” in a weakness in the overlying rock, forming a magma chamber. This pressure first forms a bulge (a “dome”), then the crust of the bulge begins to crack (surface cracks). If these cracks deepen enough to reach the magma chamber, an eruption can occur. They are also responsible for the hydrothermal features seen in the national park.

The Plume

The mantle plume goes down at least 600mi (~966km), but recent seismic evidence discovered by researchers at the University of Texas indicates it may go as far down as the outer core/mantle boundary, some 1,800mi (~2,900km) down. The plume apparently angles sharply south-southwest from the megacaldera, and the base of the plume can be found under the California/Mexico border. It is very roughly cylindrical; early estimates indicated it was some 215-300mi (346-483km) in diameter, but more recent estimates say it is at least 400mi (644km) wide. It is 2,050mi (3,300km) long, and up to 1,500ºF (816ºC) at the base, near the core.

The Magma Chambers

There are a couple of different reconstructions from seismic & other data that indicate the possible shape & size of the magma chamber. More, seismic tomography indicates there are TWO, a shallow and a deep chamber, with the deep chamber likely directly linked to the mantle plume.

The deep magma chamber, connected to the upper mantle plume.

Note state lines and park/caldera outlines on top and bottom of cube, for scale.

Credit: https://www.sciencedaily.com/releases/2009/12/091214075225.htm

According to the USGS, “The shallower magma storage region is about 90km (56mi) long, [and] extends from 5-17km (3-10.5mi) depth.”

In turn, and from the same source, “The deeper magma storage region extends from 20-50km (12-31mi) depth, contains about 2% melt, and is about 4.5 times larger than the shallow magma body.”

If we assume — based on those measurements — average values for length, width, and height, such that the smaller chamber is a rough prolate ellipsoid of approximate dimension 7.5x7.5x56mi (12x12x90km), then it has a rough volume of ~13,000cu. mi. (~54,000km3). This then gives a volume for the deeper chamber of 58,500cu. mi. (~244,000km3).

However, as it turns out, generally speaking, magma chambers don’t induce eruption until they have surpassed the 50%-full mark. And that’s a whole heck of a lot of magma, AND we’re only at 2%. On general principles, I think we’re good.

The Geysers/Hydrothermal Features

The geysers, hot springs, mud pots, fumaroles, and the like are fed by ancient rain- and meltwaters — the local ground water, in essence, except it is often coming from depth — that seep through the network of cracks and fractures in the rock. They are heated by the chambers and gradually rise, eventually forcing their way to the surface to form geysers and all the other hydrothermal features common to Yellowstone and other such similar volcanic landscapes.

Riverside Geyser. Credit: National Park Service.

A fumarole (steam vent) field in Yellowstone, where dangerously superheated steam emerges in the geyser basins. Credit: National Park Service /Jim Peaco

WARNING: most of the hydrothermal features in Yellowstone, and other active megacalderas, are DANGEROUSLY HOT. Not infrequently, these are superheated waters, meaning their temperatures may well be above the boiling temperature of water; this is especially true for geysers and fumaroles, but many — most — hot springs are also near boiling. More, at least at Yellowstone, they also are prone to being highly acidic. It is NOT AN EXAGGERATION in the least to say that entering one of these features means an instant, horrible death; it has happened many times. Worse, sometimes bodies are not recovered, simply because the water has become strongly acidic thanks to the sulfuric gases dissolved in it; the corpses simply dissolve. When the bodies are recovered, they are often in very poor condition. DO NOT EVEN THINK OF TRYING to use one as your personal hot tub, and DO NOT LEAVE THE TRAILS/WALKWAYS, because the high mineral content can form what LOOKS like solid ground, but is really a skim of mineral deposits floating on the water’s surface.

Is There Any Danger of Eruption?

Yes, because this is an active volcanic feature; there has been some uplift in areas of the park, especially under the lake, but given the nature of the feature, and the high levels of seismic activity that occur there normally, that isn’t necessarily anything to worry about. The “uplift” feature on the lake floor seems to be part of an underwater field of hydrothermal vents, fissures and faults, not unlike those found on ocean floors, so it is likely that the uplift is a result of gases and expanding hot water underneath. Certainly a good deal of the seismic activity in and around the caldera has to do with this same hydrothermal activity; this expansion, when the water is in the natural cracks of a rock stratum, will force the cracks wider until the stone eventually breaks. This fracture does create small quakes.

So-called “quake swarms” in the region are NOT in the correct area to indicate magma on the move. They also don’t have the “long-period harmonic” vibrational component to indicate flowing magma. More, detailed geophysical studies show no evidence of either magma chamber inflating.

To obtain a copy of Kiss Your Ash Goodbye: The Yellowstone Supervolcano by Stephanie Osborn, go to: bit.ly/Kin-KYAGTYSV.

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

Excerpted from Kiss Your Ash Goodbye: The Yellowstone Supervolcano, © 2018

By Stephanie Osborn

http://www.stephanie-osborn.com

Images in this article are public domain unless otherwise noted.

 How many supervolcanoes are there in North America?

There are ~170 active volcanos in the United States of America, most in Alaska and Hawaii, though there are quite a few along the West Coast states.

There are only an estimated 4-5 supervolcanoes in the USA. These include the Yellowstone Caldera (VEI:7-8), Mt. Mazama/Crater Lake (considered dormant) (VEI:7), Valles Caldera (VEI:7), Long Valley Caldera (VEI:7), La Garita Caldera (likely extinct) (VEI:8), with all except the last potentially capable of erupting. There were more, but they appear to be extinct. Extinct is relative, however; most show some degree of geothermal activity in the area.

Mt. Mazama/Crater Lake (VEI:7)

The central feature of Crater Lake National Park is Mount Mazama in southern Oregon. It is a composite volcano (a composite of alternating layers of ash/cinder and lava) in the Cascade Volcanic Range of the Pacific Northwest, which are fed by the subduction and subsequent melting of the Pacific Ocean floor tectonic plates.

Prior to the caldera-forming eruption, Mazama stood at least ~12,000ft (3,700m) in altitude. Post-eruption, it now has a maximum height of 8,934ft (2,723m) at Mount Scott (2mi/3km east of the caldera rim), which is a parasitic cone on the flank of the volcano. [Yes, that’s right, the caldera rim is some 800ft+ (240m+) lower.]

The caldera rim proper ranges from 7,000-8,000ft (2,100-2,400m) altitude, and is 5x6mi (8.0x9.7km) across. The bottom of Crater Lake goes 2,148ft (655m) down; it is the deepest lake in the U.S. The lake IS the caldera, so that's at least how far it collapsed. Some say that the bottom is near the base of the mountain, others that it goes even deeper.

Crater Lake, the water-filled caldera of Mt. Mazama.

In relatively continuous eruption since 420,000 years ago, things changed around 30,000 years ago, when the chemistry of the melt feeding the magma chamber apparently began to change from a relatively basaltic, runny magma to a much more viscous, silica-rich melt. As this melt grew thicker, the eruptions became more violent.

The catastrophic eruption occurred 7,700 years ago, and was observed by the local Klamath indigenous people, who “recorded” it in myth. It “...started from a single vent on the northeast side of the volcano as a towering column of pumice and ash that reached some 30mi (50km) high. Winds carried the ash across much of the Pacific Northwest and parts of southern Canada...As the summit collapsed, circular cracks opened up around the peak. More magma erupted through these cracks to race down the slopes as pyroclastic flows. Deposits from these flows partially filled the valleys around Mount Mazama with up to 300ft (100m) of pumice and ash. As more magma erupted, the collapse progressed until the dust settled to reveal a caldera, 5mi (8km) in diameter and 1mi (1.6km) deep.” ~USGS website

Subsequent eruptions from vents inside the caldera created what became Wizard Island, as snow- and glacier-melt slowly filled the depression. Eventually eruption ceased, and the ruins of Mt. Mazama began to resemble the beautiful Crater Lake we know today.

Mt. Mazama is officially considered dormant by the U.S. Geological Society.

The Valles Caldera (VEI:7)

Sometimes called the Jimez Caldera, this supervolcano is located in northern New Mexico, 55mi (90km) north of Albuquerque. It is named for the numerous grassland valleys (Spanish: valles) contained within the circular caldera, which is about 13.7mi (22km) in diameter. It is similar to Yellowstone in that the caldera also contains hot springs, fumaroles (steam vents), gas vents, and volcanic domes, in addition to meadows and streams.

The Valles Caldera as viewed from the rim. 

Note the volcanic domes dotting the floor.

Credit: National Parks Service.

Geologically, it is one of the best-studied calderas in the U.S. There are at least two known calderas on this site, the Valles, and the older Toledo Caldera. The nearby and associated Cerros del Rio volcanic field is older still, indicating multiple supereruptions at this site. Overall, these and related nearby volcanic features are included within the Jemez Volcanic Field & Mountain Range, which stretches across three counties in New Mexico.

Several layers of silica-rich lava and tuff (welded ash) in the region are ample proof of the eruptions, the most recent of which was some 50-60 thousand years ago and resulted in the current Valles caldera. Previous eruptions date back at least 14 million years.

The cause of the vulcanism seems to be the intersection of the Rio Grande Rift (a continental rift zone, running N-S from central Colorado state, USA, to Chihuahua state, Mexico) and the Jemez Lineament (a series of faults running E-W 600mi (965km) from Arizona east possibly as far as western Oklahoma). The Valles Caldera does not, therefore, appear to be due to a solitary mantle hotspot as such, but to rifting occurring in the middle of the continental plate, though this rifting may be from convective uplift in the mantle.

The Long Valley Caldera (VEI:7)

The Long Valley Caldera is in central California along and slightly east of the westernmost Sierra Nevada Range. It and the adjacent Mammoth Mountain/Mono-Inyo complex are around 55mi (89km) northeast of Fresno, California.

Part of the Long Valley Caldera, 

looking east from the north rim.

The caldera is ~20mi (32km) long, 10mi (16km) wide, and up to 3,000ft (910m) deep. It generated a massive supereruption some 760,000 years ago, producing the Bishop Tuff formation. The grand total of ejecta was some 150cu.mi (625km3), after which the surface sank nearly a mile (1.6km) into what had been the magma chamber.

The cause of the supereruption is unexplained; it is not fueled by a mantle hotspot, nor is it provided melt via subduction.

More, while it is adjacent to still-active Mammoth Mountain and the Mono-Inyo crater chain, and at least appears to be associated with them, the magma chemistries are very different, indicating they do not share a common melt system, and are NOT associated. This is an interesting puzzle.

“The caldera remains thermally active, with many hot springs and fumaroles, and has had significant deformation, seismicity, and other unrest in recent years.” ~USGS website

The activity is sufficient to run a geothermal power plant located there. But how much of this activity is due to the Mono/Mammoth complex and how much to the caldera source is not fully understood. Smaller eruptions have occurred around the caldera on a semi-regular long period, but the lava extruded has apparently been increasingly crystalline in nature, which may indicate that the magma source is cooling significantly.

La Garita/Creede Caldera (VEI:8)

The La Garita caldera-forming eruption is estimated as one of the largest eruptions on Earth. It lies in the midst of a huge region in the Rocky Mountains called the San Juan Volcanic Fields. The town of Creede sits on what would have been the north caldera rim, with Pueblo, Colorado 110-115mi (km) east-northeast; Colorado Springs ~120mi (193km) northeast; Denver ~150mi (240km) north-northeast.

This region became active some 35-40 million years ago, with an exceptional period of activity from 30-35 million years ago. At the tail end of this flurry of vulcanism, the La Garita supereruption took place, roughly 27 million years ago. It ejected some 1,200cu.mi. (5,000km³) of material, which became known as the Fish Canyon Tuff. This ash deposit covered an area of AT LEAST 11,000 sq.mi. (30,000km²) in a layer whose average depth was 328ft (100m). This tuff is surprisingly uniform chemically, indicating it was ejected all in a volume.

The resulting caldera was a monster 22mi (35km) wide, and anywhere from 47-62mi (75-100km) long. It is no longer recognizable as such to the untrained eye, however, as a single resurgent dome (Snowshoe Mountain) has filled it.

La Garita Caldera (red dotted outline), 

with resurgent dome (Snowshoe Mountain) inside it. 

Credit: http://elements.geoscienceworld.org/cgi/content-nw/full/4/1/17/FIG4

The energy of the eruption was some 5,000x the largest nuclear device ever detonated on Earth, the Tsar Bomba, a 50MT explosive. This places the La Garita supereruption at 250 GIGATONS of energy. The area devastated would have encompassed a substantial portion of what is modern-day Colorado, not counting ash fall.

Vulcanism in the San Juan Volcanic Field as a whole, including the La Garita supervolcano, apparently ended 2.5 million years ago. It is considered extinct.

What’s the strongest supervolcano ever known?

The biggest known eruption in geologic history IN THE USA — some say in the world — was the Fish Canyon eruption in the La Garita megacaldera.

The biggest known eruption in geologic history in the WORLD was the Guarapuava-Tamarana-Sarusas eruption in South America. The eruption occurred ~132 million years ago, produced an estimated 2,100 cu. mi. (8,600 km³) of ejecta, and was probably at least the equivalent of the La Garita eruption.

To obtain a copy of Kiss Your Ash Goodbye: The Yellowstone Supervolcano by Stephanie Osborn, go to: http://bit.ly/Kin-KYAGTYSV.

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

Excerpted from Kiss Your Ash Goodbye: The Yellowstone Supervolcano, © 2018

By Stephanie Osborn

http://www.stephanie-osborn.com

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?

NO.

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: http://bit.ly/Kin-KYAGTYSV.