Rock Talk By Larry Larason

Big Bangs

I recently watched a two part show on television that claimed the eruption of a proto-Krakatau [aka. Krakatoa] in 535 CE affected world climate to the extent that agricultural failures caused the Dark Ages in Europe and the fall of civilizations such as Teotihuacan. The show was based on a book by David Keys, now, unfortunately, out of print. I should point out that some geologists and most historians find Key's scenario dubious. There is evidence of an eruption that affected world climate, but one event does not steer history as much as Keys proposes. Still, how could one volcano cause such a disruption of human affairs?

Because it was a supervolcano. Researchers have found a caldera about 50 miles in diameter in the Sunda Straits of Indonesia. A crater that size means the eruption of proto-Krakatau would have ejected several hundred cubic kilometers of ash that would shroud the sun for more than a year.

Such caldera eruptions are definitely of a scale to affect global climate. Even the amount of ash and sulfur the Philippine volcano, Mt. Pinatubo, put into the atmosphere in 1991 cooled the Earth for three years. Pinatubo emitted about 5 cubic kilometers [1.2 cubic miles] of ash, so it is considered a moderately large eruption. Tambora erupted in Indonesia in 1815, putting 150 cubic kilometers [nearly 36 cubic miles] of ash into the air, which brought about "the year without a summer" in the northern hemisphere.

Most television shows about volcanic catastrophes concentrate on Yellowstone National Park. It is of interest because it definitely is still active and expected to explode again sometime in the future, although not in our lifetimes. The last time Yellowstone erupted 600,000 years ago, it blanketed the entire western US as far east as the Mississippi River with ash. Yet, despite the threat it poses, Yellowstone ranks number ten in a recent list of the world's largest calderas.

Here in the American Southwest we've had some big bangs in the past. Four of the world's top twenty calderas are in our own backyard. If I ask you to name one you might think of Valle Grande in the Jemez Mountains. It's big, but it does not make in into the top twenty. I'll name our winners below, but first, what makes these eruptions so powerful?

No two magma bodies have exactly the same chemical composition, but they come in a range of types from basaltic, containing about 50 percent silica, to rhyolitic, with nearly 70 percent silica. While basalt is liquid enough to flow, as it does on Hawaii, rhyolite is so viscous that it tends to build up pressure and explode. Almost all super volcanoes spew rhyolite or dacite [about 65% silica]. When such a volcano blows, it doesn't usually build a cone; instead it leaves a hole in the ground when the roof of the emptied magma chamber collapses. Much of southeastern Arizona and a lot of southwestern New Mexico are buried under rhyolite and dacite. This indicates that there have been a lot of big bangs in that region. In the Mogollon Mountains and the Boot Heel area there are at least 30 known calderas. Geologists did not begin to recognize these features until the middle of the last century. More will probably be discovered in the future as there are still some tuff deposits that have not been traced back to their sources.

Three of the top twenty largest known calderas are in southwestern New Mexico. The Emory and Bursum Calderas rank at numbers twelve and thirteen, respectively. The Emory Caldera sits between Hillsboro and Pinos Altos. At 55 by 25 kilometers, it is twice the size of Valle Grande. It erupted 33 million years ago, spewing between 1400 and 2000 cubic kilometers of ash. The 28 million year old Bursum Caldera, located east of Glenwood, is smaller than Emory at 40 by 30 kilometers, and the estimate of ash it spit out is 1400 cubic kilometers. Number 15 in the list of top twenty is the Soccoro Caldera. Smaller than the other two, it erupted between them in time.

We move north to find our next humongous eruption. The La Gariita Caldera in the San Juan Mountains of Colorado formed 27.8 million years ago and is 22 by 47 miles in size. Once a candidate for the largest known eruption on Earth, it is now considered to be number 11 – just smaller than Yellowstone. In a period of 3-7 days La Garita spewed more than 1200 cubic miles of ash. It took nearly 30 years for geologists to realize how large La Garita was, because subsequent eruptions obscured this crater. Over the 1.5 million years after the big explosion, another 7 calderas exploded, all within the original one. The youngest of these is the Creede Caldera [about 10 miles in diameter], where mineralization led to a great deal of mining.

The large calderas in the Southwest probably had big effects on the Earth's climate, but they erupted before anyone was around to notice. Should a monster eruption occur today, it could shut down our civilization for years.

Here's one example of what such an eruption could do: A KLM jet, Flight 867, was approaching Anchorage, Alaska on December 15, 1989. They were warned that Redoubt Mountain had been showing signs of an impending eruption, but their flight path took them 200 miles from the volcano. However, the mountain did blow, and a cloud of volcanic ash flew up seven and a half miles above sea level and drifted into the airliner's path. The pilots thought they were flying into a normal cloud. But once inside the plume they soon realized their mistake for the acrylic windshields became opaque, blasted by the shards of volcanic glass. The ash got into the fuel, the hydraulic fluids, and the plane itself, while St. Elmo's fire, caused by the electrostatic charge in the tiny particles, danced on the wings. Ash shorted out the electrical systems. When the cabin lights went off, the 231 passengers must have thought they were on the flight from, or to, Hell. Jet engines run hot enough to re-melt volcanic rock, and soon they were shutting down because they became coated with glass. It all ended well for Flight 867 – the pilot managed to re-start two engines, drop out of the volcanic cloud, and land safely in Anchorage. The Boeing 747 required $80 million to repair. The cloud continued moving south, and two weeks later, 3000 miles from its origin, it sand-blasted the wings of a Navy DC-9 over El Paso, Texas.

So imagine a monstrous eruption filling the sky with ash. What would it do to our transportation systems? It would be suicidal to fly in such conditions. A super volcano might eject so much ash that it would take years to settle out of the air. Today we eat "2000 mile salad" that has traveled far from where it is grown. Given enough ash, it might also clog the engines of ground based haulers. If it darkened the sun and cooled the surface, there might not be much food to be hauled.

The ancient calderas demonstrate that we don't need to invoke an asteroid strike to cause world-wide catastrophe; the Earth can create its own.