One of the great joys of being a geologist is that every summer I try to escape the heat waves of July and August by doing field research in some place with a nicer summer climate. In the late 1990s until 2003, I was funded by the NSF and the Petroleum Research Fund of the American Chemical Society to do paleomagnetic dating on the Cenozoic marine rocks of the Pacific Coast. This meant several weeks each summer in coastal Oregon, Washington, or California, to work during their dry season when the outcrops along creek beds were at their most exposed—and timed for the lowest summer tides, so we could also sample outcrops along beach cliffs. But in the past 10 years, I’ve been doing research nearly every summer up in the high elevation (camping at 8800 feet) of the San Juan Mountains of southwest Colorado. The summer weather here is delightful: dry and in the low 80s at the hottest in the daytime, cooler in the mountains, and nothing like the 100-degree heat waves we hear about on the news scorching the rest of the country.
This particular summer we finally visited one of the great geologic sights of North America, the nearly inaccessible Wheeler Geologic Area in the La Garita Wilderness, north of South Fork and Creede, Colorado. It is one of the few remaining exposures of one of the largest supervolcano eruptions in the last 65 million years. First discovered by Capt. George M. Wheeler, who surveyed it for the U.S. Army in 1874, it is a spectacular cliff amphitheater with huge erosional “hoodoos” (pinnacles and columns) of volcanic rock. It was made Colorado’s first national monument in 1908, but then demoted in 1950 when the government decided not to spend money to make it more accessible, and instead incorporated into the La Garita Wilderness Area. It rivals more famous parks, such as Bryce Canyon in Utah, for its scenic beauty. The big difference, however, is accessibility. Whereas Bryce is visited by millions of people each year, and had roads, trails, railings on the overlooks, and many tourist facilities, Wheeler is almost completely undeveloped. First you must drive almost 20 miles up a decent gravel road from 8000 feet to 11,000 feet to the site of an old abandoned sawmill near Pool Table Mountain. Then there is another 13 miles over one of the worst “roads” I’d ever traveled on, followed by a hard hike at over 12,000 feet in elevation, to reach it. We brought along four-wheel ATVs to attempt the trip, since the “road” is brutal to vehicles and even a Jeep has trouble on the numerous deep flooded washouts and piles of huge rocks that litter the “road”.
We unloaded our three ATVs and our Jeep and headed off, and I soon discovered the drawbacks of sitting behind the driver on the rear seat. While the driver is braced in a low stable position and can see what is coming, the passenger is riding high over the rear axle with almost nothing to hang on to or brace against, and no view of what’s ahead. Because my wife has more experience on these machines than I (she was born a Kansas farm girl, learning to drive farm machinery at age 8), I got to ride in back. For the next 2 hours over 13 excruciating miles, I was thrown back and forth like a rag doll, one of the most joint-jolting, bone-jarring, groin-crushing, whiplashing experiences I have ever endured. At times, she’d race through the boulder fields or jump over rocks like Evel Knievel. I was literally airborne until I pulled myself back on the seat. The San Juan Mountains typically get hard afternoon thunderstorms nearly every day, so the “road” had many flooded potholes and piles of unmelted hail, and we were spattered in mud from head to toe after we drove through a few of them.
Finally, we reached the parking area at the trailhead, and most of our party stayed behind after our quick lunch. But my wife and I and our guide began the 2-mile hike up to the Wheeler Geological Area, and soon found how hard the climb was at an elevation of almost 13,000 feet. Both of us had to stop every few minutes because we were sucking air and completely out of breath, and I began to realize that I spend too much time writing books and blogs at sea level. We we finally reached it, however, the view was indeed spectacular. We could see the amazing razor-sharp pinnacles and “hoodoos,” cut out of soft volcanic ash that had fallen in thick blanket across the region.
In fact, the entire San Juan Mountains is formed from a large volcanic field that erupted over the course of a few million years, from about 25 to 32 million years ago. The famous old mining town of Creede, Colorado, is on the rim of a giant caldera (collapsed volcano) that erupted about 26 million years ago, and you can still see the rocks of the collapsed caldera in the middle, a circular “racetrack” valley like a “moat” that surrounded the core of the volcano (and filled with amazing lake sediments full of fossil insects and plants), and the outer rim of the caldera, where the town of Creede lies. (Its wealth in silver, gold, lead, and other ores due to late-stage fluids mineralizing and percolating through cracks in the collapsing caldera after it began to cool). There are many other similar large calderas across the San Juan Mountains, all of slightly different ages.
But these eruptions are dwarfed by the eruption of the La Garita caldera in northeastern part of the San Juan Mountains in Colorado. It was once a huge hole in the ground, 35 by 75 km in size (22 by 47 miles), which forms a large irregular oblong valley in the San Juan Mountains, now largely eroded away, overprinted by later eruptions, or covered by thick forests. Some of the caldera and its ash deposits can be seen in the Wheeler Geologic Area, although the entire northeastern San Juans are composed of La Garita volcanics, including huge ignimbrite flows with gigantic columnar jointing. (The scenery of this region will be featured in the 2013 Lone Ranger movie with Johnny Depp and Armie Hammer, which just finished filming here in June). In some places, the caldera has been covered up and partially filled by eruptions of the smaller but younger Creede caldera and other younger calderas. When La Garita exploded 27.8 million years ago, it blew over 5000 cubic km (1200 cubic miles) of material all over the United States, enough material to entirely fill Lake Michigan. It deposited a widespread ash layer known as the Fish Canyon Tuff, which can still be seen up in the Arkansas River Canyon, 100 km northeast of the caldera, and beneath the surface of the Alamosa area, 100 km east of the volcano. There are thin layers of the Fish Canyon Tuff as far east as eastern Nebraska and Kansas. At one time it was one of the largest volcanic deposits in the world, and probably covered much of the Rockies and the western and central Plains of the U.S. It is calculated that the energy released by the La Garita explosion is 10,000 times more powerful than the largest nuclear device that humans have ever detonated.
La Garita appears to have been the largest explosive volcanic eruption documented in the past 65 million years, but it pales in comparison to the eruption of the Siberian basalt flows. These occurred about 250 million years ago, in concert with the biggest mass extinction of all time. Although these events did not produce ash clouds that covered the planet, they did release huge amounts of mantle gases that may have triggered a massive “super-greenhouse” global warming event. The thickness and volume of lava flows is truly staggering, estimated to be almost 4 million cubic km of lavas, or about a thousand times as large as any of the eruptions we have discussed so far.
All of these supervolcanoes erupted long before humans evolved, but there are immense eruptions that did affect humans. In a few weeks, I’ll blog about the immense Toba eruption that occurred in Indonesia 78,000 years ago, which nearly wiped out humans on this planet. This eruption was thousands of times larger than the Tambora eruption of 1815, which cause the planet to experience a “year without summer”. Toba in turn dwarfs the gigantic eruption of Krakatau in 1883, the largest eruption witnessed in historic times, and the first to be documented worldwide by the newly established network of telegraph cables around the world.
Although Indonesia has a lion’s share of the world’s deadliest volcanoes, North America was home to many supervolcano eruptions as well. One that is still a threat is the Yellowstone caldera in the heart of our first national park. The hot springs and geysers and fumaroles of Yellowstone are all evidence that an enormous mantle hotspot lies not far beneath the crust, and the ring shape of Yellowstone is actually due to collapse of a giant volcano into a caldera. Based on the dates of the volcanic flows all around Yellowstone, it last erupted about 640,000 years ago, ejecting over 1000 cubic km of material that blanketed the entire western half of the United States. This deposit is known as the Lava Creek Tuff. Thin layers of this ash can be detected in Ice Age deposits as far east as Louisiana and Alabama, over 3000 km (1000 miles) away. Another large eruption from the same hot spot occurred on the Idaho side of Yellowstone, when the Island Park caldera exploded about 2.1 million years ago, and spewed out over 2500 cubic kilometers of ash and pumice known as the Huckleberry Ridge Tuff, which blew east as far as Missouri and as far south as Texas.
Another active but slightly smaller eruptive source is the Long Valley Caldera, which underlies Bishop and Mammoth Mountain, on the east flank of the Sierras in California. About 760,000 years ago, it exploded and shot 600 cubic km of material into the air, which blanketed the entire western half of the United States as far east as Oklahoma. Its biggest eruption is known as the Bishop Tuff, which is found all around the northern Owens Valley, and can be traced to Ice Age outcrops in Kansas and Nebraska. Geologists have been nervously monitoring this caldera for decades, and every time it shows signs of awakening, they issue alerts. Sadly, the businesses in Bishop and Mammoth Mountain don’t want scientists raining on their parade, and often ignore or denounce the warnings that their lives are being threatened. As I pointed out in a previous post, this is a common behavior of people who arrogantly think the earth is made for them, and complain when they are warned of natural disasters and fail to respond. But as the historian Will Durant pointed out, “Civilization exists by geologic consent, subject to change without notice.”
Finally, after hiking back to the ATVs and another 13 bone-jarring miles back on the same “road”, we reached the trucks and trailers, and returned home. We picked a good day, because the previous afternoon there was a huge hailstorm, and the normal pattern for the Rockies is a regular afternoon thunderstorm each day. I’m planning on spending the next two days in my sleeping bag, trying to recover from the Ride from Hell. I could use something strong, maybe some morphine…
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