No Longer a Hot Spot

Here in the San Luis Valley (like much of the world) we have some awesome geology. The west side of the valley — the San Juan Mountains — shows evidence of a gigantic volcano. The La Garita Caldera volcanic event was:

“…the second greatest of the Cenozoic Era. The resulting Fish Canyon Tuff has a volume of approximately 1,200 cubic miles (5,000 km3), rating it an 8 on the Volcanic Explosivity Index. By comparison, the May 18, 1980, eruption of Mt. St. Helens was 0.25 cubic miles (1.0 km3) in volume.

By contrast, the most powerful human-made explosive device ever detonated, the Tsar Bomba, had a yield of 50 megatons, whereas the eruption at La Garita was about 5,000 times more energetic. However, because Tsar Bomba’s reaction was complete within seconds, while a volcanic explosion can take seconds or minutes, the power of the events are comparable if measured within the respective bounded timeframes. It is the most energetic event to have taken place on Earth since the Chicxulub impact which, at 240 teratons,[7] was approximately one thousand times more powerful than La Garita.”

The La Garita was enormous, obviously and all around the west side of the San Luis Valley are smaller (but still very large) calderas such as the Bonanza Caldera, parts of the main event, with a caldera some 15 miles (24 km) across (Crater Lake is 5 miles across).

On the west side of the San Luis Valey, there are signs everywhere on the of all this ancient volcanic activity. There are lots of small, pointy piles of rocks eroded from some long ago splooches of magma and canyons with beautiful rock formations, such as Penitente Canyon. (Featured photo, taken by me in March 2017) 

The San Juan volcanic field is part of the San Juan Mountains in southwestern Colorado. There are approximately fifteen calderas known in the San Juan Volcanic Fields; however, it is possible that there are two or even three more in the region.

The region began with many composite volcanoes that became active between 35 and 40 million years ago and were particularly eruptive in the time period around 35-30 million years ago. Around this time the activity changed to explosive ash-flow eruptions. Many of these volcanoes experienced caldera collapse, resulting in the fifteen to eighteen caldera volcanoes in the region today.

Wikipedia “San Juan Volcanic Field”


Map of the Caldera Rim and surrounding area. Map provided by Bill Hatcher.

The other side of the Valley is pretty interesting geologically, too. There is Great Sand Dunes National Park and the sharp peaks of the Sangre de Cristo Mountains, much younger than the San Juans.

Dusty T. Dog and my neighbor at the Sand Dunes, October 2017

I haven’t explored much in “my” valley yet. I hope that this summer, when the blessed snow has melted and the ticks and mosquitoes are rife, I can get out on the trails and see what is in my reach.

24 thoughts on “No Longer a Hot Spot

  1. Maybe a decade ago, I discovered that the place I grew up in Queens, New York was the end of the line of movement of the giant glaciers of North America. Literally, Hillside Avenue — where you could catch a bus to somewhere else — was where the glacier stopped. When they had to dig, they left a couple of weeks for paleontologists and archaeologist to come and poke around before the foundation was laid.

    I thought that was fascinating. That was why above Hillside Avenue were hills, but below it, it was flat. Which was why the trains and wide roads were all down there, whereas I live up in the hilly area and it was all small twisting roads and you could find arrowheads in the woods near the house.

    I wrote about it and I thought everyone would find it fascinating. Mostly, no one much cared. It turns out that geology is not everyone’s hobby. But how could they NOT be interested? To this day, the lack of curiosity astounds me. I wish I’d known this when I was young enough to dig and see what more was in our little patch of woods.

    Most of that open wooded area is gone now and I haven’t been back in a long time. I’ve seen Google pictures of it and it was not encouraging. Go and explore while you can! You never know when your town will be “discovered.”

    • I think it’s a huge shame that geography is no longer taught in the schools — it is truly fascinating, and provides so much info about the world we live in!

  2. I’m so very excited for you that you can now explore your valley. I hope Bear enjoys hiking as much as she enjoys the snow!

  3. Supervolcanoes are cool beans. They ought to remind us just how trivial and transient humanity is.

    Everyone maunders on and on about the Yellowstone Caldera. Here in Cali we have the Long Valley Caldera still steaming and smoking and just as “overdue”. Nobody pays it any attention because it isn’t in a big national park. It is jealous!

    LOL! If I worried about all the “end of the world is overdue” scenarios, that’s all I’d do.

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