Tuesday, July 12, 2011

A Convergence of Wonders, Day 10: Exploring a Real Hot Spot

It was day 10 of our field studies journey through the Pacific Northwest and Northern Rocky Mountains. We had finally arrived at one of the flagship destinations for our trip: Yellowstone National Park! One of the crown jewels of our national park system, home of Old Faithful...we set aside two days to explore the park, and I did something slightly devious to my students. I spent the entire day looking at "not geysers"!

Frankly, I'm not sure any of the students noticed the distinction. That's the thing about Yellowstone; it is so full of wonders and curiosities that it would be a unique landscape to explore even if there were no geysers to be seen at all. We spent our day exploring the northern loop of the park, from Canyon Village and the Grand Canyon of the Yellowstone to Mt. Washburn and then to Mammoth Hot Springs. We stopped in at Norris Geyser Basin at the end of the day, but were distracted by a wolf.
It's funny about furry charismatic animals; they seem to trump the attention of everyone, especially geology students. Our first stop of the day had nothing to do with geology at all, it was to photograph an elk. One cannot travel fast in Yellowstone National Park, because there are constant traffic jams caused by tourons (tourist morons, including ourselves...) looking at bison, elk, bear, moose, beaver, or wolf. I was immune to the effect. I took only ten pictures of this particular elk...
We explored several overlooks at the Grand Canyon of the Yellowstone and Lower Yellowstone Falls. The colorful walls of the canyon are composed of weathered rhyolite just over 500,000 years old. This canyon has been cut very rapidly! 
Standing at the brink of the 308-foot-high Lower Fall of the Yellowstone, one can feel the powerful surge of the water as it disappears over edge. It's not hard to believe that such a deep canyon could be carved by a river like this.
The most astounding story to be learned at Yellowstone was never envisioned by those who established Yellowstone as our first national park in 1873 (Yosemite was set aside nine years earlier, but as a state park). We drove to the high ridge near Dunraven Pass and Mt. Washburn to have a look around.

There is a fable about several blind men who were touching an elephant and disagreeing about its nature, whether it was like a rope (the tail), a snake (the trunk), or a tree (the leg). Yellowstone's most striking feature was so big that it was decades before geologists recognized it for what it was: a vast caldera, the site of three of the largest volcanic eruptions to ever hit the North American Continent in recent geologic time (2.1 million, 1.3 million, and 640,000 years before present).

How big is the caldera? The snowcapped mountain in the distance in the picture below is Mt. Sheridan. We were standing on the side of  Mt. Washburn; there is a forty mile gap between the two. The mountain ridge that connected the two peaks sank into the caldera during a single massive eruption that put 600 cubic miles of ash into the atmosphere. The ash spread for thousands of miles. A repeat eruption would be devastating to civilization (despite what the television tells you, such eruptions are not imminent, nor are they overdue; we would see signs of caldera unrest dozens, if not hundreds of years beforehand). The source of these eruptions is thought to be a mantle hot spot, although there are several alternate interpretations.
Volcanism has been influencing events in the Yellowstone region for a long time. In early Cenozoic time, around 50 million years ago, the region was rocked by numerous eruptions that produced volcanic mudflows (lahars) that overwhelmed forests of sequoia/redwood trees (You thought they were found only in California didn't you? They were once much more widespread). Each time a forest was destroyed and buried, a new forest would grow in the sediments. That forest was destroyed in turn, and over time dozens of layers containing petrified logs accumulated. These forests are generally hard to access by road or trail, but one solitary upright trunk can be seen in the north part of the park. There used to be three here, but souvenir collectors carted off the others, which explains the prison in which the trunk resides.
We moved on down to Mammoth Hot Springs (with a few bear diversions), and took a look at the strange edifice of calcium carbonate. Most of the hot springs and geysers of Yellowstone do not produce massive mineral deposits, but they happened here, outside the boundaries of the caldera. Most of the water emerging from geothermal features in the park is charged with silica, but at Mammoth, the water has passed along faults that traversed Paleozoic limestones. The acidic waters dissolved the calcite making up the limestone, and when the waters emerged here, the calcite came out of solution.
The hot springs are constantly growing and changing location, so the deposits never look quite the same. Heat-tolerant bacteria provide a splash of color to the otherwise snow-white rock.
Spelunkers (cavers) will recognize many of the deposits, as they are similar to flowstone features found in many limestone caverns, especially the rimstone pools seen in the pictures below. Exploring Mammoth Hot Springs is kind of like exploring an inside-out cavern...
 The rimstone pools were especially photogenic...
There were several killdeers strolling across the pools...
We drove on in the late afternoon to Norris Geyser Basin, but I didn't get any pictures. We were distracted by the whole charismatic mammal thing anyway, the subject this time being a Canis lupus enjoying a meal in a meadow north of Norris.
You can see the whole wolf drama in this post.

We were done with another day...tomorrow would include the famous geysers (maybe), and Grand Teton National Park.

I've been posting regularly for the last ten days, but I am hitting the road again tomorrow, so the next few posts will be at unpredictable intervals, as web access will be tricky. Take care, all!
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