Waiting for the big kaboom. After a bit of hiking and wildlife viewing, we drove on to Roosevelt Lodge for lunch. It was okay, I remember, but nothing outstanding; the main interest was the Lodge itself, an aggressively rustic structure built sometime in the 1920's, with massive tree trunks (bark still on) used as the primary construction material. Seemed kind of dude-ranchy to me; we quickly moved on after eating. We drove south, and up, eventually crossing Dunraven Pass; then down, into thick unburned forest, until we ran into an obstacle:

It's the 1000-foot deep Grand Canyon of the Yellowstone, displaying the reason for the park's name, the distinctly yellowish rock.

The canyon is impressive, but it's obvious that much the rock isn't very hard; it erodes away easily, and at one point we noticed a viewpoint that had been moved because its old location had become unstable.

Just at the limit of the view up the canyon, you can see a hint of something more impressive; travelling up to the next viewpoint reveals it:

It's the distinctively asymmetric Lower Falls, the biggest-volume waterfall in the Rocky Mountains, just a touch over 300 feet high. The Yellowstone River cuts its way through the yellow rock.

The rock is welded tuff and rhyolite, both products of violent, or even explosive, volcanism. That Yellowstone was the product of volcanism was obvious to the earliest close observers of the region – a large plateau of volcanic rocks, that happened to sit right on top of the surrounding sedimentary and metamorphic Rocky Mountains, and if you weren't paying attention, just looked like more mountains. But the ability to make fine distinctions between different sorts of similar volcanic rocks, and to associate those rocks with kinds of eruptive events, was pretty limited until about 60 years ago, and the Yellowstone plateau was generally thought to be a huge mass of not-particularly-distinctive overlapping ancient lava flows, probably the outflow of the Absaroka Mountains to the north and east.

It wasn't until the early 1960's and the advent of good rock-dating techniques that geologists noticed that the rocks of the central plateaus are far, far younger than the volcanic rocks that make up the surrounding mountains. The Absarokas and neighboring highlands to their northwest and south are about 50 million years old; but the central plateaus are between 650,000 and 2 million years old. About the same time, geologists began to realize the significance of all the welded tuffs in the central plateau area – almost without exception, they are produced by extremely violent, explosive eruptions. The vast layers of tuff implied massive and cataclysmic eruptive events, dwarfing any during historic time. The 35-mile-wide rolling upland in the middle of the mountains wasn't the product of flows from surrounding volcanoes filling up the valleys between them – it was the result of the original volcanoes being blasted out of existence and replaced by incandescent ash, which cooled, mostly, into the present landscape.

Further investigation revealed that the welded tuffs fell into three distinct age groups: 650,000 years ago; 1.3 million years ago; and about 2 million years ago, each age group associated with a specific caldera within the overall plateau region. Other rhyolite and basalt flows in the caldera are intermediate in age, and geologists have inferred from them a pattern to the cycles of eruption: thick flows of rhyolites, followed by a caldera-forming explosive eruption with its accompanying ash and tuff, followed by flows of rhyolites and basalts, followed by relative quiet. Based on that, it's assumed that when Yellowstone becomes volcanically active again, it will start with lava flows, rather than blowing up all at once.

Looking at a topographic map of the region, a noticable large-scale feature of the Yellowstone area is the lack of mountains to the southwest of the plateaus – the land slopes down evenly to the Snake River plain, which forms a distinctive low-lying region surrounded by mountains. The plain is also full of volcanic rocks. Further investigations indicated that hugely destructive eruptive events occurred across what is now the plain, receding further into the past as you travel to the south and west of Yellowstone. Yellowstone appears to be the current location of a stationary mantle hot spot that is, in effect, cutting a big trench across the North American Plate as the plate above it moves to the south and west, leaving low volcanic plains behind. The older history of the hot spot is in dispute; some geologists have tried to link it to the massive Columbia flood basalts, which dumped vast – vast! – quantities of volcanic rock across much of the Northwest.

[All trip entries]

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Posted by David Fleck at 05 June 2007 07:58 AM