Inside USGS, No. 5, Pleistocene Glaciations of Greater Yellowstone

Video Transcript
Download Video
Right-click and save to download

Detailed Description

Dr. Kenneth Pierce studied the geology and geomorphology of the greater Yellowstone area for nearly his entire career with the U.S. Geological Survey. From 1965 to present, Dr. Pierce has mapped glacial deposits, pioneered Quaternary dating techniques, conducted research on the Yellowstone Hot Spot, studied the geothermal areas, explored the geology of archaeological sites around Yellowstone Lake, and led field trips into the Park. In this video, Dr. Pierce discusses glaciation at Yellowstone, focusing on why glaciations have occurred and differences between the extents of Pinedale and Bull Lake. Dr. Pierce also talks about his work with obsidian hydration rinds and how the study of basal shear stress helped to determine the age of the Bull Lake glaciation and led to special recognition of his efforts through the Kirk Bryan Award. 

Details

Image Dimensions: 1280 x 720

Date Taken:

Location Taken: Vancouver, WA, US

Transcript

Pleistocene Glaciations of the Greater Yellowstone Area TEXT: INSIDE USGS Number 5 Ken Pierce U.S. Geological Survey Pleistocene Glaciations of the Greater Yellowstone Area TEXT: Pleistocene Glaciations of the Greater Yellowstone Area DR. PIERCE: My name is Ken Pierce, I'm a geologist with the U.S. Geological Survey, actually presently retired and emeritus with the U.S. Geological Survey, and my field of focus-of specialization, is in what's called Quaternary geology and geomorphology. TEXT: Why is the glacial story of Yellowstone so unique? DR. PIERCE: The glacial story in Yellowstone is really unusual for the Rocky Mountains. Yellowstone is cold, high and snowy. And all three of these features are very important to Pleistocene glaciations. The reason it's cold is because it is high, it stands 8,000 feet and higher. The reason it's snowy is that moisture can come way inland from the Pacific Ocean to Yellowstone by traversing the Snake River Plain. It moves up the Snake River Plain and Yellowstone has an area we call the Yellowstone Crescent of High Terrain. It's the Yellowstone Plateau and the surrounding mountains. Moisture-bearing storms come up the Snake River Plain, rise up onto this plateau and onto the surrounding mountains. And in rising upward, there is a lot of orographic snowfall in the present and particularly in the Pleistocene. Yellowstone glaciation started out by ice build-up in this Yellowstone Crescent of High Terrain and flowing out onto the Yellowstone Plateau. And built-up on the Yellowstone Plateau to an extent that the Plateau became above the snowline. The build-up reached about 11,000 feet which made Yellowstone one of the highest places in the whole region. And that led to the extensive glaciation. TEXT: What clues did you find in the landscape that help tell the glacial story? DR. PIERCE: Some of the high moments in doing the geology of Yellowstone don't come up very often but it's really wonderful that they do come up and it changes how you're thinking about the landscape. One of them was in the Washburn Range. On the top of Mount Washburn and traversing the whole crest of the Washburn Range, I found there were glacial striations indicating first in one area called Observation Peak, flow from the Bear Tooth Uplift south across the Washburn Range onto the Yellowstone Plateau, indicating ice was coming first from the high Bear Tooths and building up on the Plateau and then other striations at this location indicating northward flow across the crest of the Washburn Range. So this indicated two different things. One, the ice built up on the high terrain outside of Yellowstone, flowed into Yellowstone. It then built up on Yellowstone and flowed at a very high level northward to form the Yellowstone outlet glacier. And this combination of observations was critical to understanding how glacial build-up occurred in Yellowstone and how it changed through time. TEXT: Where did the ice flow? DR. PIERCE: When ice built up at the full extent of glaciation, the oldest part of the glacial sequence we have around the whole periphery of the Greater Yellowstone Glacial System is at the mouth of the Clark Fork River, at edge of the Bighorn Basin, where the glacial deposits are about 21,000 years old. At that time, the ice was primarily coming probably from the Bear Tooths. Then the ice advanced onto the Yellowstone Plateau and was building up on the Yellowstone Plateau and going over the top of the Washburn Range and way down the Yellowstone Valley to deposit the 8-Mile and Chico moraines, and that was around 19,000 years ago. And then ice continued to build up on the Yellowstone Plateau, building to the southwest, toward the source of moisture. The culmination of the glaciation that went down into Jackson Hole was about 17,000 years ago. One other thing that happened, very much like this same thing, is ice from the Bear Tooths advanced up the Yellowstone River to the Canyon Village area and deposited thick glacial moraines in that area that have granitic erratics from the Bear Tooth mountains in this thick glacial till that is kind of amorphous but very thick. And at this time, it also dammed the lake in Hayden Valley. And this is one of the main reasons that Hayden Valley exists as a sagebrush grassland, with its charismatic megafauna, particularly the bison, that area is on lake sediments in Hayden Valley. Whereas all the area around there is rhyolite and lodgepole pine, which doesn't have nearly the interest as the Serengeti of Yellowstone in Hayden Valley. TEXT: How is the most recent glaciation (the Pinedale) different from the older Bull Lake glaciation? DR. PIERCE: The Greater Yellowstone Glacial System shows some major contrasts in the extent of glaciation between the last, or the Pinedale Glaciation and the next to last, or Bull Lake Glaciation. And this changes around the periphery of the Greater Yellowstone Glacial System. For example, in Jackson Hole, the Bull Lake glaciation was very large, it filled all of Jackson Hole with ice. Whereas the Pinedale only came half the way south into Jackson Hole. The same thing happens over on the southwest side, in the Ashton area, the Bull Lake is much bigger than the Pinedale. The same happens in the west Yellowstone area, on the west side of Yellowstone, the Bull Lake is much bigger than the Pinedale. But if you go north from Yellowstone, the Pinedale overrode the Bull Lake glaciation. Or if you go northeast and east, it seems also the same thing happens, the Pinedale was about the same size as the Bull Lake, and not nearly like it is in southern Jackson Hole. There obviously is a major difference in the extent of glaciation. And I think that the Yellowstone hot spot may be the reason. On the leading margin of the Yellowstone hot spot, the terrain is uplifting. From the next to last glaciation, it's lower and it rises during the last glaciation. So the Pinedale is big in that setting. Whereas on the trailing edge of the Yellowstone hot spot, the Bull Lake glaciation occurs and the landscape lowers at the time of the Pinedale glaciation. This uplift that is occurring on leading and subsiding on training edge, may explain this large difference between the Bull Lake and Pinedale on the leading edge in which the Pinedale is overridden the Bull Lake and on the trailing edge in which the Bull Lake is much bigger than the Pinedale. TEXT: How did obsidian contribute to your understanding of when the Bull Lake glaciation occurred? DR. PIERCE: Another high moment occurred partly in the field in collecting obsidian samples in what are called the Bull Lake glacial moraines around west Yellowstone, Montana. I collected these samples and then worked with a colleague Irving Friedman to measure the hydration on the obsidian samples that were related to the glacial abrasion of these obsidians by scraping of glaciers as they were depositing the moraines. I measured the hydration on the moraines and they all came out around 14 micrometers in thickness. This, in combination with some other observations, indicated that the Bull Lake glaciation in the West Yellowstone area was about 140,000 years old. And this was a great departure from what we had thought the Bull Lake was. We, and particularly my boss Gerry Richmond, thought it was around 60,000 years old, and it really was more than twice that old, near 140,000 years old. That indicated that the Bull Lake glaciation correlated with the Illinoian glaciations of the mid-continent with the Riss glaciation of Europe and with Marine Isotope Stage VI which is a major cold interval that shows up by analysis of deep sea cores. TEXT: And your work on the Bull Lake glaciation led to the Kirk Bryan Award? DR. PIERCE: In the study of glaciation in Yellowstone, I mapped the northern third and the area outside of Yellowstone. My supervisor Gerry Richmond mapped the southern part of Yellowstone, particularly the area with the volcanic rocks. In the process of doing the mapping and the glacial studies, we ended up with two very different understandings of the glaciation. One was that the Bull Lake glaciation was 150,000 years old. The other was that glaciers built up in Yellowstone and went over the top of Mount Washburn. Both of these differences we couldn't resolve by studies in the field. And I kept wondering, is there some way I can compare the glacial reconstructions that will help resolve this difference? I learned there is a branch of science called glaciology, which is very different from glacial geology. Glaciology is the physics of glaciers, it has to do with like things like basal shear stress. So I learned about basal shear stress and quite a few features of glaciology particularly from John Andrews and Mark Myers (with the U.S. Geological Survey in Tacoma). I was able to use basal shear stress to indicate that my reconstruction made sense and that there were problems with Gerry's reconstruction. I really have to thank Gerry Richmond for making it necessary for me to learn a whole new field of science. And by using this field, to apply it to my study of northern Yellowstone. After I published this paper in 1979, I received, in 1982, the Kirk Bryan Award, which is the highest award in my field of geology, particularly for applying this glaciology concept to the reconstruction in Yellowstone. So Gerry Richmond really did me quite a favor by challenging me and making me prove my point. TEXT: How do glacial deposits of the past affect the geothermal system today? DR. PIERCE: Quite a few of the thermal areas in Yellowstone are also in areas where quite a thickness of glacial deposits filled the lowland areas. The sands and gravels are probably pretty important to forming the complex plumbing system of geysers like Old Faithful and Steamboat Geyser. The porosity and the open parts of the sands and gravels sort of permit the thermal waters to heat up and get stored, and then triggering causes one of the big eruption of geysers. The thermal areas they sometimes occur outside the area of glacial deposits but quite a few of the most important ones are in areas where there are a lot of glacial sands and gravels and other glacial deposits, including lake sediments and glacial till. TEXT: Do you have favorite stories from your times in Yellowstone? DR. PIERCE: The times I've spent in Yellowstone have been wonderful. There are some times that were a particular strain. And this is kind of an odd situation. I was able to drive a government vehicle up to the top of Mount Washburn, I hiked several thousand feet down from where I parked my vehicle, mapping all the time these beautiful glacial striations that indicate a very high ice cap in Yellowstone flowing north across the Washburn Range. And then I noticed a thunderstorm was coming up - a big one. And so I started hiking up as fast as I could. The storm was coming very fast. I was hiking up and my lungs were bursting and I kept hiking up. I knew I was in a dangerous position, on a ridge where lightning was likely to strike. I was totally exhausted and completely out of breath by the time I reached my vehicle, got inside, shut the door, and then I was safe from lightning. But it was popping all around. DR. PIERCE: Another time, we were going along the crest of the Washburn Range. And that area is at timberline or above Timberline. And there are narrow bands of small trees. I walked through one of these bands of small trees and there on the other side was a grizzly bear - standing full upright, looking at me. I thought he was 10 feet tall. I knew I was in a bad position. With me, was a person on horseback (I wasn't on horseback). The highest thing in the landscape was the back of this horse. And so I got up on the back of the horse with him and this horse was not afraid at all of the grizzly bear, it kind of challenged the bear. The bear looked us over and went off. It was a teenage bear, and I don't think it was acting particularly aggressive but it was a lot bigger than I was. DR. PIERCE: My favorite part of the area is hiking up onto a bedrock knoll several hundred feet up, a kind of a sheep's back in the glacial geologic terrane, and I very much enjoy hiking up there particularly with our dogs and getting on the crest of this thing and looking around the 360 degree compass at a lot of very interesting and geologically diverse terrain. TEXT: Special thanks to Students of Montana State University Linda Pierce Jake Lowenstern Images by Dan and Lin Dzurisin Video by Liz Westby Guest appearance by Jack TEXT: Interview with Ken Pierce Produced by Liz Westby 2016 TEXT: For more information on Yellowstone visit the Yellowstone Volcano Observatory volcanoes.usgs.gov/observatories/yvo/ USGS