Ken Pierce U.S. Geological Survey
Heavy Breathing of the Yellowstone Caldera TEXT: Heavy Breathing of the Yellowstone Caldera 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. Quaternary geology you might think of it as
the time of the ice ages, it’s the last few million years, it’s the very tag end of geologic
time, but it’s also the most important part of geologic time because it affects how we
live on the Earth and the landscapes around us.
The other part of my field of focus is geomorphology, which is the study of the form of the land
and that has a lot to do with what geologic processes are acting on the land, why the
landscape looks the way it does, what were things that happened in the past, or in the
present or in the possible future that relate to the form of the land. TEXT: How would you describe the Yellowstone
caldera? DR. PIERCE: The center of Yellowstone is a
large caldera and calderas around the world are noted for being restless, various kinds
of activity going on including uplift, subsidence, volcanic eruptions, all kinds of things happen.
And Yellowstone is no different, it’s a restless caldera. GRAPHIC TEXT: Hot Spot Geoid and Swell
Map of major geoid anomaly in the western United States.
Warmer colors indicate higher altitude anomalies, culminating in red at Yellowstone. Volcanic
fields (circles) along the hotspot track are shown with their starting ages. The dashed
white circle shows the inferred position of the present hotspot based on a rate of plate
movement of 25 km/m.y. The dotted line shows the inferred margin of the Yellowstone hotspot
swell. TEXT: Why do you say the Yellowstone caldera
is restless? DR. PIERCE: Yellowstone has been deflated
and inflated in the last part of Holocene time or what we might call post-glacial time.
And one of the things that I’ve been working on is documenting changes in the Yellowstone
caldera through time. Part of the geology of Yellowstone is set up really nice for this.
There is Yellowstone Lake and a channel of the Yellowstone River that goes across what
is the threshold in the center of the caldera. If the caldera inflates, this makes Yellowstone
Lake get higher, if the caldera deflates, this makes Yellowstone Lake get lower. And
one of the connections between Yellowstone Lake and this threshold area is a channel
of the Yellowstone River. At present, it’s acting like a large pool, as an extension
of Yellowstone Lake. But in the past, when the caldera subsided, this area has acted
as a channel of the active Yellowstone River and it actually was so vigorous it was undercutting
its banks, it’s acting like a vigorous, normal river.
If you go into an area just beyond the threshold of Yellowstone Lake in this pool section of
the Yellowstone River and auger down, you go through slack water lake-type deposits,
sand deposits and then you go into a gravel, indicating the river was very active at that
time. If we use these same cores to date the time that the Yellowstone River was acting
like a river instead of a pool, we find that this was about 3,000 years ago. So 3,000 years
ago, Yellowstone was deflated, and now it has inflated. And this process of deflation,
inflation I call heavy breathing of the Yellowstone caldera because it’s a big process. The actual
volume of material involved is quite large. TEXT IN GRAPHIC TEXT: Elevation of Yellowstone
shorelines (S) near Fishing Bridge from 14,000 years ago to present. Transgression indicates
a rise in lake levels, resulting in slack water in the outlet reach. As lake levels
subside, the Yellowstone River re-establishes its channel. The historic pattern shows inflation
and deflation. Ages are in thousands of years (ka) from radiocarbon dating of projectile
points (indicating human occupation on the shoreline) and other geologic relationships. TEXT IN GRAHIC: LiDAR (Light Detection and
Ranging) showing the outlet reach of the Yellowstone River from Fishing Bridge to Le Hardys Rapids.
Although the river is now essentially a continuous, low-velocity pool in the outlet reach, steep,
high cutbanks on the outsides of the meanders indicate that the older more sinuous channel
contained an energetic river. The dotted lines are old shorelines. TEXT: Is there a pattern to the inflation
and deflation? DR. PIERCE: One of the people who surveyed
Yellowstone is Dan Dzurisin. And he has shown that both the caldera when it inflates, it
swells up, and when it deflates, it’s a mirror image of the inflation. So that we know that
in the short term, the pattern has been very, very symmetrical – inflation and deflation,
which again, suggests some symmetrical process. TEXT: What do you think causes the caldera
to inflate and deflate? DR. PIERCE: Whether this is actually the intrusion
of magma that somehow is subsiding after that or some process related to geothermal features
such as inflation above a geothermal seal – a breaking of the geothermal seal and subsidence
as the hot fluids escape, is part of process. TEXT: Could it be magma? DR. PIERCE: If an intrusion of a magma is
thought to be the process, it inflates. Then how do you get the deflation? You can’t stuff
the magma back down the hole it came up. It’s going to stay where it is. It’s more likely
that if magmas are coming in, it keeps uplifting, maybe pausing with uplift.
One other thing that we can tell from Yellowstone, is that instead of inflating and then inflating
and then inflating, we can tell that this didn’t happen because shorelines of Yellowstone
Lake are very close to horizontal. So whatever the process of inflation and deflation is,
it’s not resulting in a net uplift of the center of the Yellowstone caldera. The most
likely cause for that kind of thing is geothermal fluids which can come into the system, escape
out the sides of system, come into the system again and escape out to the side. TEXT: The research gives us a better understanding
of Yellowstone. When did you join the USGS? DR. PIERCE: I started with the U.S. Geological
Survey in Kentucky where I was mapping the Ohio River deposits for the U.S. Geological
Survey’s effort to map the entire State of Kentucky in detail. After I had been there
for about two years, I got a phone call saying how would you like to come up to Denver and
work on the geology of Yellowstone? And we’ll give you a week to think this over. And I
said, I don’t need a week to think it over, I’m really ready to go. TEXT: How did you get involved with Montana
State University? DR. PIERCE: When I was in graduate school,
I thought that I would go into teaching because that’s the main exposure that I had to other
geologists – the professors. I started with the Geological Survey thinking, indeed, I’m
going to get some research done and then I’ll be better qualified for a teaching position.
Well, it never worked out that I went into teaching.
I moved up to Bozeman, Montana, with the U.S. Geological Survey in 2000. And since I’ve
been here, I’ve been affiliate faculty with the Department of Earth Sciences at Montana
State. And I really enjoyed being able to work with graduate students, to give lectures,
to lead field trips at Montana State. It’s very rewarding for me to be able to interact
with students. Many have graduated and gone into good careers in geology. And it is very
rewarding to be part of that story. DR. PIERCE: We’re actually standing on a small
alluvial fan. The stream came out of the mountainside there and when it got to this flat floor valley
it built a small alluvial fan. Alluvial fans are one of the easiest things to recognize
in the landscape because they are conical shape; they occur where a steep stream course
gets to a flatter place. And they have this conical fan shape. My daughter is also a geologist.
She’s at Boise State, and she and her students were studying alluvial fans and they formed
an alluvial fan club. TEXT:
Special thanks to Students of Montana State University
Linda Pierce Jake Lowenstern Images of Yellowstone and Tetons by Dan and
Lin Dzurisin Video by Liz Westby TEXT:
Interview with Ken Pierce Produced by Liz Westby
For more information on Yellowstone visit the
Yellowstone Volcano Observatory volcanoes.usgs.gov/observatories/yvo/ USGS 5