| Yellowstone: | What Lies Beneath? |
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that could send mankind the way of the dinosaur. Jackson geologist Dave Adams examines this theory.
Image # STS058-085-091 courtesy Earth Science and Image Analysis Laboratory, NASA Johnson Space Center
The entire Greater Yellowstone Ecosystem is visible in this October 1993 Space Shuttle photo. Yellowstone and Jackson lakes are easily recognizable, as are Jackson Hole and the Tetons. Palisades Reservoir on the Snake River appears at the bottom.
Yellowstone is impressive.
Not just as the park exists today, but as it must have looked 2 million years ago, when massive explosive eruptions were sending thousands of cubic kilometers of molten rock skyward and frying all life forms in the adjacent countryside. Earth scientists continually tell us that geologic change tends to be imperceptibly slow. Yet here is Yellowstone, looking anything but gradual.
Each eruption involved the emplacement of large amounts of rhyolitic magma in the upper crust creating a dome hundreds of meters high and tens of kilometers across. Rhyolitic magma (essentially molten granite) is very sticky and does not easily flow. As such, pressure in the system continued to build until fractures developed around the dome’s perimeter, and the contents of the magma chamber were explosively ejected, essentially obliterating all the landscape within a hundred kilometers. Because the lava was so viscous and contained so much gas under pressure, the eruption was particularly ill mannered.
Unlike the relatively peaceful lava flows and fountains seen today on Hawaii, the result was a billowing, frothing mass of white-hot bubbles, which almost immediately shattered and produced clouds of red-hot ash. This ash was lifted high into the atmosphere and carried by prevailing winds, delivering a thick blanket of abrasive gray dust to areas as far away as eastern Kansas. Near Yellowstone, ash that settled while still hot welded into a layer of rock many meters thick. As the magma was removed, the dome collapsed to form a caldera, which over time was partially filled by basalt flows. It would have been enough to ruin anyone’s vacation.
The caldera from the most recent Yellowstone eruption is about 50 kilometers across and is partially occupied by Yellowstone Lake. Geologists have identified a chain of progressively older calderas trending to the southwest along the Snake River Plain, ending with the McDermitt volcanic field’s creation in southeastern Oregon approximately 16 million years ago. Current theory explains this trend as the result of the North American tectonic plate’s gradual passage over a deep-seated, stationary, hot mantle plume, suggesting that in the future the chain of calderas should continue into Montana and beyond. However, geophysicists have yet to “see” such a plume, and new models are being proposed where the caldera chain may have an end as well as a beginning.
The last three Yellowstone eruptions occurred at 2.1, 1.2 and 0.6 million years ago, prompting some to suggest that we are “due” for another eruption any day now. Old Faithful may be predictable, but there is no evidence that Yellowstone has any regular eruption schedule. More to the point, there is really very little that is sudden about a caldera-forming eruption. Judging from evidence of increasingly sporadic volcanism during the 600,000 years since the last eruption, we can draw at least two conclusions. First, the system is probably still cooling down, not heating up. And, second, when and if it does heat up again, we should expect to see many thousands of years of precursory minor eruptions and some major-league dome building. The domes and craters recently discovered on the floor of Yellowstone Lake are not even minor-league in comparison, involving the effects of groundwater circulation systems perhaps only a few hundred meters in depth; while the magmatic system extends to depths perhaps exceeding 200 kilometers.
In short, a major Yellowstone eruption is somewhat like a brontosaurus. If it sits on you, it’s going to hurt. But the chances of it sneaking up on you are fairly slim. So sleep well. And pick up one of the following excellent books on the subject: Windows into the Earth: the geologic story of Yellowstone and Grand Teton National Parks, by Robert Smith and L.J. Siegel; Interpreting the Landscape: Recent and ongoing geology of Grand Teton and Yellowstone National Parks, by John Good and Ken Pierce; and, of course, the classic, Creation of the Teton Landscape, by David Love, John Reed, and Ken Pierce.
Longtime valley resident Dave Adams has been doing research on the local geology for about 15 years, honing his interest with a Master of Science in Geology from Montana State and the pursuit of a PhD in Geophysics from the Univ. of Oregon.
Copyright 2004 by FPI (Focus Productions, Inc)., P.O. Box 1930, Jackson, Wyoming 83001. All rights reserved. No part of this publication may be reproduced in any form without written permission from the publishers.
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