|View of Red Mountains across Riddle Lake, Yellowstone National Park|
As they say in the real estate biz – location, location, location.
It took me a while to figure most of this out. Early on in my illustrious Yellowstone rangering career I learned that not only the Red Mountains but also Mt. Washburn can be thought of as a set of “bookends” on the rim of the caldera, or collapsed crater. The Red Mountains are just beyond the southern rim, while the more well known (and more hikingly accessible) Mt. Washburn and the Washburn Range are just beyond the northern rim. During each eruption of 2.1 million years ago and 640,000 years ago (there was also an eruption around 1.3 million years ago outside the present park boundaries) several hundred cubic miles of magma or molten rock were violently ejected from a shallow subterranean magma chamber, after which the roof of the caldera collapsed in on itself. These explosions, lasting perhaps several hours at most, were powerful enough to blast away a substantial chunk of each mountain’s real estate. Clearly, the Red Mountains and Mt. Washburn just happened to be in the right place at the right time.
|View south across caldera from Mt. Washburn; Red Mountains on middle horizon with Tetons beyond|
Although Mt. Washburn also has an igneous history, it predates the Yellowstone eruptions by some 50 million years. What we see at Mt. Washburn is an assortment of leftover dikes and sills that once composed its original igneous internal plumbing.
Things That Make You Want To Go “Hmmmm…”
Eventually all this got me to wondering. What about the Red Mountains, the other bookend to the south? Where did they come from? Are they volcanic? Everything else around here seems to be. How old are they? I needed to find out so I could tell curious visitors more than they thought they ever wanted to know and watch as their eyes glazed over. I studied the geologic map for months and finally had a small eureka! moment. For answers, however, I needed to look beyond Yellowstone.
|The normal-faulted Teton Range|
|Heart Lake is near the base of Mt. Sheridan and the Red Mountains|
The Rocky Mountains are a product of compression of the Earth’s crust – squooshing, to use the more technical geologic term. The Tetons, on the other hand, are fault block mountains, tilted by normal faulting as a product of Basin and Range extension.
|Heart Lake patrol cabin, in the shadow of Mt. Sheridan and the Red Mountains|
So here we start to figure out the enigma that has squashed and stretched my brain for several seasons of rangering. The Yellowstone Plateau (the central part of the Park, where the volcano blew) straddles the continental divide and is part of the middle Rocky Mountains, a product of tectonic compression that usually involves thrust faulting. However, the Red Mountains are a tilted fault block range formed by tectonic extension and normal faulting.
|Geologic map courtesy of USGS. Yellow dashed line added.|
In the geologic map above, the yellow dashed line indicates the approximate boundary of the caldera with the Red Mountains a bookend on the south. There are normal fault segments all over the place (those little lines with balls on one side indicate the direction of downward movement of the fault). Notice the areas of brown. “Pzs” (dark brown) indicates Paleozoic sedimentary rocks. “TMzs” indicates younger Mesozoic and Paleocene sedimentary rocks. It would be nice if the geologic periods were further described but they are not. The shades of purple (Tyha, Tyhb, Tyhc) indicate that these older rocks were covered by the younger Huckleberry Ridge Tuff, the ash flow sheet deposited during the eruption 2.1 million years ago.
White Gulch hot springs and Heart Lake geyser basin stink, bubble and steam near the northern base of the Red Mountains. Like Washburn hot springs, these thermal features exist due to their position along the ring fracture or boundary of the collapsed caldera.
Sources (besides stuff that has piled up in my head over the years):
Christiansen, R.L., 2001, The Quaternary and Pliocene Yellowstone Plateau Volcanic Field of Wyoming, Idaho, and Montana, US Geological Survey Professional Paper 729-G
Lageson, D.R. and Spearing, D.R., 1988, Roadside Geology of Wyoming, revised 2nd ed.