Springtime in the Caldera (Plants & Rocks)

There was a time when western Utah was Hell right here on Earth. That was roughly 18 to 40 million years ago, when catastrophic volcanism raged across the Great Basin at a scale beyond anything we can imagine (1). Hundreds of super volcanos loaded with viscous gas-filled magma erupted explosively, sending volcanic ash high into the atmosphere to be carried by wind hundreds or even thousands of miles before returning to Earth. When an eruption column collapsed, or the side of a volcano blew apart, incandescent clouds of magma, rock fragments and gas raced down the slopes and across the surrounding land, incinerating everything before coming to rest to form thick layers of welded rock called ash-flow tuff, or ignimbrite.

Today, a Great Basin geotripper with guidebook in hand is frequently led to the remains of those hellacious times. But there are no tidy circular craters with sparkling lakes. The super volcanos produced calderas—giant irregular cavities that formed when so much magma was expelled so quickly that the volcano collapsed (2). Calderas are so big—up to 30 miles across—that they’re hard to see, especially in the Great Basin where faulting has further jumbled their structure. But enough of their outpourings is visible to confirm their catastrophic impact.

Calderas of the Tertiary ignimbrite flareup; only the larger ones are shown. Modified from DeCourten 2003 (highly recommended).

Last month, I visited the Joe Lott tuff (3) in western Utah. This is ignimbrite—welded ash and rock fragments that raced out of a super volcano during it’s biggest eruption, the one that caused it to collapse to form the Mount Belknap caldera. In Fremont Indian State Park at Vantage Point 14, directly across from a convenient pullout (unless wet), a spectacular outcrop of Joe Lott tuff looms over the Clear Creek Road. This particular flow was thick, and cooled slowly enough that columnar joints formed. 

Joe Lott tuff, bottom cooling unit (flow): elongate pumice fragments indicate welding.

The columnar layer is just one of four flows making up the Joe Lott tuff. An estimated one hundred cubic miles of ash-flow material was ejected in all, “in a more or less continuous sequence as shown by the lack of cooling breaks [between layers]”. Not a good time to visit!

Three flows after the first and biggest one are visible here: the “Swiss cheese” and layers above and below.

But that was 19 million years ago. Now it’s a beautiful spring day, and we can enjoy not just volcanic drama but fresh leaves and flowers—a real treat after our long Laramie winter!

Wax Currant, Ribes cereum.

Currants, Oregon grape and water birches grew along the creek and trail above the Castle Rock Campground.

Oregon Grape or Creeping Mahonia, Berberis repens.

Above and below: Water birch, Betula occidentalis.

Skunkbush was widespread and in full bloom; the leaves will appear later.

Above and below: Skunkbush or Fragrant Sumac (Rhus aromatica, formerly R. trilobata).

Along the trail south of the campground, I had my first face-to-face encounter with the little flowers of Oregon Boxwood—pretty cute with their red petals and yellow stamens. Another tiny charmer is Blue-eyed Mary. My photos don’t do her justice.

Above and below: Oregon Boxwood, Paxistima myrsinites.

Above and below: Blue-eyed Mary, Collinsia parviflora.

All the Gambel Oaks (Quercus gambelii) I saw were still leafless … until I reached Vantage Point 14 with its massive south-facing wall of columnar tuff. Here the oaks were covered in fresh yellow-green leaves glowing in the sunshine.In a side draw to the west, a column had “recently” fallen, showing that the Joe Lott tuff is white when fresh. When I made my way into the nook for a closer view, I saw that someone else had passed this way—were they also interested in this white rock? If so, it must have already fallen 800 years ago.Notes
(1) This claim is based on the following, from DeCourten and Biggar 2017 (p. 34): “The scale and violence of the volcanic blasts that buried the ancient landscape under thousands of feet of ash are hard to imagine, primarily because no volcanic eruptions ever witnessed by humans come close to rivaling these prehistoric paroxysms.”(2) Modern day examples of calderas include Yellowstone, Long Valley in eastern California, and Valles Caldera in northern New Mexico.(3) Geologists assign names to distinctive rock units. The Joe Lott tuff was named after a nearby creek and old trail, which were named after pioneer Joe Lott who homesteaded and farmed on Clear Creek.Sources
Budding, K, et al. 1987. Petrology and chemistry of the Joe Lott Tuff Member of the Mount Belknap Volcanics, Marysvale volcanic field, west-central Utah. USGS Prof. Paper 1354.DeCourten, F. 2003. The Broken Land; adventures in Great Basin geology. Salt Lake City: University of Utah Press.
DeCourten, F., and Biggar, N. 2017. Roadside Geology of Nevada. Mountain Press.
Fremont Indian State Park Museum. Trail Guide, Vantage Point #14, Geology (available at Visitor Center; free, return after use).
Ornduff, RL, Wieder, RW and Futey, DG.2006.Geology underfoot in southern Utah. Mountain Press.