Greater Yellowstone Area Eocene to Recent Hydrothermal Springs

The Gravelly Range spring deposits depicted in this photo are late Eocene (probably 34-36 million years in age).

Geologic field work is always fun, but especially so when it turns up something unexpected. Working on Eocene to Recent geology and vertebrate paleontology in the Gravelly Range, southwestern Montana promised to be enthralling because the volcanics, sedimentary units, and vertebrate fossils are at elevations of about 9,000 feet. But to come across extensive, unmapped calcareous spring deposits of probable Eocene age is topping off research efforts.

At this point, I’ll just say that our field team is still at work on the Tertiary spring deposits. We’ve found numerous leaf impressions including those of ginkgo, palm, metasequoia, Fagopsis (extinct member of Beech family), and alder – just to name a few. We’ve shown the plant assemblage collected to date to several paleobotanists, and, at least for age, their take is that the assemblage is probably latest Eocene in age, and bears many similarities to Florissant, Colorado fossil plant assemblages.

Palm frond impression from Gravelly Range spring deposit.
Ginkgo leaf impression from a Gravelly Range spring deposit.
Alnus cone from a Gravelly Range Spring deposit.

The spring deposits in the Gravelly Range are extensive, covering an area roughly 2 miles in length with deposits up to 120 feet in thickness. The springs are best characterized as travertine, although the spring systems’ edges contain clastic fluvial units and both the springs’ edges and pools have features such as plant impressions, root systems, and small travertine balls.

Gravelly Range Eocene spring deposit. Field backpacks in lower left corner for scale.

Because the Gravelly Range is so close to Yellowstone National Park, it is extremely interesting to compare its Eocene spring deposits to hydrothermal units at both the currently active Mammoth Hot Springs (which probably began its activity about 7,700 years ago), and to the fossil travertine found just north of Gardiner, Montana, that formed about 19.500 to 38,700 years ago (Fouke and Murphy, 2016: The Art of Yellowstone Science: Mammoth Hot Springs as a Window on the Universe).

The Gardiner travertine is fairly well exposed because it has been extensively quarried for several decades. Of interest for comparison are numerous plant impressions that occur within microterracettes. Fouke and Murphy (2016) suggest that these may be impressions of sage brush. A photo of the quarried wall with the plant impressions is shown below.

Plant impressions in Gardiner travertine. These impressions may be from sage brush. The travertine in this quarry face is estimated at about 30,000 years in age.

Other features in the Gardiner travertine, now partly covered by graffiti, include a quarry wall that shows terracettes and microterracettes that are outlined by darker lines within the travertine. These features are probably indicative of a proximal slope facies.

Gardiner travertine with its slope facies depicted well in smooth quarry face. The dark, irregular lines delineate terracettes and microterracettes.

Jumping forward in time to the extensive spring deposits of Mammoth Hot Springs (just within the northeast park boundary of Yellowstone National Park), is mind boggling. As in any comparison with rocks as old as Eocene to active deposition, one realizes how much detail is lost over time. But it is still worthwhile to try to compare spring features, so I’ll show a few photos of the Mammoth Hot Springs that may match up with various features of the fossil springs.

Branch and plant fragments in the process of becoming calcified at Mammoth Hot Springs – main terrace.
Calcified plant debris – Mammoth main terrace.
Terracettes – Mammoth main terrace, proximal slope facies.
Trees engulfed by prograding spring activity – Mammoth main terrace.
Travertine balls in small pond – Mammoth main terrace.

Suffice it to say, that the upcoming field season should be a good one, with more work to be done on the Gravelly Range spring deposits. And – it’s always fun to get a trip in to Yellowstone!

Yellowstone’s Firehole Lake Drive Reopens

Last Thursday (July 10),Yellowstone National Park (YNP) temporarily closed the 3.3 mile-long Firehole Lake Drive, a paved road that traverses some of Lower Geyser Basin. Melting asphalt on a part of the road near the start of the loop drive became a “soupy mess”, according to Dan Hottle, YNP spokesman. Hottle told Live Science that Firehole Lake Drive’s surface reached 160° Fahrenheit (70° Celsius) on Thursday, roughly 30° to 40° F (17° to 22° C) hotter than usual. Hot gases from area thermal activity that were trapped by the asphalt road surface and warm weather combined to cause the road damage.

YNP said that the road would reopen soon and sure enough, by the time I was there on Monday (July 14), the road was driveable. One of the YNP information rangers at Canyon Village told me that the road repairs included road crews removing damaged pavement and applying a mixture of sand and lime to soak up some of the thick bubbly road oil.  The road section was then graveled so that the hot gases could better escape a more permeable road surface.

I drove over a part of the Fire Hole Lake Drive that was repaired due to melted asphalt last Sunday, soon after the road was reopened.  The damaged road section is now graveled. Note the absence of steam rising from the road surface - even though it was cool and rainy that day.
I drove Firehole Lake Drive loop last Monday, shortly after it was reopened, and stopped to photograph some of the damaged road. The section of the road that contained the melting asphalt is now graveled, and judging by the absence of steam rising off the road (the day was cool and rainy, so I expected to see some steam billowing above the road surface), it looks like the YNP road fix is working.

Thermal activity affecting YNP roads and parking areas is not uncommon. During my Monday travels in Yellowstone, another Canyon area YNP ranger told me that about 10 years ago, a new thermal feature melted a small part of the Mud Volcano parking lot. This area is now fenced off, but the rest of the parking lot is still used. YNP spokesman Hottle also informed Live Science that YNP has closed Firehole Lake Drive in the past for repairs due to heat damage, but that these closures are not frequent.

A small part of the parking lot at Mud Volcano fell victim to thermal activity several years ago.
A small part of the parking lot at Mud Volcano fell victim to thermal activity several years ago.

And – just for some perspective on this latest road meltdown: the YNP website home page says “Yellowstone contains approximately one-half of the world’s hydrothermal features. There are over 10,000 hydrothermal features, including over 300 geysers, in the park”. Given the profusion of thermal activity, I’m not surprised that a small section of asphalt melts once in a while. I guess I’m amazed that the YNP can keep park infrastructure maintained such that millions of people can visit the park every year.

Glacial Geology Field Tripping in the Northern Yellowstone Area

Living near Yellowstone National Park has its advantages – and the best of these is being easily able to go on field trips to the Park area. A field trip opportunity came up last week when the Rocky Mountain section of the Geological Society of America came to Bozeman, Montana, for its annual meeting. One of the meeting field trips was the “Glacial and Quaternary geology of the northern Yellowstone area, Montana and Wyoming”. The trip was led by Ken Pierce, Joe Licciardi, Teresa Krause, and Cathy Whitlock. Having spent much time in the Yellowstone area, I was ecstatic about going along to find out about recent geological work. I won’t elaborate on the specifics of the trip, but for those interested in more than the photos posted below, the field trip guide is available in The Geological Society of America Field Guide 37, 2014, p. 189-203. It’s worth a read!

A few of the stops on the trip:

Paradise Valley – Chico Moraines and Chico Outwash (45.3402 N, 110.6967 W)

Chico moraine boulders have an average cosmogenic age of 16.1 +- 1.7 10BE ka.


A succession of outwash terraces border the melt-water channel which is now the Chico Hot Springs road.

North Gardiner Area – Giant Ripples (45.0551 N, 110.7659 W)

Giant ripples occur on a mid-channel bar a few miles north of Gardiner, Montana.
Cosmogenic ages on the flood deposit boulders of the giant ripples average 13.4 +- 1.2 10Be ka.

Northern Yellowstone NP – Blacktail Deer Plateau (44.9577 N, 110.5652 W)

The Blacktail Plateau is capped by moraines of Deckard Flats age - 14.2 +- 10Be ka.
The Blacktail Plateau is capped by moraines of Deckard Flats age – 14.2 +- 10Be ka.

Northern Yellowstone NP – Phantom Lake Ice-Marginal Channel (44.9554 N, 110.5289 W)

The ice-marginal channel that Phantom Lake lies in was cut into volcanic bedrock during the Pinedale glacial recession. The lake is dammed on its down-stream end by a post-glacial age alluvial fan.
The ice-marginal channel that Phantom Lake lies in was cut into volcanic bedrock during the Pinedale glacial recession. The lake is dammed on its down-stream end by a post-glacial age alluvial fan.

Northern Yellowstone NP – Junction Butte Moraines (44.9128 N, 110.3854 W)

The Junction Butte moraines have an age date of 15.2 +-1.3 10Be ka. Large  boulders of Precambrian crystalline rocks and several ponds typify the morainal surface.
The Junction Butte moraines have an age date of 15.2 +-1.3 10Be ka. Large boulders of Precambrian crystalline rocks and several ponds typify the morainal surface.

Yellowstone and Super-Eruptions

Comparison of eruption sizes  using the volume of magma erupted from several volcanoes (From USGS "Questions about Supervolcanoes":
Comparison of eruption sizes using the volume of magma erupted from several volcanoes (From USGS “Questions about Supervolcanoes”: ).

I give much thought to supervolcanoes – mainly because I live next to Yellowstone National Park and consequently spend much time in the Park. So when I saw today’s Nature publications about the cause of super-eruptions, naturally I wanted to read them.

I’ll first start with a definition for a supervolcano, and for that I’ll use one given by the U.S. Geological Survey:

The term “supervolcano” implies a volcanic center that has had an eruption of magnitude 8 on the Volcano Explosivity Index (VEI), meaning the measured deposits for that eruption is greater than 1,000 cubic kilometers (240 cubic miles). The VEI scale was created as a general measurement of the explosivity of an eruption. There are multiple characteristics used to give an eruption its VEI allowing for the classification of current and historic eruptions. The most common criteria are volume of ejecta (ash, pumice, lava) and column height. All VEI 8 eruptions occurred tens of thousands to millions of years ago making the volume of ejecta or deposits the best method for classification. An eruption is classified as a VEI 8 if the measured volume of deposits is greater than 1,000 cubic kilometers (240 cubic miles). Therefore a supervolcano is a volcano that at one point in time erupted more than 1,000 cubic kilometers of deposits.

Now to today’s online Nature publications for the cause of the eruption. There are two publications and each research team uses a different technique which results in finding two distinct causes for eruptions.

In the “Frequency and magnitude of volcanic eruptions controlled by magma injection and buoyancy, Lucca and others use thermomechanical numerical modeling of magma injection into Earth’s crust and Monte Carlo simulations to observe:

We find that the rate of magma supply to the upper crust controls the volume of a single eruption. The time interval between magma injections into the subvolcanic reservoir, at a constant magma-supply rate, determines the duration of the magmatic activity that precedes eruptions.

Malfait and others, in their “Supervolcano eruptions driven by melt buoyancy in large silicic magma chambers publication, state:

Here we use synchrotron measurements of X-ray absorption to determine the density of silica-rich magmas at pressures and temperatures of up to 3.6 GPa and 1,950 K, respectively. We combine our results with existing measurements of silica-rich magma density at ambient pressures, to show that magma buoyancy can generate an overpressure on the roof of a large supervolcano magma chamber that exceeds the critical overpressure of 10–40 MPa required to induce dyke propagation, even when the magma is undersaturated in volatiles. We conclude that magma buoyancy alone is a viable mechanism to trigger a super-eruption, although magma recharge and mush rejuvenation, volatile saturation, or tectonic stress may have been important during specific eruptions.

As I said earlier, my proximity to Yellowstone has certainly made me take note of research relating to supervolcanoes. So I’m always glad to find ongoing work on them as well as their triggering mechanisms. Hopefully, better overall understanding of supervolcanoes will expand our capability to predict their super-eruptions.