Wednesday, October 8, 2025

Drunk on Volcanology - Old Faithful Ale


The next Drunk on Geology is for the Old Faithful Ale from Grand Teton Brewing out of Victor, ID. 

To fully describe the geology of Old Faithful, and why it falls into the "Drunk on Volcanology" group, there is a bit of background geology that is needed. Old Faithful is located towards the center of Yellowstone National Park. Yellowstone itself is the volcano of which Old Faithful not only sits on, but is powered by. While I am going to give a summary overview of the geology of Yellowstone here, you can find I had done a much more in-depth look at the Geology of Yellowstone National Park here.   

Yellowstone's magma plume below the surface of the Earth. Image courtesy of National Geographic.

As you can see in the image above, Yellowstone National Park is a volcano with a rather large magma chamber located below it. This magma chamber also extends significantly across the surrounding areas as well. The Yellowstone volcano is a type of volcano known as a hotspot. A hotspot is a volcano that starts off as a plume of magma that emanates from deep within the Earth, in the mantle. This plume of magma then rises through the crust and heats up the rocks on the surface. What makes a hotspot truly unique is that the plume of magma is fairly stationary as the crustal plates then move over it, creating a string of volcanoes. The Hawaiian Islands are a good example of this hotspot string of volcanoes. 

Movement of the North American plate across the Yellowstone Hotspot. Image courtesy of NPS.gov.

You can see this movement of the Yellowstone Hotspot by a trail of eruptions that move across the northwestern United States in the map above, specifically creating the topographic feature known as the Snake River Plain. And if you didn't already figure it out, the currently location of the Yellowstone Hotspot, is ... Yellowstone National Park. 



Within Yellowstone National Park there are many geological features that are tied to the Yellowstone Hotspot volcano. We are going to focus on Old Faithful here. Named in 1870, Old Faithful is what is known as a geyser. By definition, a geyser is:
A type of hot spring that intermittently erupts jets of hot water and steam, the result of ground water coming into contact with rock hot enough to create steam under conditions preventing circulation. 
Dictionary of Geological Terms 3rd Ed.   
Old Faithful erupting. View is facing south towards the Old Faithful Lodge.  

Below the surface of a geyser there are a series of cracks and fractures in the ground. These are typically referred to as the "plumbing" of the geyser. Geysers work when rain and snow percolate into the ground, creating ground water. This groundwater is heated up by the presence of a heat source, the Yellowstone magma chamber in this instance. This heated water then rises through these cracks and fissures in the ground. As the hydrothermal waters heat up and rise, they slowly dissolves the surrounding silica within the rhyolite rocks. 

Plumbing beneath Old Faithful. Image courtesy of Smithsonian Magazine.

For Old Faithful, the majority of the cracks and fractures that make up its plumbing lie within glacial sands and gravels, not within historic lava flows that cover much of the surrounding country side. The dissolved silica within the super heated waters starts to precipitate out of the hydrothermal fluids, stabilizing, and slowly constricting the cracks and fissures that make up the network. As the water is heated up, it also expands. However, since the cracks keep the heated water contained, the water is not allowed to expand, resulting in water that has become "super heated" (a phenomenon where water can surpass the boiling point but remain as water and not turn into steam). 

Eruption of Old Faithful. View is facing north, away from the Old Faithful Lodge.

As the water moves upwards through the plumbing network, eventually the water reaches near the surface where there is no more overriding pressure from the surrounding rocks and the water is allowed to expand. Since it is super heated, the expansion immediately causes the water to turn to steam. It is this sudden expansion and steam production that produces the semi-regular geyser eruptions. The regularity of the eruptions is due to the complexity of the fracture network, the ground water inflow, and how many external vents there are. The more vents connected to a system the less regular the system is likely to be. Since Old Faithful's plumbing network is not connected to any other geysers, this isolation is likely what leads to the regularity of eruptions. 

Old Faithful eruption. View from the Visitor's Center.

Within the Old Faithful system, the cracks and fissure plumbing network expands over 650 feet and holds more than 79 million gallons of water leading to ~8,000 gallons of water released per eruption shooting over 100 feet in the air. Although known for the regularity of the eruptions, the interval between eruptions is actually fairly variable, with eruptions occurring every 60 to 110 minutes. This variability is due to several factors including earthquakes altering the geyser "plumbing", seasonality of water supply, and continuous changes to the cracks and fissures due to mineral precipitation and collapse. 

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